Tape Guide (Q and A) (Mar. 1971)

Monitor Level Drop

Q. When I record monophonic FM on both tracks of my tape machine at the same time, or when-I take a mono source and split it so as to send the same signal to Channel A and Channel B, upon balancing the input by meters with the monitor switch in the source position and then switching to tape monitor, I find the overall level dropped considerably and Channel A 2-3 dB below Channel B. When I use the output control to obtain the same listening level in the tape monitor and source monitor position, a comparison indicates an obvious deficiency of bass in the tape monitor position. I have made the above checks with several different brands and types of tape, and all gave comparable results. Please advise how to solve the problem.

-John D. Moss, Hartselle, Alabama.

A. A difference of 2-3 dB in level between channels is not "considerable" but just about on the edge of significance so far as the human ear is concerned. This difference could be due to a difference in calibration of the meters, to a difference in amplification by electronic stages, to a difference in efficiency of the two sections of the record head, to a difference in output of the two sections of the playback head, and perhaps to other factors. One would have to make a careful check, with suitable test equipment (signal generator and VTVM) to discover where the difference originates and what to do about it. What to do about it depends in part upon the adjustment facilities available inside your tape machine. Some machines contain fairly elaborate internal controls in order to permit balancing channels in recording and playback.

The above answer also applies to your problems of a drop in level when switching to tape monitor (tape playback). Some machines contain internal controls for balancing input and output levels.

In the absence of suitable internal controls to achieve balance between channels and between input and output, you have to rely upon your external controls (for recording and playback) to achieve such balance.

The bass deficiency when listening to tape playback may be due to faulty playback equalization. Playback equalization can be checked with a test tape and a VTVM connected to the output of your tape machine. Proper equalization will result in flat playback response.

Red vs. Black Oxide

Q. I have often wondered why all prerecorded tape is of the red oxide variety.

This dirties up equipment much faster than black oxide tape. I assume that the reason why black oxide tape dirties up equipment less is that the oxide is much harder. If the oxide is harder, then the commercial studios probably stay away from it because it wears out heads and parts faster. Any comments you have on the subject will be appreciated.

-M. Glen Blair, Idaho Falls, Idaho.

A. I am not familiar enough with the physical properties of black oxide tape to comment on it as a cause of wear of heads and other tape machine parts. If you are correct that it causes more wear than red oxide, this would be a good reason for staying away from it, because high quality tape heads are very expensive items.

Furthermore, my understanding is that the two oxides have quite different magnetic properties, involving frequency response, bias requirements, noise, signal level, drive requirements, etc.; and that it has generally been found that red oxide is superior for most audio use.

Whistling Recorder

Q. I am writing to you about the problem of recording from FM stereo. 1I get a barely audible whistle at 7 1/2 ips, but at 3 3/4 ips the whistle is very audible. Since my tape machine makes very listenable tapes at 3 3/4 ips, I of course wish to use this speed. I know from repairing several friends' machines that machines with 100 kHz bias have little or no troubles with "birdies." However, my recorder uses 80 kHz bias. I assume that the extra treble boost at 3 3/4 ips compared with treble boost at 7.5 ips boosts the 19 or 38 kHz present in the tuner output and therefore accentuates the whistle at the lower speed. (This whistle results from mixing of the recorder's bias frequency and the tuner's 19 or 38 kHz output HB.) I am satisfied that the tuner is OK as I have tried two other tuners with the same result. Can you advise me regarding a filter to be used ahead of the high-level inputs of my tape recorder?

-Clarence Geidenberger, Jr., Newark, Ohio.

A. You might contact your local audio store or some of the audio mail order houses to inquire what they have in the way of a multiplex filter. If you wish to construct your own filter, you can try a simple circuit consisting of a capacitor and inductor in series, tuned to 19 kHz, and placed across the input jack of your tape recorder. (Of course, one such filter for each input.) This is the method used in some tape machines. Tandberg, for example, in its Model 64 uses a 2200 pfd capacitor in series with what is apparently a 34 mhy inductor. You can perhaps obtain a TV width coil tunable in the range of 10 to 20 mhy. If so, and assuming you adjust the coil to about 15 mhy, you would use a capacitor of about 5,400 pfd. If you use an inductor of about 10 mhy, the capacitor would be about 7,000 pfd. In short, the product of inductance in mhy times capacitance in pfd would be about 70,000 if you are tuning to 19 kHz.

Large-Reel Adaptor

Q. I have a Revox tape deck and have some questions regarding the use of 10 1/2" reels on this machine. I have noticed that many of the 10.5 inch plastic reels used on this machine are warped, sometimes severely so. Is this a common problem, and if so would you recommend the use of these reels? I would prefer to use the large-hub metal reels, but many people have told me that no satisfactory adaptor exists which would permit the use of these reels on my Revox. Apparently many of these adaptors simply do not perform well, and numerous problems are encountered when they are used. Is this really the case? If not, which adaptors do function properly, and where can these be obtained? What would you recommend as the best overall method of using 10 1/2" reels on this machine?

-Stephen L. Siegel, Chicago, Illinois.

A. I think that your question on how to handle the problem you describe should really be addressed to the manufacturer of your tape machine. I agree that use of a metal reel is preferable, but do not have the background of experience which would indicate the proper adaptor to use with the Revox (if there is such an adaptor). Why not write to Revox?

Equalization

Q. My question concerns equalization when playing records through the amp in my tape recorder. The only equalization in my recorder amp is for the two speeds 3 3/4 and 7 1/2 ips. I play records through my recorder amp by plugging the turntable outputs into the microphone jacks of the unit. How can I get phono equalization without buying a separate amp? Or wouldn't equalization make a great deal of difference?

-Neil Passage, March AFB.

A. When played with a magnetic cartridge, a phonograph record requires a substantial amount of bass boost and a considerable amount of treble cut in playback in order to provide flat response. To give you an idea of how much equalization is needed-and therefore how important it is-the RIAA standard in this country requires bass boost, relative to 1,000 Hz, reaching about 17 dB at 50 Hz; and treble cut of about 14 dB at 10,000 Hz.

It would be possible to introduce such equalization into the microphone section of your recorder. But this would be a task for someone with the necessary technical competence; and it would mean giving up the use of microphone with your recorder, unless you went through the additional complexity of a switch to defeat the phono equalization. A simpler course would be to purchase one of the relatively inexpensive phono preamps that are available. Connect the output of your turntable to such a preamp, and connect the preamp to the high-level input of your tape machine.

Magnetization

Q. Ever since I got my first tape recorder in 1954, I have had a fear of a magnetized head ruining my tapes. It's gotten so that I'm almost afraid to play my good tapes for fear of erasing the high frequencies, adding to background noise, etc. I was once under the impression that a strong signal would magnetize the heads. I now wonder if the signal from the bias isn't so strong that some accidental over-recording wouldn't make any difference. When they say "high frequencies are affected," how high? Does the head become magnetized from just playing? Most important: Are the warnings appropriate to someone who plays his recorder maybe 20 hours a month and records maybe 3 hours, and uses the demagnetizer regularly, or are they meant for someone who lets the machine go for years without attention? I feel foolish wanting to demagnetize the heads every time I turn the recorder on. On the other hand, I worry that my "un-scratchable" tapes will be ruined by "a worn needle," as it were.

-Laird Brown, Dayton, Ohio.

A. The usual rule is to demagnetize the heads after every 8 hours of use.

Don't worry about the bias signal. It is the audio signal-asymmetrical-which tends to magnetize the heads. The stronger the audio signal, the greatest the tendency. Magnetized heads do have an increasing erasure effect as frequency rises. Abrupt changes of current through the heads tend to magnetize them. Some tapes machines have built-in provision for self-demagnetization of the heads.

Cupped Tape

From time to time readers have inquired what they might do about cupped tape. Following are some comments by another reader who has found a solution.

"I use 1 mil polyester tape exclusively. I once received an order of this tape which was cupped until it couldn't be recorded properly with a single-motor recorder of mine. I decided to experiment with two rolls of this cupped tape.

"I tension wound the two rolls in fast forward on a powerful 3-motor recorder by holding a gloved finger on the edge of the play-out reel to create extra tension.

I did this with the thought that perhaps by leaving the tape in this tight-wound condition for a period of time, the un-cupped side would be forced to stretch, and this stretching would eliminate the cups. The tape was left in the tight-wound condition for a year. To my surprise this worked better than I expected. When I examined the tape, I found that the cups had been completely eliminated. Yet to be determined is how long of a waiting period is really needed.

"This idea is presented with some warnings. It shouldn't be tried with underpowered single and 3-motor recorders, because motors and rotating parts could be damaged. It should be avoided if a recorder is driven with a synchronous single motor. Even with powerful 3-motor recorders, care should be used in not slowing the motors too much, as they operate at full voltage in the fast wind modes and could burn out."

-M. Glen Blair, Idaho Falls, Idaho.

(Audio magazine, Mar. 1971; Herman Burstein)

= = = =