Tape Guide (Q and A) (Oct. 1973)

By Herman Burstein

Pan Pot

Q. I would like a control that would permit me to use one microphone on Channel A or Channel B or both. I guess this would employ a pot which would gradually allow the signal to pass from Channel A, to AB, to B.

-Louis Hone, Montreal, Canada

A. If you have a stereo preamplifier cable of being fed from a mike, you can use its balance control as a panpot.

Set the preamplifier to Morro and feed the mike in the normal manner, with the gain advanced to provide reasonable output with the balance control in the mid position. The outputs of the preamplifier can then be fed into your stereo recorder. The position of the balance control of the preamplifier will determine the apparent location of the microphone in the sound field.

All of this assumes that your preamplifier has provisions for a microphone. The microphone should not be connected to a phono input because the frequency response of such an input is not flat. There will be considerable bass boost and treble cut.

An alternative arrangement would consist of using a Y connector. One of the ends of the Y would be terminated in a receptacle which could accept your microphone. The other two ends of this Y connector would be terminated in plugs which could be connected to your recorder or mixer. Thus, we have a system whereby one microphone will feed into both channels of your recorder. By adjusting the relative levels of the inputs controls for these channels, we have a method for adjusting the effective position of the microphone in the sound field.

I think it would be difficult for you to obtain a special pan pot for use directly with high impedance microphones. Further, I do not like the idea of connecting a potentiometer directly into a microphone circuit. The adjustment of this potentiometer would result in the introduction of noise.

R.f. Interference

Q. Is there a practical method of shielding an audio system from r.f. interference? I am about 45 miles from a defense radar installation whose signals modulate very distinctly when my audio gear is operated in any mode.

I also pick up radio dispatched commercial services. The problem, which is shared by my friends, is most noticeable on the high gain inputs such as phono and microphone. The noise is very bothersome when I am recording a program, because then I get the extraneous signal on the tape, and in addition pick up more interference during playback.

-Stephen Bruce, Mt. View, Calif.

A. To eliminate r.f. interference, you might try using small capacitors (roughly around 10 to 50 pfd) between the input point of the audio signal and ground. However, the interference might also be getting into your equipment at following stages, and it may be necessary to follow the same procedure at these later stages as well.

Car Speaker Sound Level

Q. I have an 8-track car stereo tape player. There are two 4-inch speakers in the front and two 6x9 speakers in the rear. The rear speakers are louder than the front ones. Could you tell me what is wrong?

-Marion Segler, Gary, Indiana.

A. The difference in level appears to be due to the fact that the rear speakers are more efficient. To cut down the level of the rear speakers, you can use L-Pads, obtainable from your local audio store or from electronic mail order houses. These pads usually come with instructions for mounting.

Meters, Response & Heads

Q. How does one set the VU meter so that it is reading the recording level properly? For my tape recorder, the adjustment can be made by turning a pot, but I have noticed that the reading varies with frequency even though the input to the circuit is kept constant.

What is the best frequency to use? What should one see on a 'scope connected to the output of the tape system when the recording level is set too high? What should be considered to be good frequency response (± 1 dB) of a good quality tape deck? Is there a book that explains interchangeability of tape heads? Are they like phono cartridges in that the electrically similar types can be interchanged (e.g. one magnetic cartridge can be interchanged with another magnetic)?

-William R. Lafferty, Wilmington, Del.

A. An accepted way of setting the VU meter is to have the pointer 0 VU at a recording level such that a 400 Hz signal results in 1% harmonic distortion on the tape. Alternatively, you can purchase a test tape which contains such a signal, equate your recording signal level at 400 Hz with the test signal, and adjust the VU meter to read 0. To equate the two signals, you adjust the recording signal until it produces the same playback output as does the test signal. If the recording level is too high and you are recording and playing a low-frequency signal, you should be able to see a distorted waveform on the 'scope. However, if your signal is a high-frequency one, you will still get a sine wave on the 'scope, because the harmonic distortion products are outside the capability of the tape system.

A high quality tape system should be able to provide response flat within approximately 1 dB between about 40 and 16,000 Hz at 7 1/2 ips, and between about 40 and 10,000 Hz at 3 3/4 ips, along with high S/N and low distortion. The upper response limit can be considerably extended at both speeds if one is willing to let distortion rise and/or signal-to-noise ratio decline.

Your question about interchangeability of tape heads might better be addressed to a manufacturer of heads, such as Nortronics. In a general manner, I can say that tape heads are not interchangeable in the way that phono cartridges of the same type are.

Tape Squeal

I have been a victim of this horrendous defect of Scotch 203 for a long time and with three entirely different tape recorders. The squeal is due to "sticktion" of the lubricated tape surface to the small-diameter tape guides after they warm up a little. The following measures help: (1) Use a quiet blower (Rotron and similar) to keep tape contact surfaces cool. (2) Wrap a layer or two of Teflon adhesive tape around all tape guides on the feed-reel side and between heads. (3) Try tape of other makes, although I must admit Scotch 203 is a beautiful tape.

-Alex Azelickis, Morton Grove, Ill.

To other readers who have also written in with advice more or less along the above lines, let me express thanks and apologies for not being able to print their exact comments.

Misalignment Spillover

Q. I have a Revox 6-36 stereo tape recorder. I use this monophonically.

When I record on channel 1 (outside tracks 1 and 4) the machine records a sputtering sound on the adjacent tracks (inside tracks 2 and 3); thus when I record on track 1, the sputtering sounds are recorded on track 2. What could be the cause?

-John Napoliello, Philadelphia, Pa.

A. It may be that there is noise in the channel 2 electronics, and that due to adjacent channel "spillover" or to vertical misalignment of the record head with respect to the playback head, this noise is being recorded on the adjacent channel.

Receiving Dolby Broadcasts

Q. Please explain how I can hook up a Dolby B decoder to my FM stereo tuner or receiver in order that I can receive Dolby-processed FM broadcasts.

-Michael Libretto, Brooklyn, New York

A. To receive Dolby-encoded FM broadcasts, the simplest approach is to substitute the Dolby decoder for a tape recorder, making use of the "tape out" and "tape monitor" jacks on your equipment. This will eliminate some of the problems which could come up when using a separate FM tuner rather than an integrated receiver.

The jacks on your Dolby decoder normally used in conjunction with the output of your tape recorder are connected to the "tape out" jacks of your sound system. The jacks on your Dolby decoder which normally go to a power amplifier input, high level aux. input and the like, now go to the "tape monitor" jacks of your sound system. If your Dolby processing equipment is equipped with separate recording facilities, they may be disregarded at this time.

The only problem is that you will have to wait for a time when the station employing the Dolby system is transmitting calibrating tones for Carefully mark the calibration knobs so that you can accurately reset them whenever you wish to use the decoder with your FM equipment. In this way you can use your Dolby equipment for any other purpose and at any time you can once again listen to Dolby-processed FM broadcasts without waiting for calibration tones.

If you plan to record such broadcasts, feed the tape machine with no Dolby circuits used. The programs are already encoded so no further encoding is required or desirable. However, the tape must be played with the Dolby decoding circuits activated. Be sure to record at proper levels, established during the calibration of your tape machine and Dolby processor. This will allow you to decode the broadcasts properly when playing back the tape.

Microphone Connection

Q. I have a pair of relatively expensive microphones with 50,000 ohms impedance, which I purchased for use with the Miranda Nocturne Tape Recorder. I have added a Concord Mark III deck, with an input impedance of 20,000 ohms. Its manufacturer recommends a mike with an impedance of between 6,000 and 20,000 ohms and an operating level between -70 and -50 db.

Would either the mikes or the deck be harmed by using the mikes directly with the Mark III? If not, what would I lose in quality?

-George W. Ferguson, Metuchen, New Jersey

A. I doubt very much that any of your equipment could be harmed by the connection you describe. If you connect your high impedance microphones to an input impedance of 20,000 ohms, I think that the principal adverse effect will be a loss of sensitivity.

However, your microphones may have enough sensitivity so that, together with the gain of the tape deck, the loss is tolerable. There may also be an increase in distortion; offhand, I cannot tell whether this would be a noticeable increase, although I am inclined to doubt that it would be.

(Audio magazine, Oct. 1973; Herman Burstein)

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