Audioclinic (Feb. 1983)

Local Power Generation and Audio Gear

Q. I live in a remote area which is not serviced by grid power. My options, therefore, are solar, electric or wind generation--with power in the form of 12-V d.c. I know that I can use most auto stereo gear, but is there 12 V d.c. equipment designed for home use?

-Steve Verchinski, Albuquerque, N. M.

A. I know of no equipment made for home use and designed for 12-volt d.c. operation. There is probably little point at present for producing such products because there will be little call for them. While I have seen a trend toward the use of independently generated electrical power, I doubt that there is a real market out there for such products.

Fortunately, automotive audio products are quite good, so you can use them without sacrificing much. Of course, their mounting arrangements are not exactly suited for the living room.

When you have 12 volts, you have the means of obtaining 120-V a.c. You need a device known as a converter, which is designed to produce the de sired 60-Hz, 120-V a.c. It is common for such converters to be used with small power plants-wind, solar, hydro, etc. This equipment may be a good option for you, because it will permit you to use standard audio products. I do not know how good the accuracy of the 60-Hz frequency is. Therefore, I suggest that the products you select should not be frequency-dependent. This means that you should use turntables and tape machines which employ servo-controlled motors. Digital clocks or timers must be of the sort which utilize their own internal crystal oscillators, rather than relying on the powerline frequency to supply the impulses which activate the counters in the digital clocks.

The power-handling capability of the converter must be somewhat greater than your total power demands.

[In my experience of using inverters in a 120-V d.c. dormitory, years ago, vibrator-type inverters have fairly accurate speed regulation and are okay for powering turntables, but their output waveforms are too spiky and noisy for use with amplifiers or other electronic gear. Motor-generator inverters are okay for amplifiers, but not for frequency-dependent turntables, as their speed varies with the load they're driving. -I.B.]

Three-Speaker Car Installation

Q. I recently helped a friend install a new cassette/stereo system in her car. She only had one front and two rear speakers, and her stereo unit did not have a fader. It did have three speaker leads, left, right and common.

I used an add-on fader, connecting both its common terminals to the stereo system's common. The rear speakers were connected normally, but I connected both of the fader's left and right "front" leads to the front speaker.

Is there any problem with this? If so, how can it be corrected without adding another front speaker?

-John A. Richard, Baltimore, Md.

A. You have one definite and one potential problem. You will definitely lose stereo separation as you fade in the front speaker. And you might possibly damage the amplifier; consult the stereo system's manufacturer on that.

You could get the same results, with out the problem, by connecting only one channel to the front speaker and switching the receiver to mono when listening to it.

If you don't have a stereo/mono switch or prefer stereo from the rear combined with a front center-channel, then install resistors in series with the front speaker. Try four or five ohms, but, again, consult the receiver maker.

This will limit the loss of separation and protect the amplifier, but it will also lower the volume available from the front speaker as well as the power available to the system when the front speaker is in use.

Transformer and Pre-Preamplifier

Q. I have a moving-coil cartridge and a step-up transformer designed to be used with it. I also have a preamplifier. I plan to purchase a pre-preamplifier kit and use both the pre-preamplifier and the transformer at the same time. Is this all right?

-Name withheld

A. If you purchase the pre-preamplifier kit, I see no need for the transformer, since the purpose of the pre-preamplifier is to render the use of a step-up transformer unnecessary. Using the new unit will show you, in all likelihood, that the transformer is not needed. If the new pre-preamp produces enough signal level to drive your system adequately, do not use the transformer, as that may overload either the pre-preamplifier or the preamplifier proper.

Sound Reinforcement

Q. A friend and I would like to combine our systems for use at various events. I have a 70-watt receiver which has jacks that let me use the preamplifier and power amplifier sections independently; my friend has separate components.

We considered using one preamplifier, routing the signal to the two power amplifiers by the use of two Y-shaped patch cords, and driving one pair of loudspeakers with each amplifier. Be cause we plan to play dbx-encoded discs at very loud sound levels, we would like to combine the outputs from the two amplifiers to drive one pair of speakers. How can we accomplish this?

-William B. Walkup, Meadow Bridge, W. Va.

A. The use of Y-connectors is fine for driving two separate power amplifiers from a single preamplifier, provided each amp drives its own speakers.

The power increase which may be possible by combining two amplifiers to drive a single pair of speakers is only 3 dB at best. If the two amplifiers are from different manufacturers, have different power ratings and have no "bridging" arrangements, you probably cannot combine them in any way without risking damage to either or both.

I would suggest, therefore, that each amplifier should drive its own pair or pairs of speakers. If you arrange these speakers properly in the areas to be covered, this will permit listening at a somewhat lower sound level than might otherwise be possible if the listening area is to be fully covered by one speaker pair. This arrangement would enable those participating in your events to carry on conversation, if this activity is appropriate. I have at tended all too many social functions where conversation would have been great except that the audio got in the way. I should have stayed home!

More on "Ground to Earth"

In reference to the question "Ground to Earth" (Audio, Nov. 1981) there is a word of caution you may wish to ex tend to your readers. Many high-gain, multi-component systems may exhibit excessive hum because of poor house ground, even if they have a three-wire a.c. system. Due to age or corrosion, the wire ground of a house may begin to float several ohms above true ground. This could cause a hum problem as well as a voltage difference which could be carried on the case of the component. Running a true earth ground, i.e. a water pipe, may remedy the hum/voltage problem but inadvertently create a dangerous ground condition from a safety point of view. Be cause the component system ground would then be the best ground on the circuit, any large discharge on the circuit would ground through the stereo system! As you can see, this could be dangerous to the operator as well as to the components themselves.

-Doug Leu, Minneapolis, Minn.

Frequency Range of Program Sources

Q. Is it possible to check the frequency range of a tape or a disc with a gauge? Can this gauge be directly connected to the output of a tape recorder or preamplifier?

-Rudolf Quaas, Pocatello, Idaho

A. A device known as a real-time analyzer will show you the frequency content of signals presented to it. An RTA shows signal level in each of several frequency bands. Most common for home use are 10-band models, with bands an octave wide, usually costing just over $200. Models with 31 or so bands, each 1/3-octave wide, are more informative but more expensive.

If you have a personal computer.1/3-octave analyzer. Radio Shack has a $20 analyzer cartridge for their Color Computer (limited to 12.5 kHz, alas); Eventide Clockworks, in New York, makes 31-band models for the TRS-80 Model I, Pet and Apple II.

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

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