Tape Guide (Q and A) (Nov. 1970)

MAY I PLEASE again remind readers that the basic purpose of the Tape Guide is to answer queries about the electronic and magnetic aspects of tape recorders, about the use of tape machines, and about problems of tape machines in general.

Too many letters still come in asking for recommendations or other comments bearing on specific items of tape and other audio equipment; the policy of AUDIO prohibits me from making such recommendations and comments. Readers are referred to the Equipment Profiles for such views.

Also there are too many queries about the idiosyncrasies of specific models of tape machines; I am not and cannot be familiar with all makes of tape machines, and therefore cannot answer most of these questions. Finally, there are too many questions which should be addressed to manufacturers or their representatives, as for example, those on specifications or maintenance of a particular model.

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Mr. Roger Odom, Engineering Manager of SPARTA Electr. Corp., Carmichael, California, has sent in a very interesting suggestion about better erasure and lower noise:

"Few, if any, home-entertainment or semi-professional recorders develop a strong enough field to erase a tape thoroughly. The previous recording may be wiped out well enough, but the residual tape noise will be much higher than need be due to incomplete erasure. Use of a good-quality bulk eraser can result in several dB or more of increased S/N, but only if the recorder's erase head is suitably disabled so as not to undo the good of thorough bulk erasure. Of course this won't help in a system whose performance is limited by circuit noise, but this should not be the case in any modern recorder of reasonable quality . . . My recorder has an erase-defeat switch, and I bulk-erase. I can easily detect where I have recorded-over, using the internal erase head, by the increased noise level. Please try it some time."

Tape Deck Amplifiers

Q. One thing that puzzles me about tape decks is that they have built-in amplifiers. I want to get a tape deck and run it through my audio preamp and power amplifier. Do the tape deck's built-in amplifiers have any effect on the performance of the preamp and power amp? Is it possible to obtain a tape deck that doesn't have built-in amplifiers, or are the built-in amplifiers a necessity? Also, if the built-in amplifiers are a necessity, would my audio preamp/power-amp combination improve the quality of the sound from the tape-deck amplifiers?

( Curtis T. Gross, APO San Francisco, California )

A. For recording, a tape machine must have a special preamp that includes an oscillator. None of the audio preamps customarily available provides this facility, nor the equalization that is required in recording ( treble boost of a specified kind). For playback, a tape machine must have a preamp to provide equalization and amplification. Many audio pre amps provide this playback facility, so that output of the tape playback head, if desired, may be connected directly to the audio preamp. ( However, this cable should be as short as possible to avoid treble loss; and it must be routed carefully to avoid hum pickup. ) Playback quality is governed by the amplification stage nearest to the playback head. Whether you are better off connecting the playback head directly to an external preamp, or going through the playback preamp of the tape machine, depends on the respective quality of each preamp. In your case, if you are using a very-high-quality external preamp, you might be better off bypassing the tape deck playback preamp. On the other hand, in a good tape machine the playback preamp contains equalization adjustments to compensate for the deviation from "ideal response" of the particular head. The external preamp ordinarily provides no such adjustment, and therefore may not give as good a frequency response.

Test Tones

Q. I have some test records with 1600-Hz standard lateral test tones recorded at velocities of 5 cm. per sec. rms and 7 cm. per sec. peak, respectively, which are almost the same. Are these test tones equivalent to the standard level tones on tape recorder alignment tapes and therefore useful for level adjustments when copying records, or does such a standard exist? These records have worked for me for copying some orchestral records, but result in level settings either too high or too low on others or on other types of music, such as classical vocal.

Your comments will be appreciated.

H. Michael Lafleur, Lafayette, Louisiana.

A. There is no direct relationship between the standard lateral test tone for records and the standard level tones for tapes. The latter are related to a prescribed amount of distortion recorded on the tape at a prescribed frequency. The NAB magnetic tape standards can give you more information on this.

In setting level for recording a tape, one cannot follow a purely mechanical procedure based on a reference level tone. Much depends on the nature of the sound to be recorded; more specifically, on the extent to which peaks exceed average level. If amplitude of the sound is relatively uniform, with peaks seldom more than 10 dB above average level, one can safely record (without excessive distortion) at a higher level than' if there is, say, a 20-dB distance between peaks and average level.

Faulty Playback

Q. I have a problem with my tape decks on recording. This is my second deck, and both of them have given me the same difficulty. When I am taping, the meters show that the signals are going through, but on playback I can hear only one channel. This happens off and on, which is the reason I bought a second deck. Also, on tapes I recorded a while back, I can hear a mixed up sound on one channel and the recording on the other channel. If I rewind, sometimes this problem clears up. Or if I take the tape off and play it at a later date it is okay. My amplifier plays both tapes and discs well, and there is always sound in both speakers.

( Clyde M. Thomas, Paraiso, Canal Zone)

A. Your trouble might lie in a faulty cable or other poor connection between the output of your tape machine and the input of your amplifier. Interchange the left and right output cables of the tape machine to verify this. If the sound goes dead on the opposite channel than previously, the trouble is in the cable.

If the trouble is not in a cable, you can likely pick up a clue as to where the trouble does lie by reversing left and right connections-for example, connecting the tape left output jack of the tape machine to the right input jack of the amplifier. In such a case, if the trouble switches channels, suspicion points at the left output circuit of the tape machine.

Similarly, you can make further tests to narrow down the source of difficulty to the tape machine or to the amplifier; and to either the playback or recording mode.

Sibilant Distortion

Q. My tape deck produces a type of sound distortion which is independent of the recording level, but dependent on the tape speed. The most extreme effect of the distortion is in nearly all sung words containing "s." The fairly short, sharp sound of the "s" in the source becomes a much-lengthened "ssss" (somewhat like a hiss) on the tape. Most extreme at the slowest speed, the effect is still detectable at 7.5 ips. When a cymbal is recorded, it loses some or all of its clarity, depending on tape speed. What is the reason for this distortion?

(Conrad M. Swartz, Minneapolis, Minnesota )

A. My guess is that the "distortion" you describe may be due to a peak in treble response. Such peaks are unlikely to be severe, if they exist at all, at 7 1/2 ips inasmuch as flat response of 15,000 Hz or higher is readily attainable today at this speed. But it still is not really easy to obtain response to 15,000 Hz at reduced speeds without giving up something in the way of noise and/or distortion. To maintain extended treble response at reduced speeds, a good deal of treble boost is employed in recording-substantially more than at 7 1/2 ips. This treble boost may encompass a peak in response in the vicinity of 5000 to 8000 Hz, unless the equalization circuit is very carefully designed. The peaked response characteristics, frequently encountered at 3% ips and lower speeds in many tape recorders, could cause the "ssss" you describe.

The large amount of treble boost can also produce distortion in two ways: (1) It may overload the tape. (2) It may overload the record amp of the tape machine. Another possible cause of the "ssss" is distortion in the record amp due to a leaky capacitor, faulty resistor, and so on.

Test Tapes

Q. I am interested in obtaining a test tape to test and align my tape deck. Can you suggest an inexpensive test tape that will fulfill the job as a good substitute for the standard test tapes?

(Laudie J. Doubrava, Fairfield, California)

A. I am sorry, but I have no substitute test tape to recommend. A great deal of ingenuity, work, and care go into the making of a good test tape; hence its expensiveness. I would be disinclined to trust an inexpensive test tape, because this would seem to reflect insufficient care in producing a product of very high precision for it to serve its intended purpose.

Head Life

Q. Could you hazard a guess as to how long it might take before a properly installed replacement head begins to show visual signs of wear? That happened within three hours after I installed a new head in my recorder, and this leads me to believe that something must be wrong.

(Robert Pearson, Chicago, Illinois)

A. I am sure it should be much more than three hours. Are you by any chance mistaking accumulation of tape oxide for visual head wear?

Measurement Levels

Q. Why is frequency response of a tape recorder measured well below 0 VU rather than at a higher level?

-R. B. Rose, APO San Francisco, California.

A. To measure a tape machine's frequency response, frequencies are recorded at a level typically 15 to 20 dB below 0 VU to avoid tape saturation at high frequencies. The record amplifier supplies a substantial amount of treble emphasis, and recording in the vicinity of 0 VU would therefore result in tape saturation at the high end of the audio range. Correspondingly, in playback there would be an erroneous indication of inadequate treble response. The reason I say erroneous is that natural sound generally contains high frequencies of substantially lower magnitude than mid-frequencies, thereby avoiding tape saturation at the high end. However, in testing record-playback response, one feeds in high frequencies of the same magnitude as mid-frequencies.

(Audio magazine, Nov. 1970; Herman Burstein)

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