Tape Guide (Q and A) (Dec. 1972)

Tape Squeal

From time to time readers write in to complain about tape squeal, usually occurring with just certain combinations of tape and tape machine. That is, a given tape will squeal with some machines and not others and a given machine will squeal with some tapes and not others. Following is a helpful commentary by Roger K. Odom, Engineering Manager, SPARTA Corp., 1931 Shelfield Dr., Carmichael, Calif.:

In every case I have experienced, squeal can be directly related to the back-tension of the feed reel, that is, if the tape-tension is properly adjusted the squeal will disappear. The basic solution is to reduce tension on the tape. The cause is a bit more complex and results, in part, from manufacturers' attempts to reduce head wear and increase tape life and response. It is directly related to factors such as the elasticity of the tape, the coefficient of friction in the tape-to-head contact, the geometry of the tape transport, and the damping (if any) between the feed reel and the tape head. Almost always it occurs in transports which rely upon back-tension in the feed reel to avoid the added head wear of pressure pads.

Hot Chassis

Q. I have a TV set without a transformer, one whose chassis can be hot. At some time in the future I want to connect the signal from the TV volume control to my high fidelity system. If the chassis is hot and I reverse the plug in the house receptacle, will that remove the danger? The service manual says to use an isolation transformer when the set is serviced Can an isolation transformer be left permanently plugged into the power line? Would this remove all danger?

-John J. Kiesel,. Valley Stream, N.Y.

A. Reversing the plug may or may not eliminate the danger of electrocution to someone touching the audio system when the TV is connected to it.

I wouldn't take the chance. The fact that the service manual says to use an isolation transformer when servicing the set is evidence of this. The isolation transformer can be left permanently plugged in and would remove the danger of electrocution. When the TV set is off, the transformer of itself will consume negligible current. (When you are away for a prolonged time, it might be a good idea to disconnect the transformer from the a.c. line.)

Receiving Microphone

Q. Here's a strange one for you. The other day when using my Uher 4000-L portable tape recorder on playing back some music, I noticed a few extra notes which weren't on the guitar I was playing when I made the recording. Curious, I pushed the record button and waited The VU meter wiggled a little. Playing back what should have been random room noise, instead I heard a series of tones in random three-note segments of different pitch. I recognized this as what I used to hear on channel 23 of a C.B. set I owned about five years ago. Channel 23 is used for remote control, and I had been told that what I heard was the city utilities being remotely controlled.

The microphone I am using is an Electro-Voice 676. Since I don't have the phantom note problem when recording on line input, I suspect the mic. What do you suspect?

-Howard Mills Lang, Panorama City, Calif.

A. I am more inclined to suspect the first recording stage than the microphone. On the other hand, conceivably it is the microphone or its cable that is picking up the C.B. broadcast. Disconnecting the mike should tell you; if you still get the phantom notes, the fault is not the mic's. If the fault is not the mic's, you may be able to solve the problem by placing a small capacitance--a few pf-between the input of the first recording stage and ground. Should the mic be the source of the phantom note pickup, I suggest that you query the manufacturer of the mic for a solution.

HF Bias Oscillator

Q. In going through some audio magazines I noticed a statement that lower bias noise is possible by increasing the bias frequency. I am thinking of building a new bias oscillator to operate in the range of 200 kHz to 500 kHz, and would appreciate your comments. I would keep the old 80 kHz oscillator to power the erase head.

-Joseph Feng, Kenilworth, Ill.

A. By increasing the bias frequency above 80 kHz I doubt that you will decrease noise except to minimize the chance of "birdies" due to interaction between the multiplex subcarrier and the bias frequency. The principal avenue to decreasing noise due to the bias frequency is to purify the oscillator waveform, which is achieved through excellence in the design and components of the oscillator circuit.

Another avenue, I am informed by a correspondent, is to eliminate the erase head (meanwhile using a substitute resistive load on the oscillator) and do your erasing by bulk eraser.

Tape Standards

Q. I would like to standardize on a particular tape. In an article the author indicates that Sony SLH-180 tape is particularly good, and therefore I think perhaps I should use that as my standard tape. But I don't know if my tape deck is adjusted for that tape. I don't know what is involved in adjusting my tape deck to the Sony tape. Can I make the adjustments myself? I don't care for the idea of lugging the deck to a repairman, unless really necessary.

-Peter A. Brodney, New York, N.Y.

A. Chances are good that your tape machine and the Sony tape are compatible. Tape machines, particularly those of good quality and reputable manufacture, are adjusted so they will operate properly with quality tapes in widespread use. New tapes coming on the market, in order to compete, must have characteristics essentially similar to other tapes that have already arrived; people won't buy non-compatible items. I suggest that you try the Sony tape and ascertain for yourself whether it creates problems, which are most likely to arise, if at all, in connection with frequency response. If such a problem does come up, it would be necessary to have an authorized service dealer make the required adjustments.

Unless you are technically well qualified, I don't think you should play around with internal adjustments of your tape recorder.

Preamp Needed?

Q. I plan to buy a tape recorder to make on-location recordings of local music groups. From these tapes I will make phono records. I don't need a preamp, do I? I can just play the phono records using the preamp in the tape recorder, can't I?

-Craig Sutherland, Troy, Mich.

A. Your tape machine's playback preamp provides different playback equalization than required when playing a phono disc. Standard tape playback equalization at 15 and 7 1/2 ips consists of bass boost commencing (3 dB up) at 3,180 Hz and ending (3 dB below maximum) at 50 Hz.

RIAA phono playback equalization consists of bass boost commencing at 500 Hz and ending at 50 Hz and of treble cut commencing at about 2,120 Hz. So the answer is that you need a separate preamp for phono disc playback.

Recording a Center Channel

Q. I am going to record a rock group with my stereo tape recorder. My problem revolves around recording the vocalist. Is it possible to split the output from his mic and feed this signal into both channels without affecting the frequency response or other characteristics of the mic?

-Harold R. Petrie Jr., Troy, N.Y.

A. I think it is possible to split the signal as you indicate without problems. To minimize the possibility of high frequency loss (particularly if the mic of medium or high impedance) or hum pickup, it is advisable to make the split at the tape recorder end of the cable rather than at the mic.

Storing Tape

Q. I've read several times that to lower print-through on recorded tapes, you should fast wind the tape at least once before playing it, and for that reason to store the tape tail out. I've tried without success to accomplish this. At the end of playback, the tape doesn't wind up on the original reel.

-Francisco X. Ramos, Summerland, Calif.

A. If you store a tape tail out, this means you have to rewind it before playing, which puts it on what we'll call reel 2. Then you play it from reel 2 onto reel 1 (the original reel), and you leave it on reel l for storage. In short, the tape does get back onto the original reel.

Four-Channel Conversion

Q. Quadraphonics being so much in the news lately has brought the following idea to me. To utilize existing equipment, I have concentrated on using the two-channel capability of my Ampex tape deck. I have decided to add a supersonic frequency-modulated carrier to each channel. This means that the bias has to be set higher than at present.

I hope that with a tape speed of 15 ips, a bias frequency of 200 kHz, and special high frequency equalization, a supersonic band of 25-40 kHz of reasonable uniformity can be recorded.

Using 32.5 kHz as the center frequency, a 7.5 kHz signal would generate maximum deviation. The tape deck would not need to have the $100 four-channel head and would provide twice the tape playing time of the four-channel system.

Do you think such a system is workable?

-Thomas Hillard, Downers Grove, Ill.

A. Offhand your idea seems workable. The chief question concerns tape speed. I wonder. how many home users are willing to employ 15 ips, considering what this entails in the way of more tape and/or larger tape reels and decks. (In fact, I wonder how many are willing any longer to use 7.5 ips, considering how well one can do today at 3 3/4 ips with a high quality machine.) The extra cost of a 15 ips machine might easily exceed the extra cost of a slower-speed machine with the $100 four-channel head, particularly taking into account the extra electronics for filtering, FM modulation and detection, etc. There is a further question as to whether a bandwidth of 7.5 kHz for two of the channels is adequate.

Hum at One Speed

Q. I own an Allied TD-1030 Tape Deck. Some time ago, I accidentally unplugged one of the output cables while the machine was running. At 3 ¾ ips, the speed at which the machine was operating when the accident happened, there is now a loud hum superimposed on the recorded signal for the channel affected. However, at 7 1/2 ips there is no such hum. My obvious question is what would cause hum to be present at one speed but not at another?

-John Streby, Flint, Mich.

A. The only explanation that occurs to me is coincidence. Something may have occurred which is unrelated to the unplugging of the cable except that it happened at the same time. For example, a defect may have occurred in the switching mechanism that comes into play when changing speeds. An open switch terminal could pick up hum. As most servicemen can tell you, strange and baffling coincidences are not uncommon.

(Audio magazine, Dec. 1972; Herman Burstein)

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