AUDIO QUESTIONS and ANSWERS: Advice on readers' technical problems, by LARRY KLEIN
By Larry Klein, Tech editor
Four-channel Cassettes
Q. I seem to remember that you wrote in a Q & A not too long ago that four -channel matrix (SQ or QS) cassettes wouldn't decode reliably if they are made on one cassette ma chine and played back on another. Yet Ralph Hodges in his Angel cassette story in the October issue wrote about four-channel SQ prerecorded cassettes and said that they decoded very well. Who's right?
CHARLES BROGAN; Patterson, N.J.
A. We both are. Decoding problems can arise because the phase relationships between the two channels-which embody the rear -channel encoding-have to be maintained accurately in recording and playback in order for the decoder to reconstitute the original four channels properly. The normal and random phase differences between the duplicator's recording -head gaps and the playback-head gaps in the home machines would be enough to confuse the decoder--at least to some degree. If you were to copy an SQ disc onto a cassette, the same head gaps in your machine would usually serve both the record and playback functions and no discrepancy would occur other than that caused by tape skewing within the cassette itself.
The fact that the Angel discs and prerecord ed cassettes were indistinguishable from each other in respect to the rear-channel effect I don't find to be terribly significant since, as the article stated, the rear channels were simply supplying an ambiance (hall sound) signal with no specific directionality. In such a case, I would suggest that it really doesn't matter audibly if the directionality of the left and right rear channels are confused or lose separation. And, as a matter of fact, my experience has been that the recent artificial synthesized reverberation or ambiance time-delay systems such as the Audio Pulse Model One or the Sound Concepts SD-50 can do a far more satisfactory job of providing "hall sound" on stereo material than any commercial four-channel disc or tape that I have heard up to this time.
FM Hiss
Q. I have a stereo tuner and I usually hear . hiss in the stereo mode. Since I hear no hiss in mono I assume that the noise is generated by the multiplex circuits. Can a noise -reduction unit be connected between my tuner and amplifier to reduce hiss?
CHARLES ALBERT
San Antonio, Tex.
A. First of all, the hiss is not generated by . the multiplex circuits in your tuner. From your description, it appears that the stereo broadcasts reaching your tuner are too weak for your tuner to be "quieted." With stereo FM reception, it takes more signal at the antenna terminals to achieve as low a noise level as heard in mono FM, which is why your mono reception is hiss -free. It would make sense for you to: (1) check your tuner's sensitivity specifications to determine if they are adequate for your location; (2) determine if your specific tuner lives up to its specifications-it may need overhaul or alignment; and (3) determine if your antenna is adequate for tuner and location (your dealer or the tuner's manufacturer should be able to answer all three of these questions for you.) I suspect that your investigations will lead you to the conclusion that as far as FM hiss is concerned, it will make more sense to get a better tuner and/or antenna system than a noise-reduction unit.
Old Kit Overhaul
Q. I was recently given an old stereo tube Q power amplifier that I would like to use with a reverb unit for my rear channels. The former owner of the amplifier said that he built it from a kit and it was working fine when he retired it in favor of a transistor job about five years ago. However, when I hooked it up it seemed very sensitive to vibration: tapping the chassis caused hum and crackling in the speakers. Also, the input -level controls and switches are very noisy when rotated. Is the unit worth fixing, or should it be junked?
ALLAN THOMSON; Los Angeles, Calif.
A. It's hard to know. A Dynakit or a Heath-kit is usually worth fixing; some other brands/units may not be. Your logical first step is to test the tubes. When each tube is in the tester, tap it gently several times. If the tester reading changes or the "short" light flashes, reject the tube. Also be prepared to replace any tube that tests very "weak." Each pair of the large output tubes should also show the same approximate readings on the tube-tester meter to ensure proper output -stage balance.
The second step, which will probably eliminate most of the hum and static, is to tighten all screws and nuts holding down the printed -circuit boards, the terminal strips, input jacks, transformers, and so forth, to the chassis. In most kit amplifiers, these nuts and bolts loosen with time--or perhaps they were not adequately tightened originally. Proper electrical grounding of many sections of the circuit depends on having these components in firm mechanical--meaning electrical--contact with the chassis.
You will probably find two or more tubular filter capacitors (cylindrical objects, perhaps 2 to 3 inches tall) with one end fitted into cut outs in the chassis or in a metal or plastic sup porting plate. If screws are holding the mounting plates, it's important that they be tight. If the capacitors are held in place by having their prongs twisted through slots in the metal chassis, be sure they are securely locked and soldered into place.
As for the noisy controls, a shot of spray from a TV-tuner cleaner spray (available from any radio or electronics parts store) will cure that problem. The spray hose should be positioned so that the fluid gets into the control.
After spraying, rapidly rotate the control several times. It might not be a bad idea to spray a little cleaner into the tube sockets also to help remove any oxidation from the tube pins and the socket contacts. You should also inspect all the soldered connections carefully.
Any that look suspicious--particularly those that have several wires going to one lug or sol der point-should be re-soldered.
If the above suggestions do not solve the problem and you are not competent to go further on your own, a professional audio repair man is your best bet. However, I would suggest you get an estimate on the job first, since his legitimate service charges may exceed the value of the amplifier to you.
R.C.A.F. vs. FM
Q. The other day I was listening to my favorite FM station while doing my Royal Canadian Air Force exercises in the center of the living room. I noticed that every time I kicked my legs, I would hear a blast of interference; when I was motionless everything sound ed fine. Since I presume I do not generate electrical signals which would interfere with the reception over a rather good system, I hope you can explain this phenomenon to me.
ELLEN MILHAN; New York City, N.Y.
A. As you've discovered, the type of exercise performed in proximity to a tuner can affect its reception. FM generally will be unaffected by less vigorous activity such as Yoga procedures that require, for example, relaxing one's pancreas. (I was unable to do that trick in a class I once attended since at the time I was not sure just where my pancreas was located.) Although you are not generating interfering signals, you are interfering with the FM signal reaching the tuner's antenna terminals for the same reasons that a TV picture will sometimes be affected when a viewer moves about the room.
If you are tuned to a weak or distant station--or if your antenna is somehow inadequate--movement in a room may at times re duce the FM signal strength sufficiently for the tuner to fall below its quieting level. This will produce the "blast of interference" you heard. The cure? Make sure that your antenna is properly oriented for best reception of the desired station and that both its leads are connected. If you are using a 300-ohm flatline folded-dipole antenna such as is supplied with many tuners and receivers, try reorienting it.
If you've simply taped it into place stretched out against the back of the equipment cabinet, there's perhaps a 50 per cent chance that you can improve its signal pickup by reorientation. Sometimes you can get improved results simply by letting it hang loose behind your cabinet or shelves.
Matrix Phase Shift
Q. Proper phase relationships are an important factor in the acoustical reproduction of a signal, as evidenced by their consideration in speaker design and installation. Yet it is precisely these phase differences that are used to carry four -channel information in the SQ matrix process. Can we not therefore expect some sort of deterioration when playing un SQ recording through a two -channel sound system, compared with the results obtained from a conventional stereo record?
KENNETH SAX; Chicago, Ill.
A. As is my practice when dealing with matters outside my area of know-how (which seems to contract as the audio field expands), I forwarded Mr. Sax's question to Benjamin Bauer who, as the inventor of the SQ system, should, I felt, have a ready answer, he did. Mr. Bauer writes as follows:
"The debate as to the effect of phase on the quality of Sound has been going on since a century ago, when Helmholtz pronounced that phase has no effect on sound. The very existence of this debate over so long a period suggests that the effect, if any, is tenuous. We have established that carefully tailored phase-shift networks have no audible effect whatsoever on the quality of signal. I refer specifically to the so-called all-pass "psi-networks" in which the function of phase angle vs. log frequency is a straight line. Such networks have been used for years in recording and broad casting without any listener's even knowing it. Nevertheless, prior to using them in quadraphony, we tested them with sine waves, square waves, impulsive sounds, speech, classical music, and popular music using a panel of expert listeners. Our panel was un able to hear any difference with or without these networks in the circuit.
The relative phase shifts between channels of a stereo or a matrixed quadraphonic pro gram are in a different category. They do cause changes-sometimes beneficial, some times deleterious-in the sound of the pro gram. In the SQ system the front channels, which carry the sounds of the front stage, are recorded strictly in-phase as in a conventional stereo disc, and this accounts, in part, for the ideal compatibility of the SQ system. The back channels are recorded at a relative 90-degree phase angle which gives them a feeling of depth, and this accounts for the improved quality of SQ records even when reproduced on a stereo system.
It should be noted that the phase shifts in loudspeakers resulting from improperly designed crossover networks or drivers are a different matter, and the nature of their phase -shift functions is taken into consideration by experienced speaker designers."
Also see:
EQUIPMENT TEST REPORTS: Hirsch-Houck Laboratory test results on the: JVC JR-S600 AM/FM stereo receiver, Marantz Model 1250 integrated stereo amplifier, KLH Model 354 speaker system, and Micro Seiki DDX-1000 turntable and MA-505 tonearm
Source: Stereo Review (USA magazine)