Audioclinic (Q and A) (Apr. 1973)

by Joseph Giovanelli

Frequency Response of Two Channels

Q. Can you tell me why there should be a difference in frequency response between one channel and another in any good amplifier? I have seen such differences mentioned in equipment reviews.

-Michael Visser, Hatfield, Pennsylvania

A. It is possible for each channel of a stereo amplifier to have a bit different gain from its mate. That difference can cause the overall frequency response of one channel to be depressed or raised above that of the other channel. Further, because of differences in transistors and somewhat different component values, the outer ends of the frequency response (outside the audible range) can vary a bit from one channel to the other.

When I speak of differences in components values between the two channels, I refer to the fact that there is a certain amount of error in the value of a given component. There is a nominal value printed on a component, but there are manufacturing tolerances.

These little variations in the actual value of the components and slight differences in gain of the transistors can and will affect the performance of an amplifier. As you will observe, however, these differences are too small to be heard. The use of large amounts of fuseback keeps such frequency errors to a very small minimum.

Interference From a Power Transformer

Q. I own a high-powered transistorized audio amplifier. It causes trouble because the power transformer buzzes audibly into the listening area and it radiates into the AM broadcast band loud enough to spoil AM reception.

I tried tightening the bolts on the transformer laminations, but with no improvement. I also tried line filters to keep the `hash" off the broadcast band, but again, with no success. Can you come up with a solution to the noisy, buzzing transformer or a solution to the buzzing "hash" which it produces on the AM band?

-Dr. J. W. Welch, Wall Lake, Iowa

A. Power transformers can make noise which can be heard as mechanical vibration. These vibrations are often amplified by the cabinet in which the equipment is housed--because it acts as a sounding board. This aspect of your problem can often be relieved by mounting the equipment on some kind of shock mount. The receiver should be floated on foam whose consistency is not such as to completely collapse under the weight of the equipment, but the foam should compress to some extent. Under these conditions, the vibrating chassis cannot make the cabinet in which the equipment is housed act as a sounding board.

The radio interference problem is something else again. I suggest that you feed the radio into some other amplifier and note if the interference is still heard. (Of course, if the radio is a portion of the amplifier that you are using, this is not practical.) If you do not hear the radio interference when the amplifier is turned off, but do hear it as soon as the amplifier is once again turned on, you will know that the amplifier is, indeed, generating the noise. This fact still does not mean that the power transformer is responsible. It is far more likely that the noise is produced by the diodes in the power supply as they rapidly switch on and off. Such switching transients can often be reduced by placing a capacitor across the terminals of each diode. These capacitors must have a voltage breakdown rating high enough so that they will withstand the full peak voltage which will appear across them when the diode is not conducting during half of each cycle. (This peak will equal the voltage stored in the filter system, plus that of the power transformer secondary, or secondary center to either end--depending on the nature of the power supply circuit employed.) I suspect you may need to use a 0.05 uF ceramic capacitor across each diode. Also, it is a good idea to use a 100 K ohm resistor across each diode, along with the capacitor. The wattage will depend on the conditions of the particular circuit, but 2 watts is an absolute minimum resistor value.

I see no way that a power transformer can produce this radio interference, at least not for long. Such radiation from a transformer would have to be the result of internal arcing which would ultimately cause a complete failure of the transformer.

(Audio magazine, Apr. 1973)

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