Archive-name: AudioFAQ/part4
Last-modified: 1995/12/7
Version: 2.01

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

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

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

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

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

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

	Biamping also allows you to use high-quality electronic 
	crossovers and drive the speaker's drivers (the voice coils) 
	directly, without the series resistance and non-linear 
	inductance of a passive crossover. Biamping which uses the 
	speaker's crossover is therefore much less desirable. Replacing
	a good speaker's crossover with an electronic crossover has 
	advantages, but involves some very critical tradeoffs and tuning 
	which is best left to those well-equipped or experienced.
	
	See also section 16.0 below, on wire and connectors in general.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	Although tubes and transistors clip differently, a good 
	amp will keep all devices from ever clipping, so this 
	difference should be moot.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	Briefly, Class A amps sound the best, cost the most, and are the 
	least practical. They waste power and return very clean signals.
	Class AB amps dominate the market and rival the best Class A 
	amps in sound quality. They use less power than Class A, 
	and can be cheaper, smaller, cooler, and lighter. Class D amps 
	are only used for special applications like bass-guitar amps and 
	subwoofer amps. They are even smaller than Class AB amps and 
	more efficient, yet are often limited to under 10kHz (less than 
	full-range audio). Class B & Class C amps aren't used in audio.
	
	In the following discussion, we will assume transistor output 
	stages, with one transistor per function. In some amplifiers, 
	the output devices are tubes. Most amps use more than one 
	transistor or tube per function in the output stage to increase 
	the power.
	
	Class A refers to an output stage with bias current greater than 
	the maximum output current, so that all output transistors are 
	always conducting current. The biggest advantage of Class A 
	is that it is most linear, ie: has the lowest distortion.
	
	The biggest disadvantage of Class A is that it is inefficient, 
	ie: it takes a very large Class A amplifier to deliver 50 watts, 
	and that amplifier uses lots of electricity and gets very hot. 
	
	Some high-end amplifiers are Class A, but true Class A only 
	accounts for perhaps 10% of the small high-end market and none 
	of the middle or lower-end market.
	
	Class B amps have output stages which have zero idle bias 
	current. Typically, a Class B audio amplifier has zero bias 
	current in a very small part of the power cycle, to avoid 
	nonlinearities. Class B amplifiers have a significant advantage 
	over Class A in efficiency because they use almost no 
	electricity with small signals. 
	
	Class B amplifiers have a major disadvantage: very audible 
	distortion with small signals. This distortion can be so bad 
	that it is objectionable even with large signals. This 
	distortion is called crossover distortion, because it occurs at 
	the point when the output stage crosses between sourcing and 
	sinking current. There are almost no Class B amplifiers on the 
	market today.
	
	Class C amplifiers are similar to Class B in that the output 
	stage has zero idle bias current. However, Class C amplifiers 
	have a region of zero idle current which is more than 50% of 
	the total supply voltage. The disadvantages of Class B 
	amplifiers are even more evident in Class C amplifiers, so 
	Class C is likewise not practical for audio amps.
	
	Class A amplifiers often consist of a driven transistor 
	connected from output to positive power supply and a constant 
	current transistor connected from output to negative power 
	supply. The signal to the driven transistor modulates the 
	output voltage and the output current. With no input signal, 
	the constant bias current flows directly from the positive 
	supply to the negative supply, resulting in no output current, 
	yet lots of power consumed. More sophisticated Class A amps
	have both transistors driven (in a push-pull fashion).
	
	Class B amplifiers consist of a driven transistor connected 
	from output to positive power supply and another driven 
	transistor connected from output to negative power supply. 
	The signal drives one transistor on while the other is off, 
	so in a Class B amp, no power is wasted going from the 
	positive supply straight to the negative supply. 
	
	Class AB amplifiers are almost the same as Class B amplifiers 
	in that they have two driven transistors. However, Class 
	AB amplifiers differ from Class B amplifiers in that they 
	have a small idle current flowing from positive supply to 
	negative supply even when there is no input signal. This idle 
	current slightly increases power consumption, but does not 
	increase it anywhere near as much as Class A. This idle current 
	also corrects almost all of the nonlinearity associated with 
	crossover distortion. These amplifiers are called Class AB 
	rather than Class A because with large signals, they behave like 
	Class B amplifiers, but with small signals, they behave like 
	Class A amplifiers. Most amplifiers on the market are Class AB.  
	
	Some good amplifiers today use variations on the above themes. 
	For example, some "Class A" amplifiers have both transistors 
	driven, yet also have both transistors always on. A specific 
	example of this kind of amplifier is the "Stasis" (TM) amplifier 
	topology promoted by Threshold, and used in a few different 
	high-end amplifiers. Stasis (TM) amplifiers are indeed 
	Class A, but are not the same as a classic Class A amplifier.
	
	Class D amplifiers use pulse modulation techniques to achieve 
	even higher efficiency than Class B amplifiers. As Class B 
	amplifiers used linear regulating transistors to modulate output
	current and voltage, they could never be more efficient than 
	78%. Class D amplifiers use transistors that are either on or 
	off, and almost never in-between, so they waste the least amount 
	of power. 
	
	Obviously, then, Class D amplifiers are more efficient than 
	Class A, Class AB, or Class B. Some Class D amplifiers have 
	>80% efficiency at full power. Class D amplifiers can also have 
	low distortion, although not as good as Class AB or Class A.
	
	Class D amplifiers are great for efficiency. However they are 
	awful for other reasons. It is essential that any Class D amp 
	be followed by a passive low-pass filter to remove switching 
	noise. This filter adds phase shift and distortion.  It also
	limits the high frequency performance of the amplifier, such
	that Class D amplifiers rarely have good treble. The best 
	application today for Class D amplifiers is subwoofers. 
	
	To make a very good full range Class D amplifier, the switching 
	frequency must be well above 40kHz. Also, the amplifier must be 
	followed by a very good low-pass filter that will remove all of 
	the switching noise without causing power loss, phase-shift, or 
	distortion. Unfortunately, high switching frequency also means 
	significant switching power dissipation. It also means that the 
	chances of radiated noise (which might get into a tuner or phono 
	cartridge) is much higher.

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

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

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

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

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

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

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

11.20 What is amplifier "bridging" or "monoblocking"?  How do I do it?
	When you're told a stereo power amplifier can be bridged,
	that means that it has a provision (by some internal 
	or external switch or jumper) to use its two channels 
	together to make one mono amplifier with 3 to 4 times the
	power of each channel.  This is also called "Monoblocking" 
	and "Mono Bridging".
	
	Tube amps with multiple-tap output transformers are simple to
	bridge.  Just connect the secondaries in series and you get 
	more power.  The ability to select transformer taps means that 
	you can always show the amplifier the impedance it expects, so 
	tube amp bridging has no unusual stability concerns. 

	The following discussion covers output transformer-less amps.
	Bridging these amps is not so simple.  It involves connecting 
	one side of the speaker to the output of one channel and the 
	other side of the speaker to the output of the other channel.  
	The channels are then configured to deliver the same output 
	signal, but with one output the inverse of the other.  The 
	beauty of bridging is that it can apply twice the voltage to 
	the speaker.  Since power is equal to voltage squared divided 
	by speaker impedance, combining two amplifiers into one can 
	give four (not two) times the power.
	
	In practice, you don't always get 4 times as much power.  This
	is because driving bridging makes one 8 ohm speaker appear like 
	two 4 ohm speakers, one per channel. In other words, when you 
	bridge, you get twice the voltage on the speaker, so the 
	speakers draw twice the current from the amp.
	
	The quick and dirty way to know how much power a stereo amp can 
	deliver bridged to mono, is to take the amp's 4 ohm (not 8 ohm) 
	power rating per channel and double it.  That number is the 
	amount of watts into 8 ohms (not 4 ohms) you can expect in mono. 
	If the manufacturer doesn't rate their stereo amp into 4 ohms, 
	it may not be safe to bridge that amp and play at loud levels, 
	because bridging might ask the amp to exceed its safe maximum
	output current.  
	
	Another interesting consequence of bridging is that the amplifier
	damping factor is cut in half when you bridge. Generally, if you
	use an 8 ohm speaker, and the amplifier is a good amp for driving 
	4 ohm speakers, it will behave well bridging.
	
	Also consider amplifier output protection. Amps with simple 
	power supply rail fusing are best for bridging.  Amps that rely 
	on output current limiting circuits to limit output current
	are likely to activate prematurely in bridge mode, and virtually 
	every current limit circuit adds significant distortion when it 
	kicks in. Remember bridging makes an 8 ohm load look like 4 ohms,
	a 4 ohm load look like 2 ohms, etc.  Also, real speakers do not 
	look like ideal resistors to amps.  They have peaks and dips in 
	impedance with frequency, and the dips can drop below 1/2 the 
	nominal impedance.  They also have wildly varying phase with 
	frequency.
	
	Finally, some amplifiers give better sound when bridged than
	others. Better bridging amps have two identical differential 
	channels with matched gain and phase through each input, left
	and right, inverting and non-inverting.  Simpler bridging 
	amplifiers have one or two inverting channels, and run the
	output of one into the input of the second. This causes the
	two outputs to be slightly out of phase, which adds distortion.
	There are also other topologies.  One uses an additional stage to
	invert the signal for one channel but drives the other channel
	directly. Another topology uses one extra stage to buffer the
	signal and a second extra stage to invert the signal. These are 
	better than the simple master/slave arrangement, and if well
	done, can be as good as the full differential power amp.

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; Unitrode Corporation  |  Internet: neidorff@uicc.com
7 Continental Blvd.                 |  Voice   : (US) 603-429-8541
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-429-8564