Audio FAQs - Part 4

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.

11.2 Can amplifier X drive 2 ohm or 4 ohm speakers? How do I raise the impedance of a speaker from (say) 4 ohms to 8 ohms?

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

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

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.

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.

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.

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

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

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.

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.

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.

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.

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

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

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

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

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

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

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

Lets first list some commonly used active electronic components and their good and bad attributes.

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

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

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

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

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

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

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

Transistors also have wide swing before saturation. Transistors are subject to a failure mode called second breakdown, which occurs when the device is operated at both high voltage and high current. Second breakdown can be avoided by conservative design, but gave early transistor amps a bad reputation for reliability.

Transistors are also uniquely susceptible to thermal runaway when used incorrectly. However, careful design avoids second breakdown and thermal runaway.

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

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

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

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

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

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

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

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

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

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

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

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

Some people claim that tubes require less or no feedback while transistor amps require significant feedback. In practice, all amps require some feedback, be it overall, local, or just "degeneration".

Feedback is essential in amps because it makes the amp stable with temperature variations and manufacturable despite component variations. Feedback has a bad reputation because a badly designed feedback system can dramatically overshoot or oscillate. Some older designs used excessive feedback to compensate for the nonlinearities of lousy circuits. Well designed feedback amps are stable and have minimal overshoot.

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

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

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

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

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

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

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

For more information on tubes, get one of the following old reference books, or check out 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

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

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

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

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

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

Some of the better op amps for audio as of today include (* means highly recommended):

Single	Dual
AD845*	AD842
AD847	AD827
AD797*	NE5535
NE5534	NE5532
OP-27	AD712
LT1115*	LM833
AD811	OPA2604*
AD841	OP249*
HA5112*
LT1057
LT1028
AD744
SSM2016

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

This can be confused because if TI copies a Signetics op amp, they may assume the Signetics prefix, or they may use their own. Fortunately, if the part numbers are the same, circuitry is almost exactly the same, as is the performance. (Note: almost)

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

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

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

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.

Toroid Corp of Maryland (Toroidal power transformers) (+)
(also sells without secondary, ready to finish)

BORBELY AUDIO, Erno Borbely (JFET & tube preamp kits, MOSFET & tube power amplifier kits. Also audiophile components)

Elektor Electronics (How it works and you-build articles)
(no longer published in US. Still available in Europe)

Enhanced Sound: 22 Electronic Projects for the Audiophile
(Some basic projects and some "how it works")

Some of the above titles, as well as a catalog of technical books, are available from:

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

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

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

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

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

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

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

Class B amps have output stages which have zero idle bias current. Typically, a Class B audio amplifier has zero bias current in a very small part of the power cycle, to avoid nonlinearities.

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

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

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

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

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

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

Class D amplifiers use pulse modulation techniques to achieve even higher efficiency than Class B amplifiers. As Class B amplifiers used linear regulating transistors to modulate output current and voltage, they could never be more efficient than 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 not as good as Class AB or Class A.

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

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

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.

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.

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.