Archive-name: AudioFAQ/part4
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)
supp...@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: BorbelyAu...@t-online.de or EBorb...@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.l.kl...@wdc.com or barrykl...@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: neido...@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.
