Audioclinic (Q and A) (Aug. 1970)

by JOSEPH GIOVANELLI

An Interesting Hiss Problem

Q. While evaluating receivers, I came across one which I liked very well except for a "hiss" which is heard while listening to recorded tapes, records, and radio. It is still heard when both the high-pass and the low-pass filters are switched in.

Could you tell me the reason for this?

-SP/5 Michael J. Bass, APO S.F.

A. All electronic circuits generate some noise. We hope that this noise is well below the signal level, so much so that it is not noticeable. Most of the noise produced by today's equipment is produced by the phonograph preamplifier circuitry, but not all of it. When the phonograph circuitry is the dominating noise source, the noise will be affected by volume controls, tone controls, and filters.

This is because this source of signal must pass through these portions of the equipment on its way to the loudspeaker. There is a considerable number of additional circuit elements between the filters and the loudspeakers, all of which will generate some noise. Because this noise, or "hiss" is produced at points located in the chain after the volume control, this control will not be effective in reducing it.

Similarly, if the noise is generated after the filters, you can see that they can't have any effect either.

I will have to digress just a moment so that you can gain the rest of the picture.

Loudspeaker systems vary in efficiency. One type of speaker system might produce room-filling volume with one or two watts fed into it. Another speaker might require 30 watts to produce the same amount of sound. These are the extremes, but what I am now coming to is that if the amplifier is producing noise in stages which are located after the filters and volume control, this noise will be heard more when listening to some speakers than with others. If you heard the receiver under discussion through relatively high-efficiency speakers, it is quite likely that if you use low-efficiency speakers the noise would not be apparent. If the noise was heard when listening to relatively low-efficiency speakers, there is probably something wrong with the particular receiver. This would be especially obvious if you heard the hiss from only one channel.

Listen to a second sample of the product. If the noise is absent, you will know that there definitely was something wrong with the first sample. Of course, you must listen to the receiver with the same speaker. If the speaker is of low efficiency and the noise is still apparent, then we can only conclude that there is a design fault in the receiver, and you should pass it by, even though it is good in other particulars.

Reproducing Pathe Records

Q. I am a Pathe Record Collector. How can I play these records on my high fidelity music system?

-Robert Moenning, San Pablo, California.

A. Some of the earlier Pathe records, black with no center paper label, start from the inside and are vertically cut. Their nominal speed is 90 rpm, but they do vary in speed. Therefore, you will need a way to determine when the record is being reproduced properly. One guide could be the key in which the piece was written. You would have to hope that it was not transposed by the performer. The speed of later records was reduced to 80 rpm.

From their initial recordings up to at least 1917, the stylus tip radius used for playing back these records was 8 mils spherical. I was not able to learn just how long after 1917 this tip size was used. I would assume, however, that, for as long as the disks were vertically cut, the standard of groove size did not change.

You need considerable force in order to prevent the stylus from skating out of the record grooves. Because the average stereo pickup does not track at heavy forces, you will need to obtain an extra stiff stylus assembly for this application. You must also have this stylus tipped with an 8-mil diamond or sapphire.

The cartridge must be wired for monophonic operation, but with one channel reversed, so that only vertical motion will be reproduced. Failure to reverse this channel will result in the almost complete cancellation of the desired program.

(Audio magazine, Aug. 1970)

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