Audioclinic (Jan. 1983)

Selectable Phono Capacitance

Q. I am aware that a phono cartridge requires a particular value of capacitance for optimal response; mine requires 200 to 300 pF. My receiver has a load-capacitance selector, with a choice of 100/250/450 pF; obviously I have set the selector at 250 pF. I am curious, however, to know if the 250-pF load is in addition to that of the tone-arm wiring and interconnecting cables, or does the 250-pF load take into consideration the arm wiring and cables? If it does not, I would change the capacitance load to 100 pF.

-Bill Landiuk, Calgary, Alberta

A. The load-selector switch on your receiver is designed to provide capacitance in addition to any other capacitance which may be present. Unless you have relatively low capacitance cable, you should find that by switching to the 100-pF setting, you have better highs with less peakiness.

Tuner and Turntable Volume Difference

Q. My tuner is two or three "notches" louder than my turntable. Is there something wrong?

-Glenn L. Brass; New York, N.Y.

A. There is no problem with your tuner producing more volume than your turntable. The difference in volume between these two sources will vary, depending on the volume of the recording being played and upon the voltage produced by your particular phonograph cartridge. Years ago, equipment was supplied with level controls for each input so that these loudness differences could be taken into account.

Relating Response to Bandwidth

Q. Please explain the difference between frequency response and power bandwidth in an amplifier.

-John Potter; Oakdale, N.Y.

A. Frequency response refers to the measured amplitude or output of a device over various frequencies. In other words, if a range of frequencies is fed into the gear at equal amplitudes, the output of the unit will contain these frequencies at equal amplitudes throughout the measured frequency range-if the unit is perfect. Such a system is said to have a flat frequency response.

Amplifiers usually have a wider frequency response at low output levels than at or near their maximum rated power. Therefore, amplifier specifications may show both low-level frequency response (usually at 1 watt output) and the bandwidth over which the amplifier can deliver its full power.

For example, an amplifier putting out 1 watt might have a bandwidth from 25 Hz to 20 kHz. The same amplifier at 10 watts might have a bandwidth of only 30 Hz to 17 kHz.

Noisy Volume Control

Q. When I adjust the volume control of my receiver up or down, there is a static sound coming through my speakers. This occurs whether the program source is FM, AM, phono or tape.

I have bypassed all of my music enhancers, thereby narrowing the discrepancy down to the control itself. After I adjust volume, the static sound stops. Can I correct this problem at home, or must I take my receiver to a repair shop?

-Donald F. Bullock; APO San Francisco

A. The problem with your volume control is that it is either internally oxidized or worn out. Most likely it is oxidized, which sounds terrible, but is simple to cure if you can obtain contact cleaner, sold for cleaning volume controls. First take the receiver apart and look at the control. Often there is a small opening around each of the two terminal lugs (to which the wires are connected). Squirt the contact cleaner into both of these openings, but be careful to use this cleaner sparingly.

If there are no openings around the lugs, remove the volume control knob and tip the unit so that the control shaft faces up. Try to squirt some cleaner where the shaft enters the body of the control.

In either of these cleaning methods, after the cleaner has been sprayed into the control, rotate the shaft up and down a few times so that the cleaner will be dispersed over the sliding contact surfaces within the control. Keep the amplifier turned off during this process.

In cases where the problem is not severe, noise can be reduced or eliminated for a time merely by rotating the control rapidly a few times, without using any cleaner.

If none of the above helps, the control may require replacement. There is also another, though rare, possibility. A coupling capacitor associated with the control may be leaky, thereby putting d.c. voltage on the control. In this event, the capacitor must be replaced.

Low-Frequency Filters

Q. I own a preamplifier with a filter that inserts a 12 dB per octave roll-off below 15 Hz in the phono preamplifier section. My speakers' low-frequency response is flat to 44 Hz. I hear no difference with the filter engaged or disengaged and assume this is because the filter operates far below my ability to hear such low frequencies. Is this correct?

-Steven T. Johnson; Rushville, Nebr.

A. Because your speakers are not designed to reproduce frequencies flat below 44 Hz, their bass response would be so far down at 15 Hz that you would not hear a difference with the filter switched in or out. You might, however, notice some cone movements with the filter out and less with it switched in. This would be most noticeable at high volume levels, on a quiet passage of a disc.

I recommend that you leave the filter switched in, which will avoid problems in the event that the tonearm is dropped. A transient produced under such conditions would probably result in damage to your speakers as well as to your amplifier.

Speaker Doubling

Q. Please explain "doubling" in reference to loudspeakers.

-James D. Gibbs; Cedar Rapids, Iowa

A. Doubling is basically harmonic distortion, and it occurs when a speaker is fed a signal lower than it can reproduce, though it will sometimes occur with high-power signals. Instead of the fundamental tone, the speaker produces its second harmonic, that is, a tone double the original frequency, hence the name. The reason doubling occurs with high-power signals is that harmonic distortion rises with increasing input level.

(adapted from Audio magazine, Jan. 1983; JOSEPH GIOVANELLI)

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