Many prospective buyers of hi-fi components may not be aware of an important change that has come into the power ratings of amplifiers (and amplifier sections of receivers) as a combined result of FTC rulings and a new technical standard promulgated by the Institute of High Fidelity. Certain older ratings are out; the power rating that is "in" now is the continuous average (minimum sinewave) power output delivered into a "rated load," over a "rated bandwidth," and at a "rated maximum total harmonic distortion."

This figure often is called "rms" power, but strictly speaking, "rms" (root mean square) applies to current or voltage. Nonetheless, when it is used for power ratings, it is intended to mean the same as the longer phrase "continuous average power."

The "rated load" is usually 8 ohms, the nominal impedance of most loudspeakers. Some amplifiers also have ratings for alternate loads, such as 4 ohms or 16 ohms.

The "rated bandwidth" is normally the accepted full audio range, from 20 Hz to 20,000 Hz (also written as 20 kHz).

Distortion levels can be whatever the manufacturer chooses. Naturally, the lower this figure, the better. In practice, few hi-fi units these days are rated for more than 0.5 percent distortion, and many boast far lower distortion than that.

For a stereo amplifier, the power data is given for a single channel, but the data must be derived from a test in which both channels were energized simultaneously.

Some years back it was possible to advertise inflated power ratings by measuring power over a limited portion of the total audio range, or by ignoring the rise in distortion when an amplifier was forced to produce power above its clipping level, or by measuring peak values of sine waves instead of effective "rms" values, or by shutting down one channel of a stereo amplifier while operating only the other channel. Today's hi-fi power ratings - regulated by law and endorsed by an industry standard - are both honest and realistic in describing the "work" an amplifier can do.

As a concession to the fact that on occasion an amplifier may be called on to produce unusually high power beyond its clipping level for a very brief instant, there's a new rating you may come across called "dynamic headroom." This figure, normally 1 or 2 decibels, simply indicates the amplifier's ability to deliver a fraction of a second's worth of power above its normal continuous-power level.

The frequency response of an amplifier is typically wider in scope than its power response. Frequency response measurements are made at a very low power output level (1 watt is common since it approximates the normal average level in actual use in home music systems). Departures from flat or level response are shown by plus or minus decibel values, which these days are generally no greater than 1 dB from at least 20 Hz to 20,000 Hz.

Some recent inquiries:

Q: I recently inherited a load of old 78-rpm discs, which I can't copy onto tape, let alone play, since my turntable does not have that speed. I can't even find a player that runs at 78 rpm. Any ideas?

A: Although scarce, turntables with the 78-rpm speed still are available. Some low-priced automatics are made by Zenith (the MC 9020, $100) and by Garrard (the 720C, $70; the 730M, $90). Higher-priced single-play models are offered by Dual (the CS604, $270; the CS621, $300). The Thorens TD126C at $750 is another. For even more you can get the Technics SL-1000 ($1,400) or the SP-10, same unit without its own arm or base ( $800). Lenco has three units (L55S, $185; £75S, $200; £78S, $220) that are unique in that their speed is continuously variable so that you can modify musical pitch if necessary.

Q: You recently suggested corrective techniques to avoid echo when making a live recording. Is there a source of detailed procedures for these techniques?

A: Because these techniques are relatively new, and exist more in the day-to-day practice of recordists than in terms of an organized body of theory, there is little formal instructional material - yet. My guess is that there will be more in the near future. Meantime, you might try to get a copy of "Multitrack Primer" from TEAC Corp., 7733 Telegraph Rd., Montebello, Calif. 90640. Query also the publication "Modern Recording," 14 Vanderventer Ave., Port Washington, N.Y. 11050. And often the manufacturer of your own recorder can supply special information.

Q: Will adding a separate mat on top of my turntable improve the sound of records?

A: A mat made of conductive fibers can reduce static charges on records and thus reduce dust attraction and noise caused by dust. The mat also may reduce reasonances and turntable rumble. However, if the mat is 3/16-inch thick or thicker, it can change the vertical tracing angle of the pickup. If so, the tone-arm should be raised (at its pivot) accordingly - ideally, the arm should be parallel to the record surface.