FAQ: rec.audio.* Systems 7/07 (part 2 of 13)
neid...@ti.com
Last-modified: 2007/07/12
Version: 2.16
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 probably also
has selector switch(s), tone controls, and a volume control. A
receiver may have outputs for two speakers, or for more.
Most modern receivers do not have phono preamps, but some are
still available for those who love vinyl. Many receivers contain
surround sound processors.
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
(DVD, 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) http://www.adcom.com
Audio by Van Alstine
Audio Research http://www.audioresearch.com
B & K http://www.bkcomp.com
California Audio Labs (CAL) http://www.calaudio.com/
Carver (some made in Japan)
Jeff Rowland http://www.jeffrowland.com
Krell http://www.pcnet.com/~krell
Mark Levinson http://www.madrigal.com/MLHP5.htm
McCormack
McIntosh http://www.mcintoshlabs.com
Proceed http://http://www.madrigal.com/PROHP2.htm
PS Audio http://www.psaudio.com
Spectral http://www.spectralinc.com
Sumo (Power amps, preamps, CD transports, D/As)
Wadia http://www.wadia.com/index.html
9.11.1 Any information on equipment made in other countries?
Thanks to Stephane Tsacas, we know:
Australia:
Krix Loudspeakers http://www.krix.com.au
Metaxas http://www.metaxas.com
Canada:
Bryston http://www.bryston.ca
Coincident http://www.coincidentspeaker.com
Energy Speakers http://www.energy-speakers.com
Newform http://www.barint.on.ca/newform
Paradigm http://www.paradigm.ca
Psb Speakers http://www.psbspeakers.com
Sonic Frontiers http://www.sonicfrontiers.com
Waveform http://www.waveform.ca
Czech Republic:
KR Enterprise http://www.kr-enterprise.com
Denmark:
Bang & Olufsen http://www.bang-olufsen.com
Bow Technologies http://www.bowtechnologies.com
Bruel & Kjaer http://www.bkhome.com
SEK Acoustics http://www.adpointer.net/sekacoustics
France:
Audax http://www.audax.com
Audio Aero http://www.audioaero.com
Audioreference http://www.audioreference.com
Cabasse http://www.cabasse.com
Confluence http://www.a-t.fr/confluence
J-M Reynaud http://www.charente-fr.com/jm-reynaud
Kora http://www.kora.net/index.html
JM Lab http://www.focal.tm.fr
Triangle http://www.triangle-fr.com
Verdier
YBA http://www.phlox-electronique.fr
Germany:
Lehmann audio http://www.lehmannaudio.de
Steinmusic http://www.steinmusic.de
Italy:
Audio Analog http://www.hi-fi-forum.com/audio_analogue.htm
Pathos http://www.hi-fi-forum.com/pathos.htm
Korea:
Pulsus http://www.pulsustech.com
Netherlands:
Final http://www.hi-fi-forum.com/final.htm
Philips http://www.philips.com
New Zealand:
Perreaux http://www.perreaux.com
Plinius http://www.pliniusaudio.com
Norvegia/Norway:
Electrocompaniet http://www.electrocompaniet.no
Tandberg http://home.sol.no/~johandor
Switzerland:
Nagra http://www.nagra.com
Goldmund http://www.goldmund.com
Revox http://www.revox.ch
Lenco
AMT
UK:
Audio Note http://www.audionote.co.uk
Cambridge Audio http://www.cambridgeaudio.com
Newtonia http://www.newtonia1.freeserve.co.uk
Quad http://www.quad-hifi.co.uk
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.
9.14 What do they mean when they say "It sounds warm?"
There are many subjective terms used to describe slight
differences in frequency response, distortion, noise, etc.
Thanks to Bruce Bartlett and Pro Audio Review, we present this
Sound Quality Glossary. This glossary puts a meaning behind
many different, common terms. There is no guaranty that people
mean the same thing when they use these terms. However, these
definitions give insight into why a system sounds the way it
does and may also help bridge the communications gap.
Airy: Spacious. Open. Instruments sound like they are
surrounded by a large reflective space full of air. Good
reproduction of high-frequency reflections. High-frequency
response extends to 15 or 20 kHz.
Bassy: Emphasized low frequencies below about 200 Hz.
Blanketed: Weak highs, as if a blanket were put over the
speakers.
Bloated: Excessive mid-bass around 250 Hz. Poorly damped low
frequencies, low-frequency resonances. See tubby.
Blurred: Poor transient response. Vague stereo imaging, not
focused.
Boomy: Excessive bass around 125 Hz. Poorly damped low
frequencies or low-frequency resonances.
Boxy: Having resonances as if the music were enclosed in a
box. Sometimes an emphasis around 250 to 500 Hz.
Breathy: Audible breath sounds in woodwinds and reeds such as
flute or sax. Good response in the upper-mids or highs.
Bright: High-frequency emphasis. Harmonics are strong relative
to fundamentals.
Chesty: The vocalist sounds like their chest is too big. A bump
in the low-frequency response around 125 to 250 Hz.
Clear: See Transparent.
Colored: Having timbres that are not true to life. Non-flat
response, peaks or dips.
Crisp: Extended high-frequency response, especially with
cymbals.
Dark: Opposite of bright. Weak high frequencies.
Delicate: High frequencies extending to 15 or 20 kHz without
peaks.
Depth: A sense of distance (near to far) of different
instruments.
Detailed: Easy to hear tiny details in the music; articulate.
Adequate high-frequency response, sharp transient response.
Dull: See dark.
Edgy: Too much high frequencies. Trebly. Harmonics are too
strong relative to the fundamentals. Distorted, having unwanted
harmonics that add an edge or raspiness.
Fat: See Full and Warm. Or, spatially diffuse - a sound is
panned to one channel, delayed, and then the delayed sound is
panned to the other channel. Or, slightly distorted with analog
tape distortion or tube distortion.
Full: Strong fundamentals relative to harmonics. Good
low-frequency response, not necessarily extended, but with
adequate level around 100 to 300 Hz. Male voices are full
around 125 Hz; female voices and violins are full around 250
Hz; sax is full around 250 to 400 Hz. Opposite of thin.
Gentle: Opposite of edgy. The harmonics - highs and upper mids
- are not exaggerated, or may even be weak.
Grainy: The music sounds like it is segmented into little
grains, rather than flowing in one continuous piece. Not liquid
or fluid. Suffering from harmonic or I.M. distortion. Some
early A/D converters sounded grainy, as do current ones of
inferior design. Powdery is finer than grainy.
Grungy: Lots of harmonic or I.M. distortion.
Hard: Too much upper midrange, usually around 3 kHz. Or, good
transient response, as if the sound is hitting you hard.
Harsh: Too much upper midrange. Peaks in the frequency response
between 2 and 6 kHz. Or, excessive phase shift in a digital
recorder's lowpass filter.
Honky: Like cupping your hands around your mouth. A bump in the
response around 500 to 700 Hz.
Mellow: Reduced high frequencies, not edgy.
Muddy: Not clear. Weak harmonics, smeared time response, I.M.
distortion.
Muffled: Sounds like it is covered with a blanket. Weak highs
or weak upper mids.
Nasal: Honky, a bump in the response around 600 Hz.
Piercing: Strident, hard on the ears, screechy. Having sharp,
narrow peaks in the response around 3 to 10 kHz.
Presence: A sense that the instrument in present in the
listening room. Synonyms are edge, punch, detail, closeness and
clarity. Adequate or emphasized response around 5 kHz for most
instruments, or around 2 to 5 kHz for kick drum and bass.
Puffy: A bump in the response around 500 Hz.
Punchy: Good reproduction of dynamics. Good transient response,
with strong impact. Sometimes a bump around 5 kHz or 200 Hz.
Rich: See Full. Also, having euphonic distortion made of
even-order harmonics.
Round: High-frequency rolloff or dip. Not edgy.
Sibilant. "Essy" Exaggerated "s" and "sh" sounds in singing,
caused by a rise in the response around 6 to 10 kHz.
Sizzly: See Sibilant. Also, too much highs on cymbals.
Smeared: Lacking detail. Poor transient response, too much
leakage between microphones. Poorly focused images.
Smooth: Easy on the ears, not harsh. Flat frequency response,
especially in the midrange. Lack of peaks and dips in the
response.
Spacious: Conveying a sense of space, ambiance, or room around
the instruments. Stereo reverb. Early reflections.
Steely: Emphasized upper mids around 3 to 6 kHz. Peaky, nonflat
high-frequency response. See Harsh, Edgy.
Strident: See Harsh, Edgy.
Sweet: Not strident or piercing. Delicate. Flat high-frequency
response, low distortion. Lack of peaks in the response. Highs
are extended to 15 or 20 kHz, but they are not bumped up. Often
used when referring to cymbals, percussion, strings, and
sibilant sounds.
Thin: Fundamentals are weak relative to harmonics.
Tight: Good low-frequency transient response and detail.
Tinny, Telephone-like: Narrowband, weak lows, peaky mids. The
music sounds like it is coming through a telephone or tin can.
Transparent: Easy to hear into the music, detailed, clear, not
muddy. Wide flat frequency response, sharp time response, very
low distortion and noise.
Tubby: Having low-frequency resonances as if you're singing in
a bathtub. See bloated.
Veiled: Like a silk veil is over the speakers. Slight noise or
distortion or slightly weak high frequencies. Not transparent.
Warm: Good bass, adequate low frequencies, adequate
fundamentals relative to harmonics. Not thin. Also excessive
bass or midbass. Also, pleasantly spacious, with adequate
reverberation at low frequencies. Also see Rich, Round. Warm
highs means sweet highs.
Weighty: Good low-frequency response below about 50 Hz.
Suggesting an object of great weight or power, like a diesel
locomotive.
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.
--
Bob Neidorff; Texas Instruments | Internet: neid...@ti.com
50 Phillippe Cote St | Voice : (US) 603-222-8541
Manchester, NH 03101 USA
Note: Texas Instruments has openings for Analog and Mixed
Signal Design Engineers in Manchester, New Hampshire. If
interested, please send resume in confidence to address above.
neid...@ti.com
Last-modified: 2007/07/12
Version: 2.17
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 $40 Grado Prestige Black is a great value for any home user.
However, some users comment that it can pick up hum from some
turntables. For the purist, there are many other 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.
10.6 Will phono cartridges still be around ten years from now?
Ten years ago, I wrote that cartridges will become scarce. I was
wrong. Today, many manufacturers to make many common, good, and
audiophile cartridges, including well respected makers like Grado,
Ortofon, Rega, Shure, and Sumiko.
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.blooberry.com/cdfaq/
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.
All sampling techniques (so also D/A techniques) require an
analog reconstruction filter following the converter. This
filter inherently adds phase shift, frequency response ripple
and high frequency roll-off, depending on the characteristic of
the reconstruction filter (which depends on the position of its
poles and zeros).
An oversampling data converter generates a higher output
sampling rate than a simpler converter, so you can use a more
simple reconstruction filter, which is cheaper and more stable
in time and temperature and produces less noise. Also, modern
oversampling systems include digital filters which compensate
the response of the analog filter in the passband, so you can
achieve systems with an overall performance of 20 Hz...18 kHz
+/-0.05 dB. Also deemphasis is mostly done in the digital
domain.
So the "sound" of a CD player is more than just the number of
bits. It's the quality of the converter, the filter requirements
imposed by that converter, the quality of the filter, and of
course, the quality of the following analog components. Power
supply quality and clock jitter also influence the sound.
10.18 What is the best under-$200 CD player?
In this price range, most manufacturers give you more features
than construction quality or sound quality. If you want a
particular feature, then use that to guide your purchase. If
you are after the best possible sound quality, let your ear
be your guide. Sound quality still varies among models. Don't
trust reviews or advice alone.
10.20 What is the best under-$500 CD player?
Some recommend Rotel. Others recommend Marantz, NAD, or Yamaha.
The industry has made major gains in terms of sound consistancy
in the past years. However, models change every year and there
are models with design flaws. Let your ear be your guide. Also,
don't forget to check quality of construction. In this price
range, you should get more than a flimsy box and more durable
mechanisms than in the <$200 price range.
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.
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 Meguier's 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, the collected
singles CD release by The Sisters Of Mercy at 79:30, 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)
10.36 Why is CD digital data written in 44.1 kHz samples?
The rate of 44.1 kHz was picked to be compatible with existing
50 Hz and 60 Hz video-based digital audio storage, where an
integral number of frame buffers could fit in a single
horizontal scan. Quote from Watkinson and Rumsey, "Digital
Interface Handbook" 2.7.6 Choice of Sampling Rate:
"In 60 Hz [525 line, 60 Hz vertical refresh) video there are 35
blanked lines, leaving 490 lines per frame, or 245 lines per
field for samples. If three samples were stored per line, the
sampling rate becomes 60*245*3=44.1 kHz. In 50 Hz video [625
line, 50 Hz vertical refresh), there are 37 lines of blanking,
leaving 588 active lines per frame, or 294 per field, so the
sampling rate becomes 50*294*3=44.1 kHz. The sampling rate of
44.1 kHz came to be that of the Compact Disk. Even though CD
has no video circuitry, the equipment used to make CD masters
is video based and determined the sampling rate."
The length of 74 minutes is determined by the physical nature
of the reading system. It's based on the encoding method, the
wavelength of the laser used (different wavelengths are
incompatible with current CDs) and the necessary support
information. During the development of the CD, von Karajan was
alledgedly asked how long a CD must be, to which he responded
it must be long enough to hold HIS performance of Beethoven's
9th symphony, but the parameters had pretty much already been
nailed down at that point.
10.37 What's the latest on DVD and DAD?
Check out the articles in The Absolute Sound on the subject,
from issue 112, which is also on the web:
http://www.theabsolutesound.com/dadforum-1.htm
http://www.theabsolutesound.com/dadforum-2.htm
http://www.theabsolutesound.com/dvdhope.htm
10.38 What's the latest on the MiniDisc(tm)?
Check out the MiniDisc(tm) organization web site for a minidisc
FAQ and other MiniDisc(tm) information.
http://www.minidisc.org
10.39 How can I record an LP or tape onto a CD?
That's a complex question, but basically, get a sound card for
your computer, get some cheap software for your computer, and
follow some of the advice at:
http://homepages.nildram.co.uk/~abcomp/lp-cdr.htm
neid...@ti.com
Last-modified: 2007/07/12
Version: 2.17
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?
Most amplifiers can drive load impedances that are too high or
too low by a factor of perhaps two, since they will be designed to
cope with speaker impedances changing with frequency by that much
or more, but you lose safety margin, so keep the volume down.
Driving too low a load impedance increases the current in the
output transistors at a time when the voltage across them
is high, so extra heat is a risk as well as extra current.
The distortion will almost certainly be higher, but the point
at which the transistors burn out may not coincide with the
distortion getting significantly worse. Unless you are an
electronics engineer and open the box, measure the heatsinks,
and do the calculations, you can't tell if it is safe just
by listening.
Also, amplifiers with transformer output stages (most tube amps)
can be damaged with too HIGH an output impedance, e.g. an open
circuit.
If the manufacturer recommends a range of impedances it is safest
to abide by that.
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?
One source is to buy a cheap boom box and only use the
amplifier. Another source is to buy a car stereo booster and
get a 12V power supply for it. Here are some companies that
sell amplifier modules and kits:
http://www.ilpelectronics.com
http://www.quasarelectronics.com
http://www.aussieamplifiers.com
http://www.partsexpress.com
Others sell amplifier hybrids that require a few extra parts
but contain most of the parts required like the STK084:
http://www.ampslab.com/trans_stk084.htm
Finally, you can build a great amp pretty easily if you are handy,
but it probably won't be that cheap. AudioXpress (Old Colony)
sells some amp kits. These kits have been built by satisfied
rec.audio.* posters. (See 11.15, 11.16, 11.17)
http://www.audioxpress.com
11.8 Is the stuff sold by Carver (or brand XXX) 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.
That same logic applies to every manufacturer. Beware marketing
hype and prejudice. Don't believe what others say or bold claims
in reviews or advertisements. Trust your ears.
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.
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.
Electrolytic supply capacitors will fail after enough time at
temperature. They will last longer if turned off between use.
However, like tubes, capacitors can last tens of years of
continuous use, as can power transformers, semiconductors, and
the like. Better quality electrolytic capacitors are rated for
operation at 105 degrees C. If you're replacing the
electrolytic capacitor in a power supply, look for capacitors
with this higher temperature rating, rather than 85 degree C
capacitors.
Electrolytic 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. What follows
are some generalizations. There may be exceptions to these
generalizations, but they are based on solid facts.
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. This
accounts for some of the characteristic "tube amplifier sound".
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
virtually zero input current. MOSFETs have lower gain than
bipolar transistors, 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 as
susceptible to thermal runaway or second breakdown when
compared to bipolar transistors, although a badly designed
MOSFET circuit can still self-destruct. 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
now available as P and N-type devices. 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 audioXpress Magazine (see the
magazine section of the FAQ for more info on audioXpress).
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?
In the late 1980s, it was common for mid-range audio to use
discrete transistors and a few carefully placed op amps. In
the 2000s, integrated circuits are much more sophisticated
and highly integrated. The idea of swapping out an inferior
op-amp for a better part as an easy way of improving sound is
far less meaningful today than it was in the 1980s.
There are many good op amps available today. Some are
engineered for use in audio. If you want to build something
for yourself, such as a filter or buffer, select a quality
op-amp that is meant for audio use. Also, pay careful attention
to the power supplies and grounding. Remember that all op-amp
circuits process signals with respect to ground, whether they
have a ground terminal or not.
But if you have a modern piece of equipment, don't waste your
time trying to replace the op amps in it with better parts.
You may make things worse, rather than better.
As an alternative, you could consider replacing ceramic or
electrolytic capacitors in the audio paths with quality film
capacitors. This is a safer idea and more likely to improve
the sound. For supply bypassing, ceramic capacitors are OK,
but they are bad if used in between stages or as part of a
filter or equalization network. Electrolytic capacitors
are also poor if used in the signal path. You can improve
the sound by adding a large value film capacitor in parallel
with the existing electrolytic capacitor.
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
937-222-0173
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)
Toroid Corporation of Maryland
2020 Northwood Drive
Salisbury, MD 21801 USA
410-860-0300
Fax 410-860-0302
USA Toll Free 888-286-7643
sa...@toroid.com
http://www.toroid.com
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 Kits, div. Classified Audio Video Inc. (kits from
Erno Borbely designs)
sup...@audiokits.com
http://www.audiokits.com
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
BORBELY AUDIO, Erno Borbely (JFET & tube preamp kits, MOSFET &
tube power amplifier kits. Also audiophile components)
Angerstr. 9
86836 Obermeitingen, Germany
Tel: +49/8232/903616
Fax: +49/8232/903618
E-mail: Borbel...@t-online.de or EBor...@aol.com
http://www.borbelyaudio.com
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
Old Colony Sound (+) (See 11.15)
PAiA Electronics (?) (Musician-related kits)
3200 Teakwood Lane
Edmond OK 73013 USA
405-340-6378
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
Available only from author direct at
barry....@wdc.com or barry...@coxnet.net
Howard D Sams & Co ISBN 0-672-21833-X
Electronics World
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
Everyday Practical Electronics
audioXpress 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)
Silicon Chip Magazine
http://www.siliconchip.com.au/
The Technique of Electronic Music, by Thomas H Wells
Schirmer Books ISBN 0-02-872830-0
Vacuum Tube Amplifiers, MIT Radiation Lab series
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
even smaller than Class AB amps and more efficient, because
they use high-speed switching rather than linear control.
Starting in the late 1990s, Class D amps have become quite
good, and in some cases rivaling high quality amps in sound
quality. 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 switching 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 71%. 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 theoretically not as good as Class AB
or Class A.
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. If the switching frequency is
high enough, then less filtering is required. As technology
improves, industry is be able to make higher switching
frequency amplifiers which require less low-pass filtering.
Eventually, Class D amplifier quality could catch up with Class
A amplifiers. Some believe that it already has.
Some people refer to Class E, G, and H. These are not as well
standardized as class A and B. However, Class E refers to an
amplifier with pulsed inputs and a tuned circuit output. This
is commonly used in radio transmitters where the output is at
a single or narrow band of frequencies. Class E is not used
for audio.
Class G refers to "rail switched" amplifiers which have two
different power supply voltages. The supply to the amplifier
is connected to the lower voltage for soft signals and the
higher voltage for loud signals. This gives more efficiency
without requiring switching output stages, so can sound better
than Class D amplifiers.
Class H refers to using a Class D or switching power supply
to drive the rails of a class AB or class A amplifier, so that
the amplifier has excellent efficiency yet has the sound of a
good class AB amplifier. Class H is very common in professional
audio power amplifiers.
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.
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.
--
Bob Neidorff; Texas Instruments | Internet: neid...@ti.com
50 Phillippe Cote St. | Voice : (US) 603-222-8541