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

Deck Specs

Q. I am planning to buy a Sony tape deck but have some apprehension because its signal-to-noise ratio is specified as only 50 dB. I would appreciate any information you can supply.

-B. Greenberg, Manlius, N.Y.

A. If the S/N of a tape recorder is 50 dB based on 1 percent harmonic distortion, this implies that the ratio is in the range of 56-58 dB at 3 percent harmonic distortion, which is quite good. I don't know whether the Sony's 50 dB spec is based on 1 or 3 percent distortion. If on 1 percent, then it would appear to be a good performer in terms of S/N.

Re-biasing

Q. (1) I have two Sony two-head tape recorders. Using low-noise tape, the treble seems overly bright. I've been told that two-head machines are very difficult to re-bias. Can they be re-biased? If so, how? (2) What is cupped tape? (3) If a 7-inch reel of 1-1/2 mil tape holds 1,200 feet, wouldn't the double-play tape be 3/4 mil rather than 1/2 mil?

-David Rowland, Geneva, N.Y.

A. (1) Two-head machines can be re-biased, although the procedure is more tedious than in the case of a 3-head machine. Using the latter, the tape can be monitored as it is recorded, so that the result of bias (and other) adjustments can be immediately checked. With a 2-head machine, it is necessary to change bias, record audio signals, rewind the tape, and check the result of the bias change in playback. If the result is not satisfactory (flat response being desired), one has to go through the foregoing procedure several times until it is accomplished. When shifting from conventional to low-noise tape, an increase in bias is required; otherwise treble is exaggerated.

(2) Cupped tape has a curl in the long direction. If laid on a flat surface, the tape does not lie flat but is U-shaped.

(3) The terms 1-1/2 mil, 1 mil, and 1/2 mil as applied to tape refer to the thickness of the base of the tape. Adding the thickness of the magnetic coating, the so-called 1-1/2 mil tape has a total thickness of about 2 mils, while the total thickness of so-called 1/2 mil tape is about 1 mil.

Tape Hiss

Q. I have an Ampex tape deck and have the following three problems:

(1) When the machine is on but the tape is stationary and the tape monitor switch is depressed, there is a noticeable hissing sound. The hiss in creases when the tape is moving and decreases somewhat when the tape equalization switch is set to 7-1/2 ips rather than 3-3/4 ips. How can this noise be eliminated? (2) When the tape is running, the deck makes a sound much like a fan, which is quite distracting and drowns out low volume passages of music. (3) When I record using microphones, the play back sound has very heavy bass.

-Garry Ballek, Chicago, Ill.

A. (1) It appears that the playback hiss is due to the playback amplifier in the tape deck. If the tape hiss, which comes up when the tape is moving, appreciably increases the total hiss, this suggests that the electronic hiss is at a reasonably low level and will be swamped by the audio signal and tape hiss. If the total hiss in playback is substantial, this suggests that you are recording at too low a level or playing back at an exaggeratedly loud level.

To reduce electronic hiss would re quire redesign and/or use of a noise-reduction unit such as the Dolby B.

Perhaps some improvement can be achieved by introducing low noise resistors in the first stage of the playback amplifier. (2) Consult Ampex about a suitable method of reducing mechanical noise. Perhaps seating the deck on a soft pad might help (but be careful not to block any air passage required beneath the deck in order to afford ventilation). (3) The heavy bass response may be due to the particular microphones you are using, or the way you use them. The proximity effect of bass accentuation occurs if you are too close to the mikes. Perhaps you don't have the mikes directly facing you, so that they don't pick up enough high frequencies. There may be a fault in your method of connecting the mikes to the deck; a long length of cable when using high impedance microphones will attenuate treble, causing a bassy effect. There may be a fault in the input circuit of your tape deck.

What Bias Frequency?

Q. A question on Tandberg biasing. I understand that bias frequency should be set to about five times the highest audio frequency to be reproduced. Using a cross-field head, the Tandberg 3000X and 5000X decks can reproduce to about 26 kHz, but the bias is only about 85 KHz. Why wasn't a higher bias used? Also, won't the FM multiplex carrier beat with the bias?

-Jeffrey Ahl, Ithaca, N.Y.

A. There is very little in the way of audio signal above 15 kHz. So, as a practical matter, Tandberg has probably found that bias frequency raises problems of loss of bias current due to head inductance and winding capacitance. Also, with increasing frequency, bias assumes the properties of a radio wave and gets into places where it isn't wanted. As for the bias beating with the FM multiplex carrier, this is taken care of by filtering out the carrier.

Demagnetizer Warning

Q. My head demagnetizer carries a warning that the demagnetizing poles should not come into contact with the surface to be demagnetized. Is this warning solely due to the damage possible by physical contact or does touching the surface to be demagnetized negate the demagnetization? Can a head demagnetizer be used to degauss a color TV?

-Ronald Slakie, Tacoma, Wash.

A. The warning has in mind physical damage. The usual head de magnetizer is too weak for color TV.

(Audio magazine, Herman Burstein)

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