Audioclinic (Jun. 1980)

Updating a Stereo Console

Q. I converted an old console in a few respects, hoping to improve it. Included in this conversion are a magnetic cartridge and a preamplifier.

The sound that resulted is extremely boomy! In the magnetic preamplifier I included an equalizer for the RIAA curve. Could the amplifier stages in the console be something other than "flat" for use with the original ceramic cartridge? Even with the bass turned down as far as possible, the rumble is bad. How can I solve this problem?

-Nano Brenes, Brooklyn, N.Y.

A. There are a number of possibilities why the conversion of your stereo console was not successful. First, perhaps the new preamplifier is not correctly equalized; this unit might be bass heavy. To check this out, obtain an RIAA test disc and make a frequency run on the preamplifier, measuring at the output of the device. Check both channels.

It is possible that the tonearm is contributing to the boomy sound because of resonance problems. I suggest that you try this cartridge/preamplifier combination with a tonearm known to be relatively free from audible resonances. You will then know if the arm or something else is at fault.

It may be that the original ceramic cartridge was bass shy, which was compensated for in the original design of the console. It may also be that the designer figured that the user would want a "boomy" sound. To check these out, sweep the console with an audio oscillator. To do this, first disconnect the preamp. Connect the output of the oscillator to the point which was fed by the ceramic cartridge or by the new preamplifier.

If you do find that the amplifier is not flat, you will have to trace out the circuit to see if you can determine where and how the bass is boosted.

The circuits responsible for this will have to be either removed or modified so that the amplifier has flat response as measured at its speaker terminals.

Of course, a resistor load must be substituted for the speaker when making such measurements.

You indicated that you hear rumble all the time. Perhaps the turntable is not a good one, in which case you will not cure the rumble even when producing a more reasonable frequency response than may now exist. It may even be that you are hearing hum.

Changers used with ceramic pickups do not necessarily employ well-shielded motors; they are unnecessary for ceramic cartridges. Therefore, there could be excessive hum fields being induced into the magnetic cartridge.

The simplest cure for this is likely to be the use of a turntable designed with magnetic cartridges in mind.

Discs and Films Still With Us

Q. With today's technology it seems that both phonograph records and motion picture films should have died away. They have not. Why?

-Name withheld.

A. As for motion picture films, they are with us because of their comparative ease of editing, as opposed to the new and complex problems related to the editing of video tape. Further, movie theater owners are reluctant to replace existing, functional, optical equipment. Doing so would require a tremendous investment and also the training of projectionists to work with a completely different technology.

Disc recordings remain with us for a number of reasons. First of all, the disc-manufacturing process is such that discs can be made more quickly and more cheaply than can tape formats. High-quality, open-reel tapes have been produced, but the public never accepted this medium to any very great degree; it was, therefore, largely withdrawn from the market. It appears that the cassette format is gaining good public acceptance, but it will still be some time before the best mass-produced cassettes can sound as good as or better than the best mass-produced disc recordings.

Remember, too, that if you are looking for a particular selection on disc, you simply examine the disc, count the bands, and then set the stylus down on the proper one. There are now automatic scanning mechanisms for doing this, making the process even more efficient. While it is true that there are some means now available for locating a particular selection on a tape, these systems are still not as quick and efficient as finding one's place on a phonograph record.

Power Amplifier "Burn-Out"

Q. Having worked in a factory service department of an established high-fidelity equipment manufacturer, I am amazed how some solid-state power amplifiers are "blown up." The driver and output transistors are internally shorted. Many power amps seem to "blow up" or "fry" because of an accidental dropping of the tonearm onto a record, and yet these same types of amplifiers, however, are used by rock bands for sound reinforcement. In such an application extreme transients occur, often driving these amplifiers into clipping, yet they do not fail.

What is it about a dropped tonearm that causes these amplifiers to "go?" What are some other causes that can do this to amplifiers?

-Robert C. Doak, Glassboro, N.J.

A. The dropping of a tonearm onto a disc results in a tremendous pulse of low-frequency signal. This is not simply a transient pulse; it is virtually d.c.

and is present for a comparatively long time at the point of driving the amplifier into clipping. This is a far more serious problem than what would happen in a rock performance. There, the amplifier may run at a higher power level, but the nature of the program is such that the duty cycle is not as great as it would be in the case of the cartridge being dropped onto a record.

Much of this and similar problems would be eliminated if a subsonic filter were used.

Any other cause of this kind of amplifier failure would have a similar nature, capable of producing virtual d.c. at high levels for a few microseconds.

Rapidly tuning an FM receiver could produce a similar situation, especially if the tuner has a really large output coupling capacitor.

Added to this might be a case where an amplifier does have a few coupling capacitors in association with resistors which could produce large time constants. A very high pulse of signal could cause such a network to take a charge. This charge would have to bleed off slowly, keeping the amplifier biased into very heavy conduction. LII If you have a problem or question about audio, write to Mr. Joseph Giovanelli at AUDIO Magazine, 1515 Broadway, New York, N.Y. 10036. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Audio magazine, Jun. 1980; Joseph Giovanelli )

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