AUDIOCLINIC (Nov. 1981)

Patch as Patch Can

Q. Is there any point in wiring an entire system with low-capacitance patch cords or am I wasting my money on a marginal improvement?

-Sp-4 Darrel Babin, APO N.Y.

A. Unless audio cable runs are long, 50 feet or so, capacitance has little effect on performance. Of course, the exception to this is phonograph interconnections. Phonograph cartridges often must "see" some given amount of capacitance. Therefore, the kind of cable you use in this case is, in part, dictated by its capacitance and by the amount of capacitance your phonograph cartridge requires.

It is often true that low-capacitance cables do not have good shielding and thus are more subject to the pickup of r.f. interference than some of the high-capacitance cables.

Obviously, as patch cords are relatively short, there is no need to use low-capacitance cables for this purpose.

Correspondence Course in FM Stereo Reception

A reader on the West Coast wrote both to me and to the chief engineer of an FM station to which he listens. He complained that despite the installation of a directional antenna, complete with coaxial cable, his stereo FM reception was noisy.

This seemed hard for me to believe, inasmuch as he was located relatively close to this desired station. I suggested that my correspondent check his antenna installation once again, looking for defective balun transformers, poorly soldered connections, etc.

I thought that this was the end of the matter. My correspondent, however, kindly forwarded the reply from the broadcast engineer. Inasmuch as his diagnosis of the situation proved to be correct, I am including it here with the hope that it will be of help to others who may at some time be afflicted with this interesting and annoying situation.

-J.G.

"Trying to diagnose reception problems at a distance is like trying to diagnose illness over the telephone. I believe your problem is caused by receiver frontend overload from the very high-power FM transmitters located on Wolf back Ridge, behind Sausalito.

"The cure for this problem is to continue to use your outdoor antenna with an attenuator inserted in the line next to the receiver. You should use the balun at the receiver and at the antenna, with RG/59 coaxial cable between the baluns.

"The value of the attenuator must be determined experimentally. The idea is to reduce the strength of the unwanted signals to a value which will no longer overload the receiver without reducing the desired signal to a point where it becomes noisy because of lack of strength.

"Plug-in attenuators for use with 75-ohm cables are available from electronics dealers who supply cable television equipment. I suggest the use of a 20-dB attenuator to begin with. If you locate a dealer with a stock of attenuators, arrange exchange privileges until you locate a value which works."

-Fred Krock, Orinda, Cal.

Punch and Tweeter Show

Q. My left channel has more "impact" or "punch" than my right channel. Conversely, my right channel sounds "thin" when compared to the left channel. What can I do to get better stereo imaging, with equal punch from both channels?

- Lawrence A. Joe, Toronto, Ont., Canada

A. The difference in punch between the left and the right channel may be resulting from room acoustics problems.

Without moving the speakers, interchange the cables between the two speakers so that the cable which was intended to serve the left channel now serves the right, and vice versa. If the difference in sound quality still remains with the left channel still having more punch--you have ruled out all possibilities except differences in the speakers or anomalies in room acoustics.

To determine which of these conditions is the culprit, interchange the left and right speakers, being careful to place them in the exact positions their "partner" previously occupied. If the lack of punch still occurs in the left channel, the problem is caused by room acoustics. If the right channel now has the problem, you will know that the problem lies with the speakers--tweeter balance being the most likely problem.

Hopefully your speakers will have means for making such an adjustment.

If the quality shifted to the right channel when you made the original cable interchange, both speaker problems and room acoustics are ruled out. You must look elsewhere in your system for the answer.

If you interchange the two channels from the turntable and the problem shifts to the opposite channel, you will know that there are differences in the phono cartridge from one channel to the other.

If the problem does not shift with the interchange of phonograph channels, you are left with possible differences in the tone controls, phono stage or amplification. You will then need to measure the electrical performance of each channel for further evaluation.

Ground to Earth

Q. I have several questions concerning audio system grounding.

1. When using a number of separate components, is it better to (a) run a separate chassis ground wire from each component to a central grounding point, and from that point to ground or (b) run a single chassis ground wire from chassis to chassis, and from there to ground?

2. Should the main earth grounding wire be tied to the ground on a three-prong grounded outlet or would it be better to run the main earth ground wire to its own separate earth-grounding point, such as a cold water pipe?

-David Yandle, Berkeley, Cal.

A. There are no hard and fast rules about grounding. I have seen cases where no grounding whatsoever is the way to keep hum at a minimum and therefore suggest starting out that way. If you find, for example, that the tape machine produces excessive hum, run a wire between the tape recorder chassis and the preamplifier or receiver chassis.

One thing is virtually certain, however. The phonograph should be grounded to the preamplifier or receiver, preferably at a point near the phono inputs.

If the main unit in your sound system has a three-wire power line system, no earth grounding is likely to be required.

Chances are good that you will not need to run wires between the rest of the components and the amplifier, or whatever serves as your main control center.

When it is necessary to run grounds between pieces of equipment, each one is to be run to the control center. My own rule is that if they are not required to reduce hum, do not use them.

It is only where you do not use a three-prong outlet that you may need to consider some kind of earth ground. Using both an earth ground as well as that which automatically occurs when using a properly installed, three-prong outlet will likely add hum and, in some cases, produce rather severe voltage differences between the earth ground and that taken as ground by the three-prong outlet's own ground.

Switching to Drive

Q. I would like to know if there will be any problem involving impedance, performance degradation, chances of equipment damage, etc. when driving the power amplifier sections of three inexpensive receivers, equipped with preamplifier outputs and power amplifier inputs, from the preamplifier outputs of one of the receivers, feeding through an equalizer. In essence, I would be using the second and third receivers as power amplifiers only.

I know this could be accomplished via the tape monitor loops but I wish to control all six channels of amplification (only two channels of program source) with one volume control to avoid building or purchasing any special equipment.

-H. Schulman, Detroit, Mich.

A. There is no problem feeding three power amplifier sections in three receivers from the outputs of one receiver's preamplifier. You will have to work out Y connectors or some similar means in order to accomplish your purpose of having all amplifiers controlled by the same volume control on the "master" receiver.

I suppose that, if you have long cable runs between the preamplifier and the inputs to the receiver, there could possibly be some loss of highs which you could make up by the use of the equalizer. I hope that the equalizer has a low-impedance output so that this will shunt out virtually all capacitive reactance of the interconnecting cables.

Your idea will work whether or not the equalizer is interposed between the preamplifier's outputs and the various power amplifier inputs. In any case, no damage will be produced by so arranging your gear.

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(Audio magazine, Nov. 1981, JOSEPH GIOVANELLI)

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