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

Input Matching

Q. I have a transistor tape deck with a microphone input impedance of 3,000 ohms, and the manual says a microphone with impedance between 2,500 and 5,000 ohms should be used. However, I have not been able to find a mike with impedance in this range. If I use a low impedance microphone with a step-up transformer, can I expect as good quality as if I had a low impedance input in my tape recorder?

-Grant C. Downey, Sheppard AFB, Texas.

A. You do have a problem, inasmuch as low impedance mikes are typically in the range of 50 to 600 ohms, while high-impedance ones are typically in the range of 10,000 to 25,000 ohms.

Therefore it appears that you will have to use a low-impedance mike with a step-up transformer. Moderately priced microphones of good quality and low impedance are available, and a suitable step-up transformer of good quality will not impair its performance.

Visual Volume Level

Q. I have a six foot stack of LP's which I plan to put on tape. I feel I need a piece of equipment that optically registers the volume level. Could you recommend such a piece of equipment?

-Edmund L. Vincent, Warwick, R.I.

A. The record-level indicator of your tape recorder should do the job, that is, enable you to compensate for different volume levels of different records.

Reducing Hiss

Q. I am primarily concerned with recording live music on my Ampex PR-10 tape machine. I am experiencing a slight hissing noise in the background of my tapes. I have tried different types of recording tape and have settled for Scotch 201. I have aligned the bias for this tape and keep the heads demagnetized.

The recorder is a stereo model, and both channels exhibit the noise. I am almost sure the noise originates in the preamps, as I can turn the power amplifier up in the idle state and hear what sounds like "shot" noise in transistors. Although the noise is not a serious problem, I would like to do all I can to eliminate it as I hope to use my Ampex for master tape recording. I estimate my S/N ratio to be about 52 to 55 db, and I would like to improve this to about 58 to 60 db.

Could I change the 12AX7 tubes in the preamps to ECC83, 12AY7, or 7025 types? How about changing the resistors in the preamps to low-noise types? I have tried using Scotch 120 (high output) tape, but noticed a slight loss of high frequencies, although the increased S/N sounds great.

-William O. Stottlemyer; Trezevant, Tenn.

A. It is not clear from your query whether the hiss originates predominantly in recording or playback. Sometimes hiss in recording is due to excessive attenuation of the incoming signal (to avoid overloading the first amplification stage). In any case, I am sort of doubtful that changing the 12AX7 for an equivalent type of tube will produce significant improvement. One might suspect an individual tube to depart from the average for its type in terms of noise, but this seems not to be your problem inasmuch as you have the same hiss on both channels. However, I only doubt; I am not sure; changing to an ECC83 or 7025 might yield a tiny improvement. Better yet, if possible, you might try using selected tubes of one of the types you have mentioned; this means having a handful of tubes from which to choose. I am more inclined to think that using high quality low-noise resistors will produce desired results.

Deposited metal-film resistors are worth trying. Use these both in the plate and cathode circuits of the first stage. Sometimes the use of low-noise resistors in the second stage may also improve matters.

Balanced and Unbalanced Lines

Q. I would like to know the principal of balanced lines as used with mikes. To what should the second wire be connected if not to ground?

-Harvey W. Kunz, Old Tappan, N.J.

A. In balanced line circuits, the microphone feeds the primary winding of a transformer. Neither end of this winding is grounded. You can see that if the two leads from the microphone are connected to such a transformer, the signal will still see a complete path even though the primary is not grounded. This causes a voltage to be developed across this winding when the mike is driven. This voltage is then induced into the transformer's secondary winding and the signal is on its way to the audio circuits.

What I have described is the typical balanced line circuit. It will produce less hum than the conventional unbalanced system. This is reasonable when you consider that any hum voltage which is induced into the cable will be induced into both conductors.

The direction of the induced hum voltage will be the same for both conductors. In order that a signal can appear across the primary of the input transformer, there must be a difference in polarity between the two hum voltages, just as is produced by the microphone. Thus, while hum voltage is present, it is not heard to any extent because it is cancelled out.

The chances are that is no other precautions are taken, some hum will be present. The cable is, therefore, shielded, with one end of the shield connected to the microphone case. The other end is grounded to the mike preamplifier. The shield does not carry signal. It acts to minimize the amount of hum voltage induced into the cable.

The unbalanced line has only one conductor. Any humable to penetrate the shield will not be cancelled in the manner of the balanced line system. The hum will appear as such in the output of the recorder, public address system or what have you.

The unbalanced line is what you are likely to be most familiar with; it is used to interconnect program sources in high fidelity music systems. The shield actually carries the signal.

Mono on Four-Track Machines

Q. I have several hundred 78 rpm records and would like to record them on tape. Would I be able to record four single tracks on a stereo tape machine? Would you advise the 33 1/3 ips speed or 7 1/2 ips?

-William C. Day, Springfield, Ohio

A. Most quarter-track stereo tape machines permit you to record four mono tracks. With a reasonably good quality tape recorder, the 3 3/4 ips speed should be adequate for the purpose you have in mind. In fact, the reduced treble response at 3 3/4 ips compared with 7 1/2 ips may be advantageous in cutting noise.

(Audio magazine, Aug. 1972; Herman Burstein)

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