From: andrew@alice.att.com (Andrew Hume)
Newsgroups: rec.audio
Subject: rec.audio FAQ
Keywords: faq
Date: 22 Dec 92 18:20:52 GMT
Article-I.D.: alice.24467
Organization: AT&T Bell Laboratories, Murray Hill NJ


hi netters,

	bob neidorff is having news problems and so i am posting this for him.
there will be three parts.

	just a reminder, DON'T HIT 'R'!! reply to bob or rec.audio as
per the instructions below.

			andrew hume

-----------------------------------------------------------------
Many have asked about/for a list of frequently asked questions for 
newsgroup rec.audio.  Thanks to the large efforts of a few generous
people and the additional small efforts of many, we are able to present
this first pass effort.  

We welcome all comments, general, specific,  positive and negative, about 
any part of the faq.  I suggest that specific comments, such as:
	Corrections,
	Requests for new questions,
	Additions to present questions, and
	Suggested new questions and answers
be directed to neidorff@uicc.com.  From there, they will be carefully
read and incorporated into the next release of the faq, if practical.

I suggest that general comments, such as comments on:
	Format,
	Length,	
	Organization, and
	General subject matter
be posted directly to rec.audio for discussion.

I also encourage people to direct general feelings to neidorff@uicc.com.
We want to hear things like "I liked the part about..." or "I don't like...",
and feel that this kind of comment is best handled directly.
==============================================================================
This is Part 1 of 3 parts of the faq for newsgroup rec.audio, dated 15 Dec 92.

1.0 Contents:

    Part 1:
	1.0	Contents
	2.0	Organization
	3.0	Purpose
	4.0	Omissions
	5.0	Credits
	6.0	Errors and Corrections
	7.0	Disclaimer
	8.0	Copyright Notice
	9.0	Speakers
	10.0	Amplifiers
    Part 2:
	11.0	CD Players, CDs, Turntables, and LPs
	12.0	High Fidelity Systems
	13.0	Listening Rooms and Houses
	14.0	Recording
    Part 3:
	15.0	Mail Order
	16.0	Wire
	17.0	The Press
	18.0	Retail
	19.0	Miscellaneous
	20.0	Net Protocol

2.0 Organization:

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

This FAQ is split into three individual postings.  This FAQ also references a 
frequent posting of sources for audio via Mail Order.  That list is maintained 
by nau@SSESCO.com (William R. Nau).  Contact Mr. Nau directly for information 
or corrections to that posting.

3.0 Purpose:

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

4.0 Omissions:

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

5.0 Credits:

This FAQ is the work of many people.  Allow me to thank everyone who helped
now.  We appreciate all of your contributions.  However, remember, all blame
and criticism should be directed to Bob Neidorff at neidorff@uicc.com.  Some
of the contributors to this FAQ are listed below.  Others have made great
contributions, but are no less appreciated.

	andrew@research.att.com (Andrew Hume)
	jj@research.com (jj)
	neidorff@uicc.com (Bob Neidorff)
	DPierce@world.std.com (Richard d Pierce)
	P.Smee@bristol.ac.uk (Paul Smee)

6.0 Errors and Corrections:

Some errors are intentionally included in this FAQ.  (It's easier to say that
and later admit we were wrong, than to claim that this is a best effort).
If you have a correction to the FAQ, additional information, or a new topic
for the FAQ, please send e-mail to neidorff@uicc.com describing your thoughts
in detail.  Please include your e-mail address in your submission, so that we
can stay in touch.  Every submission will be considered for inclusion in the
next release of the FAQ.

7.0 Disclaimer:

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

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

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

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

8.0 Copyright Notice:

The information contained here is collectively copyrighted by the authors.
The right to reproduce this is hereby given, provided it is copied intact,
with the complete 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.

9.0 Speakers:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	Consult the manual which comes with the subwoofer.

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

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

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

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

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

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

	The speakers go under names like "Acoustic Monitor DB IV",
	"Acoustic Linear," "Pro-Poly," "Audio Reference 4350" and so on. 
	They all "feature" things like "liquid cooled 3" tweeter", poly-
	cone 12" woofer, fantastic (but impossible) frequency response,
	98 db/watt sensitivity, and so on.

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

	Stay away, you're getting ripped off.

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

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

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

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

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

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

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

9.9 Where can I read more about speaker building?
	Speaker Builder Magazine
		Audio Amateur Publications
		PO Box 494
		Peterborough NH  03458 USA
		603-924-9464
	The Loudspeaker Design Cookbook, Fourth Edition
		by Vance Dickason (C) 1991
		ISBN 0-9624191-7-6
		$29.95 + $2.00 S&H from
		Old Colony Sound Lab
		PO Box 243
		Peterborough NH  03458-0243 USA
		603-924-9464
	High Performance Loudspeakers by Martin Colloms

9.10 Where can I buy speaker drivers?
	A & S Speakers (Broad line)
		3170 23rd Street
		San Francisco CA  94110 USA
		415-641-4573
	Audio Concepts (Their own kits plus drivers)
		901 South 4th Street	
		LaCrosse WI  54602 USA
		800-346-9183
	Phil Baker (Surplus cabinets only)
		546 Boston Avenue
		Medford MA  02155
	Carbonneau
		609 Myrtle NW
		Grand Rapids MI  49504 USA
		616-456-9528
	Drexler Audio Systems (Bandor Speaker Distributor)
		14 Rose Lane
		Rosemont PA  19010
	Gold Sound (Broad line including pro speakers)
		PO Box 141
		Englewood CO  80151 USA
		303-789-5310
	Just Speakers
		3170 23rd Street
		San Francisco CA  94110 USA
		415-641-9228
	Madisound (Broad line)
		8608 University Green
		Box 4283
		Madison WI  53711 USA
		608-831-3433
	McGee Electronics (Polydax, Pyle, Motorola, EV, MTX)
		1901 McGee Street
		Kansas City MO  64108-1891 USA
		913-491-5287 or 800-876-2433
	Meniscus
		2442 28th Street SW Ste D
		Wyoming MI  49509 USA
		616-534-9121
	Parts Express
		340 East First Street
		Dayton OH  45402-1257 USA
		513-222-0173
	Solen Electronique
		4470 Avenue Thibault
		St.-Hubert, QC  J3Y 7T9 Canada
		514-656-2759
	Speakers Etc.
		1828 West Peoria Avenue
		Phoneix AZ  85029 USA
		602-944-1878
	SRS Enterprises (Pyle, Pioneer)
		318 South Wahsatch Avenue
		Colorado Springs CO  80903 USA
		719-475-2545
	Transducer Technology (Cabasse, Accuton, Vieta,
			Audiom by Focal, Stage, Tekton)
		4320 Spring Valley Road
		Dallas TX  75244
		214-991-6994
	Zalytron (Broad line including kits)
		469 Jericho Turnpike
		Mineola NY  11501 USA
		516-747-3515

9.11 Where can I buy loudspeaker kits?
	Audio Concepts, Inc.
		901 South Fourth Street
		La Crosse WI  54601 USA
		608-784-4570
	Mahogany Sound
		2610 Schillingers Rd #488
		Mobile AL  36695 USA
		205-633-2054
	Also see above, under suppliers for speaker drivers.

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

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

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

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

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

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

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

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

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

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

	In addition to speaker manufacturers, there is also a company which 
	sells universal rebuild kits for $27.95 per pair, that contain new 
	foam, a special glue, and instructions.  If you have a blown or 
	distorted voice coil, this still won't help. A few netters have used 
	rebuild kits from this company successfully. Contact:
		Stepp Audio Technologies
		PO Box 1088
		Flat Rock NC  38731 USA
		800-747-3692

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

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

	You can get a lot done with a simple spreadsheet and the equations and
	tables in a book like The Loudspeaker Design Cookbook (see above).

	Active Filter Designer by Fernando Garcia Ciesca
		$17.50.  Crossover design.
	BoxModel by Robert Bullock
		$50.00.  Sealed, vented, and passive radiator
		modeling below 400Hz.
	CACD by Scientific Design Software
		Crossover optimizer.
	CALSOD by Witold Waldman
		$65 basic model, $300 complete.
		Crossover optimizer.
	Crossover Design by G. R. Koonce
	Driver Eval by G. R. Koonce
		$25 for both.  Simple programs do what they say.
	Filter Designer 1 by SpeakEasy
		$195 Crossover design program
	Loudspeaker by Maximum Effort Software
		$65 Box design for many different enclosures
		plus crossover network design
	LEAP by Audio Teknology
		$295 to $895 depending on options.
		Complete multi-way design program.
	Low Frequency Designer by SpeakEasy
		$195 Design many different box types.
	LMP by Ralph Gonzales/Sitting Duck Software
		$49.50 Crossover characteristic display
	MacSpeakerz by True Image
		$249 Apple Macintosh box and crossover design
	ModelONE by TDR
		$288 Does auto sound design knowing the
		patterns of various car interiors
	PXO by Robert Bullock
		$50 simple crossover design program
	Speak by DLC Designs
		$395 Does box design and crossover design
	Speaker Designer by Stuart Bonney
		$19.50 Low-frequency box design.
	Two-Way Active Crossover Design by Gary Galo
		$20 Simple crossover component calculation
	XOPT by Peter Schuck
		$199 Complete filter design; rumored to have bugs

10.0 Amplifiers
	Note: A receiver contains an amplifier, so the following questions
	apply to both receivers and amplifiers.

10.1 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. The most plausible explanation
	involves magnetic induction of noise in the relatively low current
	HI cable from the relatively high current signal in the LO cable.
	Accordingly, Vandersteen recommends the two cable pairs for a channel
	be separated by 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. Bimaping 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.

10.2 Can amplifier X drive 2 ohm or 4 ohm speakers?
	Almost any amplifier can drive almost any load if you don't turn the
	volume up too high.  Tube amplifiers are one exception.  Some amplifiers
	will 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 10.3.

10.3 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 10.2 above)

	The second is that in the case of 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. 

	Many amplifiers have output connectors for two pairs of speakers.
	In general, these amplifiers also have a speaker selector switch.
	These amplifiers almost exclusively connect the speakers in parallel
	when both are selected.  It is common for these amplifiers to specify
	8 ohm speakers only.  The reason that they specify 8 ohm speakers is
	because the amplifier is built to drive either 4 or 8 ohms, and two
	sets of 8 ohm speakers loads the amplifier as if it were one set of
	4 ohm speakers.  It is probably perfectly safe to connect one set
	of 4 ohm speakers to an amplifier with two sets of outputs, provided
	that you NEVER use the second set for any set of speakers.

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

	More importantly, human hearing is sensitive to the log of the power
	coming out of the speaker.  This means that the perceived difference
	between an 80 watt amplifier and a 100 watt amplifier, all else equal,
	is very small. (under 1dB!)

	There is a wide variation in the "efficiency" and "sensitivity" of
	the various speakers available.  I have seen good speakers with 
	under 80dB per watt efficiency and have also seen good speakers with
	over 96dB per watt efficiency, measured one meter from the speaker.
	This difference of 16dB 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 88dB 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. 

10.5 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 digital voltmeter and a test recording or signal 
	generator.

10.6 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.

10.7 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 cheap.  (See 10.14, 10.15, and 10.16.)

10.8 Is the stuff sold by Carver really awesome?
	Some people really like it.  Others believe that it doesn't perform up
	to specifications.  Still others believe that it isn't all that it is
	hyped up to be. 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.

10.9 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.

10.10 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.

10.11 Do I need a preamp?
	If you have a turntable, you MUST have a (real) preamp, or something 
	with a phono preamp section.  

	If you don't have a turntable, then the only value in a preamplifier
	is to select the signal source (tuner, CD player, etc) and adjust the
	volume.  If you only listen to CD's, skip the preamp entirely.  Get a 
	CD player with a level control and connect it straight to a power 
	amplifier.

	If you have a receiver, then the odds are that you already have all of 
	the functions of a preamp.

10.12 Should I leave my equipment on all of the time, or turn it on and off?
	In general, it doesn't matter.  Either is fine.

	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, as
	long as they are not left off for many years.  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 may 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.

10.13 What about swapping op-amps?
	Many amplifiers, receivers, preamplifiers, CD players, and mixers use
	integrated circuits (ICs) called operational amplifiers (op amps) as
	audio amplifiers.  The first op amps had a reputation for 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 in your gear.  It is
	dependent on the individual design, layout, etc.

	As technology and IC design expertise marches on, audio op amps are
	getting better, and swapping is getting less and less fruitful.  New
	op amps continue to surface which will displace yesterday's best, but
	sound surprisingly similar to a straight wire.

	All of that said, some of the better op amps for audio as of today
	include (* means very highly recommended):
		Single		Dual
		AD847		AD827
		AD797*
		NE5534		NE5532
		OP-27		AD712
		LT1115*		LM833
		AD811		NE5535
		AD841		AD842
		OPA2604*	OP249*
		HA5112*
		LT1057
		LT1028

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

	Op amp part numbers start with a manufacturer's prefix:
		Analog Devices uses AD
		Burr Brown uses OPA
		Motorola uses MC
		National uses 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, the circuitry is almost exactly the
	same and so is the performance. (Note: I said almost)

	The next thing in the part number is two, three or four or five digits.
	This is invariably the key to the part. There are some op amps with
	different numbers that are electrically the same, but if the numbers
	are the same, the parts are almost surely the same.  For example, an
	LM357N and an LM357J are electrically identical and will 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.

10.14 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
	that they have a catalog available.

	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
	DigiKey Corporation (Full Line of Parts) (+) (C)
		701 Brooks Avenue South
		PO Box 677
		Thief River Falls MN  56701-0677 USA
		800-344-4539
	Maplin (European Parts) (?) (C)
		+44 702 554161 (For Orders Only)
		+44 702-552911 (Customer Service)
	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
	MesaBoogie (Tubes, instrument speakers) (?)
		707-778-8823
	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)
	Parts Express (Speakers, Cables, Connectors) (+) (C)
		340 East First Street
		Dayton OH  45402-1257 USA
		513-222-0173
	Radio Shack (Parts, Low-End Audio) (+) (C)
	Old Colony Sound (Audio parts and audio kits) (+) (C)
		PO Box 243
		Peterborough NH  03458-0243 USA
		603-924-9464
	Tanner Electronics (Surplus Parts) (+)
		214-242-8702
	Triode Electronics (Tubes, transformers, boxes) (?)
		2010 Roscoe Street
		Chicago IL  60618
		312-871-7459

10.15 Where can I buy audio amplifier kits?
	Alas, Heath is no longer making Heathkits. Alternatives:
	Old Colony Sound (See 10.14)
	Hart Electronics (UK)
	Hafler
	Crimson (UK)
	Mark V Electronics
		8019 E Slauson Ave
		Montebello CA  90640 USA
		800-423-3483
		213-888-8988

10.16 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
	Elektor Magazine
		Audio Amateur Publications
		(see above)
	"Audio IC Op-Amp Applications" by Walter Jung
		OpAmp Technical Books
		213-464-4322
	"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
	"IC Op-Amp Cookbook" by Walter Jung
		OpAmp Technical Books
		213-464-4322
	Journal of the Audio Engineering Society
		Audio Engineering Society
		60 East 42nd Street
		New York City NY  10165-0075 USA
		212-661-2355
	Electronic Music Circuits, by Barry Klein
		Howard D Sams & Co ISBN 0-672-21833-X
	The Technique of Electronic Music, by Thomas H Wells
		Schirmer Books ISBN 0-02-872830-0
	Wireless World
	Radio-Electronics
	Popular Electronics					
	Analog Devices Audio/Video Reference Manual
	Radiotron Designer's Handbook, Fourth Edition (old)
	Vacuum Tube Amplifiers, MIT Radiation Lab series

-- 
Bob Neidorff; Unitrode I. C. Corp.  |  Internet: neidorff@uicc.com
7 Continental Blvd.                 |  Voice   : (US) 603-424-2410
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-424-3460
Newsgroups: rec.audio