Tape Guide (Q and A) (Nov. 1976)

Increased Bias

Q. Please give me a procedure for increasing the bias of my tape deck in order to use low-noise tapes. I have an oscilloscope and VTVM, but no signal generator. What other adjustments must be made in order to take full advantage of low-noise tape? How would increasing the bias affect playback of already recorded tapes?

-Michael Gray, Rolla, Mo.

A. You should have a signal generator in order to adjust bias. One way of making the adjustment in changing to low-noise tape is to increase bias as much as possible while at the same time maintaining treble response as flat as possible. That is, increase bias in small increments, at the same time checking treble response in playback.

If you have no signal generator but trust your ear to tell whether treble is flat, you can dispense with the generator. Optimally, in changing from conventional to low-noise tape, one should not only increase bias but also reduce treble boost somewhat and increase the amount of audio signal fed to the record head. Changing the bias does not affect playback of previously recorded tapes; bias enters only into recording.

Binaural Recording

Q. I have become interested in binaural recording, that is, two-channel recording for earphone listening.

Could you please advise me on the correct recording and microphone placement techniques, type of microphone, and whether or not 1 should use a dummy head with microphones placed in its "ears?"

-Joseph Hardy, Fort Worth, Tex.

A. I doubt that it is absolutely necessary to use a dummy head for binaural recording. You can use two directional microphones placed approximately ears' distance apart. Or you can use a mid-side type of microphone (two microphones in one case), such as made by Neumann (Telefunken), this last is rather expensive. If you employ separate microphones, you have the advantage of being able to experiment with distance between them and with angle of placement.

Also, see the May, 1976, issue of Audio for building your own inexpensive binaural mikes, and the September 1976 issue for corrections to the original article.

Deck Modification

Q. While the playback response curve of my tape deck is quite good (within 2 dB of flat), the record-playback response has a peak of over 8 dB at 12,500 Hz relative to 1,000 Hz. I would suspect some modification of the equalization is indicated. I would appreciate any suggestions.

-G. F. Maughmer, Esconcido, Cal.

A. I would not be inclined to modify the playback response, since this is quite good. Before doing anything about the record equalization, I suggest that you increase the bias to the record head. This will reduce the treble response and at the same time result in less distortion. To the extent that you do modify the record equalization, try to move the treble peak to a higher frequency, perhaps 20,000 Hz or so, which may result in reasonably flat response within the audible range, say out to 15,000 Hz.

Reverb S/N Ratio

Q. I have a Dokorder tape recorder and an Akai tape deck fed to my Ken wood receiver through a Pioneer re verb amplifier. I usually use my reverb amplifier with the reverb control backed all the way down. The result seems to be a signal boost which I have interpreted as an increased signal-to-noise ratio. Is this assumption correct?

-Ronald Slakie, Tacoma, Wash.

A. It is not possible for your reverb unit to increase the signal-to-noise ratio. What may be happening is that the reverb unit causes some treble attenuation, thereby giving the impression of less noise.

Tape Deck Friction

Q. I have a TEAC 4020 tape deck. On fast forward and reverse, electrical sparks shoot onto the tape from underneath the reel. On one occasion the tape almost caught fire, and I stopped the reels immediately. On normal forward and reverse, there are no electrical sparks or fire. What is wrong?

-P. Kramen, Brooklyn, N.Y.

A. It seems that the brakes or something else is scraping. At high speeds, such scrapping will, of course, result in more friction, heat, and greater possibility of fire than at normal operating speed. You should have your tape machine promptly checked by an authorized service station.

Head Cleaners

Q. On the recommendation of the service man at the store where I purchased my Revox tape deck, 1 have frequently used isopropyl alcohol to clean its heads and tape path. But the technician at another dealer says not to use isopropyl alcohol. He says it causes the head laminations to come unglued, and he recommended denatured grain alcohol. The best I could do was to obtain denatured wood alcohol. Also, Nortronics tape head cleaner is available, but expensive. What is the best head cleaner? What is the chance that my heads have been damaged?

-Eric Church, Tolono, Ill.

A. A number of tape recorder manufacturers recommend isopropyl alcohol, although some do not. I do not know what is absolutely the best for your deck; it would be advisable to consult the manufacturer or the instruction manual. Chances are that you have not harmed the heads or else you would probably have known by now. Manufacturers are alert to the common use of isopropyl alcohol, and they would be unlikely to use heads that readily fall apart upon such use, otherwise they would issue a prominent warning against such use.

Playback Equalization

Q. I am contemplating the purchase of a tape transport, including head, and a separate playback amplifier--Shure or Pioneer. Don't tape machines need different playback equalization at different speeds? Don't playback preamps such as the Shure only provide equalization for 7 1/2 ips? Is there a preamp that would permit using all speeds (1-7/8, 3 3/4, and 7 1/2 ips)? Is this speed equalization what is meant by NAB specifications for the preamp? Have I got the whole thing all mixed up?

-David Ohde, Weaverville, Cal.

A. Yes, tape recorders require different playback equalization at different speeds, and the equalization generally offered in inexpensive playback preamps is usually the NAB standard for 7 1/2 ips. It may be that some more recent units that have come on the market offer variable playback equalization. Please query your audio dealer on this. If you have equalization for one speed, namely 7 1/2 ips, then for the slower speeds you could touch up the equalization-that is, cut the bass a bit and boost the treble a bit until the sound seems satisfactory.

Channel Switch

Q. I have a TEAC 4020 tape deck, which has automatic reverse in playback only. I record monophonically on Track 1 (left channel) from left to right. When I switch to Playback there is no music, just a buzzing noise from both the right and left speakers.

When I finish recording Track 1, I flip over the reel to record Track 4. On playback in the reverse mode, all the music of Track 1 comes through the speakers. The same problem applies to Track 4. This problem does not exist on the right channel which works perfectly.

-P. Kramen, Brooklyn, N.Y.

A. Your playback problem on the left channel appears to be due to either a defect in one section of the playback head or a defect in the switching system.

Jammed Cassettes

Q. I have a problem with my cassette machine. Often when I attempt to play, rewind or advance the tape, it will not move in either direction. I have tried the major brands of cassettes, and it usually happens with the C-60 and C-90. Is the problem with the cassettes or with the machine? Will a jammed cassette damage the machine? Is it possible to modify the motor to make it more powerful or install a new, more powerful motor?

-Sheldon Lichtman, Fort Bragg, N.C.

A. The problem could be either in the machine or the cassette, but experience indicates that some cassettes give more trouble than others. The longer-playing cassettes with thinner tape are apt to be more troublesome.

Follow your own experience in learning which cassettes are most trouble free with your particular machine. But if there is no brand of cassette which prevents frequent jamming, your machine should be taken to an authorized service station. If the unit is left in the jammed state for even a few minutes, this may damage the motor.

Thus, your motor may have already been damaged and may not have as much power as a new one. Consult the manufacturer with respect to replacement.

( Audio magazine, Nov. 1976, Herman Burstein)

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