Audioclinic (Aug. 1982)

Noisy Pink Noise

Q. I play records fairly loudly. With most conventionally made recordings, this represents a volume control setting of 26 to 24 dB. At this level the background noise from the amplifier is acceptably low.

With direct-to-disc and half-speed mastered recordings, however, I generally need to increase the volume control setting to its -18 or -16 dB positions to get equivalent sound levels. At these settings, noise is clearly audible across the room. I wonder if a substantially higher powered amplifier would alleviate the problem.

-Name withheld

A. Is the noise you speak of really produced by the sound equipment or is it present on the discs themselves? To determine which of these conditions exists, set your controls as you would when playing, say, a half-speed mastered disc with the volume control at its-16 dB setting. Do not put the arm on the disc. If noise is present to the degree you have described, it would appear that the phono section of your equipment does not have a good enough signal-to-noise ratio for your needs. If, on the other hand, no noise is present, or very little, you will know that this noise is generated from the discs themselves.

Assuming that your noise problem is in the phono stages, a more powerful amplifier will not help matters. You need a phono section having a better signal-to-noise ratio than your present one has. Alternatively, you need a cartridge capable of supplying more signal to your phono stage.

Dear Mr. Giovanelli:

Thank you for your response to my letter. You may be interested to know that, upon receipt of your answer, though I had previously determined the noise to be from my equipment as opposed to being produced from the discs, I decided to recheck. It occurred to me to switch my equalizer/ analyzer out of the system. The noise was dramatically reduced.

After much switching, l isolated the problem to be from the pink-noise connections which feed into the AUX inputs of my amplifier. As soon as these connections were unplugged from the equalizer/analyzer, the sound with the equalizer switched in was as quiet as that produced by the amplifier alone.

I did not think that my equalizer/analyzer produced pink noise in its play mode, but apparently it does

All Ohms Not Equal

Q. I plan to install a biamp system in my car. I have 8-ohm woofers in the rear deck and 4-ohm tweeters in the dash, but I have been told that all speakers should be of the same impedance. Will this impedance difference be detrimental to the operation and performance of my system?

-R. E. Fortwon, Panama City Beach, Fla.

A. In the case of a conventional passive crossover network, it is true that it is best for the speakers to be of the same impedance. In the case of a system driven by a separate amplifier for each speaker, crossover will not be affected by the impedance of the individual speakers. Therefore, since the amplifier can handle the 4-ohm load, there is no problem with your plans to bi-amplify your auto system.

The Sounds of Silence

Q. What is meant by muting?

-Daniel Walter, Brooklyn, N.Y.

A. Muting refers to a silencing. Most notably, a muting circuit results in silence when tuning from one FM station to the next one. Without such circuits, there is a large amount of noise between stations. Because some people find this noise to be annoying, manufacturers provide a means of suppressing sound produced by the tuner or tuner section of a receiver in the absence of a desired station.

Muting can also refer to the absence of sound during the change cycle of a record player. The signal from the cartridge is shorted out so that the raising and lowering of the tonearm produces sound as would normally be produced when the stylus leaves or touches the disc. Transients of this sort can be a danger to speakers and some amplifiers, so muting in this instance also serves the practical purpose of protecting components.

Muting is also used in tape machines so that no sound is produced except when the tape is actually playing. The high-speed "chatter" of the tape passing by the heads during fast winding is eliminated.

Unmasking Ambience

O. It is well known that connecting a third speaker to the "hot" terminals of the R and L amplifier and the ground of neither creates a differential which uncovers previously masked ambience. This signal, however, is 6 dB too loud, and placing a 10 to 20 watt variable resistor in series with the speaker will allow this speaker to be so balanced that L and R difference information will not be obtrusive. True ambience then can be produced by this third, or rear, speaker.

After experimentation, I find that this same signal can be produced in four channel headphones by removing the common ground from one pair.

If I remove the ground lead from the phones as I described and then use a Y-connector to feed the phones from a stereo receiver, what value of resistor will I need in order to drop the "rear" channel 6 dB? Would a variable resistor be better in order to allow for a fine balance adjustment? If so, what value? Is any possible damage likely to occur to the amplifier as a result of this arrangement?

-Jon A. Hand, Pulaski, Tenn

A. You will need a variable resistor of at least 25 ohms to drop the signal 6 dB and allow for some fine balancing. I suspect that it may be necessary to drop the signal by more than 6 dB because of the dramatic spatial dimension always present with headphones. You may, therefore, wish to use a 50-ohm resistor. The wattage of this resistor can be low, no more than 10 watts.

Another reason for using a variable resistor is that the amount of recoverable ambience will vary from one program to another. You well might wish to have some means of adjusting for such differences.

I see no possible damage to your equipment through this arrangement.

In the case of a power amplifier, the headphones are usually fed by way of a rather large series resistor. Hence, no overload or overall impedance reduction is possible.

(Source: Audio magazine, Aug. 1982; Joseph Giovanelli )

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