Audioclinic (Aug. 1983)

Image Rejection

Q. The specs supplied with my tuner boast of a 114 dB image rejection.

The tuner was, however, tested by an independent laboratory whose findings were that the tuner only has a 92 dB image rejection.

Upon writing to this component's manufacturer, I learned that measurements of this kind, if not taken in a "screen" room, can be misleading.

What is a "screen room"? What are your thoughts on this?

- Lowell Knapp, Charleston, W. Va.

A. The question is almost academic. Your tuner's 92 dB image-rejection ratio is excellent in itself. Therefore, unless you are plagued by interference from overflying airplanes (when listening to a weak signal at the high end of the band), the matter of image suppression will be of little consequence.

It should also be taken into account that the better the specification for a piece of equipment, the harder it is to measure this high performance. This is true not only for image rejection but for signal-to-noise ratio, distortion, tuner input sensitivity, etc.

An r.f.-free screen room is a room, or perhaps a "cage" within a room, screened with copper and very well grounded. This keeps any external radio-frequency sources from entering the area and possibly finding their way into the equipment under test, thereby producing erroneous measurements. With external sources eliminated, the only source of signal is that fed to the tuner's antenna terminals from appropriately calibrated signal generators.

Pipe Organ Twice Over

In the February 1982 "Audioclinic," I think that you misinformed a reader when he questioned you on the subject of how low a pipe organ can play. You state that most pipe organs can pro duce frequencies down to 32 Hz, corresponding to a 16-foot pedal pipe, and that some other instruments can even get down to 16 Hz.

If the reader was attempting to determine what the lowest sound on a disc recording of an organ could be, the word "most" should not have been in italics; almost any concert organ worthy of being recorded will have 32 foot pedal pipes.

If he was simply requesting information about pipe organs in general, you should have also added that there are pipe organs that can create frequencies lower than 16 Hz-specifically, the largest organ in the world, housed in Convention Hall in Atlantic City, New Jersey. To really be accurate, the term "largest" is a toss-up between this instrument and the John Wanamaker organ in Philadelphia. The Atlantic City organ has a total of 33,114 pipes and plays in a room the size of four football fields. This instrument has a pedal stop whose largest pipe is an unbelievable 64 feet long; the frequency it produces is 8 Hz! (I will grant you that it is more "felt" than heard, but its effect is quite noticeable.)

-Daniel J. Conrad, San Francisco, Cal.

As an organist and audio enthusiast who owns several hundred recordings of pipe organs around the world, I was interested in your response to reader James D. Gibbs in the February 1982 issue concerning the lowest frequency of an organ. Two minor points are here for your consideration:

To say that "most" pipe organs do not have 32-foot stops is a bit misleading because "most" pipe organs never get recorded at all. Of those deemed worthy and important enough to re cord, a substantial number will have one or more 32-foot stops.

Although I would not expect anyone but a specialist to be aware of it, there is at least one organ in the world that has an actual 64-foot stop! This amazing stop (part of an altogether amazing instrument) is in the Town Hall organ in Sydney, Australia.

-Daniel E. Gawthrop, Orem, Utah

"Needle Talk"

Q. Please explain the cause of "needle talk" in record players. In one case I've encountered, the acoustical output from the pickup is very notice able. Is it possible to reduce or eliminate it?

-W. Moroney, Melbourne, Australia

A. The mechanical motion of a stylus in a groove sets up vibrations in the stylus and record. These vibrations in turn move the air, generating sound waves. Any empty spaces in the phono cartridge or headshell will act as resonators, making this "needle talk" or "stylus talk" more audible.

The lower the stylus mass, the less such sounds will be produced. [In my experience, the higher the compliance, the less "talk," too. –I.B.] So changing the cartridge may help.

I suppose one could fill air spaces in the shell with caulking compound [if you didn't mind the extra mass-1.81. I have known some cartridge manufacturers to advocate mounting their cartridges solely by such silicon caulking compound, reducing mechanical coupling between cartridge and shell. This can sometimes eliminate dips and peaks in the region of 4 to 6 kHz.

And, of course, closing the turntable’s dust cover will muffle sounds.

Combined Use of Receiver and Amplifier

Q. If I upgrade to a better amplifier, can I run the tuner section of my present receiver to the tuner or AUX input of the new amp? Can I still use the amplifier section of the present receiver to power a time-delay system?

-Roger Ross, Peshastin, Wash.

A. If you upgrade to a separate amplifier, you can connect your old receiver to the new amp as you have suggested. If you take your signal from the receiver's "Tape Out" jacks, only the new amplifier's tone and volume controls will affect the sound. If you take the signal from the receiver's "Preamp Out" jack, both the amplifier's and receiver's tone and volume controls will affect the sound from your tuner section.

You should be able to use the amplifier section of your present receiver to power the speakers used with a time-delay device. If you use the receiver's "Tape In" jacks and not its "Amp In" jacks, you will be able to adjust volume and tone to achieve the best sonic effect.

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

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