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

Microphone Overload

I would like to comment on the letter from John D. Moss concerning microphone overloading (col. 2, page 16, October, 1970). The question of whether a particular microphone will overload the inputs of a tape recorder or amplifier depends upon the output level of the microphone and the gain of the microphone preamplifier. In the case of the AKG C-451 microphone, which I assume is the one which Mr. Moss is interested in, the manufacturer states that the output level is -39 dB referred to 1 mw/10 dynes/cm^2. This reference level corresponds to a level of 94 dB SPL at the microphone's diaphragm.

Most microphone preamplifiers made in the United States for use with dynamic microphones feature a first-stage gain of from 40 to 60 dB. It can be seen that a preamp with 60 dB of amplification will produce an output from the AKG microphone of +21 dBm for 94 dB SPL. Even those amplifiers capable of such an output level would be operating dangerously close to severe distortion. And when this microphone is used in a close-microphoning situation where sound levels can exceed 120 dB, the preamp would need to be able to deliver an output of better than +47 dBm. I know of no equipment capable of withstanding anything approaching such a level.

There are two solutions to the problem. One is to use a preamplifier with lower gain, say 40 dB. This would in all probability be sufficient for any situation short of very close pickup of very loud instruments. The other possibility is to reduce the output level of the microphone with a resistive pad While the in-line pad might seem to be a simpler and more flexible answer to the overload problem, it is undesirable from the standpoint of its effect on the system's signal-to-noise ratio. The thermal noise level for a 20 kHz bandwidth is approximately -131 dBm, so that a perfect amplifier providing 60 dB of gain would have a signal-to-noise ratio of -71 dBm. Typical amplifier circuits will add from 4 to 12 or more dB of noise, so that the signal-to-noise ratio can turn out to be only -59 dBm. Since a resistance pad in the output of the microphone reduces the level, for example 10 dB, it also has the effect of reducing the possible signal-to-noise ratio by the same amount.

It would be much more desirable to take advantage of the microphone's full output by using a lower-gain preamplifier. A preamp with only 40 dB gain, when fed from a high-output microphone, is capable of a noise ratio of up to -87 dBm (for a circuit adding 4 dB of noise itself). Microphones with exceptionally high output levels, such as the AKG and Neumann models, can often make use of input preamps with as little as 20 dB gain. The advantages for the signal-to-noise ratio in such a situation are obvious.

So in answer to Mr. Moss, I would say that if the input stage of his preamp provides more than about 40 dB of gain ahead of the volume control, he is headed for trouble unless he pads down the output of this microphone.

The exact amount of loss required in the pad would depend upon the maximum sound pressure levels he expects to encounter. The loss should be kept as low as possible so as not to degrade the signal-to-noise ratio any more than necessary.

I have been concerned with the problem of noise vs. maximum signal level for some time.

I am currently using a mixing console with 40 dB microphone amplifiers.... I am hoping in the near future to be able to make use of a new console using operational amplifiers with variable gain over a range of 20 to 45 dB and a lower equivalent input-noise level than the present system. This console, in combination with the Dolby A system and the new higher output Scotch #206 tape, should produce a signal-to-noise ratio that would have seemed impossible only a short time ago.

-Charles B. Dawes, Evanston, Ill.

Reel Conversion

Q. Can you tell me if it is possible to convert a 7" reel tape recorder to a 10 1/2" one? I realize that in most cases the conversion could not be made within the limits of the tape recorder deck, but if a 10 1/2 " reel is to be accommodated, could it not be done by using the existing reel pin as a drive? It appears that if a pulley, mounted on the existing tape drive pin, were used to drive another pulley of exactly the same size on a reel pin secured in an area outside the tape deck, then a 10.5" reel could be driven by the machine. The 10 1/2" reels would have to be in the same plane as the existing reels.

With a 1:1 pulley drive there would not be any changes in drive speeds, and all functions would remain equal, including fast wind/rewind. I don't believe that any permanent change need be made to the tape recorder. The reel pins outside the deck would be ball bearing mounted to prevent any undue load. I know that there are 10.5” reel machines, but they are out of reach of my pocketbook, and the solution I propose may solve my problem in recording classical music.

(T. L. Fick, Philadelphia, Pa.)

A. I can offer only slight comment on your proposal for an "outrigger" arrangement, inasmuch as the niceties of good tape drives are rather outside my sphere.

The chief trouble you may encounter, I believe, would be in maintaining low wow and flutter. A pulley drive between reel pins would not necessarily prevent wow and flutter from intruding. Also, there is the question of torque of the feed and takeup reels. While the torques may be satisfactory for the 7" reels, I am not sure they would be so for the 10.5" reels on a 1:1 pulley ratio. Some commercial tape machines have attempted outrigger devices such as you propose, but I understand that these have not solved all problems.

Playback Equalization

Q. I am faced with the problem of incorrect playback equalization when playing directly from the tape head into my external preamp. I would like to modify the turnover and rolloff controls of this preamp to conform with the RIAA standard tape playback equalization curves for 7.5 and 3.75 ips. I have enclosed' the equalization curves and circuits for my preamp. Since the only equalization I use, other than tape, is RIAA for records, I would like to modify any of the other equalization positions for tape playback, one for 7.5 ips and one for 3.75 ips. The preamp manual states that for best sound in most cases, when using the tape-head position, one should set the turnover (low-end) and rolloff (high-end) switches to the tape and flat positions. I assume that this meets the playback standard for 7.5 ips.

(Major Russell W. Kemp, APO San Francisco )

A. It is difficult to advise on specific values of resistors and/or capacitors for equalization of a tape playback-head signal, because in part the equalization must allow for departure of the head from "ideal" performance. Typically, playback heads tend to exhibit some high-frequency loss, and some departure in the bass region-usually somewhat exaggerated response. The 7 1/2-ips playback curve of your preamp comes very close to "ideal." For an "ideal" playback head, a playback curve containing a total of 36 dB bass boost is required between about 10 and 20,000 Hz. The curve you sent me shows boost approaching a total of about 34 dB. Hence you are only about 2 dB away from correct equalization at 7.5-ips for an "ideal" head. Allowing for the fact that your playback head may show some bass emphasis at very low frequencies, you may be still closer to correct equalization. If desired, you could use your bass tone control to supply a slight extra amount of bass boost in playback.

At 3.75-ips, "ideal" playback equalization would involve about 31 dB of bass boost. If you combine your RIAA playback curve at the low end (turnover) with a flat setting at the high end (rolloff), you will again find yourself within about 2 dB of correct equalization for an "ideal" head.

Your alternative course is to experiment with capacitor values similar to those already in the turnover circuit to achieve still more exact equalization. Then you would have to use a standard test tape, and measure playback response.

Low Treble Level

Q. My problem occurs when I am recording a long-playing record from my changer, through my preamp, and into the tape deck. I record at 7.5 ips, and I find that when I play the tape back through my system the recording is very bassy and boomy. I have to turn the loudness switch off and roll the bass controls of each channel almost completely off and turn the treble controls almost all the way up.

Also, does it make any difference to the recording whether I have the preamp tape-input switch on tape or on input? Also, I wish to record my 45-rpm records. Can I do this at 3.75 ips, or should I use 7.5 ips? Lastly, I wish to record some old 78 rpm records. How can I do this (equalization, and so on)?

( Stephen J. Newhouse, New Hyde Park, N.Y.)

A. Absence of adequate treble may be due to excessive bias current, azimuth misalignment of the record and playback heads with respect to each other, a worn and therefore widened gap in the playback head, faulty equalization in recording and perhaps in playback, collection of tape oxide, dirt, or other matter on the playback head, and an excessively long cable between the tape machine output and the input of the following amplifier.

Another possibility is that you are feeding an already equalized playback signal (containing lots of bass boost) into the tape-head input of your preamp, instead of into the tape machine input of your preamp. The tape head input supplies a great deal of bass boost and is intended for a signal direct from the playback head.

It should not matter whether your tape-input switch is in one position or the other.

For best quality, record your 45's at 7.5 ips.

Recording old 78's satisfactorily is a matter of equalization supplied by your preamp and is not a question involving your tape machine. Play around with the phono equalization and tone control settings of your preamp until things sound right. You might try recording at 3% ips rather than 7% ips as a means of reducing noise without appreciable affect on the desired audio signal.

Excessive Noise Level

Q. I own a quality tape deck, and have been bothered by a high level of tape noise (hiss) during quiet passages of music. I listen to classical music which has been recorded from phono discs. If I play a prerecorded tape, the noise level is quite acceptable.

From this I conclude that the problem occurs when recording a tape. If a virgin reel of tape is put on the deck and the volume of the amplifier driving the speakers is set so that the noise level is audible when the tape is playing, the following is observed with no signal fed into the deck: (1) an audible increase in the noise level after recording a section with the record level at its lowest point, but with erase and bias still applied; (2) a marked increase in the noise level above that in Point 1 when the level control is at its maximum position; but not noticeable increase at 2/3 of maximum. I measured the bias frequency to be about 50 KHz, and the waveform appeared to be sinusoidal, but I did not measure the distortion of the waveform. I would like to know: (1)

Is this the normal bias frequency for this type deck? (2) If it is, do you think the cause of the noise is the low bias frequency? (3) If I replace the bias oscillator will the record head in this tape desk function properly at a bias frequency of about 80 KHz? Will the erase head work at that frequency?

-Dale Simonich, Champaign, Illinois.

A. A bias frequency of 50 KHz seems low for a high quality tape recorder. A rule of thumb is that the bias frequency should be at least 5 times that of the highest audio frequency to be recorded. Hence quality machines ordinarily use bias frequencies of about 75 KHz and upward.

I don't think that the cause of noise is the bias frequency. Rather, I think it is the bias waveform, i.e. distortion in the waveform. Slight amounts of bias distortion which are invisible to the eye on an oscilloscope are nevertheless sufficient to cause appreciable noise. Careful design and high quality parts go into an oscillator circuit that will produce minimum distortion and therefore noise. Nevertheless, even the best tape machines add some noise during recording owing to imperfection of the bias waveform.

If you change the bias frequency, chances are that you will have to increase the amount of bias supplied to the record head, because the impedance of the head rises with frequency. Similarly for the erase head.

(On the other hand, there is some chance that for one head or the other you might have to decrease the amount of bias if it happens that the inductance of the head and the capacitance in series with the head are resonant at the bias frequency.)

Bias Application

Q. I am a little confused by something I read about setting bias. I interpreted this to say that bias is set in the playback mode. I had always considered that bias was only used in the record mode. Is bias actually used in playback also?

(G. McWherter, Harlingen, Texas )

A. Bias is used only in recording. Bias may be indirectly measured in the course of playback, by adjusting bias while recording until maximum playback output is obtained at a frequency such as 500 or 1000 Hz. (It is further customary to increase bias until playback output drops about 3 dB, thereby making frequency response less subject to moderate variations in bias.)

(Audio magazine, Jan. 1971; Herman Burstein)

= = = =