Audioclinic (Feb. 1984)

Welcome to 1984

Writing this at the end of 1983, I want to take just a line or two to wish all of you the best for 1984. I wish, too, to thank the many of you who have taken the time to write just to tell me that my answers to your letters were of help. You are really quite a nice bunch of people. I get far more letters of this sort than I do the kind telling me to leave town.

I have received a couple of letters from those who are concerned that perhaps they really do not get personal attention. I guess this is because I use a word processor to answer my mail these days. I have no staff, so I must do all of the typing, and I'm not a really good typist. I look better, though, with the chance to fix things before the hammers hit the paper; it's the only reason that I have done this. I still read each letter and study it to see how I can be of help. None of that has changed.

Because of the subjective nature of questions dealing with specific products, I find it best not to handle them. I do not believe it is proper for me to foist my personal, subjective opinions on you. Given all of this, I look forward to working with all of you again this year.

-J.G.

MM Cartridge with MC Input

Q. I own an integrated amplifier which has provisions for using a moving-coil cartridge. My moving-magnet cartridge has a 3.5-mV output. If the MC arrangement is just a step-up transformer, why does my sound level drop when I switch from MM to MC?

-Ron Webb, Tempe, Ariz.

A. In one way or another, the moving-coil circuitry in a receiver adds gain to the phono circuitry to make up for the tiny voltage produced by moving-coil cartridges. The input impedance of such a circuit is usually low, and, hence, the output from your MM cartridge will be shorted out by this low impedance. This would result in a lower signal than you had expected when attempting to use your MM cartridge with your MC circuitry.

I have never tried such an arrangement and cannot say for certain what will happen in every case. I believe that the actual degradation of performance will depend on the details of the MC. circuit in any given instance.

There is always the possibility, too, that something is amiss with the MC circuitry. You'll never know this unless you obtain a moving-coil cartridge.

Record Timing

Q. Am I correct in assuming that there is no standard for noting times of individual "cuts" or for complete record album sides? Do such times include music only or also the silent time between the cuts?

-Rudi Schmid, Kensington, Cal.

A. When timing a single selection on a record album, the timing starts at the instant that the first note plays and ends at the instant that the last note finishes. The silence before or after that selection is not included.

When timing a complete album side, the timing begins at the instant that the first note begins and concludes the instant the last note ceases. This, of necessity, includes all music and all silences between selections.

"Wide" Stereo

Q. A couple of years ago, I bought my mother an AM/FM stereo and cassette unit which possesses a feature I had never seen before. There is a button which, when pressed, turns the regular stereo into "wide" stereo. It was amazing to hear the improvement. Please explain how this "wide-range" stereo button works.

-Bruce DeQuasie, Parkersburg, W. Va.

A. There are a number of variations on the idea of enhancing the spatial qualities of stereophonic program sources. Essentially, what is done is to take some right-channel signal, change its phase, and introduce a small portion of that signal into the left channel. Similarly, the left-channel signal is introduced into the right channel. (Some more elaborate schemes introduce time delay before feeding signals into their corresponding, opposite channels.)

Amplifier Overload with Bass and Treble Boost

Q. I have read that, when incorporating a mixer with bass and treble controls into a sound system, excessive boost may cause the power amplifier to "clip," depending on the power of the amplifier. If the mixer has the capabilities of a 12-dB boost in both bass and treble, would these boosts require more power from the amplifier, perhaps resulting in overload?

-Michael P. Conner, St. Louis, Mo.

A. The same factors apply to boosting frequencies with a mixer as apply to boosting them with an equalizer or with an amplifier's tone controls. Treble frequencies require so little power to reproduce that they will probably never be boosted enough to overdrive the amplifier. [They can, however, be boosted enough to burn out tweeters, if the material being reproduced is rich in high frequencies or harmonic distortion. -I.B.] Bass, however, requires much more power. The low frequencies already account for most of the power being delivered by the power amplifier. Even so, the possibility of overdriving the power amplifier depends upon how close the system was to being overdriven before the boost was added. If the program being fed into the system was deficient in bass, chances are that the added boost would not result in the amplifier's being driven to the point of clipping.

If the amplifier has lots of reserve power, it is possible that even when the program has lots of bass, boost still won't be sufficient to cause problems.

Too Much Signal

In your June 1983 column, you replied to a reader whose tuners received one station with a loud hiss when in the basement, but received it without hiss upstairs. Your reply appears to have missed the point.

The reader is obviously plagued by an excess, rather than a lack of signal, as shown by the three clues he cites--signal strength meter well above normal, worse hiss when an FM distribution amplifier is used, and drastically diminished hiss when a knife switch in the antenna lead-in is opened.

The phenomenon here is well known to manufacturers of CAFM processing equipment for CATV systems. Excessive signal level from a grouping of FM stations will cause intermodulation distortion at the sensitive front-ends and front mixers of FM tuners. The multiple products produced (nF1 ± nF2 ± nF3 .) effectively appear as a raised r.f. noise floor.

The problem becomes worse on some of the newer synthesized tuners, because the preselect filtering formerly available from multiple-gang tuning capacitors is no longer available.

(Some better quartz tuners, however, do use stages of varactor pre-selection). There are several solutions to the reader's problems. First, do not use a distribution amplifier or preamp. (In any case, you are unlikely to find one with a lower noise figure than the first r.f. amplifier of your tuner.) Second, use coax all the way; if the antenna is a 300-ohm type, use an outdoor 300/75 ohm transformer with it.

Use a 75-ohm splitter between the two floors, and always keep both lines terminated by the 75-ohm inputs on the tuners. Last, try a resistive 75-ohm attenuator between the antenna lead and the "noisy" tuner. Use only as much attenuation as necessary to get good performance. (This creates the same effect as the reader's partially opened knife switch, but in a predictable form).

These problems can occur when a sensitive tuner is hooked up to a CATV FM feed, and receives 30 to 40 FM channels at once, all at substantially higher levels than off-air signals. Again, an attenuator (e.g., 20 dB) right at the tuner input can help.

-Gilles Vrignaud, Ouante Corp., Santa Clara, Cal.

Amplifier Linearity

Q. If a power amplifier is to be linear, should not its output be proportional to its input? I was looking at a test report for a power amplifier, and something was wrong. If 100 mV produces 1-watt output, why is not the input required to produce the rated 200-watt output 200 times that input voltage, or 2.0 volts-rather than the 1.4 volts shown in the test report? Would not a linear input/output curve be preferable?

-Robert Malanchuk; Columbus, Ohio

A. You are certainly correct that a linearity should exist between the change in output power and the change in input voltage which produced it. Bu: that linearity is not a one-to-one equivalence, as we are comparing two different units. Power consists of voltage and current. If voltage alone is considered, we see from Ohm's Law (P = E^2 R) that power varies as the square of the voltage across a fixed resistance. Put another way, to increase power output 200 times, we must multiply the voltage input by the square root of 200, which is approximately 14. Multiplying 14 x 100 mV, we get 1.4 V, the figure quoted in the amp specs you were discussing.

(adapted from Audio magazine, Feb. 1984; JOSEPH GIOVANELLI)

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