Tape Guide (Nov. 1978)

Bias Adjustment

Q. I have read several times about adjusting the bias on a tape deck to the specific recording tape being used.

I would like to' know what the term bias means in this context and how one goes about adjusting the bias on the machine being used.

- James Leone, Rochester, N.Y.

A. Bias is a high frequency current, usually in the range of 60 to 150 kHz which is fed to the record head along with the audio signal, or is sometimes fed to a separate (crossfield) head mounted opposite the record head.

The purpose of bias is to reduce distortion and maximize the level of the signal recorded on the tape. All better tape machines contain a control, often internally, for either increasing or decreasing bias.

The usual procedure specified by the manufacturer is to record and play back a tone--typically specified as 500 Hz or 1 or 2 kHz--then adjust the bias of the tape until the output, as measured in playback, is at maximum.

Some manufacturers advise a further increase in bias until output at the specified frequency drops ' dB. For further information, you should consult the service manual put out by your deck's manufacturer.

Cassette Buzzing

Q. I have a pronounced buzzing or humming in the left channel of my cassette deck which occurs only in re cording because prerecorded cassettes do not produce this noise. If the left channel control is turned down, the noise disappears. The noise doesn't appear to be a 60 Hz-hum as the pitch is too high.

- F. Keuchmann; Maywood, Ill.

A. It may be that you are picking up a harmonic of the 60-Hz line frequency emanating from either the power transformer or the motor. You might check the leads to these components to see if they come very close to the left channel record amplifier. Also, your problem might lie in a poor switch or jack contact, so clean these switches and jacks with a suitable cleaning fluid.

Since turning down the volume control eliminates the buzz, this indicates that the problem occurs prior to this control, hence you should also check the circuitry in the early stages of the record amplifier.

Response Perception

Q. Which is a better frequency response: 30-22,000 Hz, ±3 dB, or 50 16,000 Hz, ±3 DB?

- Walter Mattox, Jr., Atlanta, Ga.

A. On paper, 30-22,000 Hz within 3 dB certainly looks better than 50 16,000 Hz. But to the human ear, except for rare cases, I doubt that the difference really matters. In order to extend response to 22k, a tape deck had to make sacrifices in terms of signal to-noise ratio and/or distortion. In other words, in settling for slightly more limited response, the 50-16,000 Hz deck may give better all around performance.

Recording Hiss

Q. I seem to have a problem when recording. There is a hiss as loud as the music which occurs at all tape speeds. What is wrong?

- Jerry Ubels; New West Minster, B.C., Canada.

A. It seems that you have either or both of the following problems: 1) Noisy components such as resistors or transistors in the record and/or play back amplifier, 2) A mis-calibrated record-level indicator so that you are recording at too low a level, thereby accentuating the level of tape and amplifier noise relative to the desired audio signal.

You can tell whether your problem is essentially in recording or playback by playing either a prerecorded tape or one made on another machine. If the hiss is much lower with such tape, then your problem lies in recording. If the hiss remains the same, then the difficulty is in the playback amplifier.

Tape Changes

Q. What does one have to do to one's tape recorder in order to use chromium dioxide tapes?

- John Reeds, W. Lafayette, Ind.

A. To use chromium dioxide tapes in a tape deck set for conventional tape (ferric oxide), changes are needed in bias, treble equalization in the record amplifier, and in the amount of signal fed to the record head.

Specifically, these are about a 50 percent increase in bias current, a reduction in the treble boost, and more cur rent has to be supplied to the erase head, as the CrO2 tape is more difficult to erase.

Distorted Recorder

Q. I wonder why the recordings on my tape deck come out distorted. I set the VU meter level so it doesn't go over the red distortion line?

-Buzz Hammond, Mt. Vernon, Oh.

A. Possibly the VU meter is mis-calibrated so that it doesn't read 0 VU until the signal level is more than enough to cause tape distortion. Another possibility is that insufficient bias current is being fed to the record head … if the treble response seems over-bright, this indicates that bias is too low. Another possibility is distortion in the re cording and/or playback amplifiers. If tapes recorded on another machine sound clean in playback on yours then the problem is in recording rather than playback.

Question of Length

Q. Please indicate the pros and cons of tape lengths ranging from 1200 to 3600 feet on a seven-inch reel?

- Walt Mattox, Jr., Atlanta, Ga.

A. It is generally not advisable to buy more than 1800 feet of tape on a seven-inch reel. Therefore, one should buy either the 1 1/2-mil tape (1200 feet) or 1-mil tape (1800 feet). A thinner tape tends to raise problems with squeal and print-through.

(Source: Audio magazine, Nov. 1978; by Herman Burstein)

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