Newsgroups: rec.audio,rec.music.info,rec.answers,news.answers From: neidorff@uicc.com Subject: FAQ: rec.audio (part 1 of 4) Message-ID: Date: Mon, 18 Oct 1993 14:36:18 GMT Approved: rec-music-info@cp.tn.tudelft.nl Archive-name: AudioFAQ/part1 Last-modified: 1993/9/21 Version: 1.9 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: 10.13 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 nau@SSESCO.com (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. We appreciate all of your contributions. However, all blame and criticism should go 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.att.com (jj) neidorff@uicc.com (Bob Neidorff) Rick Oakley (no internet access) DPierce@world.std.com (Richard d Pierce) P.Smee@bristol.ac.uk (Paul Smee) gmw1@cunixa.cc.columbia.edu (Gabe M. Wiener) 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. 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 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 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. 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. 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? 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 0-9624-191-7-6 $29.95 + $2.00 S&H from Old Colony Sound Lab PO Box 243 Peterborough NH 03458-0243 USA 603-924-9464 High Performance Loudspeakers by Martin Colloms 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 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 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 (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 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. (Wide range of kits. Catalog available) 901 South Fourth Street La Crosse WI 54601 USA 608-784-4570 Fried Products (Parts kits starting $550. Catalog available) (Emphasizes high-end transmission line speakers) (Parts kits have plan, crossover, and driver) Conshocken Road Conshocken PA 19428 USA 215-277-9004 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 Another vendor of speaker repair parts is: 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 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 speaker programs for speaker design and speaker design hardware, such as measurement systems, get the archive "audio.speaker" in directory "dist" on node "research.att.com" (also known as 192.20.225.2). 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. 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. 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 1 dB!) 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. (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? 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.