AUDIOCLINIC (Feb. 1987)

Who? Where?

I have noticed a growing tendency for correspondents not to write their addresses-and sometimes even their names-on the letters they send me. I suppose they reason that putting this information on their outer envelopes, or the self-addressed stamped envelopes they enclose, is sufficient.

The problem is, I don't work at Audio magazine's office, where I'd get these letters one at a time. Instead, pack ages of letters are forwarded to me. I open a number of letters at a time, then try to sort them by date so I can answer the earliest ones first. (I answer the undated letters last.) Often, the external and/or return envelopes become separated, and I must rely on names and addresses in the letters them selves-so please include that information.

One more thing: If you want a reply, you'll get it faster if you enclose a stamped, self-addressed envelope and put your ZIP code on it. Typing envelopes and looking ZIP codes up in a directory takes time which would be better employed in answering your questions.

Thank you for your help!

-J. G.

Bi-amplification Hookup

Q. What hardware is required to hook up a biamplified system?

-David C. Bennett, Elizabeth City, N.C.

A. First, you need speakers with accessible woofer and tweeter terminals which bypass any internal crossovers.

Then you need a preamplifier, one amplifier channel for each woofer and tweeter (four in all, for stereo), and an electronic crossover. The amplifiers can be stereo or mono models; you can even use the preamp and power amp sections of an integrated amplifier or receiver if the unit has preamp-out and amp-in jacks.

The crossover frequency should be set to match the requirements of your woofers and tweeters. The preamp output is connected to the crossover input. The crossover's high-frequency (or high-pass) output should be fed, via the less powerful amplifier channels, to the tweeters. The low-frequency (low-pass) output should be fed, via the other amplifier channels, to the woofers. The crossover will usually have a level control for the high or low outputs to match the woofer and tweeter levels. If not, use the amplifiers' in put gain controls for this.

Breaking Glass

Q. Why does glass shatter when op era singers sing?

-Brenden Mesch, Plano, Tex.

A. If such a thing can happen, think the scenario would run this way: The glass used is likely to be thin.

Tapping it would produce a ringing sound whose frequency is the resonant frequency of the glass. (All bodies capable of vibrating will do so at some specific frequency, known as the resonant frequency.) If a source produces a sound at the resonant frequency of a glass pane (or of any other body, such as a drum head or taut string), the glass (or other body) will be made to vibrate. This phenomenon is called "sympathetic vibration." If the source frequency and the driven body's resonant frequency match precisely, very little acoustical energy is required to keep the driven body (the glass, in this case) vibrating.

If sufficient energy is imparted to the vibrating glass, the elastic limit of the material will be exceeded and the glass will shatter.

Whether an opera singer (most of whom have wide vibrato-meaning that their output frequencies are not constant) can impart sufficient energy to a glass to cause it to shatter is, to me at least, suspect.

Bypassing Preamp Stages

Q. I recently obtained a Compact Disc player. I tried operating it through the balance and volume controls of my preamplifier, bypassing the earlier portions of the circuit. The results were not successful. The slightest raising of volume from minimum resulted in a large amount of output which sounded harsh as well as loud. Undaunted, I lowered the output from the disc player, but this did not help significantly.

The volume pot is 50 kilohms per section; the balance control is a 20 kilohm pot. I can't imagine why this would load down the CD player, but would pots of different values solve the problem?-Name withheld A. I do not believe that different values for either the balance or the volume control can help your situation. My suggestion is for you to restore your preamplifier to its original state. I suspect that the output impedance of your player is much lower than the d.c. resistance of the pots; thus, the player may not be properly loaded in your current system.

I wonder if the volume circuit that you are working with is a feedback control rather than the more conventional signal attenuator. The low impedance of the player may change the operation of the circuit to such an ex tent that feedback is virtually shorted out, leading to uncontrollable volume adjustment.

In summary, I can only repeat that you will obtain less distortion and better overall performance by restoring the circuit to its original state.

Problems with an FM Stereo Adaptor

Q. I recently equipped my 1962 tube FM tuner with a modern, multiplex stereo adaptor. For about five minutes after a cold start, sound is great! Then I hear a high-pitched whistle. This whistle gradually descends in pitch to be low 1 kHz. If I wait long enough, the tone fades to inaudibility. Of course, switching to "mono" at the preamplifier kills the tone entirely. What's happening?

-John H. Lowry, Terre Haute, Ind.

A. The only likely cause of the problem you have described is an unstable 19-kHz oscillator in the adaptor. I have noticed such symptoms in instances where the oscillator is misaligned; otherwise, I have not seen one of these oscillators drift.

It's a stab in the dark, but inasmuch as you say that your adaptor is a modern unit, it is probably designed to work with low-level signals. Tube-type detector circuits usually generate quite high voltages, on the order of 1V or so.

The detectors in recent tuners develop one-tenth of this value. Thus, it may be that your tuner feeds more signal into the adaptor than it was designed to accept. Try padding the adaptor down with a voltage divider having a 10:1 ratio. Perhaps it will then behave.

(Source: Audio magazine, Feb. 1987, JOSEPH GIOVANELLI)

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