AUDIOCLINIC (Mar. 1987)

Volume Settings with Remote Control

Q. I am using an equalizer in the tape loop of my integrated system.

This equalizer is equipped with a remote control which enables me to ad just the volume of the entire system via the equalizer. At what approximate set ting should I place the volume control on the integrated amplifier to achieve the best sound from the system overall.

-Bill Jacques, Phoenix, Ariz.

A. I believe the exact setting of the volume control on your integrated amplifier is not critical. I suggest that you set it to a point where, with the equalizer's remote volume control adjusted to its maximum, the program level from the loudspeakers is just a bit too high. I suggest this setting to account for variations in volume among various pro gram sources.

Feed-Through Capacitors

Q. I obtained a service manual for my CD player in order to gain an understanding of its operation. I am puzzled about the nature and use of one component, shown to be a "feed-through" capacitor. The circuit boards are surrounded by a perforated cage, which I assume is used to shield the player's circuitry against r.f.i. or to pre vent r.f. from escaping. The feed through capacitors are mounted on the back of the cage. Within this cage are soldered wires which are really the audio output connections from the player.

The capacitor extends through the cage, and the output cables are soldered to the capacitor terminals, which are on the outside of the cage.

These feed-through capacitors are definitely different from the output coupling capacitors, which are located on one of the circuit boards. Why are there feed-through capacitors when there are already output coupling capacitors?

-Kenneth Beers Jr., Akron, Ohio

A. Feed-through capacitors are used as r.f. bypass capacitors. In your player, they are used to prevent any r.f. from the player from entering your audio system. They do not block d.c., as output coupling capacitors do. The terminal which connects the output signal from the circuit board to the terminal on the outside of the cage is really one straight piece of wire. Therefore, capacitance is not formed between the two terminals. Rather, it is formed be tween that wire and the surrounding conductor, which serves to mount the capacitor to the cage and also serves as its ground point.

Now that you understand how such capacitors are constructed, you can realize that their capacitance is very low, measuring only a few pF. Because of the low output capacitance of most CD players, the capacitive reactance offered by the feed-through capacitor won't affect the frequency response.

Separate Mono Amps

Q. What hardware is required for a system using separate monophonic power amplifiers for each of the two channels, as opposed to a stereo power amplifier? How must such a system be interconnected?

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

A. There is no special arrangement needed just because a system includes two separate mono power amplifiers rather than a single stereophonic power amplifier. Make the connections in the same way that you would when connecting a stereo amplifier to the rest of your system. All you need are interconnecting cables. In the case of a stereophonic amplifier, the cable connecting it to the preamplifier may be a "two-in-one" type. Be cause of the physical separation be tween the mono power amplifiers, you may need two separate cables to make connections to them. If you are not involved with exotic bridging or special phase-reversing systems, this is all there is to it.

Amplifier Hum, Speaker Connections

Q. When I first turn on my music system, there is no hum; after a few minutes of operation, there is. This hum can be affected by the adjustment of the bass tone control. I have checked the tubes in my system, and they are all okay. Do you know what the problem is?

Also with regard to this music sys tem, there are, on the back of the power amplifier, terminals to which the loudspeakers are to be connected.

These are marked "Main," "8 Ohms," and "16 Ohms." My speakers are marked "Plus" and "Minus." I have another set of speakers, marked "Red" and "Black." How would I connect either of these pairs of speakers to this amplifier?

-Bobby Ball, Dewitt, Va.

A. First, let's look at the hum problem. Sometimes, as tubes warm up, their electrical characteristics change.

It may be that such changes are causing the hum. Many amplifiers and pre amplifiers have hum-balancing adjustments. If your equipment has such a control, turn it a bit one way or the other and see whether this eliminates the hum.

Not having heard the hum, I have no way of knowing whether it is 60 or 120 Hz. If it is 60 Hz, then it is most likely associated with the filament circuitry as just discussed. If it is 120 Hz, then the power supply's electrolytic capacitors are involved. Such capacitors can change value when voltage is applied.

It is not common, but I have seen it happen. If the amplifier is old, replace all of the filter capacitors in its power supply. Don't forget those interstage decoupling capacitors.

Your letter does not say if this is an integrated power amplifier. If you are using a power amplifier with a preamplifier, disconnect the power amp from the preamp and note if the hum still occurs. If it does not, you should concentrate your energies on the preamplifier, performing the same maintenance as described for an amplifier.

As for loudspeaker connections, the exact connections will depend upon the impedance of your loudspeakers. If their impedance is 8 ohms, use the 8 ohm connection. If 16 ohms, then the 16-ohm connection is to be used. In either case, consider the terminal which is marked with its impedance to be the "hot" terminal. The terminal marked "main" is the ground or common terminal.

Regarding the loudspeakers them selves, "red" or "plus" indicates the terminal to be used for the "hot" amplifier connection, and "black" or "minus" indicates the "common" connection to the power amplifier.

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

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