FAQ: rec.audio.* Intro 7/96 (part 1 of 13)

[neid...@uicc.com]

Archive-name: AudioFAQ/part1
Last-modified: 1996/9/23
Version: 2.07

1.0 Contents:

Part 1 Intro
1.0 Contents
2.0 Organization
3.0 Purpose
4.0 Omissions
5.0 Credits
6.0 Errors and Corrections
7.0 Disclaimer
8.0 Copyright Notice
Part 2 Systems
9.0 High Fidelity Systems
Part 3 Sources
10.0 CD Players, CDs, Turntables, and LPs
Part 4 Amplifiers
11.0 Amplifiers
Part 5 Speakers
12.0 Speakers
Part 6 Rooms
13.0 Listening Rooms and Houses
Part 7 Recording
14.0 Recording
Part 8 Wire
15.0 Wire
Part 9 Retail
16.0 Retail
Part 10 Mail Order
17.0 Mail Order
Part 11 Networking
18.0 Net Protocol
Part 12 The Press
19.0 The Press
Part 13 Misc
20.0 Miscellaneous

2.0 Organization:

This FAQ is divided into a preamble and a list of subjects. Each
subject is described by a list of questions and answers. Some
questions have no answers yet. That's life.

This FAQ is split into 13 individual postings. This FAQ also references
a frequent posting of sources for audio via Mail Order. That list is
maintained by n...@SSESCO.com (William R. Nau). Contact Mr. Nau directly
for information or corrections to that posting. The list is also
available via FTP in the /pub/Rec.Audio directory of ssesco.com
as file Mailorder.txt

Lines beginning with "|" are new since the last version of the FAQ.

3.0 Purpose:

The purpose of this FAQ is to address frequently-asked questions as a
whole, so that rec.audio.* volume can be reduced. Towards this end, we
assembled a list of common questions, and some general answers to these
questions. Audio is part science and part art, so some of the answers
are objectively correct, while others try to open-mindedly present both
sides of a subject.

4.0 Omissions:

Many valuable things have been left out of the FAQ. In part this is
because there is just too much to say about audio. In part, this is
because the general reader doesn't need that much detail. Also, some
things were omitted because they are too controversial or inflammatory.

5.0 Credits:

This FAQ is the work of many people. Allow me to thank everyone who
helped now. Some of the contributors to this FAQ are listed below.
Others have made great contributions, and are no less appreciated.

and...@research.att.com (Andrew Hume)
j...@research.att.com (jj)
Ken Kantor of NHT
neid...@uicc.com (Bob Neidorff)
Rick Oakley (no internet access)
DPi...@world.std.com (Richard d Pierce)
P.S...@bristol.ac.uk (Paul Smee)
ga...@panix.com (Gabe M. Wiener)

6.0 Errors and Corrections:

If you have a correction to the FAQ, additional information, or a new
topic for the FAQ, please send e-mail to neid...@uicc.com describing
your thoughts in detail. Please include your e-mail address in your
submission, so that we can stay in touch. Every submission will be
considered for inclusion in the next release of the FAQ. This FAQ will
be reposted every month.

7.0 Disclaimer:

Everyone's human. Nothing is perfect. The people who wrote the
information here put varying amounts of research into their work.
To the best of my knowledge, no one made any contribution or comment
because of a vested interest.

Audio is a very lucrative and competitive industry, filled with honest
companies, aggressive marketing people, people who stretch the truth
very thin, excellent products, and lousy products. We tried hard to
screen the hype from valuable data. If we insulted, omitted, or
otherwise disturbed you, your company, your product, or something
you feel strongly about, please let me know.

The information here attempts to paraphrase a large portion of the
information exchanged in rec.audio newsgroup discussions. At times,
people will make recommendations or suggestions to others on some of
these newsgroups. Some of these statements are included here because
we have been asked to include statements of that sort. These
statements, and everything in this document are the opinions of various
people. Nothing here is intended as recommendation or suggestion.

Further, no matter how it is worded, nothing here should be taken as
fact. The authors take no responsibility for any use of this
information.

8.0 Copyright Notice:

The information contained here is collectively copyrighted by the
authors. The right to reproduce this is hereby given, provided it is
copied intact, with the text of sections 1 through 8, inclusive.
However, the authors explicitly prohibit selling this document, any
of its parts, or any document which contains parts of this document.

[neid...@uicc.com]

Archive-name: AudioFAQ/part2
Last-modified: 1996/9/23
Version: 2.07

9.0 High Fidelity Systems
People frequently use the term "Stereo" to refer to a sound
reproduction system. To be more accurate, we will use the term
High Fidelity System to refer to a pile of equipment including
at least one source, at least one amplifier, and at least one
speaker. Common sources are turntables, CD players, tape
players, tuners, and receivers.

9.1 What is a receiver?
A receiver is a tuner, power amplifier, and preamp combined. A
common receiver has inputs for a turntable, a CD player, a tape
deck, and perhaps one or two other sources. It has selector
switch(s), tone controls, and a volume control. A receiver may
have outputs for two speakers, or for more.

9.2 What is a tuner?
A tuner is a radio reception device which can not drive
speakers. Sometimes, the radio in a tuner is higher quality
than the radio in a receiver. A tuner may or may not receive
the AM broadcast band, but 99.999% will receive the FM broadcast
band. Some also receive short wave bands, frequencies used
for long-distance rather than for local commercial broadcasts.

9.3 How should I go about selecting a system?
If you're looking to buy something, the first step is to figure
out what you can spend. If you're looking for a whole system,
this gets tricky, because you have to allocate amounts for the
different components. The most popular current rule-of-thumb
for a single source system (speakers, amp, 1 something-player)
is to divide the money about equally among the three parts. If
you want several players, you'll have to decide whether they are
all equally important, and so deserve the same amount of money;
or whether some are less important, in which case you can spend
less on them and put the savings elsewhere.

This rule isn't hard-and-fast. It's just meant as a starting
point so you don't have to listen to every possible combination
of equipment. If you are building around a CD player, you might
spend a bit less on the player and a bit more on the speakers.
If you are buying turntable (or something else which plays by
physical contact) on the other hand, it might be good idea to
put a bit extra into the player. The reason for this is that if
you skimp on the turntable, then when you come to buy a better
one you may find that your records have been worn out by the
cheap player. If you skimp on the speakers, on the other hand,
then when you can afford better speakers the music will still be
there on your records.

Another perspective says that you should spend the most you
can on your source, as the sound can never be better than
what you get off of the record/CD.

See also 12.1, 12.2, and 10.1 for information on what to listen
to and what to listen for when evaluating speakers, turntables,
CD players, tape recorders, and systems in general.

9.4 How can I improve the sound of my stereo?
The cheapest improvement you can make, and perhaps the most
effective, is to position your speakers carefully and correctly.
See 13.1, below. This will improve the frequency response
flatness, making it easier to hear every instrument and voice.
Setting speaker position correctly can also improve the
three-dimensional recreation of a stereo image.

9.5 Do I want a combo system or separate components?
Combo systems used to be cheap jokes; that's not always true
now. Some sound very nice; there are even some made by
"audiophile" companies, and they sound even nicer. They've got
lots of advantages. They take up less space. The controls tend
to be well-integrated, especially if they are remote-controlled.
Therefore, they are easy to operate; this can be a major plus if
some of the people who'll use it are afraid of, or not very good
at, technology. Also easy to set up, and don't leave millions
of wires dangling all over everywhere.

If you do go for a combo, get a brand name; either an audiophile
company, or a good "consumer electronics" company. Brand-X
combos are generally overpriced and unpleasant. If possible,
buy it where you can listen to it first, such as a "real" hi-fi
shop. Mid-range hi-fi shops sell combos, as a way of
introducing beginners to quality sound.

In most good combos, the speakers are the weak link. If you do
go for a combo, you can almost always improve the sound
drastically by buying a set of better speakers. Better speakers
start in the $100-$200 price range. Some of the best combos
come without speakers, forcing you to do this. A good combo
with replacement speakers will give you very pleasant music.

Sounds good, you say, so why do people bother with components?
Well, you can get better sound with a component system -- but
usually at the expense of convenience and size. A good
component system will normally require a mixture of boxes from
different makers to get the best results, so you've got to spend
more time listening to things. However, if you listen to your
music seriously, then the performance of a component system is
the reward for that extra work.

Components are harder to set up and operate. However, as noted,
you can get better sound. You also get more flexibility. If,
for example, you decide you want a better CD player, you just
replace the CD player. With a combo system, you've got to
replace the whole system. If your component tape deck breaks,
you can remove it from the system and take it in for repair or
replacement. With a combo, the whole system has to go in for
repair or be replaced.

When you want to add some new recording medium to your system
(laserdisc, VCR, DAT, DCC, MD, ...), if you've got components
you just go buy the appropriate box. Many combo systems do not
have places (or many places) to attach extra bits, so again you
could be looking at replacing the whole thing. With a component
system, you can add a turntable; most modern combos can't cope
with turntables any more. Do you have a record collection?

If you're really not sure, components are the safer bet; if
you're going to make a mistake, that's probably the better way
to be wrong. But, if you're sure that a combo would be best
for your needs, it can be a totally reasonable choice.

Now, some people may be tempted by one-maker 'component sets',
particularly the modern, miniature ones. They tend to be
equivalent to combos. Most use non-standard connections, rather
than the normal twin phono plug, so that it's likely you can't
swap or add components anyway. Even where they use standard
interconnects, they may rely on non-standard interconnections
for control purposes. In a few cases, they also rely on sharing
power, with a power supply in only one of the boxes and the rest
taking low-voltage connections from that. And, no one maker
makes the best everything. By default, assume that they will
have the same disadvantages (and most of the same advantages) as
combos. If it's important for it to work with "standard"
components from other makers, be sure to ask before you buy.

One-maker 'component sets' are also often of lower quality than
true individual components. Component sets are designed for
convenience and appearance, rather than sound quality.

And, if you're in doubt, go for separate components.

9.6 How can I get better FM radio reception?
A. Use a (better) antenna. (See 9.7 and 9.8 below)
B. Use a (more) directional antenna. (See 9.7 and 9.8 below)
C. Aim your directional antenna. Rhombics are ungainly to move,
but Yagis and dipoles are small enough to point right at
the station. With the dipole, to tune in a station to
the East, run the antenna North-South. With a Yagi,
point the individual elements North-South with the
smallest element on the East end.

9.7 How good are these compact FM antennas?
For receiving, small is ugly. The bigger the antenna (all else
equal) the better. Of course, all else is never equal, but
these fancy, expensive mini antennas tend to be awful. Some
compensate for their small receiving structure with a small
antenna signal amplifier. However, the quality of that
amplifier is often no better than the quality of the amplifier
in your tuner or receiver, so the antenna just gives you a
stronger signal, complete with stronger noise.

All of that said, some compact FM antennas can work better than
a simple dipole in some situations. Some have an internal
amplifier, which helps with weak signals if the input stage in
your receiver is poor. Some are directional. Some aren't. If
possible, be sure that whatever you buy can be returned for a
refund if it doesn't work out well for you.

9.8 What makes the best FM radio antenna?
Although there is no "best" antenna for everyone, one of the
most directional is the "rhombic". Being very directional, this
antenna can select one weak station out of many strong ones, or
one group of stations originating from a general direction.
In addition, very directional antennas are good at reducing
multipath interference, a problem which is more severe in
cities with tall buildings.

This antenna is very long, and made up of four pieces of wire
with feedline at one end for antenna connections and a resistor
at the other for termination. Rhombics for FM broadcast band
use are at least 15 feet (4.5 meters) long, but can be made
fairly narrow, less than 3 feet (1 meter) wide. A more narrow
antenna will be more directional. A longer antenna will give a
stronger signal.

Another very directional antenna is the "yagi", which looks just
like a common TV antenna. You can even use a common TV antenna
as a very good FM antenna. The FM and TV bands are very close
together. It has the advantages of being cheap, directional,
and easy to rotate.

One of the simplest and easiest to make antennas is the folded
dipole, made from 300 ohm twin lead. It is approx. 58" long.
This antenna is surprisingly good for receiving signals in a
moderately strong signal area. Folded dipoles come with many
tuners and receivers as a standard accessory. They are also
available for approximately $2 at audio and department stores.

Whatever antenna you have, you can often get it to work better
for specific stations by moving it. In the case of the folded
dipole, sometimes it works better vertically, and other times it
works best horizontally. Sometimes, you can get that one
elusive station to come in perfectly if you bend the two ends of
it at funny angles. Don't be afraid to experiment. One
warning. As atmospheric conditions change, the best antenna
placement may also change.

An excellent reference book on antennas is printed by the
American Radio Relay League (ARRL). It is called The ARRL
Antenna Book. Currently in its 17th edition, it is a 736
page large, illustrated paperback which includes a disk
of MS-DOS software. It costs $30 plus s/h. It has fairly
complete antenna theory, practical information such as
charts, drawings, comparisons, and tips on construction
and adjustment. ISBN 0-87259-473-4. The ARRL is founded
and chartered as a non-profit organization to better
amateur radio, and antennas are a vital part of amateur radio.
American Radio Relay League
225 Main Street
Newington CT 06111 USA
203-666-1541

Also useful:
Practical Antenna Handbook by Joseph J. Carr
Tab Books #3270/McGraw Hill - ISBN 0-8306-3270-3

9.9 What about power line conditioners?
Each home and each outlet has slightly different power line
impedance and power line noise. Each amplifier is affected by
power line impedance and power line noise differently. Power
line conditioners try to reduce this line noise. Some also
change the power line impedance in a way which is supposed to be
better. We will leave it to your ears to decide if these
devices help the sound of your system enough to justify their
expense.

9.10 How can I reduce vibration sensitivity?
Some complain that heavy foot falls will cause skipping or more
subtle sonic problems with CD players or turntables. If you
have these problems, there are a few different things which you
can try to reduce the problem. One is to add weight to the rack
which holds the equipment. Heavier things move slower. If you
can get the motion slow enough, it won't cause sonic or tracking
problems.

Another solution is to add rubber or elastomer (Sorbothane)
cushions under the CD player or turntable. This might make it
better, but might also make it worse. Experiment.

A third solution is to increase the coupling between the rack
and the floor using spikes, which concentrate the weight on
a very small area. Another way to increase the coupling between
the rack and the floor is to use a plastic adhesive like HoldIt,
sold under the UHU trade name in office supply stores.

9.11 What equipment can I buy that is 100% made in the USA?
There are many lines of equipment that are carefully hand
crafted in the USA. Unfortunately, these systems are usually
the high-end ones. Some US companies also make gear in the
far east. When in doubt, ask. Some US audio manufacturers are:
Adcom (some made in Japan)
Audio by Van Alstine
Audio Research
California Audio Labs (CAL)
Carver (some made in Japan)
Jeff Rowland
Krell
Mark Levinson
McCormack
McIntosh
Parasound
Proceed
PS Audio
Spectral
Sumo (Power amps, preamps, CD transports, D/As)
Waida

9.12 Should I buy "xxx"? Which is better: "yyy" or "zzz"?
We can provide facts and opinions (and you get to decide which
is which :-), but we can't recommend if, or which way, you
should jump, because we don't know what your priorities are.
(That won't stop us from trying, though!) For example, if you
are considering a used item at a low price vs. a new one at a
higher price, one of us might say "go for the new one because
of the warranty", when another would say that you can fix it
yourself if it breaks. They're both right.

This also applies to speakers. One may have very good, flat
bass, but only go so low, where the other may go lower, but
have less flat frequency response. Which is better? Depends
on the buyer. Good speakers are carefully designed to
achieve a balance of performance that matches the priorities
of the designer. Some designers put much of their budget into
appearance. Some designers put their budget into very high
efficiency. Others strive for the smallest box which can
deliver an acceptable low frequency performance. Do you
really want people on the network making that decision for you?

9.13 What is Surround Sound? Pro Logic?
In an effort to make movie soundtracks more dramatic and
engaging, Dolby Labs created a signal encoding which encodes
more than just two channels of audio onto the stereo signal.
Many popular receivers and home-theater systems include the
required circuitry to decode these signals. These components
are referred to as Pro Logic, Dolby Pro Logic, or Surround
Sound components. Very few audio recordings contain this
encoding, but it is very common with movie soundtracks and
some network TV programs.

Best Surround Sound reproduction requires five separate
speaker systems, but some improvement is claimed from a
surround sound receiver and three speakers over two speakers.
In its best implementation, surround sound will give a fuller
sense of being in the middle of the action. The quality of the
image is a function of the recording, the broadcast quality,
and the choice of reproduction components.

For more information on surround sound, get:
The soundtrack comes after you...
which is available by anonymous FTP from ftp.csn.org as:
/Laserdisc/ld03

COPYRIGHT NOTICE

[neid...@uicc.com]

Archive-name: AudioFAQ/part3
Last-modified: 1996/9/23
Version: 2.07

10.0 CD Players, CDs, Turntables, and LPs

10.1 What should I listen for when evaluating a turntable or CD player?
For tape decks and turntables, beware first of speed variations
(wow and flutter). A good check for this is Richard Strauss'
"Also Sprach Zarathustra" (aka: The Theme From 2001), which has
a long, low, sustained organ note that comes in well before the
main theme starts, and is held through the first movement.
Concentrate on that. Make sure it doesn't wobble or warble.
There's also a good bit at the beginning of Pink Floyd's
"The Wall", but it doesn't go on as long, so you've got less
time to think about it. Tape decks are prone to losing
high-frequency notes, so pick something you like which has lots
of treble, and make sure it is clear.

The sound of a turntable is largely bound up in the kind of
cartridge mounted on it. Make sure to listen to a table with
a cartridge similar to what you're buying, and not one in a
different price bracket. If possible, audition the turntable
with the same arm and cartridge, so that you will experience
potential cartridge/arm interactions, too. Most cartridges
work better with one arm than another. Treat the
tonearm/cartridge pair as a system, rather than independent
parts.

For CD players, try some piano music. See if the high notes
sound tinny. Also, try something which has some soft parts,
not the same as turning the volume down. Distortion for CD
players (as for other devices) is measured at a high output,
but in fact in CD players (unlike others) it's likely to be
worse in soft passages of music. Most classical recordings
contain a suitable soft passage. Most rock music won't.

Distortion in CD players, if you want to call it that, is
a function of the granulation noise, or time-delay pre-echo that
can come out of the filtering. To listen for this, use material
that is rich in high-order harmonics, such as brass music.
Unfortunately, you can't reliably predict how a CD player
will sound by looking at specifications, features, or the
technology it uses. If you want to know how a player will
sound, you MUST listen to it.

10.2 Are some discs better than others?
Some recordings are better than others. Some artists are better
than others. Some recording engineers are better than others.
Some microphones are better than others. Some music is better
than others.

Ignoring that, there is some difference between discs. Some
of the very earliest discs were badly made and deteriorated
with time. The technical problems that caused those problems
have been solved.

Some "gold" discs are available which are advertised to have
better life and quality than common "aluminum" discs. These
sell for an extra US $15 or more per disc over the cost of the
same music on a common disc. Studies have shown that there is
an advantage to glass-encased, gold platters for archiving
computer data that is not error tolerant and will need to be
stored for many tens of years. I have yet to see a similar
comparison which justified any extra effort for storing audio
recordings for 50 years. Part of the reason for this is that
audio recordings contain error correction codes, allowing a
CD player to perfectly reconstruct minor flaws. Another reason
is that CD players can effectively reconstruct badly damaged
audio data, even if some data is completely missing.

Some discs seem to have pinholes in the aluminum, which are
visible when the disc is held up to a strong light. However,
these discs play fine and last very well, so the effect of these
pinholes is probably nil. Some have performed studies counting
errors on various discs with various players. They found that,
in general, the error count was consistent from one player to
another. Also, in general, most discs have a low, consistent
error rate which is perfectly correctable using the redundant
data stored on the disc. This study did find that one group of
discs had a higher error rate than all of the rest. This group
was the promotional discs, also called "music samplers" given
away by music companies to introduce you to their family of
artists and performers. Despite these higher error counts,
these discs still played fine.

If there is no abusive handling involved, I have rarely heard of
a disc that degraded with time. Of the few that have existed,
they tended to be from one of the bad batches mentioned earlier.

There is no doubt that some discs are mastered better than
others. Some are badly mixed. Some are so badly recorded that
there is noticeable clipping. Some are made from damaged master
recordings. CD technology is no guarantee of good music or of a
good recording.

10.3 Are CDs better than LPs?
Some excellent recordings are mastered digitally, and sound
great on LPs. This suggests that there is nothing inherently
bad about digital.

Some find that LPs sound better than CDs. Advocates of LPs
claim that the digital to analog (D/A) converter in home CD
players isn't up to the quality of the information on the disc.
They also claim that the analog electronics in a home CD player
can be poor.

Some believe that CDs do not sound like LPs because the CD does
not have the frequency response errors, the distortion, or the
stereo separation problems of LPs.

In general, though, there are good and bad CD players, just as
there are good and bad turntables, cartridges, and tone arms.
Any ultimate comparison would require ultimate equipment, which
is unaffordable. In moderately priced systems, there will be
some signal damage from the turntable system and some signal
damage from the CD player.

LP lovers often learn the nuances of cartridge selection, record
care, and even turntable and tonearm adjustment. They have
found that the turntable will sound different if the arm height
is adjusted, if the cartridge angles are changed, and if the
tonearm wire is moved. CDs do not offer as many avenues for the
home experimenter.

However, Audio Amateur Magazine has published modification
projects for CD players; particularly for Magnavox 560 and
similar European players. Audio Magazine has also published
such articles.

10.4 What turntable should I buy?
Despite improvements in motor technology, most great turntables
use belt drive. Rubber roller (idler) drive sounds the worst.

Select a turntable with a very heavy platter for the least wow
and flutter. Give the platter a rap with your knuckle. It
should not "ring" like a cymbal. It should feel and sound dead.

Also look for a turntable that has good isolation from base
to stylus. With the amp on and the turntable selected, but
with the turntable motor off, put an old record on the
turntable, lower the stylus onto the record, and then tap
the edge of the base. Not too hard, you don't want to send
the arm flying. At worst, you will hear a quick 'thump'
followed by silence through the speakers; if you're lucky,
you'll hear nothing at all. If the sound continues beyond a
quick 'thump', the mechanical isolation is not great, and you
should look at some other make. When you perform this test,
be sure to unplug the turntable power cord.

If the turntable has a tonearm, try to evaluate the arm,
too. A good arm should be adjustable in height. A good arm
should allow cartridge adjustments. A good arm will be very
rigid and have no bearing play. A good arm should accommodate
a wide range of cartridges. Despite this, some arms work
better with high compliance cartridges, while others are
at their best with low compliance. Ask.

Turntables by Denon, Dual, Linn, Michell, Oracle, Pro-Ject,
Rega, Sota, Thorens, and VPI are recommended. If you want a
turntable on a budget, consider the NAD 5120 at approx. $160.

10.5 What phono cartridge should I buy for my older turntable?
The $30 Grado ZTE+1 is a great value for any home user but a
purist. For the purist, there are still many choices, both
moving coil and moving magnet. Each sounds slightly different,
and has its individual strengths. Moving Magnet (MM)
cartridges tend to have higher output than Moving Coil (MC)
cartridges, with exception. Low Output Moving Coil cartridges
require unusual preamplification. Check with a dealer before
buying one. Some candidates:
Sumiko Blue Point $125
Denon DL-160 $125
Shure VST-V $150 (MM, Std Mount)
Audio Technica AT-95E (MM, Std Mount)
Denon DL-100 $85 (MC High Output, Std Mount)
Ortofon MC-10 Super MkI $110 (MC Low Output, Std Mount)

10.6 Will phono cartridges still be around ten years from now?
Most likely, there will still be cartridges available, but not
in the variety available ten years ago. They will become
"Special Order" in some stores and unheard of in others.

"Pro" or "DJ" cartridges will stay available in good supply,
"Audiophile" cartridges will stay available and very expensive,
"Mid-line" cartridges will become very scarce, and a few
"Budget" cartridges will remain available in copious supply.
At the same time, some makers will drop their cartridge lines
completely.

10.7 Will LPs still be around ten years from now?
There is a strong movement of collectors and purists who will
keep their collections and buy good used discs. Count on these
people to keep the used disc market hot for 25 years longer.

As for new music, less is being pressed today than 20 years
ago. Many popular artists are being released on LP in parts
of Europe, but availability is dependent on country. One
person said that many new LPs are available in Spain.

LP sales have increased recently in Japan and in the UK. Polydor
is now re-releasing older recordings on vinyl, and will continue
to press them as long as it is profitable. Likewise, there are
several re-releasing projects in Japan. Some are for Jazz
collectors and others are for pure analog as well as classical
music lovers. They are selling the LPs by subscription, with
shipments every 2 or 3 months. Each release includes about 20
titles. Japan has released over 100 LPs this way last year.

10.8 What about CD green pens?
In a nutshell, save your money.

A CD player "reads" information on the disc with a laser light
beam. Some believe that if you put a green stripe on the very
perimeter of the disc, then the light beam will not reflect
around inside the disc and will more clearly pick up the data.

Scientific studies of the data coming off of the disc have
failed to show any difference between a virgin disc and a green
painted disc. I have not heard of double blind listening
comparisons that have proved that there are people who can hear
the difference, although many have performed uncontrolled tests
with positive results.

10.9 What about CD stabilizer rings?
In a nutshell, save your money.

The data coming off of the disc is a serial string of ones and
zeros. If this bit stream has jitter, then it may reach the D/A
converter out of sync. If this happens, then the actual analog
signal recreated will have jitter, and won't be perfectly true.
The vendors of stabilizer rings say that using these rings will
reduce jitter and make a more perfect signal. Vendors also
claim that the rings can increase the mass of a disc, making it
spin more smoothly, and reducing transient load on the power
supply from the motor.

Some players will not play discs that have stabilizer rings on
them. The clamp can't handle the thickness. Other players play
ringed discs, but do not play them well, because the disc motor
was not built for the added load.

With those exceptions, scientific studies of the data coming off
of the disc have failed to show any improvement going from a
virgin to a ringed disc. I have not heard of double blind
comparisons that prove that people hear the difference, either.

10.10 What about CD spray treatments (ArmorAll et al)?
In a nutshell, save your money.

Current wisdom is to avoid any disc coating or spray. Some will
definitely damage the disc.

There are many theories on what ArmorAll can do to a disc. One
is that it reduces static which will attract the delicate head
of the laser detector to the disc. Another theory is that the
cleaner will fill voids in the disc with silicone, thereby
making it easier to read by reducing diffraction effects.

Scientific studies of the data coming off of the disc have
failed to show any difference between a virgin disc and a
treated disc. I have not heard of double blind listening
comparisons that have proved that there are people who can hear
the difference.

One of the strongest proponents of ArmorAll issued a "recall" on
his advice. He now warns that ArmorAll can damage the disc. He
also advises that you can clean ArmorAll off treated discs with
Dawn dish detergent.

10.11 Are 1-bit CD players better than multi-bit players?
In a nutshell, they are virtually the same.

There are some excellent sounding 1-bit players and some
excellent sounding multi-bit players. Some feel that the 1-bit
technology has more future because it can be improved with the
rapidly improving digital technology, while the multi-bit
players improve with slowly improving analog technology.
Multi-bit also has its advocates.

All of the various D/A converters try to do the same thing, and
try to achieve the exact same ideal performance. How well they
succeed is more a function of their skill and the quality of the
parts that they buy than the technique that they use. In other
words, the architecture of a D/A converter is less important
than the quality of its implementation.

10.12 Are three lasers better than one in CD players?
Some players have one beam, some three. All use one laser diode
to generate the beam. Three-beam is just a different method for
doing track alignment. Neither is better than the other.

There are good 1-beam players and good 3-beam players.
Manufacturers want advertising claims and "More Beams Is Better"
sounded good to some marketing people. Trust your ears.

| 10.13 Is the BMG 11-for-1 deal good?
Yes. You have to put up with their frequent mailings. You can
elect the "POSITIVE OPTION" and not have to answer each mailing
to avoid an order. You should expect to pay approximately $2.00
per disc for shipping and handling in the US and more elsewhere,
but even at that price and assuming that you will buy one of
their discs for $16.00, you still do well. Assuming, of course,
| that you want at least 11 of the discs that they are offering
for sale. Some states requires sales tax on BMG sales, and some
states tax "free" discs, but the tax still is small compared to
the discount from retail.

The BMG collection contains over 2500 discs. This includes
classical, pop, jazz, and other. All BMG discs come from the
larger labels. Some rumored that BMG discs are inferior to the
discs sold in normal retail chains. This has not been
substantiated. In fact, BMG distributes their discs through
retail chains, as well as through the mail, so you may get a BMG
disc either way.

| BMG has a web site. There is also a great CD Club FAQ on the
| web. Try these sites:
| http://www.bmgmusicservice.com
| ftp://ftp.netcom.com/pub/ra/ramseyms/cd/CD_Club.FAQ

10.14 What should I do if there is a problem dealing with BMG?
The number to reach BMG is 317-692-9200. Their people have been
very cooperative with me and others. It is always good policy
to confirm any phone call with a letter, restating the problem
and the resolution you were promised over the phone. It is good
practice to write down the name of the person you speak with.
You can also contact BMG by FAX at 317-542-6090.

If BMG sends you something that you didn't order, DON'T OPEN THE
PACKAGE. Write REFUSED on the package and put it back in the
mailbox. They will accept the return and credit your account
for any charges.

BMG has hired a marketing firm to send out information on the
classical club. Call 800-264-9555, but don't expect customer
service from this number.

10.15 How do I get out of the BMG racket?
If you have taken any discs from BMG, you must either return
what you have ordered or fulfill the terms of your original
agreement. This often means buying one disc at full price and
paying for the shipping on all discs you ordered and received.

Once you have done this, you can quit the club at any time.
Take your next order form and mark it with a bold marker in
large letters "CANCEL MEMBERSHIP" and mail it to: BMG COMPACT
DISC CLUB, PO BOX 91413, INDIANAPOLIS, IN 46291 USA. It may
take a month to fully take effect, but they will honor your
request. While waiting for the cancel order to take effect, be
sure to return all future order forms marked the same way.
Otherwise, you may wind up with unwanted discs.

10.16 How do I get the most out of BMG?
Only buy one disc at full price, fulfilling your obligation.
Request the "POSITIVE OPTION" so that you save on postage. Only
buy from special fliers. Every month, except November and
December, they send out a "Two for half price then one free"
flier. They have almost all of the stuff in the regular fliers.
They even offer "Buy one get two free" sometimes. Wait for
those special deals. You can even order discs from an October
catalog using the order form that came in the February catalog.

You can get even more out of BMG by signing up, getting 8 discs
for the price of one, quitting, signing up again, etc. People
have done this successfully. BMG reserves the right to deny
membership to anyone, so you run a very slight risk of being
denied membership the 20th time. However, I have never heard
of anyone ever being denied membership for any reason.

The file CDClubFAQ.txt explains more than you ever wanted to
know about the BMG and Columbia music clubs. It is available
by FTP from: ftp.netcom.com in /pub/ra/ramseyms/cd
or by gopher at: biogopher.wustl.edu An HTML version
can be found at: http://www.eskimo.com/~bloo/cdfaq/cdindex.html
Online BMG and CH Popular Catalogs are available at:
gopher://biographer.wustl.edu or
http://biogopher.wustl.edu:70/1/audio/bmg
Online BMG Classical Catalog is available by FTP from:
ftp.gmd.de in /music/cd-catalogs
Get file bmg-classical-collection_2ed.gz

10.17 What are the differences between multibit and Bitstream/MASH
Analogue to Digital converters (16-bit vs 1-bit CD players)?

Audio data is stored on CD as 16-bit words. It is the job of
the digital to analogue converter (DAC) to convert these numbers
to a varying voltage. Many DAC chips do this by storing electric
charge in capacitors (like water in buckets) and selectively
emptying these buckets to the analogue ouput, thereby adding
their contents. Others sum the outputs of current or voltage
sources, but the operating principles are otherwise similar.

A multi-bit converter has sixteen buckets corresponding to the
sixteen bits of the input word, and sized 1, 2, 4, 8 ... 32768
charge units. Each word (ie sample) decoded from the disc is
passed directly to the DAC, and those buckets corresponding to
1's in the input word are emptied to the output.

To perform well the bucket sizes have to be accurate to within
+/- half a charge unit; for the larger buckets this represents
a tolerance tighter than 0.01%, which is difficult. Furthermore
the image spectrum from 24kHz to 64kHz must be filtered out,
requiring a complicated, expensive filter.

Alternatively, by using some digital signal processing, the
stream of 16-bit words at 44.1kHz can be transformed to a
stream of shorter words at a higher rate. The two data streams
represent the same signal in the audio band, but the new data
stream has a lot of extra noise in it resulting from the
word length reduction. This extra noise is made to appear
mostly above 20kHz through the use of noise-shaping, and the
oversampling ensures that the first image spectrum occurs at a
much higher frequency than in the multi-bit case.

This new data stream is now converted to an analogue voltage
by a DAC of short word length; subsequently, most of the noise
above 20kHz can be filtered out by a simple analogue filter
without affecting the audio signal.

Typical configurations use 1-bit words at 11.3MHz (256 times
over-sampled), and 4-bit words at 2.8MHz (64 times oversampled).
The former requires one bucket of arbitrary size (very simple);
it is the basis of the Philips Bitstream range of converters.
The latter requires four buckets of sizes 1, 2, 4 and 8 charge
units, but the tolerance on these is relaxed to about 5%.

MASH and other PWM systems are similar to Bitstream, but they
vary the pulse width at the ouput of the digital signal
processor. This can be likened to using a single bucket but with
the provision to part fill it. For example, MASH allows the bucket
to be filled to eleven different depths (this is where they get
3.5 bits from, as 2^(3.5) is approximately eleven).

Lastly it is important to note that these are all simply
different ways of performing the same function. It is easy to
make a lousy CD player based around any of these technologies;
it is rather more difficult to make an excellent one, regardless
of the DAC technology employed. Each of the conversion methods
has its advantages and disadvantages, and as ever it is the job
of the engineer to balance a multitude of parameters to design a
product that represents value for money to the consumer.

10.18 What is the best under-$200 CD player?
Once, Magnavox (and the identical Philips) CD players were known
for poor reliability. They are much better today. Some believe
that for sound, an under-$200 Magnavox is as good as any in its
price range. Others say JVC. Be sure to listen for yourself
before you buy.

10.20 What is the best under-$500 CD player?
Some recommend Rotel. Others recommend Marantz, NAD, or Yamaha.

10.21 (removed)

10.22 (removed)

10.23 How can I clean a dirty CD?
Use a drop of dish detergent and lots of clean water. Do not
rub. Never rub or wipe in a circle. If you must stroke the disc
do it with a soft cotton cloth in a straight line from the
center outwards (radially). Rinse the disc in running clear
water, shake off most remaining drops, and lightly pat dry
with a soft, clean cloth.

10.24 Can you repair a damaged CD?
If the disc is lightly scratched on the bottom, then you can
polish out the scratch and probably repair the disc perfectly.
If there are lots of scratches or deep scratches, or there is
damage on the top, you may be facing a lost cause. The music
information is immediately under the label. If you scratched
the reflective layer, the disc is normally unrecoverable.

Before trying any repair, try washing the disc with clear water
and a bit of liquid dish detergent. Do not scrub or rub hard.
Rinse the disc with clear water and shake off as much water as
you can. Finally, wipe the last few drops off with a soft,
clean cloth, in a radial direction.

SMALL scratches can be removed with a scrufty T-shirt and
toothpaste, such as Tom's Toothpaste.

You may wish to try a thin coating of Johnson's Klear floor wax
on the bottom of the CD. Often it will cover the scratches
enough to allow playing. The refractive index is pretty close
to polycarbonate, so filled scratches will be nearly invisible.

Noteworthy Music (800-648-7972) sells CD repair kits (#CDR 200,
$11.99, one shipping unit). They seem to work as advertised,
although getting the disc to the point where you can't see any
sign of the scratch does take real care and persistence.

You can buy professional plastic polishing compounds at many
hobby shops. The ones used for polishing acrylics, plexiglas,
etc. work. Ordinary lapidary jeweler's polishes also work.
You'll need a rough polish to remove the scratches, then tin
oxide to polish to a mirror finish. Telescope lens kits also
work. Novus plastic polish and cleaner has been recommended.
T-Cut, a car paintwork polish, works well for big scratches.
Reviewers at Audio Magazine recommend the "Memorex CD Repair
And Maintenance Kit" as the best tool for badly damaged CDs.
Another recommended polish is Miguires Plastic Polish #17.

Sometimes, a gentle polishing will make a disc playable
even though the scratch is not fully removed. This may be
even better than complete scratch removal because it leaves
more protective plastic behind.

10.25 Can I add digital output to a non-digital-out CD player?
Some Magnavox CD players using the Philips chip set can be
modified. Look for a SAA7220 IC. If it has one, then it can be
modified. If you have experience modifying electronic
equipment, follow this procedure:

Take pin 14 of the SAA7220 IC and remove whatever terminating
resistor is on it. Connect it through a 560 ohm resistor to the
input of a wide band pulse transformer. Tie the other end of
the primary of the transformer to ground. Pulse Engineering
PE65612, Schott Corp 6712540, and Scientific Conversions
SC916-01 all will work. Bypass the primary through a 620 ohm
resistor. Connect the output of the transformer to an RCA jack.
Do not ground either side of the RCA jack. This output is now
S/PDIF compatible. (Thanks for the tip to Positive Feedback)

10.26 What can I get in the way of a CD test disc?
Each test disc offers something different. Some discs contain
useless filler which advertises a product or shows a unique
capability, but really doesn't help you test or improve your
system.

Many use the Hi-Fi News & Record Review test discs. So far,
these have received only positive comments.

Chesky produces 2 test discs. The first, "Chesky Jazz Sampler
Volume I" contains some excellent imaging test signals (called
LEDR), some well-recorded acoustic jazz, and other test signals.
The second, "Chesky Jazz Sampler Volume II" has similar music &
different tests.

Stereophile produces three test discs.

Denon also produces two test discs. The first, "Digital Audio
Check" is more useful for home use. The second, "Audio
Technical" is more for repair shops and test-disc addicts.

If you are looking for test CDs, one source of supply that
stocks lots of different test CDs is:
DB Systems
Main Street
Box 460
Rindge Center NH 03461 USA
603-899-5121

10.27 How do the letters ADD on my CD relate to sound quality?
The simple answer to this question is that there is no relation
between the three letter code and sound quality. Those three
letters refer to the recording and mastering tools used in
making the CD.

The first letter refers to the recording process. For example,
a disc labeled ADD was ANALOG recorded, where a disc labeled
DDD was DIGITALLY recorded. Analog recording means that some
form of conventional analog tape recorder was used, whether it
be a two-track home-quality recorder or a very expensive
wide-tape, high-speed, multi-track recorder. Digital recording
could be as simple as a two-track DAT recorder, or can be a
much fancier multi-track digital recorder.

The second letter refers to the recorder used in the mixing and
editing process. Mixing and editing is the process of combining
a multi-track master recording, setting levels, editing out
defects, adjusting equalization, and creating a two-track final
tape. There are good machines available for this which are
analog and good machines which are digital.

The third letter refers to the final master, which for a CD
is always digital. I have seen discs that are labelled
as AAD, ADD, DAD, and DDD.

Future releases may not have this three letter code on them
because they don't tell you anything that is significant. Also,
some codes have been used incorrectly on some discs, which
makes the information that much more meaningless.

10.28 How can I clean LPs?
There are expensive machines for this purpose which work very
well. One popular model goes by the name Nitty Gritty. These
machines spray cleaner onto the record, work it into the
grooves, and then vacuum the cleaner and dirt out. If you are
serious about records and have lots of them, it may be a good
investment for you.

If you have a more reasonable collection, you might be happy
with a good hand washing every now and then. To give your
records a good hand washing, start by preparing this wash:
1 gallon distilled water
1 gram Alconox (a laboratory detergent)
Also, get a natural bristle brush and trim it to the correct
stiffness/bristle length so that the bristles can get into the
grooves but aren't stiff enough to scratch the record.
Some record-cleaning recipies recommend alcohol. However,
alcohol will leach plasticizer from vinyl, and eventually
degrade LPs. Alcohol will also disolve the shellac of 78s,
so should never touch a 78.

Lay the LP flat and pour a thin coat of the above fluid on it.
Brush the wash into the grooves with the bristle brush. Brush
in the direction of the grooves, going through all grooves.
Flush the wash and dirt off with cool, running tap water.
Rinse the record with distilled water and pat it dry
with a soft, clean cotton cloth.

Also consider using a carbon fiber brush every time you play
the LP. It picks up some surface dirt and removes static.

10.29 How do you set the stylus pressure correctly?
Stylus tracking force is typically adjusted at the back of the
tonearm with a knob that is calibrated in grams at the stylus
tip. With the control set to zero, the stylus should sort-of
float above the record surface. The control is then increased
to the number recommended by the cartridge manufacturer.

Do not, under any circumstances, use a lower than recommended
force, as the cartridge may lose the ability to maintain
contact with the groove wall on passages of large amplitude.
This WILL result in RECORD DAMAGE.

If you want the best possible tracking and sound quality, you
will want to fine-tune the tracking force. Use a test record
and listen very carefully, or get the help of a good dealer
with a battery of instruments.

10.30 How do you set the anti-skating on a tonearm?
If you have a recommendation or suggestion from the
tonearm manufacturer, follow their advice first.
They will give you the best starting point.

Some tonearms come with calibrated anti-skate. The manufacturer
of these tonearms has tried to calibrate the anti-skate control
so that if you match the setting of the anti-skate to the
setting of the stylus pressure, you will have nearly perfect
anti-skate. Read the manufacturer's recommendations to see if
this applies to your tonearm.

You can see gross errors in anti-skate by looking at
the stylus. If you shine a light on the front of the
tonearm while playing a record, you will be able
to see whether the stylus is centered in the stylus
holder. If the stylus is biased to one side or another
while playing a record, then the anti-skate is way off.

More subtle adjustments can be made by listening for
mistracking. If you can, obtain a record with equal
left right modulation at high frequency with ascending
modulation magnitude (volume), such as the Shure
ERA-III, IV, or V test record. They have five bands of
"greensleeves" played on flute, and you fiddle until the
audible breakup is equal in both channels, and adjust
tracking weight until it occurs in the highest band.
This is, like other cartridge and tonearm adjustments,
easier for the experienced hand than the beginner.

Some high-end dealers have electronic instruments which
allow them to accurately adjust anti-skate and other
cartridge and tonearm parameters. If you can get this
service, consider yourself fortunate.

10.31 How else do you adjust a tonearm/cartridge/stylus?
There are a few other critical adjustments. Again, a good
high-end dealer may be your best resource. Your ear may
also be your best test instrument.

You need a level turntable. Use a quality carpenter's
level. Some people like the Shure stylus force gage for
setting stylus pressure accurately. Other tools which are
well recommended are the Geo-disk, a good protractor, and
above all, the Cart-Align, which uses a very precise
etched plastic mirror for cantilever alignment.

You'll also want to set the tracking angle. It CAN be
done by eyeball, but is best done with test instrumentation
and a record. There is also the cartridge angle, tonearm
height, etc. Read the instructions which came with your
tonearm for the best specific advice for that tonearm.

Tonearm cable is more critical than any cable anywhere else
in the signal chain. Cable capacitance directly sets the high
frequency characteristics of the cartridge. In addition, the
correct grounding of the shield is essential to minimize hum.
It may be necessary to change preamp input capacitors so that
the cable/preamp combination loads the cartridge with the
right overall capacitance. Replacing tonearm cable will have
a similar effect, but may be harder to change tonearm cable
than to change preamp input capacitors. Consult the
cartridge, tonearm, and preamp manuals for specific advice.
Also refer to 16.6 for more information on tonearm cable.

An excellent article on setting up a turntable is:
Stereophile, July 1990, Pages 62-85.

10.32 Do CDs deteriorate with time? What is their life span?
A CD consists of a polycarbonate top layer, an aluminum (or
gold) metal reflective layer, a polycarbonate bottom layer,
and some miscellaneous printing ink. Of these materials,
polycarbonate seems to be extremely stable with time provided
that it is well cared for. Do not use any liquids on a CD
that contain silicones or solvents. Do not leave CDs in
sunlight or other bright light. Do not stick labels on CDs.
Do not write on CDs. Do not expose CDs to temperatures higher
than normal room temperatures. Don't leave a CD under water.
Even the top side of a CD is critical and subject to damage.

Some pressings from the early 1980s used ink which damaged the
polycarbonate top layer and eventually got into the aluminum.
These inks are not in use today. Some earlier discs were made
with imperfect sealing around the perimeter of the disc. This
was evident because the aluminum in the disc extended all of
the way to the disc edge. These discs were known to fail due
to moisture getting to the aluminum and causing it to oxidize.
Modern CD factories have solved this problem as well.

With those cautions, modern CDs will last for more than 30
years without deterioration. Most of the CDs which were
made in 1983 are still around today and still sound good.

10.33 How much music can you possibly cram into a CD?
The longest seen so far (reported by Stuart Kahler) is a
MiC bootleg of Depeche Mode "Evolution", at 81:09. Next are
'No Quarter' by Jimmy Page and Robert Plant at 79:38, an
MCA reissue of Steely Dan: Greatest Hits at 79:17 and a
Musical Heritage recording of Bach: Goldberg Variations
at 79:02. Modern CDs are pressed using tighter track spacing
than the first CDs, because modern equipment is capable
of holding tighter tolerance than the original machines.

| 10.34 What are input and output levels and impedances for signal
| sources, preamps, amps, etc?
| We have been unable to find any formal standard on this topic.
| However, there is an EIA Bulletin: EIA Consumer Products
| Engineering Bulletin No 6-A (CPEB6-A) 1974, titled "Preferred
| Voltage and Impedance Values for the Interconnection of Audio
| Products". The key word in the title is 'Preferred'.
|
| EIA CPEB6-A recommends 3mV at 47k ohms for magnetic phono
| cartridges, 250mV at less than 10k ohms for tape and preamp
| outputs, and 100k ohm minimum for tape, tuner, and amp aux
| inputs. The bulletin also has information on microphones,
| and headphones. You can order a copy through a technical
| library or directly from the EIA.
|
| 10.35 Why are turntable speeds 78 RPM, 45 RPM, etc?
| The speeds were chosen because that is the speed that resulted
| when you used standard parts. Electric motors rotate at 1800
| rpm, most shafts are 1/4". Those combinations with the proper
| gears and idlers came out to 78 rpm. In reality it's 78.26
| rpm. Tape recorder speeds evolved the same way.
|
| The 78.26 was standardized after electric recording/playback
| occured. Prior to that, speeds were "in the neighborhood of"
| 78 rpm. Some lower and some higher. 80 rpm was used in many
| recordings. (Courtesy of Bill Vermillion)

[neid...@uicc.com]

Archive-name: AudioFAQ/part4
Last-modified: 1996/9/23
Version: 2.07

11.0 Amplifiers
Note: A receiver contains an amplifier, so the following
questions apply to both receivers and amplifiers. In the
following text, "amp" and "amplifier" are used synonymously.

11.1 What is Biamping? Biwiring?
Most speakers are connected to an amplifier by one pair
of terminals on each speaker. Within these speakers, a
crossover distributes the signal (modified appropriately)
to each of the drivers in the speaker.

Some speakers are set up to be either biwired or biamped. A
much smaller number allows triwiring and triamping. The same
principles apply but use three sets of wires or three amplifiers
instead of two. Most speakers that support biamping/biwiring
have two pairs of terminals and some mechanism for shorting
the two pairs together when used in the normal way. This
mechanism is most likely a switch or a bus bar. To help
the descriptions below, I will refer to these two pairs as
LO and HI (because normally one pair connects to the woofer
and the other pair connects to the tweeter/midrange).

Biwiring means that a speaker is driven by two pairs of wires
from the same amplifier output. One cable pair connects HI to
the amp, and the other cable pair connects LO to the same amp
output that you connected the HI cable to. Biwiring is
controversial; some folks hear a difference, some do not. One
plausible explanation for this involves magnetic induction of
noise in the relatively low current HI cable from the high
current signal in the LO cable. Accordingly, Vandersteen
recommends the two cable pairs for a channel be separated by at
least a few inches. In any case, the effect appears to be small.

Biamping means that the two pairs of terminals on a speaker are
connected to distinct amplifier outputs. Assuming you have two
stereo amplifiers, you have two choices: either an amp per
channel, or an amp per driver. For the amp per channel, you
connect each terminal pair to a different channel on the amp
(for example, the left output connects to HI and the right side
to LO). In the other configuration, one amp connects to the LO
terminals, and the other amp is connected to the HI terminals.

The point of biamping is that most of the power required to
drive the speakers is used for low frequencies. Biamping allows
you to use amps specialized for each of these uses, such
as a big solid-state amplifier for the LO drivers and higher
quality (but lower power) amp for the higher frequencies.
When you have two identical stereo amps, some folks
recommend distributing the low-frequency load by using an amp
per channel. In any case, whenever you use two different
amplifiers, be careful to match levels between them.

Biamping also allows you to use high-quality electronic
crossovers and drive the speaker's drivers (the voice coils)
directly, without the series resistance and non-linear
inductance of a passive crossover. Biamping which uses the
speaker's crossover is therefore much less desirable. Replacing
a good speaker's crossover with an electronic crossover has
advantages, but involves some very critical tradeoffs and tuning
which is best left to those well-equipped or experienced.

See also section 16.0 below, on wire and connectors in general.

11.2 Can amplifier X drive 2 ohm or 4 ohm speakers? How do I raise the
impedance of a speaker from (say) 4 ohms to 8 ohms?
Almost any amplifier can drive almost any load if you don't turn
the volume up too high. Tube amplifiers are one exception.
Some amps clip if you play them too loud. This is bad and
damages speakers. Other amplifiers shutdown if they are asked
to play too loud. Many will overheat, with bad consequences.
However, in almost all cases, it takes seriously loud sound or
low speaker resistance (less than 4 ohms) to do damage. Running
two sets of 8 ohm speakers at once with common amplifiers
represents a 4 ohm load. Four sets of 8 ohm speakers makes a 2
ohm load. Two sets of 4 ohm speakers also makes a 2 ohm load.
If you stay sober and don't turn it up past the point where it
distorts, you are PROBABLY safe with most amplifiers and most
loads. See 11.3 for more information.

You can raise the impedance of a speaker by a few different
methods. However, each has drawbacks. If your amplifier won't
drive your speakers, AND you are sure that the problem is that
the speakers are too low impedance, you might try one of these
techniques.

A) Add a 4 ohm resistor in series with the speaker.
This requires a high power resistor, because the
resistor will dissipate as much power as the speaker.
Doing this will almost always hurt sound quality, too.
This is caused, in part, by the fact that speakers do
not have constant resistance with frequency. See 11.3
for more information on this.

B) Use a matching transformer. There are speaker matching
transformers which can change from 4 ohm to 8 ohm, but
a high quality transformer like this can cost as much
as a common receiver. Also, even the best transformer
will add some slight frequency response and dynamic
range errors.

C) Use two identical speakers in series. If you have two
4 ohm speakers which are the same make and model, you
can wire them in series and make an equivalent speaker
with 8 ohm impedance. The sound from that "new speaker"
will not be as precisely localized as it would from one
speaker, so your stereo image may be hurt. Also, it
requires that you buy twice as many speakers as you
might have bought otherwise. However, this technique
has one side benefit. Two speakers can handle twice the
power of one.

11.3 How do I drive more than two speakers with one stereo amplifier?
One amp can drive many speakers. However, there are two limits
to this practice. The first is that you can overheat or damage
an amplifier if you drive too low of an impedance to loud
listening levels. Avoid loading any amplifier with a lower
impedance than recommended. Adding two speakers to one amp
output loads that output with half the impedance of one speaker.
(See also 11.2 above)

The second is that with tube amplifiers, which are uncommon
in today's common system, it is important that the speaker
impedance and the amplifier output impedance be well matched.

When driving two or more speakers from one amp output, always
wire them in parallel, rather than series. Series connection,
while safe in terms of impedance levels, can hurt sound quality
by raising the impedance that the speakers themselves see.
Also, when different speakers are wired in series, amplifier
voltage will divide between the speakers unevenly, because
different speakers have different impedance-versus-frequency
characteristics.

Many amplifiers have connectors for two pairs of speakers. In
general, these amplifiers also have a speaker selector switch.
Most amplifiers connect speakers in parallel when both are
selected, although some less expensive ones will wire the
speakers in series. It is common for these amplifiers to require
8 ohm speakers only, because the amplifier is built to drive
either 4 or 8 ohms, and two sets of 8 ohm speakers in parallel
loads the amplifier like one set of 4 ohm speakers. It is
almost always safe to connect one set of 4 ohm speakers to
an amplifier with two sets of outputs, provided that you
NEVER use the second terminals for any other speakers.

11.4 How big an amplifier do I need?
Unfortunately, amplifier power ratings and speaker power ratings
are almost always misleading. Sometimes, they are factually
wrong. Speaker ratings are almost useless in evaluating needs.

To start with, sound pressure, measured in dB, often stated as
dB SPL, is a function of the log of the acoustic "sound" power.
Further, human hearing is less sensitive to differences in power
than the log transfer function would imply. This means that the
perceived difference between a 50 watt amplifier and a 100 watt
amplifier, all else equal, is very small! One columnist said
that a 250 watt amplifier puts out twice the perceived
loudness of a 25 watt amplifier, but quantitative statements
about perception should always be treated with caution.
That statement came from Electronics Now Magazine, Jan 1994,
Page 87, Larry Klein's "Audio Update" Column, which is also
good reading on the subject of required amplifier power.

There is a wide variation in the "efficiency" and "sensitivity"
of the various speakers available. I have seen good speakers
with under 80 dB per watt efficiency and have also seen good
speakers with over 96 dB per watt efficiency, measured one meter
from the speaker. This difference of 16 dB represents a factor
of 40 difference in power requirement!

So the first step in determining amplifier requirements is to
estimate relative speaker efficiency. Other factors include how
loud you will want to listen, how large your room is, and how
many speakers you will drive with one amplifier. This
information will give you a rough starting point. For an
example, a typical home speaker will produce 88 dB at 1 watt.
In an average room, a person with average tastes will be happy
with this speaker and a good 20 watt per channel amplifier.
Someone who listens to loud music or wants very clean
reproduction of the dynamics of music will want more power.
Someone with less efficient speakers or a large room will also
want more power.

Past that point, you will have to use your ears. As with all
other decisions, your best bet is to get some candidates, borrow
them from a friendly dealer, take them home, and listen to them
at your normal and loudest listening level. See if they play
cleanly when cranked up as loud as you will ever go, into your
speakers in your room. Of course, it is also important to be
sure that the amp sounds clean at lower listening levels.

11.5 Do all amplifiers with the same specifications sound alike?
Some say that they do. Some say that they don't. Some
demonstrated that many amplifier differences can be traced to
very slight frequency response difference. Let your own ears
guide you. If you want to compare amplifiers, you can do it
best in a controlled environment, such as your home, with your
music and your speakers. Also be very careful to match levels
precisely. All you need to match levels of amplifiers is a high
input-impedance digital voltmeter set to AC volts and a test
recording or signal generator. For best accuracy, set levels
with the speakers wired to the amplifier.

11.6 Is this amplifier too big for that set of speakers?
There is no such thing as an amplifier that is too big. Small
amplifiers are more likely to damage speakers than large ones,
because small amplifiers are more likely to clip than larger
ones, at the same listening level. I have never heard of
speakers being damaged by an overly large amplifier. I have
heard of 100 watt speakers being damaged by a 20 watt
amplifier, however, in really abusive hands. This will happen
because when an amplifier clips, it will generate much more
energy at high frequencies than normal music would contain.
This high energy at high frequencies may be less than the
continuous power rating of the speaker, but higher than the
actual energy rating of the tweeter. Tweeters tend to be
very fragile components

11.7 Where can I get a cheap low-power amplifier?
There are very few available. One source is to buy a cheap boom
box and only use the amplifier. Another source is Radio Shack.
A third alternative is to buy a car stereo booster and get a
12V power supply for it. Finally, you can build an amp pretty
easily if you are handy, but it probably won't be that cheap.
Mark V Electronics, for example, sells 20 watt amp kits for
under $30 and 80 watt amp kits for under $150. Sound Values
has a 60 watt amp kit complete for about $200, and Old Colony
sells some amp kits for a bit more. All three, Mark V, Old
Colony, and Sound Values kits have been built by satisfied
rec.audio.* posters, although quality of the Mark V kit is
lower than the others. (See 11.15, 11.16, 11.17)

11.8 Is the stuff sold by Carver really awesome?
There is a lot of repeated rumor and prejudice for and against
Carver equipment based on anecdotes of older Carver equipment.
Sometime in 1994, Bob Carver left the Carver Company, so it is
reasonable to expect significant changes in the company and
their product line. One of Carver's claims to fame is lots of
watts per pound of weight. As with almost everything else, the
best policy is to listen for yourself and see what you think.

11.9 What is a preamplifier?
A preamplifier is an amplifying electronic circuit which can be
connected to a low output level device such as a phono cartridge
or a microphone, and produce a larger electrical voltage at a
lower impedance, with the correct frequency response. Phono
cartridges need both amplification and frequency response
equalization. Microphones only need amplification.

In most audio applications, the term 'preamplifier' is actually
a misnomer and refers to a device more properly called a
'control amplifier'. Its purpose is to provide features such
as input selection, level control, tape loops, and sometimes,
a minimal amount of line-stage gain. These units are not
preamplifiers in the most technical sense of the word, yet
everyone calls them that.

11.10 What is a passive preamplifier?
A passive preamplifier is a control unit without any
amplification at all. It is a classic oxymoron, because it has
no capability to increase the gain of the signal. It is only
used with line level sources that need no gain beyond unity.

11.11 Do I need a preamp? Why?
The tasks of a preamp are to:
Switch between various input signals,
Amplify any phono inputs to line level,
Adjust the volume,
Adjust the treble and bass if necessary,
Present the right load impedance for the inputs, and
Present a low source impedance for the outputs.

If you have a turntable, you NEED a preamp with a phono input.
This is because the turntable has an output which is too
small for driving amplifiers and because the output of the
turntable requires frequency response equalization. You
can't connect any other source to a phono input other than a
turntable (phono cartridge). Also, you can't connect a phono
cartridge or turntable to any input other than a phono input.

Microphones also require special preamplifiers. Some microphones
also require "phantom power". Phantom power is operating power
for the microphone which comes from the preamp. Microphone
preamps are often built into tape decks and microphone mixers.

If you only have high level inputs, such as the output of a CD
player and the output of a tape deck, the main value of a preamp
is selecting between inputs and providing a master volume
control. If you only listen to CDs, it is plausible to skip
the preamp entirely by getting a CD player with variable level
outputs and connecting them directly to a power amplifier.

Some caveats apply. One, the variable outputs on a CD player are
often lower sound quality than fixed outputs. Two, some sources
have high or nonlinear output impedances which are not ideal for
driving an amplifier directly. Likewise, some amplifiers have
an unusually low or nonlinear input impedance such that common
sources can't drive the input cleanly. A good preamplifier
allows use of such devices without sacrificing sound quality.

Unfortunately, the only way to be sure that a preamplifier is
of value with your sources and your amplifier is to try one.

Almost all receivers contain a phono preamp, a volume
control, and input switching. Therefore, if you have a
receiver, you may never need a preamp.

11.12 Should I leave equipment on all of the time or turn it on and off?
Some gear draws significant electricity, so you will waste money
and fossil fuel if you leave it on all of the time. As an
example, a common amplifier consumes 40 watts at idle. High-end
gear uses far more electricity, but ignoring that, 40 watts x
168 hours x 52 weeks x US $0.0001 per watt hour (rough estimate)
is $35/year. Now add a CD player, a preamp, and a tuner, and it
really adds up.

High-end enthusiasts claim that equipment needs to warm up to
sound its best. If you care about the best sound, give your
equipment at least 20 minutes to warm up before serious
listening. Warm up will allow the inside temperature to
stabilize, minimizing offsets, bring bias currents up to their
proper values, and bringing gain up to operating level.

Either way, good gear will last a very long time. Tubes are
known to have a finite life, but good tube designs run tubes
very conservatively, giving them life exceeding 10 years of
continuous service. Some amplifiers run tubes harder to get
more power out, and thereby may be more economical to turn off
between use.

Filter capacitors will fail after enough time at temperature
with voltage applied. They will last longer if turned off
between use. However, like tubes, filter caps can last tens of
years of continuous use, as can power transformers,
semiconductors, and the like.

Filter capacitors have a funny problem that justified a simple
break-in or reforming when they are restarted after many years
of rest. It involves bringing up the power line voltage slowly
with a variable transformer. For tips on reforming capacitors,
consult "The Radio Amateur's Handbook", by the ARRL.

Semiconductors seem to fail more often because of bad surges and
abuse than age. Leaving gear off may be best for semiconductors
and other surge-sensitive gear if you expect power line surges,
as come from an electrical storm or operation of large motors.

Fuses seem to age with temperature and get noisy, but they are
so inexpensive that it should not bias your decision. However,
some are inconvenient to change, and may require opening the
case and even voiding the warranty.

11.13 Do tube amps sound better than transistor amps? FETs?
Lets first list some commonly used active electronic
components and their good and bad attributes.

TUBE: (Valve, Vacuum Tube, Triode, Pentode, etc.)
Tubes operate by thermionic emission of electrons from a
hot filament or cathode, gating from a grid, and collection
on a plate. Some tubes have more than one grid. Some tubes
contain two separate amplifying elements in one glass
envelope. These dual tubes tend to match poorly.

The characteristics of tubes varies widely depending on the
model selected. In general, tubes are large, fragile, pretty,
run hot, and take many seconds to warm up before they operate
at all. Tubes have relatively low gain, high input resistance,
low input capacitance, and the ability to withstand momentary
abuse. Tubes overload (clip) gently and recover from overload
quickly and gracefully.

Circuits that DO NOT use tubes are called solid state,
because they do not use devices containing gas (or liquid).

Tubes tend to change in characteristic with use (age).
Tubes are more susceptible to vibration (called
"microphonics") than solid state devices. Tubes also
suffer from hum when used with AC filaments.

Tubes are capable of higher voltage operation than any other
device, but high-current tubes are rare and expensive. This
means that most tube amp use an output transformer. Although
not specifically a tube characteristic, output transformers
add second harmonic distortion and give gradual high-frequency
roll-off hard to duplicate with solid state circuits.

TRANSISTOR: (BJT, Bipolar Transistor, PNP, NPN, Darlington, etc.)
Transistors operate by minority carriers injected from emitter
to the base that are swept across the base into the collector,
under control of base current. Transistors are available as PNP
and NPN devices, allowing one to "push" and the other to "pull".
Transistors are also available packaged as matched pairs,
emitter follower pairs, multiple transistor arrays, and even
as complex "integrated circuits", where they are combined with
resistors and capacitors to achieve complex circuit functions.

Like tubes, many kinds of BJTs are available. Some have high
current gain, while others have lower gain. Some are fast,
while others are slow. Some handle high current while others
have lower input capacitances. Some have lower noise than
others. In general, transistors are stable, last nearly
indefinitely, have high gain, require some input current, have
low input resistance, have higher input capacitance, clip
sharply, and are slow to recover from overdrive (saturation).
Transistors also have wide swing before saturation.

Transistors are subject to a failure mode called second
breakdown, which occurs when the device is operated at both
high voltage and high current. Second breakdown can be avoided
by conservative design, but gave early transistor amps a bad
reputation for reliability. Transistors are also uniquely
susceptible to thermal runaway when used incorrectly. However,
careful design avoids second breakdown and thermal runaway.

MOSFET: (VMOS, TMOS, DMOS, NMOS, PMOS, IGFET, etc.)
Metal-Oxide Semiconductor Field Effect Transistors use an
insulated gate to modulate the flow of majority carrier current
from drain to source with the electric field created by a gate.
Like bipolar transistors, MOSFETs are available in both P and N
devices. Also like transistors, MOSFETs are available as pairs
and integrated circuits. MOSFET matched pairs do not match as
well as bipolar transistor pairs, but match better than tubes.

MOSFETs are also available in many types. However, all have
very low input current and fairly low input capacitance. MOSFETs
have lower gain, clip moderately, and are fast to recover from
clipping. Although power MOSFETs have no DC gate current, finite
input capacitance means that power MOSFETs have finite AC gate
current. MOSFETs are stable and rugged. They are not susceptible
to thermal runaway or second breakdown. However, MOSFETs can't
withstand abuse as well as tubes.

JFET:
Junction Field Effect Transistors operate exactly the same
way that MOSFETs do, but have a non-insulated gate. JFETs
share most of the characteristics of MOSFETs, including
available pairs, P and N types, and integrated circuits.

JFETs are not commonly available as power devices. They make
excellent low-noise preamps. The gate junction gives JFETs
higher input capacitance than MOSFETs and also prevents them
from being used in enhancement mode. JFETs are only available
as depletion devices. JFETs are also available as matched
pairs and match almost as well as bipolar transistors.

IGBT: (or IGT)
Insulated-Gate Bipolar Transistors are a combination of a MOSFET
and a bipolar transistor. The MOSFET part of the device serves
as the input device and the bipolar as the output. IGBTs are
only available today as N-type devices, but P-type devices are
theoretically possible. IGBTs are slower than other devices but
offer the low cost, high current capacity of bipolar transistors
with the low input current and low input capacitance of MOSFETs.
IGBTs suffer from saturation as much as, if not more than
bipolar transistors, and also suffer from second breakdown.
IGBTs are rarely used in high-end audio, but are sometimes
used for extremely high power amps.

Now to the real question. You might assume that if these
various devices are so different from each other, one must be
best. In practice, each has strengths and weaknesses. Also,
because each type of device is available in so many different
forms, most types can be successfully used in most places.

Tubes are prohibitively expensive for very high power amps.
Most tube amps deliver less than 50 watts per channel.

JFETs are sometimes an ideal input device because they have
low noise, low input capacitance, and good matching. However,
bipolar transistors have even better matching and higher gain,
so for low-impedance sources, bipolar devices are even better.
Yet tubes and MOSFETs have even lower input capacitance, so
for very high source resistance, they can be better.

Bipolar transistors have the lowest output resistance, so
they make great output devices. However, second breakdown
and high stored charge weigh against them when compared to
MOSFETs. A good BJT design needs to take the weaknesses of
BJTs into account while a good MOSFET design needs to
address the weaknesses of MOSFETs.

Bipolar output transistors require protection from second
breakdown and thermal runaway and this protection requires
additional circuitry and design effort. In some amps, the
sound quality is hurt by the protection.

All said, there is much more difference between individual
designs, whether tube or transistor, than there is between tube
and transistor designs generically. You can make a fine amp
from either, and you can also make a lousy amp from either.

Although tubes and transistors clip differently, clipping
will be rare to nonexistant with a good amp, so this
difference should be moot.

Some people claim that tubes require less or no feedback
while transistor amps require significant feedback. In
practice, all amps require some feedback, be it overall,
local, or just "degeneration". Feedback is essential in
amps because it makes the amp stable with temperature
variations and manufacturable despite component variations.

Feedback has a bad reputation because a badly designed
feedback system can dramatically overshoot or oscillate.
Some older designs used excessive feedback to compensate
for the nonlinearities of lousy circuits. Well designed
feedback amps are stable and have minimal overshoot.

When transistor amps were first produced, they were inferior to
the better tube amps of the day. Designers made lots of mistakes
with the new technologies as they learned. Today, designers
are far more sophisticated and experienced than those of 1960.

Because of low internal capacitances, tube amps have very
linear input characteristics. This makes tube amps easy to
drive and tolerant of higher output-impedance sources, such
as other tube circuits and high-impedance volume controls.
Transistor amps may have higher coupling from input to output
and may have lower input impedance. However, some circuit
techniques reduce these effects. Also, some transistor
amps avoid these problems completely by using good JFET
input circuits.

There is lots of hype out on the subject as well as folklore
and misconceptions. In fact, a good FET designer can make a
great FET amp. A good tube designer can make a great tube amp,
and a good transistor designer can make a great transistor amp.
Many designers mix components to use them as they are best.

As with any other engineering discipline, good amp design
requires a deep understanding of the characteristics of
components, the pitfalls of amp design, the characteristics
of the signal source, the characteristics of the loads, and
the characteristics of the signal itself.

As a side issue, we lack a perfect set of measurements to
grade the quality of an amp. Frequency response, distortion,
and signal-to-noise ratio give hints, but by themselves are
insufficient to rate sound.

Many swear that tubes sound more "tube like" and transistors
sound more "transistor like". Some people add a tube circuit
to their transistor circuits to give some "tube" sound.

Some claim that they have measured a distinct difference between
the distortion characteristics of tube amps and transistor amps.
This may be caused by the output transformer, the transfer
function of the tubes, or the choice of amp topology. Tube amps
rarely have frequency response as flat as the flattest
transistor amps, due to the output transformer. However, the
frequency response of good tube amps is amazingly good.

For more information on tubes, get one of the following old
reference books, or check out Glass Audio Magazine (see the
magazine section of the FAQ for more info on Glass Audio).

The Receiving Tube Manual (annual up to 1970)
The Radiotron Designers Handbook
Fundamentals of Vacuum Tubes" by Eastman 1937, McGraw-Hill

11.14 What about swapping op-amps?
Many components use ICs called op amps as audio amplifiers.
Earlier op amps had poor sound quality, especially if misused.
Some engineers with a strong background in ICs and op amps
learned that they could improve sound if they replaced slow,
noisy, low slew-rate, or otherwise bad op amps with better ones.
Some less informed people tried doing the same thing and made
the sound worse.

One pitfall with op amp swapping is that some op amps are more
prone to unwanted oscillation than others. The faster the op
amp, the more likely it will cause an unwanted oscillation,
which will really damage the sound. For that reason, Joe may
succeed in replacing 741 op amps with 5534 op amps in his gear,
and you may fail. It is dependent on design, layout, etc.

As technology and design expertise improves, audio op amps
get better and swapping is getting less and less useful.
Newer op amps are displacing yesterday's best, and sound
surprisingly similar to straight wire.

Still, there are different op amps for different purposes.
Bipolar op amps are ideal for preamplifiers where noise is
critical. The OP-27, OP-37, LT1028, and LT1115 are very well
received for phono preamps, head amplifiers, and microphone
preamplifiers. Bipolar op amps are also more practical for
signals with low source impedance.

FET devices like the OPA604 and OPA2604 have higher slew rate,
higher bandwidth, and lower input current. These op amps are
better for line-level inputs and high source-resistance signals.
Some amplifiers, like the OP-37 and LT1115 achieve higher
bandwidth by using less internal compensation. These amplifiers
are not unity gain stable, and should not be used in circuits
with low closed loop gain or large feedback capacitors.

Some of the better op amps for audio as of today include
(* means highly recommended):
Single Dual
AD845* AD842
AD847 AD827
AD797* NE5535
NE5534 NE5532
OP-27 AD712
LT1115* LM833
AD811 OPA2604*
AD841 OP249*
HA5112*
LT1057
LT1028
AD744
SSM2016

With op amp part numbers, there is a lot of room for confusion.
Here is a guide to the numbers that is often accurate:

Op amp part numbers start with a manufacturer's prefix:
Analog Devices uses AD
Burr Brown uses OPA
Linear Technology uses LT
Motorola uses MC
National uses LF and LM
PMI uses OP
Signetics uses NE and SE
TI uses TL
This can be confused because if TI copies a Signetics op amp,
they may assume the Signetics prefix, or they may use their own.
Fortunately, if the part numbers are the same, circuitry is
almost exactly the same, as is the performance. (Note: almost)

The next thing in the part number is two, three, four or five
digits. This is invariably the key to the part. If the numbers
are the same, the parts are almost surely the same. For
example, an LM357N and an LM357J are electrically identical and
sound the same.

Next is a letter or two indicating the op amp package and
possibly how it has been tested and what tests it passed.
Unfortunately, manufacturers haven't standardized these letters.
Fortunately, you almost never care. If it is a dual-inline
(DIP) package and you are replacing a DIP, you shouldn't have to
worry whether or not it is ceramic or molded. Likewise, you
rarely care if it has 100uV offset or 4mV offset for audio.
Finally, you don't care if it wasn't tested at elevated
temperatures because you will use it in your house, inside well
ventilated gear.

So in general, an NE5532J is a TL5532N, and an AD827JN will
sound the same as an AD827LD. If you aren't sure about some
detail, call or write the IC maker and ask for a data sheet on
the parts in question. They will always send data sheets for
free, and these data sheets contain details on the various part
numbers, internal circuitry, and electrical characteristics.

11.15 Where can I buy electronic parts to make an amplifier?
There are many commercial parts distributors that sell only to
Corporations. Their prices are often list, their supply is
often good, and their service varies. Common ones are Arrow
Electronics, Gerber Electronics, Hamilton Avnet, and Schweber
Electronics. See your local phone book.

There are also distributors that cater to smaller buyers. These
typically have only one office. Some have lousy selections but
great prices. In the following list, (+) means that the dealer
has a good reputation, (?) means that the dealer has
insufficient reputation, and (X) means that some have reported
problems with this dealer. (C) means they have a catalog.

All Electronics Corporation (Surplus, Tools, Parts) (?) (C)
PO Box 567
Van Nuys CA 90408 USA
800-826-5432
818-904-0524
Allied Electronics (Full Line of Parts) (+) (C)
800-433-5700
Antique Electronics Supply (Tubes, capacitors, etc) (?)
688 First St
Tempe AZ 85281 USA
602-894-9503
Billington Export Ltd. (Valves and CRTs)
I E Gillmans Trading Estate
Billinghurst, RH14 9E3 United Kingdom
Tel (0403) 784961
Chelmer Valves (Valves)
130 New London Rd
Chelmsford, CM2 0RG United Kingdom
DigiKey Corporation (Full Line of Parts) (+) (C)
701 Brooks Avenue South
PO Box 677
Thief River Falls MN 56701-0677 USA
800-344-4539
Electromail (Wide range of parts, similar to Radio Shack)
PO Box 33, Corby, Northants NN17 9EL United Kingdom
Tel 0536 204555
Langrex Supplies Ltd. (Obsolete Valves)
1 Mayo Rd.
Croyden, Surrey, CR0 2QP United Kingdom
Maplin (General parts supplier)
PO Box 3
Rayleigh, Essex, SS6 2BR United Kingdom
Tel 01702 556751.
Marchand Electronics (?) (Crossover kits)
1334 Robin Hood Lane
Webster NY 14580 USA
716-872-5578
MCM Electronics (Speakers, A/V Repair Parts, Etc) (+) (C)
650 Congress Park Dr
Centerville Ohio 45459-4072 USA
513-434-0031 or 800-543-4330
MesaBoogie (Tubes, instrument speakers) (?)
707-778-8823
Michael Percy (Connectors, MIT, Wonder Caps, Buf-03) (+)
PO Box 526
Inverness CA 94936 USA
415-669-7181 Voice
415-669-7558 FAX
Mouser Electronics (Full Line of Parts) (+) (C)
PO Box 699
Mansfield TX 76063-0699 USA
800-346-6873
817-483-4422
Newark Electronics (Full Line of Parts) (+) (C)
Old Colony Sound (Audio parts and audio kits) (+) (C)
PO Box 243
Peterborough NH 03458-0243 USA
603-924-9464
Parts Express (Speakers, Cables, Connectors) (+) (C)
340 East First Street
Dayton OH 45402-1257 USA
513-222-0173
PM Components (High end audio parts and valves)
Springhead road
Gravesend
Kent, DA11 3HD United Kingdom
Tel (0474) 560521
PV Tubes (Valves and Transformers)
104 Abbey St.
Accrington, Lancs, BB5 1EE United Kingdom
Tel (0254) 236521
Radio Shack (Parts, Low-End Audio) (+) (C)
RATA Ltd (Audio parts and cables: Kimber, Ansar, Vishay)
Edge Bank House
Skelsmergh
Kendal, Cumbria, LA8 9AS United Kingdom
Tel (0539) 823247
SJS Acoustics (High-end parts, valves, transformers)
Ben-Dor
Lumb Carr Rd.
Holcombe, Bury, BL8 4NN United Kingdom
Sowter Transformers (Mains and output transformers)
EA Sowter Ltd. PO box 36
Ipswich, IP1 2EL United Kingdom
Tel (0473) 219390
Tanner Electronics (Surplus Parts) (+)
214-242-8702
Toroid Corp of Maryland (Toroidal power transformers) (+)
(also sells without secondary, ready to finish)
608 Naylor Mill Rd
Salisbury MD 21801-9627 USA
410-860-0300
Triode Electronics (Tubes, transformers, boxes) (?)
2010 Roscoe St
Chicago IL 60618 USA
312-871-7459
Welborne Labs (Connectors, Linear Tech ICs, Wima Caps) (?)
P.O. Box 260198
971 E. Garden Drive
Littleton, CO 80126 USA
303-470-6585 Voice
303-791-5783 FAX
Wilson Valves (Valves)
28 Banks Ave.
Golcar, Huddersfield, HD7 4LZ United Kingdom

11.16 Where can I buy audio amplifier kits?
Alas, Heath is no longer making Heathkits. Alternatives:
AP Electronics (High grade components and kits)
20 Derwent centre
Clarke St.
Derby DE1 2BU United Kingdom
Audio Note (Audio parts, kits, and high quality amps)
Unit 1
Block C, Hove Business Centre
Fonthil Rd.
Hove, East Sussex, BN3 6HA United Kingdom
Tel (0273) 220511
Audio Synthesis (Many kits from Ben Duncan designs) (?)
99 Lapwind Lane
Manchester M20 0UT, UK
061-434-0126 Voice
060-225-8431 FAX
Crimson (UK) (?)
Hafler (+) (may be out of the kit business)
Hart Electronic Kits (Audiophile kits and components)
Penylan Mill
Oswestry
Shropshire, SY10 9AF United Kingdom
Tel (0691)652894
Mark V Electronics (?)
8019 E Slauson Ave
Montebello CA 90640 USA
800-423-3483
213-888-8988
Old Colony Sound (+) (See 11.15)
PAiA Electronics (?) (Musician-related kits)
3200 Teakwood Lane
Edmond OK 73013 USA
405-340-6378
Sage Audio (Various kits UK$95 to UK$430)
Construction House
Bingley
West Yorkshire
England BD16 4JH UK
Sound Values (+) (See 11.7)
185 N Yale Avenue
Columbus OH 43222-1146 USA
614-279-2383

11.17 Where can I read more about building amplifiers, preamps, etc.?
Audio Amateur Magazine
Audio Amateur Publications
PO Box 494
Peterborough NH 03458 USA
603-924-9464
Analog Devices Audio/Video Reference Manual
Electronic Music Circuits, by Barry Klein
Howard D Sams & Co ISBN 0-672-21833-X
Electronics Australia (Magazine with audio projects)
AUD47 per year 12 issues, often discounted
PO Box 199
Alexandria, Austrailia
+612 353 9944 or +612 353 6666
Elektor Electronics (How it works and you-build articles)
(no longer published in US. Still available in Europe)
PO Box 1414
Dorchester DT2 8YH, UK
Enhanced Sound: 22 Electronic Projects for the Audiophile
(Some basic projects and some "how it works")
by Richard Kaufman
Tab Books #3071/McGraw Hill
ISBN 0-8306-9317-3
Glass Audio Magazine
Audio Amateur Publications
PO Box 494
Peterborough NH 03458 USA
603-924-9464
IC Op-Amp Cookbook, Third Edition by Walter G. Jung
ISBN 0672-23453-4, Howard W. Sams, Inc.
Journal of the Audio Engineering Society (Theory & Experiment)
Audio Engineering Society
60 East 42nd Street
New York City NY 10165-0075 USA
212-661-2355
Popular Electronics
Radio-Electronics
Radiotron Designer's Handbook, Fourth Edition (old, tube info)
The Technique of Electronic Music, by Thomas H Wells
Schirmer Books ISBN 0-02-872830-0
Vacuum Tube Amplifiers, MIT Radiation Lab series
Wireless World
Some of the above titles, as well as a catalog of technical
books, are available from:
OpAmp Technical Books, Inc.
1033 N Sycamore Avenue
Los Angeles CA 90038 USA
800-468-4322 or 213-464-4322

11.18 What is Amplifier Class A? What is Class B? What is Class AB?
What is Class C? What is Class D?

All of these terms refer to the operating characteristics
of the output stages of amplifiers.

Briefly, Class A amps sound the best, cost the most, and are the
least practical. They waste power and return very clean signals.
Class AB amps dominate the market and rival the best Class A
amps in sound quality. They use less power than Class A,
and can be cheaper, smaller, cooler, and lighter. Class D amps
are only used for special applications like bass-guitar amps and
subwoofer amps. They are even smaller than Class AB amps and
more efficient, yet are often limited to under 10kHz (less than
full-range audio). Class B & Class C amps aren't used in audio.

In the following discussion, we will assume transistor output
stages, with one transistor per function. In some amplifiers,
the output devices are tubes. Most amps use more than one
transistor or tube per function in the output stage to increase
the power.

Class A refers to an output stage with bias current greater than
the maximum output current, so that all output transistors are
always conducting current. The biggest advantage of Class A
is that it is most linear, ie: has the lowest distortion.

The biggest disadvantage of Class A is that it is inefficient,
ie: it takes a very large Class A amplifier to deliver 50 watts,
and that amplifier uses lots of electricity and gets very hot.

Some high-end amplifiers are Class A, but true Class A only
accounts for perhaps 10% of the small high-end market and none
of the middle or lower-end market.

Class B amps have output stages which have zero idle bias
current. Typically, a Class B audio amplifier has zero bias
current in a very small part of the power cycle, to avoid
nonlinearities. Class B amplifiers have a significant advantage
over Class A in efficiency because they use almost no
electricity with small signals.

Class B amplifiers have a major disadvantage: very audible
distortion with small signals. This distortion can be so bad
that it is objectionable even with large signals. This
distortion is called crossover distortion, because it occurs at
the point when the output stage crosses between sourcing and
sinking current. There are almost no Class B amplifiers on the
market today.

Class C amplifiers are similar to Class B in that the output
stage has zero idle bias current. However, Class C amplifiers
have a region of zero idle current which is more than 50% of
the total supply voltage. The disadvantages of Class B
amplifiers are even more evident in Class C amplifiers, so
Class C is likewise not practical for audio amps.

Class A amplifiers often consist of a driven transistor
connected from output to positive power supply and a constant
current transistor connected from output to negative power
supply. The signal to the driven transistor modulates the
output voltage and the output current. With no input signal,
the constant bias current flows directly from the positive
supply to the negative supply, resulting in no output current,
yet lots of power consumed. More sophisticated Class A amps
have both transistors driven (in a push-pull fashion).

Class B amplifiers consist of a driven transistor connected
from output to positive power supply and another driven
transistor connected from output to negative power supply.
The signal drives one transistor on while the other is off,
so in a Class B amp, no power is wasted going from the
positive supply straight to the negative supply.

Class AB amplifiers are almost the same as Class B amplifiers
in that they have two driven transistors. However, Class
AB amplifiers differ from Class B amplifiers in that they
have a small idle current flowing from positive supply to
negative supply even when there is no input signal. This idle
current slightly increases power consumption, but does not
increase it anywhere near as much as Class A. This idle current
also corrects almost all of the nonlinearity associated with
crossover distortion. These amplifiers are called Class AB
rather than Class A because with large signals, they behave like
Class B amplifiers, but with small signals, they behave like
Class A amplifiers. Most amplifiers on the market are Class AB.

Some good amplifiers today use variations on the above themes.
For example, some "Class A" amplifiers have both transistors
driven, yet also have both transistors always on. A specific
example of this kind of amplifier is the "Stasis" (TM) amplifier
topology promoted by Threshold, and used in a few different
high-end amplifiers. Stasis (TM) amplifiers are indeed
Class A, but are not the same as a classic Class A amplifier.

Class D amplifiers use pulse modulation techniques to achieve
even higher efficiency than Class B amplifiers. As Class B
amplifiers used linear regulating transistors to modulate output
current and voltage, they could never be more efficient than
78%. Class D amplifiers use transistors that are either on or
off, and almost never in-between, so they waste the least amount
of power.

Obviously, then, Class D amplifiers are more efficient than
Class A, Class AB, or Class B. Some Class D amplifiers have
>80% efficiency at full power. Class D amplifiers can also have
low distortion, although not as good as Class AB or Class A.

Class D amplifiers are great for efficiency. However they are
awful for other reasons. It is essential that any Class D amp
be followed by a passive low-pass filter to remove switching
noise. This filter adds phase shift and distortion. It also
limits the high frequency performance of the amplifier, such
that Class D amplifiers rarely have good treble. The best
application today for Class D amplifiers is subwoofers.

To make a very good full range Class D amplifier, the switching
frequency must be well above 40kHz. Also, the amplifier must be
followed by a very good low-pass filter that will remove all of
the switching noise without causing power loss, phase-shift, or
distortion. Unfortunately, high switching frequency also means
significant switching power dissipation. It also means that the
chances of radiated noise (which might get into a tuner or phono
cartridge) is much higher.

11.19 Why do I hear noise when I turn the volume control? Is it bad?
Almost all volume controls are variable resistors. This goes
for rotary controls and slide controls. Variable resistors
consist of a resistive material like carbon in a strip and a
conductive metal spring wiper which moves across the strip as
the control is adjusted. The position of the wiper determines
the amount of signal coming out of the volume control.

Volume controls are quiet from the factory, but will get noisier
as they get older. This is in part due to wear and in part due
to dirt or fragments of resistive material on the resistive
strip. Volume control noise comes as a scratch when the control
is turned. This scratch is rarely serious, and most often just
an annoyance. However, as the problem gets worse, the sound of
your system will degrade. Also, as the problem gets worse, the
scratching noise will get louder. The scratching noise has a
large high-frequency component, so in the extreme, this noise
could potentially damage tweeters, although I have never seen
a documented case of tweeter damage due to control noise.

Some controls are sealed at the factory, so there is no
practical way to get inside and clean out the dirt. Others have
access through slots or holes in the case. These open controls
are more subject to dirt, but also are cleanable. You can clean
an open volume control with a VERY QUICK squirt of lubricating
contact cleaner, such as Radio Shack 64-2315. Even better is a
non-lubricating cleaner, such as Radio Shack 64-2322. With any
cleaner, less is better. Too much will wash the lubricant out
of the bearings and gunk up the resistive element.

You can also clean some controls by twisting them back and forth
vigorously ten times. This technique pushes the dirt out of the
way, but is often just a short term fix. This technique is also
likely to cause more wear if it is done too often. Try to do it
with the power applied, but the speaker disconnected, so that
there is some signal on the control.

Sealed and worn controls should be replaced rather than cleaned.
Critical listeners claim that some controls, such as those made
by "Alps" and by "Penny and Giles" sound better than common
controls. Regardless of the brand, however, it is essential
that whatever control you buy have the same charcteristics as
the one you are replacing. For most volume controls, this
means that they must have AUDIO TAPER, meaning that they are
designed as an audio volume control, and will change the level
by a constant number of dB for each degree of rotation.

Badly designed circuits will wear out volume controls very
quickly. Specifically, no volume control is able to work for
a long time if there is significant DC current (or bias current)
in the wiper. If the output of the control goes to the input of
an amplifier, the amplifier should be AC coupled through a
capacitor. If there is a capacitor there, it might be leaky,
causing undesirable DC current through the volume control.

If you have a circuit with no blocking capacitor or a bad
blocking capacitor, you can add/replace the capacitor when
you replace the control. However, get some expert advise
before modifying. If you add a capacitor to a device which
doesn't have one, you will have to make other modifications
to insure that the amplifier has a source for its bias current.

11.20 What is amplifier "bridging" or "monoblocking"? How do I do it?
When you're told a stereo power amplifier can be bridged,
that means that it has a provision (by some internal
or external switch or jumper) to use its two channels
together to make one mono amplifier with 3 to 4 times the
power of each channel. This is also called "Monoblocking"
and "Mono Bridging".

Tube amps with multiple-tap output transformers are simple to
bridge. Just connect the secondaries in series and you get
more power. The ability to select transformer taps means that
you can always show the amplifier the impedance it expects, so
tube amp bridging has no unusual stability concerns.

The following discussion covers output transformer-less amps.
Bridging these amps is not so simple. It involves connecting
one side of the speaker to the output of one channel and the
other side of the speaker to the output of the other channel.
The channels are then configured to deliver the same output
signal, but with one output the inverse of the other. The
beauty of bridging is that it can apply twice the voltage to
the speaker. Since power is equal to voltage squared divided
by speaker impedance, combining two amplifiers into one can
give four (not two) times the power.

In practice, you don't always get 4 times as much power. This
is because driving bridging makes one 8 ohm speaker appear like
two 4 ohm speakers, one per channel. In other words, when you
bridge, you get twice the voltage on the speaker, so the
speakers draw twice the current from the amp.

The quick and dirty way to know how much power a stereo amp can
deliver bridged to mono, is to take the amp's 4 ohm (not 8 ohm)
power rating per channel and double it. That number is the
amount of watts into 8 ohms (not 4 ohms) you can expect in mono.
If the manufacturer doesn't rate their stereo amp into 4 ohms,
it may not be safe to bridge that amp and play at loud levels,
because bridging might ask the amp to exceed its safe maximum
output current.

Another interesting consequence of bridging is that the amplifier
damping factor is cut in half when you bridge. Generally, if you
use an 8 ohm speaker, and the amplifier is a good amp for driving
4 ohm speakers, it will behave well bridging.

Also consider amplifier output protection. Amps with simple
power supply rail fusing are best for bridging. Amps that rely
on output current limiting circuits to limit output current
are likely to activate prematurely in bridge mode, and virtually
every current limit circuit adds significant distortion when it
kicks in. Remember bridging makes an 8 ohm load look like 4 ohms,
a 4 ohm load look like 2 ohms, etc. Also, real speakers do not
look like ideal resistors to amps. They have peaks and dips in
impedance with frequency, and the dips can drop below 1/2 the
nominal impedance. They also have wildly varying phase with
frequency.

Finally, some amplifiers give better sound when bridged than
others. Better bridging amps have two identical differential
channels with matched gain and phase through each input, left
and right, inverting and non-inverting. Simpler bridging
amplifiers have one or two inverting channels, and run the
output of one into the input of the second. This causes the
two outputs to be slightly out of phase, which adds distortion.
There are also other topologies. One uses an additional stage to
invert the signal for one channel but drives the other channel
directly. Another topology uses one extra stage to buffer the
signal and a second extra stage to invert the signal. These are
better than the simple master/slave arrangement, and if well
done, can be as good as the full differential power amp.

[neid...@uicc.com]

Archive-name: AudioFAQ/part5
Last-modified: 1996/11/26
Version: 2.07

12.0 Speakers:

12.1 What should I listen to when evaluating speakers?
The most important thing is to listen to recordings that
you *know*. Any good salesman will play you recordings
that highlight that particular speaker. Do not be embarrassed
about bringing a stack of CDs with you to the hi-fi shop.

Do not spend your valuable listening time switching between a
dozen pairs every 3 seconds. If you are shopping at a quality
store, the dealer will, from the description of your room, your
size requirements, your musical tastes, and your budget, be able
to show you a couple of pairs that will be close to what you
want. Spend several minutes listening to each. When you think
you're close, don't be embarrassed about spending half an hour
or more listening to the speakers. You're going to have them in
your home for a lot longer, and many speakers will cause
"listening fatigue" after a short time. Make sure you really
like them before you hand over money.

One thing to try is well recorded "Spoken Word" records; most
people have a very good ability to tell when a speaking voice
sounds unnatural, even if they've never heard the person
speaking live. If you play an acoustic instrument, find
something that features that instrument solo, or in a small
group; make sure it really sounds like it should. Almost
everyone has heard a live piano. Piano can be very revealing.

Blues, jazz, folk, or 'easy listening' music with simple
instruments and a female vocalist is also revealing. Well done
female singing voices provide a very good test of a system's
response. Try something simple and soft, which will let you
hear any noises coming from the system; and something complex,
with lots of instruments all happening at once, to make sure the
system doesn't go muddy when things get complicated. And, of
course, try a few of your favorites, and see if you like what
happens with them.

If a sales person suggests some music to listen to, the odds are
that it isn't the most revealing. Sales people tend to suggest
things which sound great. Anything you own and like is good,
because you know it and are happy to listen to it carefully. No
matter how good the recording, if you don't like Opera, you
won't listen to it as carefully as your favorite, scratchy,
1940's rhythm and blues.

Most important is to listen to something you are familiar with.
Even if a recording is flawed (and what ones aren't?), how is it
different from your normal setup? Some of the most important
differences are "Gee, I never heard that instrument before!"

12.2 What should I listen for when evaluating speakers?
When comparing two speakers side-by-side, doing an AB
comparison, be extremely careful to match the levels before
evaluating. A slight level difference can make one speaker
sound better, even though the difference may not be perceived
as a level difference. Some claim that you will be influenced
by a difference of less than 1/2 dB!

First and foremost, the sound should be natural. If you listen
to vocals, close your eyes and try to picture someone singing in
the same room with you. Does it sound realistic? Likewise with
instruments. You selected recordings of instruments that you
like and have heard live. Do they sound like what you remember
them sounding like live?

Your very first impression should be something like "what nice
sound". If your initial gut reaction is "gosh, what a lot of
detail", the system is likely to be heavy in the treble (often
interpreted by beginners as "more detailed") and you'll probably
find that annoying after a while. If your first reaction is
"hey, what powerful bass", then the system is probably
bass-heavy, rather than ideal. The most common mistake for
beginners is to buy a system with REALLY powerful bass, because
it sounds "impressive" at first. After a while, though, you'll
get tired of being thumped on the head by your music.

Not to say that good bass and treble aren't important. But your
first realization should be that the music is all there, and
that it comes together as good music, without one particular
part trying to dominate it. Sit back and listen to it for a
bit. You should be able to pick out the individual instruments
if you want. They shouldn't force themselves on you, and you
should also be able to hear the music as a single piece, the sum
of its parts, without feeling like each of the instruments is
trying to grab your attention away from the others.

You should check how things sound with the amp turned up, and
also with it turned down to a fairly low volume level. Some
speakers which sound very nice at low levels begin to sound
confused, like they can't cope, when turned up. On the other
hand, some sound nice loud, but sound thin and bodiless when you
turn them down a bit. With the spoken word or female vocalist,
listen for "sibilance", a pronounced 'hiss' at the end of 's'
and 'z' sounds. It shouldn't be there. Most planar speakers
just can't play very loud. Whatever you hear, do some
auditioning at the maximum volume you anticipate ever wanting.

It is acceptable and sometimes desirable to switch the stereo to
mono to evaluate naturalness. Mono is a good test of both the
room and the speakers. The image should be rock-solid dead
center, and not move with signal or level. If it isn't perfect
mono, it will be nearly impossible to create a good stereo.

A speaker in a large box is capable of producing low frequencies
at higher volumes with more efficiency than a small box, but
that doesn't mean that a small box can't have great bass, it
just won't be as efficient and can't play as loud.

Good speakers can "recreate a natural stereo sound stage",
placing some instruments to the left of the left speaker, some
sounds in the middle, and some to the right of the right
speaker. Poorer speakers make it harder to localize voices.

12.3 Why use a subwoofer? Will it help? One or two?
One reason to get a subwoofer is to add bass to a feeble system.
A second reason is to move the lowest frequencies to a separate
driver, and thereby reduce a particular kind of distortion
caused by the nonlinear mixing of different sounds, called
"intermodulation distortion". A third is to increase the power
handling ability of the system and the overall reliability. All
are valid reasons, but it isn't so simple.

To improve the sound of a good speaker system, a subwoofer must
"integrate smoothly" into the system, extending the bass without
causing peaks or dips. Many subwoofers have a crossover that
goes between your amp and your main speaker which sends the lows
to the subwoofer and sends the higher frequency signals to the
main speakers. This may damage the perfect sound of a good
system, it may sound similar, or it may sound better.

Most good small speaker systems have a bass peak at resonance,
which attempts to compensate for the absence of lower bass.
Like it or not, this is the only way to make a small system
sound realistic. If the small system is done well, the
improvement you will get from a subwoofer will be small, but
still real and, to many, significant.

Correctly done, a good subwoofer will enhance the sound of a
good small-box system. Done wrong or haphazardly, anything is
possible. Even a fine large speaker system might benefit from
careful addition of a subwoofer. However, the better the
original system, the more likely it will be that a modest
subwoofer will do more harm than good.

Low frequencies travel less directionally than high frequencies,
so many people say that only one subwoofer is required for good
sound. This is true to some extent, but not completely true.

There are a few reasons for getting two subwoofers. Some feel
that you need two subwoofers to accurately reproduce the stereo
image, no matter how little low-frequency stereo information
there is. Others feel that two subwoofers are much easier to
set up in a room, less likely to excite standing waves in the
room, and give smoother sound.

A third reason is that two subwoofers can produce twice the
sound of one. Finally, even though subwoofers produce very low
frequency sound and very low frequency sound is non-directional,
subwoofers also have output at 100 Hz, and sound at 100 Hz is
directional, so two subwoofers will give a slightly better
stereo image than one. Assuming, of course, that the two are
separated by at least two feet.

Finally, even though original source signals rarely contain any
music with stereo components below 50Hz, there may be some noise
component with low-frequency out-of-phase noise. This unusual
noise might add a sense of space to a recording if it is
reproduced by a system in which the woofers are very far apart.

It is still true that a single good subwoofer, correctly added
to a system will help the sound but two will probably help more.

12.4 How do you connect a subwoofer to a stereo?
Many subwoofers contain their own amplifier and crossover.
For these, take the preamp output and feed it into the subwoofer
amp input and also into the main amplifier.

For other subwoofers, just run them in parallel with your main
speakers, or combine them into your system with your own bass
amplifier and crossover.

Some A/V receivers contain a splitter specifically for use with
subwoofers. If you have one of these, you will either want a
separate amplifier for your subwoofer or an amplified subwoofer.

Consult the manual which comes with the subwoofer.

12.5 What do I need for surround sound?
"Surround Sound" has referred to a number of different products
over the years. Many mass-fi receivers have "Surround Sound"
buttons that do little more than muck up the imaging.

In recent years the term "Surround Sound" has become synonymous
with the surround systems produced by Dolby Laboratories. Dolby
Surround comes in several flavors, such as passive surround
(which simply decodes the phase information and sends it to the
rear speakers) and the more advanced system called Pro Logic.
Pro Logic system uses computer circuitry to route directional
information to the appropriate speakers.

Generally, one needs at least two more speakers beyond the main
stereo pair. Advanced Pro Logic systems such as the Lexicon and
Fosgate can accommodate several more speakers beyond the two
additional ones (usually placed in the rear). Often one can
find Pro Logic systems with two front, two rear, two side, as
well as a center channel speaker for dialogue.

12.6 I was just approached (accosted?) by a couple of kids driving a
van that said they had some GREAT speakers to sell. They are
overstocks, used by major recording studios and DJs or even
hot, and they normally sell for $1000/pr, but they'll let
me have them for just $399. Am I getting ripped off?

Yes, you most certainly are. The speakers these people sell
are none of what they describe. They are never used in
studios. There might be one or two DJs out there that use
them because they can't afford anything else. They are not
overstocks, and in all likelihood, they are NOT HOT!.

Are they good speakers? No, they're, at best, no better than
the big boom boxes you find in $400 rack systems in department
stores. They are worth no more than what the kids paid for
them ($100/pr).

The speakers go under names like "Acoustic Monitor DB IV",
"Acoustic Linear," "Pro-Poly," "Audio Reference 4350" and so
on. They all "feature" things like "liquid cooled 3" tweeter",
poly-cone 12" woofer, fantastic (but impossible) frequency
response, 98 db/watt sensitivity, and so on. The brand
names are remarkably similar to reputable firms, but
different enough to delay law suits.

These speaker are made by a couple of manufacturers with the
intent of being sold exactly this way. They cost the kids in
the van about US $100 a pair, and the kids are given minimal
training about what kinds of stories to use, what parking lots
are the most likely to generate sales (department store parking
lots near colleges in September is a great time for these guys).
Anything over the US $100 the kids paid is pure profit.

Stay away, you're getting ripped off.

12.7 What speakers should I consider in the $XXX/pair price range?
This is probably the most commonly asked question on rec.audio,
and also the most impossible to answer. The market keeps
changing, everyone has different tastes, and no one has the time
to listen to even 10% of the products available in any country.
Also, many good products are only available in specific regions
or countries.

If you really want recommendations and are willing to listen to
the opinions of others, check the past few issues of Stereophile
Magazine. Although they are strongly biased towards very
expensive gear and have their own particular other biases, they
do steer you to some very good equipment in their
frequently-updated list of "RECOMMENDED COMPONENTS".

12.8 Can you build better speakers than you can buy?
Some people can build better than you can buy. These people are
either experts, golden ears, extremely well equipped, inspired,
or a combination of the above.

Some companies have plans available to entice you into buying
their drivers: Audio Concepts, Audax, Dynaudio, Focal, KEF, and
Scanspeak. Your success rate with these plans will probably be
very good IF your cabinetry skills are very good and IF you
follow the plans precisely. If you deviate (as everyone does),
anything is possible.

Stereophile has published three different plans designed by Dick
Olsher which are similar two-way ported systems. A recent one of
these was in Stereophile Nov '90, pages 94-127. Audio Magazine
published a plan called "The Pitts" by Ken Kantor, in Audio, Nov
'88 pages 65-71 continued in Dec '88 pages 73-77. This plan is
a two-way sealed box.

I have built one published design and one manufacturer's design.
I believe that both met my expectations. They took me a long
time to build, taught me a lot, were fun projects, and sounded
good when finished.

I also believe that a commercial system which cost what my parts
cost will never sound anywhere near as good as the one I build.
If you consider $2/hour for my time, however, building is
financial suicide.

Designing your own system is even more a can-of-worms, and
should be left to those with either a strong stomach, a very
forgiving ear, infinite resources, or excellent guidance.

12.9 Where can I read more about speaker building?
Europe's Greatest Speaker Designs
Solen Electronique
4470 Avenue Thibault
St.-Hubert, QC J3Y 7T9 Canada
Voice 514-656-2759
FAX 514 443-4949
High Performance Loudspeakers by Martin Colloms
Speaker Builder Magazine

Audio Amateur Publications
PO Box 494
Peterborough NH 03458 USA
603-924-9464

Synergetic Audio Concepts Classes and Newsletters
Syn-Aud-Con teaches classes on Audio and Acoustics
12370 W. Co. Rd. 100 N.
Norman IN 47264 USA
812-995-8212
The Loudspeaker Design Cookbook, Fifth Edition
by Vance Dickason (C) 1995
ISBN 1-882580-10-9
$34.95 + $4.45 S&H from:
Old Colony Sound Lab

PO Box 243
Peterborough NH 03458-0243 USA
603-924-9464

$30.00 + approx. $3 Shipping from:
Madisound
8608 University Green; Box 4283
Madison WI 53711 USA
608-831-3433
$30.00 + ??? S&H from:
Parts Express
340 E. First St
Dayton OH 45402 USA
800-338-0531

12.10 Where can I buy speaker drivers?
A & S Speakers (Broad line)
(Also known as Just Speakers)
4075 Sprig Drive
Concord CA 94520 USA
Voice 510-685-6400
FAX 510-603-2724
http://www.justspeakers.com
Audio Concepts (Their own kits plus drivers)
901 South 4th Street
LaCrosse WI 54602 USA
Voice 608-784-4570
http://www.audioc.com
Phil Baker (Surplus cabinets only)
546 Boston Avenue
Medford MA 02155 USA
Bandor Design & Development Studios (Aluminium coned speakers)
11 Penfold Cottages
Penfold Lane
Holmer Green
Bucks, HP15 6XR United Kingdom
Tel. (0494) 714085
DBS Audio (Speaker kits and crossovers)
PO Box 91, Bury St.
Edmunds, Suffolk, IP30 0NF United Kingdom
Tel (0284) 828926
Drexler Audio Systems (Bandor Speaker Distributor)
14 Rose Lane
Rosemont PA 19010 USA
Falcon Electronics (Drivers and cross overs)
Tabor House
Mulbarton
Norfolk, NR14 8JT United Kingdom
Tel. (0508) 78272
Faraday Sound (Concrete loudspeaker cabinets)
248 Hall Road
Norwich, NR1 2PW United Kingdom
Tel. (0603) 762967
Gold Sound (Broad line including pro speakers)
PO Box 141
Englewood CO 80151 USA
303-789-5310
Madisound (Broad line)
8608 University Green
Box 4283
Madison WI 53711 USA
608-831-3433
http://www.itis.com/madisound/
Meniscus (Broad line)
2442 28th Street SW Ste D
Wyoming MI 49509 USA
616-534-9121
Parts Express (Broad line)

340 East First Street
Dayton OH 45402-1257 USA
513-222-0173

Solen Electronique (Airborne, Audax, Ceratech, Dynaudio, Eton,
Lpg, Morel, Peerless, Scan-Speak, Seas, Solen, Vifa)
4470 Avenue Thibault
St.-Hubert, QC J3Y 7T9 Canada
Voice 514-656-2759
FAX 514 443-4949
The Speaker Co (Large range of drive units plus speaker kits)
Unit 9, Waterside Mill
Waterside, Macclesfield, SK11 7HG. United Kingdom
Tel. (0625) 500507
Speakers Etc.
1828 West Peoria Avenue
Phoneix AZ 85029 USA
602-944-1878
SRS Enterprises (Pyle, Pioneer)
318 South Wahsatch Avenue
Colorado Springs CO 80903 USA
719-475-2545
Wilmslow Audio (Kits and drive units. KEF, Dynaudio, Audax, SEAS,
Peerless, Scanspeak, Morel)
Wellington Close
Parkgate Trading Estate
Knutsford, Cheshire, WA16 8DX United Kingdom
Tel (0565) 650605
Zalytron (Broad line including kits)
469 Jericho Turnpike
Mineola NY 11501 USA
516-747-3515

12.11 Where can I buy loudspeaker kits?
Audiocab (Speaker kits and cabinets)
9 Skewbridge Close
Wooten Bassett, Swindon, SN4 7DW United Kingdom
Tel (0793) 848437
Audio Concepts, Inc. (Wide range of kits. Catalog available)
(see 12.10, above)
Fried Products (Parts kits starting $550. Catalog available)
(Emphasizes high-end transmission line speakers)
(Parts kits have plan, crossover, and driver)
1323 Conshocken Road
Norristown, PA 19401 USA
610-277-1014 or 800-255-1014
IPL Acoustics (Kits using SEAS, Morel, Audax, and Visaton)
2 Laverton Road
Westbury, Wiltshire, BA13 BRS United Kingdom
Tel (0373) 823333
Mahogany Sound (Parts kits and Woodstyle kits)
(Parts kits have plan, crossover, and driver)
(Woodstyle kits also have 3/4" MDF veneered boxes)
(Prices $150/pair to $500/pair. Catalog available)
(Two way, three way & subwoofer kits)
2610 Schillingers Rd #488
Mobile AL 36695 USA
205-633-2054
Tabula Rasa (Wide range of speaker kits)
1 Silkin Dalton Close
Broadfield, Crawley
W. Sussex, RH11 9JD United Kingdom
Tel. (0293) 531190
Also see above, under suppliers for speaker drivers.

12.12 How can I improve the sound of my speakers?
The best way to change the sound of your speakers is to change
where you put them. Ideally, the speakers should be located at
ear level, in front of you, squared off between you. It's then
a matter of fiddling with a) the angles, b) the distance apart,
c) the distance from you, and d) the distance from the wall.
Just moving the speakers around in the room or putting them onto
stands can make a major difference. For more on speaker
placement, see 13.1 below.

Other than that, speaker modifications can be a can of worms, or
can produce very subtle changes, which you might prefer. For
example, you might improve a speaker by adding some cross braces
of 1"x1" wood from left to right and from front to back. This
will stiffen the cabinet and reduce speaker cabinet wall
vibrations, which probably hurt sound quality. Alas, this will
be most effective with lower-cost and poorly built speakers.

Along similar lines, some claim success putting lead wire or
epoxy putty on thin parts of the speaker to damp out resonances.
You can try doing this to the thinner parts of the speaker
"basket" or frame, or to the front "baffle" or supporting panel.

Still another "tweak" is to add sound deadening felt pads to the
inside walls of the speaker. Instead of felt pads some advocate
sand-filled latex coatings on the inside walls of speakers.
Others advocate ceramic tiles held in place with "thinset".
Still others rave about commercial products like AC Glop,
Acoustic Magic, and Bostik Sheet. However, the people who rave
about these products tend to be the same people who sell them.

Any change along the lines of adding felt, cross-bracing, or
putty will have subtle effects on the sound.

For the brave at heart, you can replace old or cheap drivers
with better ones, but the results of this one change can be very
dissatisfying if you happen to get the wrong type of driver for
that application, and may never sound right, even if you use a
similar driver. Speaker system design is still somewhat of a
science and somewhat of an art. Throwing paint on a canvas
often makes a mess.

Whatever change you try, don't "burn your bridge" home. Be sure
that you can undo whatever change you did, just in case. Many
tweaks to good speakers, no matter how well thought through,
will correct for one flaw, but create others, or correct a flaw
that the designer had cleverly used to his advantage.

12.13 How can I replace/re-cone my old speakers?
The best chance of success is to buy an identical replacement
speaker driver from the manufacturer of the system.

Second choice is to buy the exact same driver from a
distributor. This is sometimes difficult because it is hard to
learn exactly what driver the manufacturer used. In addition,
EVEN IF the manufacturer used stock speakers, they might have
used matched pairs or selected speakers by hand for an exact set
of specific characteristics.

There are companies that rebuild drivers, but they charge quite
a bit. I have heard $75 per driver. This is rarely done for
anything but very expensive commercial drivers. Speaker
manufacturers will often sell owners the materials that they
need to repair a speaker. If you are handy with delicate
things, it is worth a try.

In addition to speaker manufacturers, there are companies which
sells rebuild kits for approximately $30 per pair, containing
new foam, a special glue, and instructions. If you have a blown
or distorted voice coil, this still won't help. A few netters
have used rebuild kits from this company successfully. Contact:
Stepp Audio Technologies
PO Box 1088
Flat Rock NC 38731 USA
800-747-3692

Two other vendors of speaker repair parts are:
Parts Express (sells 8", 10", 12", & 15" repair kits)
340 E First St

Dayton OH 45402-1257 USA
513-222-0173

Simply Speakers
P. O. Box 22673
St. Petersburg FL 33742 USA
800-767-4041 or 813-571-1245

| Also check out: http://www.decware.com/surround.htm for
| directions on replacing speaker foam.

Some speaker manufacturers have very good warranties.
Electro-Voice warranties all professional products for life.
KEF has a similarly broad warranty on their speakers. Contact
the manufacturer first.

12.14 What computer programs can I use to design speakers?
There are many useful programs available, but none are complete
without a good knowledge of speaker design. Further, you will
NEED to supplement any program with hand tweaking for the best
sound. Finally, no simulation program is ever useful without
good model parameters, and the parameters which manufacturers
give you are often imperfect, so many good designers strongly
recommend your own lab measurements. The Loudspeaker Design
Cookbook (see 12.9) tells you how to measure a speaker, and also
gives enough theory to feel confident with a good program.
You can get a lot done with a simple spreadsheet and the
equations in a book like The Loudspeaker Design Cookbook.

For more information on programs for speaker design and on
speaker-design hardware, such as measurement systems, get
the archive "sahfsd**.doc" from directory:
usenet/rec.audio.high-end/Software
on "ftp.uu.net". In addition, there are other interesting
audio-related files in that directory. Look around.
That file is also available on ftp.graphics.cornell.edu in
/pub/rahe/software

12.15 Can I magnetically shield my speakers for use near a TV?
You probably will need to buy speakers that are made with an
integral magnetic shield. Magnetic shielding is usually done
by either shielding the speaker magnet or by cancellation of the
magnetic field very close to the magnet, or by both. Shielded
speakers are NOT built by lining the enclosure with metal.
While it sounds like a good idea, it doesn't work.

A common magnet shield is a mild steel cup around the magnet.
This is the cheapest shield, and is usually fairly ineffective.
It also will interfere with the speaker's critical magnet gap,
so this type of shield can hurt speaker performance by shorting
the magnetic field and reducing the magnetic flux density in the
gap, which can reduce efficiency and affect the speaker's low
frequency performance.

Cancellation is done using a reverse-polarized magnet glued to
the back of the main magnet. If done right, it can almost
completely cancel the rear stray field. In some cases it can
also increase the magnetic flux density in the gap, which may
or may not be desirable.

12.16 What are all of these abbreviations people use for speakers?
Most of these parameters are well documented in the Loudspeaker
Design Cookbook. (see 12.9) In summary:

Fs Driver free air resonance, in Hz. This is the point at
which driver impedance is maximum.
Fc System resonance (usually for sealed box systems), in Hz
Fb Enclosure resonance (usually for reflex systems), in Hz
F3 -3 dB cutoff frequency, in Hz

Vas "Equivalent volume of compliance", this is a volume of
air whose compliance is the same as a driver's
acoustical compliance Cms (q.v.), in cubic meters

D Effective diameter of driver, in meters
Sd Effective piston radiating area of driver in square meters
Xmax Maximum peak linear excursion of driver, in meters
Vd Maximum linear volume of displacement of the driver
(product of Sd times Xmax), in cubic meters.

Re Driver DC resistance (voice coil, mainly), in ohms
Rg Amplifier source resistance (includes leads, crossover,
etc.), in ohms

Qms The driver's Q at resonance (Fs), due to mechanical
losses; dimensionless
Qes The driver's Q at resonance (Fs), due to electrical
losses; dimensionless
Qts The driver's Q at resonance (Fs), due to all losses;
dimensionless
Qmc The system's Q at resonance (Fc), due to mechanical
losses; dimensionless
Qec The system's Q at resonance (Fc), due to electrical
losses; dimensionless
Qtc The system's Q at resonance (Fc), due to all losses;
dimensionless

n0 The reference efficiency of the system (eta sub 0)
dimensionless, usually expressed as %

Cms The driver's mechanical compliance (reciprocal of
stiffness), in m/N
Mms The driver's effective mechanical mass (including air
load), in kg
Rms The driver's mechanical losses, in kg/s

Cas Acoustical equivalent of Cms
Mas Acoustical equivalent of Mms
Ras Acoustical equivalent of Rms

Cmes The electrical capacitive equivalent of Mms, in farads
Lces The electrical inductive equivalent of Cms, in henries
Res The electrical resistave equivalent of Rms, in ohms

B Magnetic flux density in gap, in Tesla
l length of wire immersed in magnetic field, in meters
Bl Electro-magnetic force factor, can be expressed in
Tesla-meters or, preferably, in meters/Newton

Pa Acoustical power
Pe Electrical power

c propogation velocity of sound at STP, approx. 342 m/s
p (rho) density of air at STP 1.18 kg/m^3

12.17 What are fluid-filled (fluid-cooled, ferro-fluid) tweeters?
These tweeters are built almost exactly the same as other
tweeters. They look and act almost exactly the same, too.
The only difference is that they have a small, controlled
amount of a special fluid inserted into the gap between the
magnet and the voice coil.

One big effect of adding this fluid to a tweeter (or to any
speaker) is that it makes the voice coil capable of dissipating
more heat. This means that the speaker can have a lighter voice
coil, for better performance, or a higher power rating for the
same voice coil. The other big effect of this fluid is to add
mechanical damping. The frequency response and transient
response of the driver will change, possibly for the better.

In addition, this fluid may help center the voice coil, may
lubricate the voice coil, and may help keep dirt out of the gap.
This fluid will not increase the magnetic field, concentrate the
magnetic field or otherwise change the magnetic circuit. Nor
will it cushion impact if the voice coil bottoms.

The fluid used for this purpose is often called "ferrofluid".
It consists of sub-microscopic particles of magnetic material
suspended in special oil. This fluid stays in the gap because
of the strong magnetic pull of the magnet. There is some debate
over whether these fluids can dry out with time. Manufacturers
claim that the oil used is non-volatile.

It is possible to use ferrofluids in mid-range drivers and
woofers. However, as tweeters tend to have the most fragile
voice coils, tweeters have the most to gain from ferrofluid.
There are various different fluids on the market, some of which
have characteristics tailored to tweeters, some to woofers, etc.

It is very risky to blindly add fluid to a driver. It may not
be compatible with the adhesives used in the driver, may not be
practical with the particular driver layout, and is impossible
to remove. Permanent driver damage is possible.

12.18 Should I use spikes under my speakers? Pennies under the spikes?
Spikes prevent speakers from rocking. They also couple the
speaker directly to the floor. Spikes will pierce carpet.
Some spikes will damage carpet. Most will just put a small
hole in the carpet which is invisible. Putting a heavy
speaker directly on carpet will cause a permanent mark on
the carpet. Spikes can prevent this.

If you have a pretty hardwood floor, then spikes will definitely
damage the finish. A rigid disc under the spike will distribute
the load and lessen the damage. Any coin should work fine. Using
a coin will not change the speaker/floor interaction. Do not use
a coin with a carpeted floor. Alternatives to spikes for wood
floors are Blu-Tack and similar products. (see 12.19)

If your floor is extremely rigid, then the spikes will make
the speaker more rigid. If the floor is more conventional,
such as a suspended floor or a wooden floor over joists,
spikes can have a positive or negative effect, depending on
the resonant characteristics of the floor/speaker system.

The counterforce resulting from a forward cone motion in a
speaker may try to move the speaker backwards, but spikes will
have little or no effect on this. Most audible effects from
spikes are due to coupling the speaker to the floor, so it
will be less likely to resonate on its stand. Some argue that
in most cases, spikes will have no audible effect at all.
Try it for yourself.

12.19 How do you couple speakers to speaker stands?
Ideally, your speakers should sit flat on the speaker stand
or floor. They shouldn't see-saw back and forth if nudged.

One good way to accomplish this is to use a small dab of
putty under each corner of the speaker. There are a few
common putties used for this, but all share the properties
of being very elastic and staying flexible indefinitely.
These putties are inexpensive, removable, and reusable.

Try either Blu-Tak, which is available in the UK from office
supply stores for cleaning typewriter elements, Faber Castell
UHU Hold-It, which is available in the US from office supply
stores for holding up pictures, DAP's Fun-Tak, which is sold
in hardware stores for holding up pictures, or Pritt Buddies.

12.20 What is a Sealed, Ported, Bass Reflex, Acoustic Suspension,
Bandpass, and Coupled Cavity Speaker? Which is better?

All are "direct radiator" enclosures, so called because the
sound is produced directly from the driver (the "radiator")
without the assistance of a contrivance such as a horn.

SEALED BOX:
The simplest direct-radiator system. The rear of the driver
sees a sealed enclosure, and none of the rear output of the
driver contributes to the sound output. Depending upon how
stiff the mechanical suspension is vs how stiff the enclosed
air in the enclosure is (and that's a function of the size of
the box), you can have either an Infinite Baffle enclosure,
in which the mechanical suspension is the dominant source of
system stiffness and the box is large; or an Acoustic
Suspension enclosures, where the air in the box is the
dominating stiffness, and the box is small.

Sealed boxes tend to be the lowest efficiency systems for a
given box size and bass cutoff frequency.

VENTED ENCLOSURES:
Also the same as Bass Reflex, Ported, or Passive Radiator.
Here, an aperture in the box provides a means for the rear
output of the cone to contribute to the total output of the
system. However, it only contributes over a very narrow range
of frequencies. In fact, in a properly designed system, the
front output of the cone is reduced at the same time the
output of port increases, so the port DOES NOT ADD to the
output of the woofer, it REPLACES the output of the woofer at
these frequencies. This, if done properly, can significantly
reduce distortion and increase power handling at very low
frequencies, a region that can be difficult for drivers.

Vented systems can be up to 3 dB more efficient than a sealed
box system that has the same bass cutoff frequency and size.

BANDPASS:
These are compound systems in that they have at least two
enclosures: one on the front and one on the rear of the driver.
The enclosure on the front, which looks remarkably like a vented
box (because it is), acts as a low pass filter, and, can couple
the output of the woofer more efficiently to the outside. They
have several useful advantages. For example, the front enclosure
can be used as a very effective acoustic crossover, filtering
out mechanical noises generated by the woofer, something
no electronic crossover can do. For very low frequencies,
such an acoustic crossover can be far less expensive and
more easily designed than an equivalent electronic crossover.

They are called "bandpass" because the combination of the rear
enclosure and the driver form the high pass portion while the
front enclosure forms the low pass section. Making the bandwidth
of the system narrower raises the efficiency of the system.

COUPLED CAVITY:
A variation of bandpass and vented systems, they are the results
of a designers attempt to solve specific problems. They consist
of two or more rear enclosures, each coupled to the next by a
vent. Each enclosure/vent combination is another resonant system,
and the combination is, essentially, a high order, multi-tuned
resonant system.

Generally, these systems have quite complex response and are
difficult to design. No comprehensive theory on their operation
exists like that for sealed, vented and bandpass systems.

12.21 What is the best material to make speaker boxes out of? Why?
An ideal speaker cabinet material would be very stiff, so that
it would not tend to move with variations in box air pressure.
It would also be very well damped, so that if it ever does
deflect from air pressure, it will come back to the original
position without resonating. It would also have a very high
resonant frequency (supersonic), so that low frequency box air
pressure would not cause it to resonate. An attractive material
is preferred, and additional credit is given for a material
which is easy to cut, glue, and finish. A great material would
be cheap, too. Finally, it would be nice if the material were
light, because we all have to move our speakers sometimes,
and it's hard to appreciate good speakers with a sore back.

With all of those attributes, it would seem that no
material is perfect. However, there are many materials that
have enough of the above good attributes to make excellent
speaker cabinets. Yet each has advantages and disadvantages.

In the list of good speaker box materials below, letters are
used to indicate which attributes the material possesses.

S = Stiff
D = Damped
H = High Resonance
A = Attractive
M = Machinable
C = Cheap
L = Light

MEDIUM DENSITY FIBERBOARD (MDF): SDMC This is the most practical
material for quality speakers. It is harder to find than plywood,
but most lumber yards can special order it. It cuts very nicely
and has a smooth surface. It takes veneer very well. However,
bring a helper when you pick the stuff up. One sheet is very
heavy. MDF is harder on tools than common wood, but easier than
particle board. This is the material that many great speaker
makers use. US $45 for a 4'x8'x1" sheet. Density: 50 lbs/cu ft.

POLYCARBONATE (LEXAN): DM A clear or solid-color polycarbonate
box can look strikingly good. However, this is not a cheap
material. To locate it, look in the classified directory under
PLASTICS. US $400 for a 4'x8'x0.5" sheet. Density: 75 lbs/cu ft.
Acrylic (Plexiglass) is cheaper than Polycarbonate, but weaker
and poorer damped (not recommended).

CORIAN, FOUNTAINHEAD, AVONITE, SURELL, GIBRALTAR: SDA Regardless
of the brand, these synthetic countertop materials come in a
wide array of colors and look beautiful. They are hard to buy,
and harder to work. They take special glue to bond and require
wet sanding with very fine paper to finish. You can tap it, but
it's too brittle for wood screws. Corian is acrylic mixed with
powdered aluminum trihydrate clay filler. Avonite, Gibraltar,
and Surell are polyester resin mixed with the same clay filler.
One user commented that Corian is easier to use and is easier
to make invisible seams than the other synthetics. US $20 per
1'x1'x0.5". Density: 100 lbs/cu ft. Available from:
Art Specialties
PO Box 215
Depew, NY 14043
800-724-4008

MARBLE: SDHA One challenge with marble speaker enclosures is
cutting holes for the drivers. A carbide bit on a router will
work, but it will dull quickly. Marble is also difficult to glue,
so bracing is difficult. But it sure is pretty when you're done!
US $25 to $45 per 1'x1'x1.25". Density: 160 lbs/cu ft.

PLYWOOD SHEETS SPACED AND FILLED WITH SAND OR LEAD SHOT: SDAMC
If you have time on your hands and want a great impractical box,
try this. Make a simple box out of common plywood. Then glue
cleats on the outside of the box to space the outside plywood
from the common plywood. Glue hardwood-veneered plywood to the
cleats and pour sand or lead shot into the spaces between the
cleats. It won't be light, but with the filler, it will be
extremely well damped. In addition, if you use strong cleats
and glue well, the box will be extremely stiff. One person used
different size Sonotubes as an alternative to plywood, and
filled the space between them with sand. Be sure to sterilize
the sand in your oven before putting it in the box.

ALUMINUM SHEETS SPACED AND FILLED WITH ALUMINUM HONEYCOMB
(Aerolam): SDHL Airplanes use this material for flooring. Next
time a plane crashes in your neighborhood, see if you can get
the wreckage for your next speaker project. You can't get a
better, light-weight material. Celestion has exploited this for
some great products. If you're really ambitious, you can make
your own sandwich out of high-quality plywood faces and a thick
honeycomb core. You will probably need an epoxy to glue the
honeycomb to the plywood. A home-brew sandwich is easier to cut
and glue than Aerolam.

FORMED CONCRETE: SDHC There are tricks to working concrete, such
as to cast braces, rebar, and steel-wire right into the mix.
Also, some concrete is better damped than other. Remember to oil
your concrete forms so that they can be removed. Most concrete
speakers use an MDF front panel, but you can pour one if you use
cardboard tubes or plywood rings to mold the concrete into the
shape of a speaker cutout. Alternately, you can make a common
veneered plywood speaker box and cast concrete inside it for
stiffening.

Any box can be improved by making the walls thicker, by bracing
the walls, and by stiffening the walls. The stiffness of a
material goes up as the cube of the thickness, so a slightly
thicker material is much stiffer. A thicker panel will also have
a higher resonant frequency because the stiffness goes up faster
than the mass.

Consider lining the inside of your speaker with ceramic tile,
attached with thinset mortar. You can get tile remnants cheaply.
They are easy to apply and can be added as an afterthought to
an imperfect box. However, be sure to attach all braces before
tiling, because it is hard to attach anything to tile.

Also consider bracing any weak parts of the box. For example,
all joints will benefit from a wooden cleat. The back of the
box will benefit from stiffeners where the speaker terminals
are attached. Most importantly, brace the front panel, or
make it out of a double thickness of material.

12.21 What is the best "stuff" to fill a speaker cabinet with?
The following discussion will focus on practical facts on speaker
cabinet stuffing and on sealed systems. Theory is limited help
in selecting speaker stuffing. Vented system do share a few of
these same issues and will also be mentioned, but the goals and
physics of stuffing a vented box are different than those of a
sealed box.

NHT speakers use polyester fill. Some use a Danish polyester that
mimics the properties of fiberglas very closely. Excluding this
special poly, there are two kinds of polyester available: pillow
stuffing, and audio-spec polyester.

Forget pillow fill. It's cheap and easy to get. If you use
enough, it will damp the midrange, and that's a lot better than
an empty box. But it has little effect on lower frequencies.
Some fabric stores sell "Super Good Stuff" from Stearns
Technical Textiles. This is a common, inexpensive material
that is said to perform as well as audio-spec polyester.

For lining the walls of a vented enclosure to reduce internal
reflections, or filling a transmission line to absorb the back
wave, highly absorptive wool or fiberglas are ideal. However,
these materials do not provide the desired results in a sealed
system. They will provide more reflection absorption than
polyester, but the latter is quite good in this regard in the
critical midrange. In a sealed system you don't want absorption
at lower frequencies anyway; you want damping and isothermal
conversion. (Author's note: I have tried "all-out" efforts
using fiberglas lining and polyester fill to achieve the
best of both worlds. I found little practical benefit over
polyester alone.)

Most professional designers agree that practical experience,
combined with trial and error is the best way to get optimum
stuffing material, quantity, and method for a given design.
This is why good designers routinely experiment with fill in
the development of a new system. If you are designing a system
that differs substantially in shape or volume or source
impedance (passive crossover) from one of known reference,
you will need to experiment to get best performance.

Adjusting the filling is the last step in getting bass right,
and is used mostly to fine-tune the system Qtc and resonance.
As increasing amounts of polyester are added to a sealed box,
the resonance and Q gradually go down. This can be shown
mathematically to be due in roughly equal parts to the effects
of simple resistive damping and isothermal conversion. At some
point, a minimum is reached, and further material reverses the
trend by taking up volume. An experienced designer can find
the optimum amount of fill in a few trials by monitoring the
impedance versus frequency curve as stuffing is added or
removed.

Filling also has the important effect of reducing internal
reflections, to reduce standing waves and comb filtering.
However, the amount of filling has comparatively little
effect on this.

12.22 What size fuse or circuit breaker should I put in my speaker to
protect it from damage?
Most modern speakers consist of a box containing more two or
more drivers interconnected through a network of inductors,
capacitors, and resistors. One fuse or circuit breaker in
series with that array can't possible protect all drivers.

Conventional circuit breakers are a very bad choice for speaker
protection. They add series resistance, series inductance, and
lousy electrical contacts, all tending to degrade performance.
Moreover, breakers have a trip characteristic that does not
match the damage mechanisms of speakers.

Fuses are a better choice, but still are not very good. This
is because speakers have complex thermal behavior. Loud
playing will warm up the voice coil making it more sensitive to
damage. No fuse takes this into account correctly. A fuse
will do a better job of protecting tweeters, but is still not
perfect.

If you want to protect a speaker with a fuse, use the lowest
current, fast-blow fuse which will not blow during normal
listening. This may trip prematurely in a very loud passage,
or may degrade sound quality, but it is your best bet for fuse
protection. For a woofer, start with a 1 Amp fuse and work up.
For a tweeter, start with 100mA and work up.

There are also cheap tweeter protectors available which contain
a light bulb and a resistor potted in a small tube. They work
pretty well, and if you reduce the tweeter network's series
resistance by a few tenths of an ohm, they are not terrible for
the sound. But they are audible and not failsafe.

[neid...@uicc.com]

Archive-name: AudioFAQ/part6
Last-modified: 1996/9/23
Version: 2.07

13.0 Listening Rooms and Houses

13.1 How should I place speakers in my room? What size room is best?
You are after two important, distinct goals: flat frequency
response and good three-dimensional image. At your disposal is
the room size, the room shape, speaker height, speaker
placement, listening position, and room treatments. Even though
good speakers are essential to good sound, room effects are also
extremely important. In many cases, the differences in room
effects will be more noticeable than spending twice as much on
speakers!

For smoothest bass response, a listening room should be as large
as possible, have dimensions as unrelated as possible, and
should be optimally damped. Although nothing is ever ideal,
there are a few room dimension ratios that are better for
listening rooms:
Height Width Length
1 1.14 1.39
1 1.28 1.54
1 1.6 2.33
If your room isn't shaped like that, don't worry. These
effects are not major.

Also for smooth bass response, woofers should be at distances
from the nearest three room boundaries that are as different as
possible. In some cases, the line dividing the listening room
into left and right halves must be considered a room boundary.
Also, for smooth bass response, the listener's ears should be
at distances from the nearest three room boundaries that are
as different as possible.

All of this is essential because a wall near a speaker boosts
the bass from that speaker at some frequencies. If a speaker
is the same distance from three walls, then some frequencies
will be emphasized much more than others, rather than slightly
more.

For best three-dimensional image, a listening room should have
good symmetry about the plane between the two speakers. This
means that if one speaker is in a corner, the other speaker
must be in a corner. If this symmetry is not right, the first
reflection from the wall behind one speaker will be different
from the first reflection from the wall behind the other speaker
and critical parts of the stereo signal will be damaged.

Also, no large object should block the path from speakers to
listener or from speaker to speaker. Speakers should be
elevated so that tweeters are at listener ear height. The
distance between speakers should be no greater than the distance
from each speaker to the listener. Finally, the tweeters should
be aimed at the listeners.

A normal box-shaped listening room with bare walls will have
"slap echo" which will reduce intelligibility. A good cure is
randomly-placed wall hangings consisting of small rugs spaced
an inch or so away from the wall to increase sound absorption.
Another cure is convex-shaped art objects on the walls to
disperse harmful reflections. If money is available, commercial
room treatments such as "Tube Traps" and "RPG Diffusers" are
also valuable, but many of the benefits of these exotic devices
are available with simpler techniques.

As a general rule, in a good room, speakers and listener can be
close to room boundaries with minimal adverse effects. In a bad
room, a good strategy is to place both speakers and listener as
far away from room boundaries as possible.

An excellent starting point for speaker placement is to measure
the listening room diagonal dimensions. Divide that measurement
by three. Put each speaker that distance from a corner, on the
room diagonals.

I----------------------------------I
I I
I L I
I I
I S S I
I I
I----------------------------------I

Place your listening position midway between the two speakers
and approximately half way from the speakers to the wall. Be
sure that there is nothing in the "triangle" formed by the
listening position and the speakers.

Try this and then move things 12" (30cm) at a time to see if
you can improve the sound. Your ears will be a better guide
than any commonly-available instruments. To keep track of
what you are doing, take notes. To remember exactly where
you put the speaker on the floor, a practical trick is to
mark the floor with a sewing needle and thread.

Some speakers want to be aimed right at the listener (toed in)
while others work best pointed straight ahead. Experiment.

13.2 How do I wire a house for sound?
A fundamental principle of physics is that the farther a signal
travels, the more the signal will be degraded. Translate this
to mean that the shorter the wire, the better. Understanding
this, the idea of running speaker cable between every room of
the house isn't as attractive as it first seems.

If you still decide to wire your house for sound, you should do
it at the same time you're wiring for telephone and electricity.
It is possible to wire a house after the walls are closed, but
it becomes very difficult.

It is economical to use common house wire (Romex, UF, NM, etc)
for speaker wire in the walls, but this may violate building
codes. Check with an electrician or inspector first. It will
also confuse future electricians, so label the wire clearly, all
along its length.

If you want to make your house like a recording studio, it is
best to use the techniques of recording studios. When studios
run long lengths of sound cable from one room to another, they
drive the cable with 600 ohm line amplifiers. They also use
shielded, twisted-pair cable. They only connect the shield at
one end of the cable. Finally, they use balanced inputs at the
other end of the cable.

13.3 Where can I read more about listening room construction and tuning?
"Building a Recording Studio" by Jeff Cooper
Mix Bookshelf
"Handbook for Sound Engineers"
"The Master Handbook of Acoustics" by F Alton Everest
"Sound Engineering 2nd Edition" by Don and Carolyn Davis;
Howard W. Sams & Co. (C) 1990
"Good Sound" by Laura Dearborn
Introductory, but clear and accurate
"Sound Recording Handbook" by John M. Woram
Howard W. Sams & Co. #22583
Excellent General Reference
"Audio Technology Fundamentals" by Alan A. Cohen
Howard W. Sams & Co. #22678
Overview of Audio Theory
"Introduction to Professional Recording Techniques"
by Bruce Bartlett
Howard W. Sams & Co. #22574
"Modern Recording Techniques" by Hubar and Runstein
Howard W. Sams & Co. #22682
"Sound Studio Production Techniques"
by Dennis N. Nardantonio
Tab Books
"The Uneasy Truce Between Music and the Room"
F. Alton Everest
Audio, February 1993, Pgs. 36-42
"Coloration of Room Sound by Reflections"
F. Alton Everest
Audio, March 1993, pgs. 30-37

13.4 What is white noise? What is pink noise?
"White noise" is characterized by the fact that its value
at any two different moments in time are uncorrelated.
This leads to such noise having a flat power spectral
density (in signal power per hertz of bandwidth), and is
loosely analogous to "white light" which has a flat power
spectral density with respect to wavelength.

Pink noise has flat power spectral density per PERCENTAGE
of bandwidth, which leads to a rolloff of -3 dB/octave
compared with white noise.

There are many reasons for using pink noise in audio testing.
One is that music has an average spectral content much closer
to pink noise than white noise. Another is that pink noise
can be readily measured with constant Q bandpass filters and
naturally leads to flat plots on logarithmic frequency scales
- which correspond to the equally tempered musical scale.

Pink noise is often used with 1/3 octave band filters to
measure room acoustics. This idea has merit since 1/3 octave
is a convenient number near the limit of our ears ability to
detect frequency response irregularities, and because
averaging measurements over 1/3 octave bands smooths out the
numerous very narrow peaks and dips that arise due to
standing waves in rooms.

Another term you'll hear about is Gaussian noise - this is
noise with a Gaussian amplitude probability density.
Gaussian noise has the amazing property that linearly
filtering it preserves its Gaussian amplitude density and
that sums of Gaussian random variables are again Gaussian.
The two terms shouldn't be confused. It is possible to have
Gaussian white or pink noise.

[neid...@uicc.com]

Archive-name: AudioFAQ/part7
Last-modified: 1996/9/23
Version: 2.07

14.0 Recording
There are more different recording systems available today than
ever before. Digital and analog are both available to the
consumer. With the advent of consumer digital recorders, used
pro analog recorders are becoming available for surprisingly low
prices. Now may be the time for you to buy a microphone and
recorder and make your first!

14.1 What is DAT? What is its status today?
DAT (Digital Audio Tape) is currently the standard professional
digital format for 2-track digital recording. DAT had a
short-lived consumer presence, but never "made it". As digital
recorders have no tolerance for clipping, using a DAT recorder
takes a slightly different knack. The results can be worth it,
however, as DAT format offers the same resolution and dynamic
range as CDs. DATs record for up to 3 hours on a tape, and can
run at three different sampling rates: 32 kHz, 44.1 kHz
(for CD), and 48 kHz (the DAT standard). Longplay mode cuts
frequency response to 14kHz but adds even more recording time.

14.2 What is DCC? What is its status today?
DCC is Philips' attempt to modernize the regular cassette. DCC
decks can play analog cassettes, and can record new Digital
Compact Cassettes. They use stationary heads (DATs use rotary
heads as do VCR's), and although they are digital, they use
lossy compression to fit all the data on the cassette. Although
DCC sound quality is far better than the 1960 standard cassette,
the DCC does not have the sound quality present in DAT or CD.
DCC may be a good choice for consumers who want to assemble mix
tapes for cars or walkmans, but is not suitable for any
professional applications.

| As of October 1996, DCC is quite affordable in price. Some
| DCC home recorers are under $200. However, blank DCC tapes
| are still hard to find and fairly expensive ($10 each for 90
| minute lengths). Also, DCC manufacturers are dropping DCC
| from their lines, indicating that it is either on the way
| out or never made it in.
|
| Although the ability to play analog cassettes is a strong
| advantage of DCC, many people have had trouble with oxide
| particles falling off analog cassettes and clogging the gap
| of the DCC head. This may be due to the extremely low
| quality of some analog cassette tapes and may be due to the
| very tiny gap of DCC heads.
|
| Caution: NEVER demagnetize DCC heads. This will permanently
| damage the heads.

14.3 What about writable compact discs? What is the status today?
| Recordable CDs are available, and costs are dropping. As
| of October 1996, blank discs are available for under
| $10 each and recorders are available for under $1000.
Note that recordable CDs can only be recorded once. Also
some CD recorders do not allow start-and-stop but require
| that the whole disk be written at the same time. This
| means that you really need a computer with a large hard
| disk to master a CD.

14.4 What are Dolby B, C, and S, HX Pro, and DBX? Are they compatible?
Dolby B, C, S, and DBX are techniques for increasing the
signal/noise ratio of recordings. All work in similar ways:
they compress the dynamic range of the sound during recording,
then expand it back upon playback. As much as we would like
it to be otherwise, you only get correct reproduction if you
use Dolby B to play back a Dolby B tape. Same for Dolby C,
Dolby S, and DBX. Dolby HX Pro is the exception.

Dolby B works mostly with higher frequencies; it increases
their levels during recording and decreases their levels, and
the levels of high-frequency noise such as tape hiss, during
playback.

Dolby B tapes can be played back without Dolby B processing,
but high frequencies are over-emphasized and the sound will
be excessively bright. This can be compensated for to some
extent by turning down the treble control. Audio novices
often remark that commercially recorded tapes recorded using
Dolby B sound dull when played back with Dolby B; this is
because they are accustomed to the boosted high frequencies
they hear when playing these tapes without Dolby.

Dolby C achieves greater noise reduction (about 8-10 db) than
Dolby B by working with a greater range of frequencies and
altering relative levels more; this means that playing Dolby C
tapes back with no Dolby processing or with Dolby B, leads to
very bad frequency response and a sound that most people find
unpleasent. Dolby C may also be more sensitive to variations
among decks in exact frequency response, alignment, etc. Some
people find that tapes recorded using Dolby C sound best only
when played back on the deck on which they were recorded.

Dolby S works with an even broader range of frequencies than
Dolby C, and achieves slightly greater noise reduction. Its
has three advantages over Dolby C: (1) many people find that
tapes recorded and played back using Dolby S sound closer to
the original than tapes done using Dolby C; (2) tapes recorded
using Dolby S don't sound awful if played back on Dolby B decks,
and (3) Dolby S seems to be less sensitive to variations among
decks.

DBX is similar to Dolby B, C, and S, but uses the same compression
on all frequencies, high and low. However, DBX is mostly used
in the professional market. Very little home DBX equipment is
available, and some of that home equipment is no better than
comparable Dolby B home systems. All DBX systems are compatible
with all other DBX systems, but incompatible with Dolby. A DBX
tape will sound terrible without DBX processing during playback.

All compression/expansion systems suffer two problems. One is due
to the fact that compressors can't compress a loud signal before
they have heard a bit of it, so that little bit of loud signal
will get through uncompressed. Likewise, quiet passages will not
be expanded until after they are detected. These delays give rise
to an audible problem often called "breathing".

The other problem inherent in all compression/expansion systems
is that if there are any frequency response errors in the tape
recorder, they will be made worse by the compression/expansion.
For example, if there is a 2dB dip in frequency response at 1kHz
in the tape recorder, this will be accentuated to a 4dB dip if
the compressor is using a 2:1 ratio. So compression/expansion
trades noise for frequency response error. For that reason and
the previously mentioned breathing, some people prefer to use
their recorder without any noise reduction at all. They prefer
a bit of noise to the other errors.

Dolby HX Pro is not noise reduction and does not use
compression or expansion. HX Pro is a technique developed by
Dolby Labs to increase tape headroom by decreasing the bias
when recording signals with a large high frequency component.
This allows better transient response, particularly on less
expensive tapes, and requires no processing when the tape is
played back. Dolby HX tapes can be played back on any system
with no decrease in quality.

| Dolby Corporation has developed other techniques and other
| acronyms for products related to surround sound. The phrase
| "contains Dolby" isn't as meaningful today as it used to be.

14.5 What is the best cassette deck under $400?

14.6 What is PASC? Can I hear the effects?
PASC (Perceptual Audio Sub-band Coding) is a data-compression
algorithm. It increases the length of recording that can be
stored in a given number of data bits by eliminating sounds that
the developers' research claims can not be perceived by human
listeners. Its most important component is the omission of
quiet sounds that occur at the same time and near the frequency
of louder sounds. It provides up to a 4x increase in the length
of recordings a given digital medium can hold; this is essential
to allow full-length digital recordings on DCC (and on MD, which
uses a different compression technique). It is not necessary
to translate CD data to analog before compressing it using PASC,
nor the reverse.

You CAN hear PASC, but it is very difficult, since it is not
a distinctive noise (like a hiss) nor a consistent diminution
(like a notch in a speaker's response), but a broad,
uncorrelated dropout in a changing collection of sounds that
are masked by sounds that you can hear very easily.

Since it is lossy, repeated PASC recording will cause
progressive loss, and this signal damage may become easily
noticeable. This is a side effect that recording companies
hope will have the effect of discouraging piracy via DCC.

| For more information on audio compression, consult these
| articles (courtesy of Jonas Palm):
|
| R. Veldhuis, M. Breeuwer, R. van der Waal, "Subband Coding of
| Digital Audio Signals Without Loss of Quality," IEEE ICASSP,
| 1989, pp. 2009-2012.
|
| J. Johnston, "Perceptual Transform Coding of Wideband Stereo
| Signals," IEEE ICASSP, 1989, pp. 1993-1996.
|
| G. Davidson, L. Fielder, M. Antill, "High-Quality Audio Transform
| Coding at 128 kbits/s," IEEE ICASSP, 1990, pp. 1117-1120.
|
| J. Princen, A. Bradley, "Analysis/Synthesis Filter Bank Design
| Based on Time Domain Aliasing Cancellation," IEEE Trans ASSP,
| Oct. 1986, v. 34 n. 5, pp. 2161-2164.
|
| P. Duhamel, Y. Mahieux, J. Petit, "A Fast Algorithm for the
| Implementation of Filter Banks Based On 'Time Domain Aliasing
| Cancellation,'" IEEE ICASSP, 1991, pp. 2209-2212.
|
| J. Johnson, "Transform Coding of Audio Signals Using Perceptual
| Noise Criteria," Journ. Acoustical Society of America, Feb. 1988,
| pp. 314-323.
|
| 2nd Draft-Proposed Standard on Information Technology Coding of
| Moving Pictures and Associated Audio, document ISO/IEC
| JTC1/SC2/WG11 MPEG 90/001, Sept. 1990.
|
| G.Thiele, G. Stoll and M. Link "Low bit-rate coding of high-quality
| audio signals. An introduction to the MASCAM system." EBU Review
| No. 230

14.7 What is SCMS? Can I hear the effects?
SCMS (Serial Copy Management System) is a copy-protection system
intended to stop rampant piracy of commercial recordings to
digital tape. SCMS allows the home taper to copy from a CD to a
digital tape, but prevents anyone from digitally copying that
new digital tape.

You CANNOT hear SCMS.

14.8 How can I bypass SCMS?
There are professional devices used by engineers to manipulate
the digital bitstream, but they cost several hundred dollars and
are not cost effective for consumers. If you need to make
perfect digital copies of digital copies, buy a professional
digital recorder. Pro models do not have SCMS, are more durable
than consumer recorders, and may have better quality electronics
than consumer models.

14.9 What's this about a tax on DAT?
Every digital audio tape recorder and every blank digital tape
sold in the USA is priced to include a "premium" or "tax". This
tax is collected by the US Copyright Office and distributed to
the recording artists and record companies that own the
copyrights to commercial music. These fees are supposed to
repay them for lost royalties.

Many believe that this "tax" is illegal, because it represents
an assumption that the buyer will use the recorder and tape to
violate a copyright, and not to record their own works. A
founding principle of the USA legal system is that everyone is
assumed innocent until proven guilty.

If you believe that this law is unjust, write your elected
representatives.

14.10 Is it legal to copy an LP, CD, or pre-recorded tape?
In the US today, it may be legal to copy LP's, CD's, etc. for
your own private use (such as to copy a CD to play on your
walkman). UK law specifically prohibits this, but it is almost
never enforced. It is definitely not legal in the US, UK, or
almost anywhere else, to copy these sources for commercial
purposes, or to give the copies to others.

It is as of yet unclear whether you own the rights to sell
or give away a copy of a recording if you made the copy on media
which was sold with an included digital audio tax.

14.11 How do I clean and demagnetize tape heads?
First, a caution: DAT recorder tape heads are VERY fragile.
Before cleaning the heads on a DAT recorder, get specific
recommendations from a very knowledgeable source that is
intimately familiar with DAT head cleaning. In the internet,
a good source is the DAT-Heads-Digest FAQ. For more information
on DAT-Heads-Digest, see section 20.2, below.

To clean tape heads, use pure isopropyl alcohol and lint-free
swabs. Wipe the metal parts of the transport with alcohol
(DON'T wipe the rollers!) and allow them to dry. Throw the swab
away after use. Be exceedingly careful when cleaning the heads
on a DAT. DAT heads are notoriously easy to misalign by
incorrect cleaning.

Practical tape head demagnetizers are available for under $10.
Try to find one with a plastic coated tip. If you can't find
one which is plastic coated. you can slip a drinking straw or
plastic tube over the tip for the same effect. This plastic
will prevent the demagnetizer from scratching the head.

Before plugging in the demagnetizer, remove all tapes from your
working area and unplug the recorder. Hold the demagnetizer
away from the recorder as you plug it in. Slowly bring the tip
of the demagnetizer up to the tape head and slide it back and
forth across each tape head for five one-second strokes. Then
pull it away from the head slowly and go on to the next. After
demagnetizing the heads, use the tip on each metal tape guide
with a similar five strokes. Last, slowly pull the demagnetizer
far away from the recorder and unplug it. Recording engineers
use a demagnetizer before each recording session.

14.12 How do I adjust a tape recorder for best results?
Adjusting a tape machine for best results usually requires
special equipment and test tapes. Unless you know what you're
doing, leave it for a pro. If you are serious about doing it,
buy the service manual for your particular tape recorder. It
will list a detailed procedure, as well as describe the correct
test tape and tools.

As for setting of record levels, it is best to experiment with
different levels on different tape brands. Different
formulation will reach saturation for different levels.
Generally speaking, the transients on a Chrome tape should peak
at about +6 dB above 0, though some formulations can take
significantly hotter signals.

14.13 Where can I get new pinch rollers or drive belts?
Projector-Recorder Belt Company
Whitewater WI USA
800-558-9572

14.14 What is a good rubber (pinch) roller cleaner?
Teac RC-1 available from
J&R Music World
59-50 Queens-Midtown Expressway
Maspeth NY 11378-9896 USA
800-221-8180 or 718-417-3737
Tascam Rubber Cleaner RC-2 available from:
Tape Warehouse
Chamblee GA
1-404-458-1679

14.15 How can I program a recorder to tape a radio broadcast?
Radio Shack and Panasonic make a clock/radio/cassette that can
be set to record at a specific time. Radio Shack also sells 120
minute cassettes, which can be used for 60 minutes per side.
The recorders are not high quality, and the long tapes are
fragile, but it works.

You can buy "appliance timers" at hardware stores that will
start and stop an appliance at a specific time. Radio Shack
sells fancier versions of the same thing for more money. Gadget
freaks love "X-10" control systems. These can be configured to
do the same thing. All require a recorder that can be left in
RECORD mode. Such recorders are identified by a "TIMER" switch
on the front panel. Many cassette decks have a TIMER switch for
use with timers.

This can be set to start a recorder at a particular time. As
the recorder will be started from a remote control rather than
by the power line voltage, no timer switch is required. Radio
Shack has a very similar product available for $99.95, may be
less on sale.

Damark also sells a learning remote with a built in on/off
timer and sleep timer. It can learn 17 commands per device on a
total of 5 devices and should be great for taping a few shows.
They also sell a 8 device remote with timer. As Damark sells
close-outs, these items may not be available in the future.
Damark
7101 Winnetka Avenue North
PO Box 29900
Minneapolis MN 55429-0900 USA
800-729-9000 or 612-531-0066

There is a similar learning remote available from MCM
Electronics. They call it a Lonestar Learning Universal
Remote, item 80-450, $24.95 on sale.

MCM Electronics (Speakers, A/V Repair Parts, Etc) (+) (C)
650 Congress Park Dr
Centerville Ohio 45459-4072 USA
513-434-0031 or 800-543-4330

Another timer-remote is the Fox 800 (approx. US $80 retail).
The Fox 800 was spotted at Damark 800-729-9000 for $30 in
March of 1995.

There is also a Carver remote with timer.

For the true nerd, there's the programmable remote sold as a
Scientific Calculator, the HP-48. Audio remote control
software for this fine adding machine exists. For more
information, consult the HP-48 FAQ. The HP-48 FAQ contains
pointers to a few remote control programs. The FAQ is archived
at site rtfm.mit.edu in /pub/usenet-by-group/comp.sys.hp48

Use a VCR for audio-only recording. Hook the audio in to the
output of a radio, tuner, or receiver. You may also have to
connect some video signal to the VCR so that the sync circuits
work correctly.

14.16 Will CrO2 or Metal tapes damage a deck made for normal tape?
No. They will work fine. They are no more abrasive than common
tape and may actually be less abrasive than very cheap tapes.
Recorders which are designed for CrO2 or Metal tape have
different bias settings and equalization settings to take best
advantage of the greater headroom and to give flat response with
these different types of tape. However, they use similar if not
identical heads as less expensive tape recorders. Almost all
tapes are in some way lubricated, and these lubricants minimize
wear and squeaking.

14.17 Why do my old tapes squeak in my car cassette deck?
One problem that will cause this is "binder ooze". The binder
is the glue which holds the oxide particles to the backing.
With time, this binder can ooze forward and actually get past
the oxide particles, so that there is sticky stuff on the
surface of the tape. When this sticky stuff goes past the
heads, it can cause a slight stick, which will sound like a
squeak. You won't feel it with your fingers, but it is there.
If you have a prized tape with this problem, consider baking
the tape in a home oven at a very low temperature, like 150F.
This might cure the problem by drying out the binder.

14.18 Is VHS Hi-Fi sound perfect? Is Beta Hi-Fi sound perfect?
The HiFi recording format is subject to two different problems:
Head-switching noise and compression errors.

To get perfect reproduction, the FM subcarrier waveform being
played back by one audio head must perfectly match the waveform
from the other head at the point of head switching if a glitch
is to be avoided. If you record and then play the tape on the
same VCR under exactly the same conditions, you have a
reasonable chance of this working. But if the tape stretches
just a bit, or you play it on another VCR whose heads are not in
exactly the same position, or the tracking is off, the waveforms
will no longer match exactly, and you will get a glitch in the
recovered waveform every time the heads switch. This sounds
like a 60 Hz buzz in the audio, which is often audible through
headphones even if not through speakers.

The same glitch will occur in the video waveform too, but since
head switching always happens during vertical retrace, you won't
see it.

The wonderful signal to noise ratio of VHS HiFi is achieved
through the use of compression before recording and expansion
after playback. The actual signal to noise ratio of the tape
itself is about 35 dB and a 2.5:1 compressor is used to
"squeeze" things to fit. Like all companders, this produces
audible errors at certain places on certain signals, such as
noise "tails" immediately after the end of particularly loud
passages.

Worse, compressors often have problems simply getting levels
right. That is, if you record a series of tones, starting at
-90 dB and working up in 1 dB increments to 0 dB, and then play
them back, you will almost invariably have level errors. The
trend from soft to loud will be there but the steps won't be
accurate. Two or three of your tones might come out at
essentially the same level, then the next one takes a big jump
to catch up or even overshoot.

For music, the result will be that the relative levels of some
instruments, passages, etc. will not be accurate.

This doesn't matter as much for movies, which tend to have
steady volume level. Also, movie enjoyment is rarely hurt by
these level errors. VHS and Beta HiFi is fine for reproduction
of movie and tv soundtracks. They are also perfectly fine for
non-critical audio applications. But VHS and Beta HiFi are not
serious competitors to DAT, CD, open-reel analog tape, or even a
high quality cassette deck.

14.19 How do HiFi VCRs compare to cassette recorders? DAT recorders?
VHS HiFi and Beta HiFi are analog recording formats which use
modulation techniques to record a video signal and a stereo
audio signal on a videocassette. The audio capabilities
typically surpass that of the "linear" audio tracks found on all
video recorders, thus the "HiFi" designation. "HiFi" is
essential for getting good sound quality on your video
recordings and out of pre-recorded videos.

HiFi is also touted as an excellent audio recorder for
audio-only (no picture) applications. On paper, the
specifications are typically superior to analog cassette but
inferior to DAT. In reality, the quality of HiFi video
recorders is better than low quality cassette recorders but not
as good as high quality cassette recorders when they are used
with noise reduction systems. In no case can a HiFi video
recorder compare to DAT. It suffers from generational loss and
audible noise.

Many people use VHS HiFi for recording radio broadcasts, since
VCRs often have built-in timers and can record for up to 9
hours. If you use a HiFi video recorder to record from an
audio-only source, beware that some decks will not function
properly without a video signal for synchronization. If you are
interested in very good quality sound, use a deck with manual
level control.

14.20 What is the difference between VHS HiFi and Beta HiFi?
To record the video and HiFi sound signals onto the same tape
area, VHS HiFi uses "depth multiplexing", while Beta HiFi uses
"frequency multiplexing". That is, the FM signal for Beta HiFi
occupies a different frequency band than do the Beta format's
luminance and chroma signals, and is simply mixed with those
signals and laid down on the tape by the video heads. In VHS
the luminance and chroma signals were too close together in
frequency for this to work. VHS HiFi uses a separate pair of
heads on the spinning head drum to record the HiFi carrier.
These heads' gaps are shaped so that the HiFi carrier is
actually recorded at a different depth in the tape than the
luminance and chroma signals.

14.21 Is there any good reason to buy a HiFi VCR for common TV shows?
If you do not own a stereo TV, the purchase of a HiFi VCR will
give you the capability to listen to stereo TV broadcasts to
your system.

14.22 What is the best cassette tape?
One simple answer to this question is that the best tape is the
tape which was used to align your tape recorder. A second
simple answer is that more expensive tapes are frequently
better in terms of quality of the backing, durability of the
oxide, accuracy of the shell and guides, and life.

Background: When you make a tape recorder, you build electronic
circuits which have specific, non-flat frequency response.
These circuits correct for the non-flat response of the tape
heads, the recording process, and the tape. These circuits can
be adjusted after the recorder is made, but adjustment is
tricky, and may or may not be successful with every tape made.
The designer of the tape recorder picked one tape as their
standard when they did the design, and built that recorder
to work well with that particular tape. It may work better
with a different tape, but it won't necessarily sound the
best with what one person calls the best sounding tape.

From a review of frequently given answers to this question,
it is obvious that almost every brand of tape has its advocates.
Many brands also have their detractors. Maxell and TDK tend to
have a strong following, but that is in part because they own a
large share of the US tape distribution market.

14.23 What is the best Reel-to-Reel tape?
See 14.22. Just as cassette tape recorders are set up
specifically for one type of tape, reel-to-reel tape recorders
are equalized and biased so that they are best with one specific
brand and model of tape. Just as more expensive cassette tapes
will last longer and have less noise than cheaper ones, you can
expect fewer dropouts, better quality control, and lower noise
from more expensive reel-to-reel tapes.

The major brands in reel-to-reel tape include Ampex, Scotch
(3M), AGFA/BASF, and Maxell.

14.24 What is Type I, Type II, Type III, and Type IV cassette tape?
These are IEC (International Electrotechnical Committee)
standards. They provide broad standards for all tapes,
and end the need to align a deck for an individual tape.
Type 1 is for normal "iron oxide" tapes (Fe2O3), Type 2
is for high-bias "chromium oxide" tapes (CrO2), Type 3
(obsolete) is for FeCr (ferric chrome), and Type 4 is
for Fe (Metal). Type 2 tapes tend to be more expensive
than type 1, and type 4 tapes are the most expensive.
This is because type 2 tapes tend to have less noise and
flatter high frequency response than type 1, and type 4
tapes tend to have even flatter highs and even less noise.

Some Type 1 tapes are more expensive than other Type 2 tapes,
and may be worth the extra price. More expensive tapes come
in better shells, have better lubrication, fewer dropouts,
smoother frequency response, and better uniformity from tape
to tape. Even though the types imply a particular tape
formulations, the type really refers to the tape performance.
For example, some iron oxide tapes have an unusual oxide
formulation with very small grains that conforms to the type
2 standard better than the type 1 standard. These tapes
will be labeled type 2, but may not have any chrome in them.

Most modern cassette recorders sense the tape type by the
holes in the back of the housing and adjust bias and
equalization to compensate for the differences. A few
top cassette recorders (the Revox and several Nakamichis)
automatically align to a particular tape by recording test
tones and then setting their own equalization.

In practice, each brand and model tape is slightly different.
For the very best recordings, adjust your recorder for the
tape you use most, or buy the tape which works best in your
recorder. Manufacturers adjust each recorder for a specific
tape at the factory. So the best tape might be the one
referenced in the recorder owner's manual. In a recording
studio, it is common to align the bias and equalization for
the specific tape used, and stick with that tape.

14.25 Why do I have hum when I connect cable to my VCR (or TV), which is
connected to my audio system?
What you are experiencing is probably a "ground loop", caused
by multiple connections from your equipment chassis ground to

building ground. Since disconnecting the cable or building
antenna from the VCR eliminates the hum, the cure is simple.
The following info talks about "the cable" but works the same
with a coax from a master antenna system.

Go to Radio Shack and buy one each of:

15-1253, "300-ohm TV-VCR Matching Transformer"
This looks like a little box with two screw terminals
and a push-on male F (coax) connector.

15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor matching
transformer"
This is a longish box or tube, with a female F
connector on one end and a bit of twin-lead coming from
the other. The twin-lead ends in a pair of what are
called "spade lugs" (shaped like U's).

Note: each of these part numbers may have a "B" or other
letter at the end. These indicate slightly different details
of functionally equivalent parts. Don't worry about it.

Connect the two spade lugs on the -1140 to the two screw
terminals on the -1253. Make sure they don't touch each other;
this shouldn't be difficult to get right. This gives you the
"isolator", with a female coax connector on one end and a male
coax connector on the other.

Just insert the isolator "in line" in the incoming cable lead.
ie treat it as you would a (very short) extension cord. You
can do this right at the back of the VCR (or whatever the cable
is hooked to).

Only one of the two units called out here (15-1140) actually
provides isolation. Two of the 15-1253 units back to back will
NOT work. Two of the 15-1140 units back to back will work fine
but will be less convenient.

If you can't find these specific parts, and want to know if the
substitutes you've found will work, test them with an ohmmeter,
measuring from either the pin or shield of the coax side to
either wire of the twin-lead side. If it's not an autoranging
unit, set the meter to its highest resistance range. You want
to see no connection (ie: infinite resistance, an open circuit)
between them. As with the parts described above, only one of
the coax/twinlead adapters needs to pass the test.

This trick runs the signal through a PAIR of baluns. This is
more than is absolutely required to solve this problem, and may
weaken the signal slightly. This should not be a problem on
most cable systems. But, some audio stores are beginning to
carry a unit made expressly for this purpose. It contains a
single 75 ohm to 75 ohm isolation transformer. This should
introduce less signal loss. It will also be better shielded
than the two baluns (see next paragraph). Under $10 would be an
appropriate price.

The back-to-back baluns may allow "ingress". That is, if you
are near to a TV transmitter, the short length of twinlead may
pick up broadcast TV signals and mix them with the cable,
causing interference. If you can find a prepackaged 75 ohm
isolation transformer as described in the preceding paragraph,
it should be better in this regard.

Mondial is selling a unit dubbed the "Magic-1"; this does the
same job but with three capacitors instead of transformers. It
is said to cause less than 1 dB of signal loss. On the other
hand, it costs about $90.

Yet another solution is to attack the problem at the line-level
audio connection between the VCR and the rest of your stereo.
Radio Shack's stereo ground isolators (270-054) are made for
this purpose. These go in the line-level AUDIO connections
between the VCR (or TV) and the rest of your sound system. If
both the line in and line out jacks on the VCR are connected to
the sound system, you'll need two of these isolators. They are
audio frequency transformers and may add some distortion and
frequency response error.