Audioclinic (April 1984)

Noise Between Stations

Q. Most FM tuners with which I am familiar produce lots of noise during the process of tuning from one station to the next. Why?

-Name withheld

A. All electronic equipment has some residual noise. An FM tuner possesses a considerable amount of r.f. and i.f. gain. Under such circum stances, even a small amount of noise can be amplified so much that it can be easily heard.

When a strong signal is received, various circuits within the tuner act to reduce the sensitivity of various stages, thereby lowering the noise in the presence of signal.

If you now tune in a weak signal, more sensitivity will be required to make this signal audible. Thus, some of the background noise, suppressed in the presence of a strong signal, begins to be heard when listening to a weaker signal.

Stereo from Mono

Q. How are the "oldie," pre-stereo recordings "electronically processed" for reissue in stereo? Such reissues usually sound artificial and unnatural (perhaps reverberant is the word I am seeking).

Playing these "fake" stereo records using the mono mode not only reduces noises such as pops and clicks, but also frequently improves the sound.

-Rudi Schmid, Kensington, Cal.

A. A number of techniques can be used to produce pseudo-stereo from old, monophonic sources. The simplest of these methods is to put more bass in one channel and more treble in the other. The perceived result is that lows will be heard from one speaker and highs from the other. (This is not a favorite technique from the standpoint of the disc mastering engineer be cause of vertical "pull-up" problems caused by a non-centered low end.) Another system is to feed the mono phonic signal into two stereo channels.

Reverberation is then added to each of these channels, with this reverb de rived from a different "chamber" for each channel. Differences in reverberation characteristics from one chamber to the next can result in a sense of space surrounding the program.

Still another approach is to send the monophonic signal directly into one stereo channel. Then the same, mono phonic signal is delayed just a bit, and sent to the other channel. This, again in conjunction with reverberation, can produce a sense of space around a performance.

Some monophonic performances are so "dry" acoustically that these techniques, when used with care, can sometimes aid in enhancing our enjoyment of a monophonic recording.

All too many of these old recordings are ruined, at least for me, just be cause of the overuse of one or all of these techniques. (I am certain there are other techniques for producing pseudo-stereo, but these are the ones which immediately come to mind.)

Lowering Turntable Speed

Q. I have an old record changer which has no "pitch control." I checked the speed of this unit and found it to be running at 35 rpm rather than 33 1/3 rpm. Is there any way of adjusting the speed?

-G. V. V., Omaha, Nebr.

A. To decrease turntable speed of a typical puck-driven phonograph re quires slightly grinding the motor shaft.

Before doing anything, I suggest that you use a strobe card to check for proper speed. (While you are doing this, you may wish to check all speeds.) It is easy to spot speed errors. If you try measuring turntable speed with the aid of a stopwatch, you may not be able to make a correct judgment as to exact speed. Whatever you do, don't grind any portion of the motor shaft unless you are sure that the phonograph is really running fast, and that the error remains even after replacing the idler puck.

To do the work, remove the platter. It is probably held in place via a C washer, which must be pried off, hopefully without its flying off into space. Hold the drive puck away from the shaft, perhaps with the aid of a rubber band.

Obtain some fine emery cloth. Examine the shaft to be ground down in order to determine the correct "step" to be worked on. Once you have determined this, turn on the motor and grind the shaft by pressing the emery cloth against it. Be sure to check the turntable speed periodically so you don't grind off too much material. It's best to be overcautious.

When the work is completed, dismount the motor and disassemble it.

Remove all emery dust which may have found its way into it, especially the top and bottom bearings. While the motor is apart, put a drop of oil into each bearing and allow it to soak into the metal. Reassemble the motor and remount it. Apply a drop of oil to the bearing of the drive puck, and to the turntable bearing. Check all other mechanical parts for proper lubrication, and grease where needed.

Replace the platter and C washer, and your changer will be ready for use.

Rear-Channel Recovery

Q. I have been using a four-channel receiver with four loudspeakers for a number of years. The front speakers are connected to the front channels in the normal manner. The rear speakers are connected to the rear amplifier in accordance with the hookup suggested by Hafler for recovery of the ambient sound in many stereo recordings.

I have found this arrangement useful in that it allows me to control volume and tone separately. This is especially helpful for balancing the system be cause my front and rear loudspeakers are not matched.

Lately I have been wondering whether an alternate hookup might improve the sound even further. My receiver has provisions for "strapping," which will more than double the power-output capacity of the front channels. I could then connect the rear speakers, together with the front-channel speakers, to the front amplifier channels (again using the Hafler circuit for the rear channels), and use a separate L-pad to provide for channel volume balance.

What are the advantages and disadvantages of both of these arrangements?

-Klaus Hieronymus, Darien, Conn.

A. Your present arrangement, using all four amplifier channels, with the rear channels as a Hafler circuit, is a good one. It is simple to achieve good channel balance between the main and de rived ambience information. Any added power you might obtain by strap ping your amplifier would, in my opinion, be insignificant. I do not like to waste power in the form of heat, and that is just what you would be doing if you used an L-pad as you have de scribed.

You make a good point about using the separate tone controls in your rear channels. The lack of speaker matching is a good reason for the extra set of tone controls; I like that idea and it would be lost with the snapping sys tem.

This arrangement can be readily modified to incorporate a time-delay unit, should you decide to add one at a later date.

(adapted from Audio magazine, Apr. 1984; JOSEPH GIOVANELLI)

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