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Contents of the AUDIOFAQ.TXT file


Archive-name: AudioFAQ/part1
Last-modified: 1994/10/25
Version: 1.22

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:
10.14 Amplifiers (continued)
11.0 CD Players, CDs, Turntables, and LPs
Part 3:
12.0 High Fidelity Systems
13.0 Listening Rooms and Houses
14.0 Recording
15.0 Mail Order
Part 4:
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 four individual postings. This FAQ also
references a frequent posting of sources for audio via Mail Order.
That list is maintained by [email protected] (William R. Nau). Contact
Mr. Nau directly for information or corrections to that posting.
The list is also available via FTP in the pub/rec.audio directory
of SSESCO.com.

Lines beginning with "|" are new since the last version of the FAQ.

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. Also, 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. Some of the contributors to this FAQ are listed below.
Others have made great contributions, and are no less appreciated.

[email protected] (Andrew Hume)
[email protected] (jj)
[email protected] (Bob Neidorff)
Rick Oakley (no internet access)
[email protected] (Richard d Pierce)
[email protected] (Paul Smee)
[email protected] (Gabe M. Wiener)

6.0 Errors and Corrections:

If you have a correction to the FAQ, additional information, or
a new topic for the FAQ, please send e-mail to [email protected]
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. This FAQ
will be reposted every month.

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 are 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 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 CDs 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 may 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. 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
auditioning at the maximum volume you anticipate 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
mono, it will be nearly impossible to create a good stereo.

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.

Good 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 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 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 beyond 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 DJs 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 DJs out there that use
them because they can't afford anything else. They are not
overstocks, and in all likelihood, 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. The brand
names are remarkably similar to reputable firms, but
different enough to delay law suits.

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 US $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 the US $100 the kids paid is pure profit.

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.

If you really want recommendations and are willing to listen to
the opinions of others, check the past few issues of Stereophile
Magazine. Although they are strongly biased towards very
expensive gear and have their own particular other biases, they
do steer you to some very good equipment in their
frequently-updated list of "RECOMMENDED COMPONENTS".

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?
Europe's Greatest Speaker Designs
Solen Electronique
4470 Avenue Thibault
St.-Hubert, QC J3Y 7T9 Canada
Voice 514-656-2759
FAX 514 443-4949
High Performance Loudspeakers by Martin Colloms
Speaker Builder Magazine
Audio Amateur Publications
PO Box 494
Peterborough NH 03458 USA
603-924-9464
Synergetic Audio Concepts Classes and Newsletters
Syn-Aud-Con teaches classes on Audio and Acoustics
12370 W. Co. Rd. 100 N.
Norman IN 47264 USA
812-995-8212
The Loudspeaker Design Cookbook, Fourth Edition
by Vance Dickason (C) 1991
ISBN 0-9624-191-7-6
$29.95 + $2.00 S&H from
Old Colony Sound Lab
PO Box 243
Peterborough NH 03458-0243 USA
603-924-9464
$25 + $2 S&H (sale price as of 3/24/94) from
Madisound
8608 University Green; Box 4283
Madison WI 53711 USA
608-831-3433
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 or 608-784-4570
Phil Baker (Surplus cabinets only)
546 Boston Avenue
Medford MA 02155
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
Madisound (Broad line)
8608 University Green
Box 4283
Madison WI 53711 USA
608-831-3433
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 (Airborne, Audax, Ceratech, Dynaudio, Eton,
Lpg, Morel, Peerless, Scan-Speak, Seas, Solen, Vifa)
4470 Avenue Thibault
St.-Hubert, QC J3Y 7T9 Canada
Voice 514-656-2759
FAX 514 443-4949
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
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. (Wide range of kits. Catalog available)
(see 9.10, above)
Fried Products (Parts kits starting $550. Catalog available)
(Emphasizes high-end transmission line speakers)
(Parts kits have plan, crossover, and driver)
1323 Conshocken Road
Norristown, PA 19401 USA
610-277-1014 or 800-255-1014
Mahogany Sound (Parts kits and Woodstyle kits)
(Parts kits have plan, crossover, and driver)
(Woodstyle kits also have 3/4" MDF veneered boxes)
(Prices $150/pair to $500/pair. Catalog available)
(Two way, three way & subwoofer kits)
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. Alas, 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 these 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 good 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
speaker driver from the manufacturer of the system.

Second choice is 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 set
of 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 are companies which
sells rebuild kits for approximately $30 per pair, containing
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

Two other vendors of speaker repair parts are:
Parts Express (sells 8", 10", 12", & 15" repair kits)
340 E First St
Dayton OH 45402-1257 USA
513-222-0173

Simply Speakers
P. O. Box 22673
St. Petersburg FL 33742 USA
800-767-4041 or 813-571-1245

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 9.9) tells you how to measure a speaker, and also
gives enough theory to feel confident with a good program.
You can get a lot done with a simple spreadsheet and the
equations in a book like The Loudspeaker Design Cookbook.

For more information on programs for speaker design and on
speaker-design hardware, such as measurement systems, get
| the archive "sahfsd02.doc" from directory:
usenet/rec.audio.high-end/Software
on "ftp.uu.net". In addition, there are other interesting
audio-related files in that directory. Look around.

9.15 Can I magnetically shield my speakers for use near a TV?
You probably will need to buy speakers that are made with an
integral magnetic shield. Magnetic shielding is usually done
by either shielding the speaker magnet or by cancellation of the
magnetic field very close to the magnet, or by both. Shielded
speakers are NOT built by lining the enclosure with metal.
While it sounds like a good idea, it doesn't work.

A common magnet shield is a mild steel cup around the magnet.
This is the cheapest shield, and is usually fairly ineffective.
It also will interfere with the speaker's critical magnet gap,
so this type of shield can hurt speaker performance by shorting
the magnetic field and reducing the magnetic flux density in the
gap, which can reduce efficiency and affect the speaker's low
frequency performance.

Cancellation is done using a reverse-polarized magnet glued to
the back of the main magnet. If done right, it can almost
completely cancel the rear stray field. In some cases it can
also increase the magnetic flux density in the gap, which may
or may not be desirable.

9.16 What are all of these abbreviations people use for speakers?
Most of these parameters are well documented in the Loudspeaker
Design Cookbook. (see 9.9) In summary:

Fs Driver free air resonance, in Hz. This is the point at
which driver impedance is maximum.
Fc System resonance (usually for sealed box systems), in Hz
Fb Enclosure resonance (usually for reflex systems), in Hz
F3 -3 dB cutoff frequency, in Hz

Vas "Equivalent volume of compliance", this is a volume of
air whose compliance is the same as a driver's
acoustical compliance Cms (q.v.), in cubic meters

D Effective diameter of driver, in meters
Sd Effective piston radiating area of driver in square meters
Xmax Maximum peak linear excursion of driver, in meters
Vd Maximum linear volume of displacement of the driver
(product of Sd times Xmax), in cubic meters.

Re Driver DC resistance (voice coil, mainly), in ohms
Rg Amplifier source resistance (includes leads, crossover,
etc.), in ohms

Qms The driver's Q at resonance (Fs), due to mechanical
losses; dimensionless
Qes The driver's Q at resonance (Fs), due to electrical
losses; dimensionless
Qts The driver's Q at resonance (Fs), due to all losses;
dimensionless
Qmc The system's Q at resonance (Fc), due to mechanical
losses; dimensionless
Qec The system's Q at resonance (Fc), due to electrical
losses; dimensionless
Qtc The system's Q at resonance (Fc), due to all losses;
dimensionless

n0 The reference efficiency of the system (eta sub 0)
dimensionless, usually expressed as %

Cms The driver's mechanical compliance (reciprocal of
stiffness), in m/N
Mms The driver's effective mechanical mass (including air
load), in kg
Rms The driver's mechanical losses, in kg/s

Cas Acoustical equivalent of Cms
Mas Acoustical equivalent of Mms
Ras Acoustical equivalent of Rms

Cmes The electrical capacitive equivalent of Mms, in farads
Lces The electrical inductive equivalent of Cms, in henries
Res The electrical resistave equivalent of Rms, in ohms

B Magnetic flux density in gap, in Tesla
l length of wire immersed in magnetic field, in meters
Bl Electro-magnetic force factor, can be expressed in
Tesla-meters or, preferably, in meters/Newton

Pa Acoustical power
Pe Electrical power

c propogation velocity of sound at STP, approx. 342 m/s
p (rho) density of air at STP 1.18 kg/m^3

9.17 What are fluid-filled (fluid-cooled, ferro-fluid) tweeters?
These tweeters are built almost exactly the same as other
tweeters. They look and act almost exactly the same, too.
The only difference is that they have a small, controlled
amount of a special fluid inserted into the gap between the
magnet and the voice coil.

One big effect of adding this fluid to a tweeter (or to any
speaker) is that it makes the voice coil capable of dissipating
more heat. This means that the speaker can have a lighter voice
coil, for better performance, or a higher power rating for the
same voice coil. The other big effect of this fluid is to add
mechanical damping. The frequency response and transient
response of the driver will change, possibly for the better.

In addition, this fluid may help center the voice coil, may
lubricate the voice coil, and may help keep dirt out of the gap.
This fluid will not increase the magnetic field, concentrate the
magnetic field or otherwise change the magnetic circuit. Nor
will it cushion impact if the voice coil bottoms.

The fluid used for this purpose is often called "ferrofluid".
It consists of sub-microscopic particles of magnetic material
suspended in special oil. This fluid stays in the gap because
of the strong magnetic pull of the magnet. There is some debate
over whether these fluids can dry out with time. Manufacturers
claim that the oil used is non-volatile.

It is possible to use ferrofluids in mid-range drivers and
woofers. However, as tweeters tend to have the most fragile
voice coils, tweeters have the most to gain from ferrofluid.
There are various different fluids on the market, some of which
have characteristics tailored to tweeters, some to woofers, etc.

It is very risky to blindly add fluid to a driver. It may not
be compatible with the adhesives used in the driver, may not be
practical with the particular driver layout, and is impossible
to remove. Permanent driver damage is possible.

9.18 Should I use spikes under my speakers? Pennies under the spikes?

|9.19 How do you couple speakers to speaker stands?
| Ideally, your speakers should sit flat on the speaker stand
| or floor. They shouldn't see-saw back and forth if nudged.
|
| One good way to accomplish this is to use a small dab of
| putty under each corner of the speaker. There are a few
| common putties used for this, but all share the properties
| of being very elastic and staying flexible indefinitely.
| These putties are inexpensive, removable, and reusable.
|
| Try either Blu-Tak, which is available in the UK from office
| supply stores for cleaning typewriter elements, Faber Castell
| UHU Hold-It, which is available in the US from office supply
| stores for holding up pictures, and DAP's Fun-Tak, which is
| sold in hardware stores for holding up pictures.

10.0 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.

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 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.

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 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 10.3 for more information.

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 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.

Many amplifiers have 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. 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.

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 high
input-impedance digital voltmeter set to AC volts 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. 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

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 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 10.15, 10.16, 10.17)

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. However, much of this
is repeated rumor based on older Carver equipment. 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? 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.

10.12 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.


10.13 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.


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