TAPE GUIDE (Dec. 1991)

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Disabling the Erase Head

Q. Tapes that I bulk-erase are quieter than those erased by my deck's erase head. Also, they don't have the low-level buzzing noise that I occasionally hear. Would a switch for the erase head's current allow me to turn it off and get a few more dB of signal-to-noise ratio when I use bulk-erased tapes? Is there a good reason why manufacturers don't include this feature?

-William Shipman; Fredericksburg, Va.

A. Yes, disabling the erase head--by switching off the oscillator current going to it-can give you a somewhat quieter recording than if the erase head is active. However, if the heavy load presented by the erase head is removed from the bias oscillator, the amount of bias current going to the record head is likely to rise. Therefore, if you plan to erase only by means of a bulk eraser, it becomes necessary to readjust the bias current going to the record head.

On the other hand, you may want the option of using either the bulk eraser or the erase head. For example, if you want to erase only the tracks on side A of a cassette, you must use the erase head; a bulk eraser necessarily erases all tracks, from both sides. In this situation, you would require a substitute load and a switching arrangement that diverts the erase current to the substitute load when the erase head is cut out. The substitute load would have to present the same impedance to the oscillator as does the erase head.

Altogether, you face a technical problem of at least moderate difficulty, although far from an insuperable one.

If I recall correctly, many years ago there was an open-reel deck which incorporated the feature you desire, using an actual erase head as the substitute load. However, this feature added significantly to cost and helps explain why we don't see it, particularly since there seems to be little demand for it in the market.

Effectiveness of Play Trim

Q. A few cassette decks include a Play Trim feature to correct for azimuth mismatch between the tape and the playback head. How effective is Play Trim? Is there a difference between two-head and three-head decks with respect to its effectiveness?

-Anthony Hudaverdi, Santa Monica, Calif.

A. Play Trim can correct for modest treble losses due to azimuth mismatch and other factors; in the range from 10 to 20 kHz, it can provide about 3 to 6 dB of treble boost.

Losses due to imperfect treble equalization in recording are apt to be quite minor, relatively speaking.

Losses due to excessive bias tend to be larger but not all that great unless the deck manufacturer or the user has not taken due care with respect to proper bias. So treble loss due to these two factors can be handled quite effectively by Play Trim in most cases, but losses due to azimuth misalignment can be quite profound even if the misalignment is quite small. To take an example, assume misalignment of only 10' (1/6 of a degree, which is too small to be discerned by eye). At 10 kHz, the resulting loss is 2.1 dB, which is within Play Trim's ability to correct. But at 20 kHz, the loss is 10.2 dB, which is well outside Play Trim's ability to correct completely.

I see no reason why Play Trim's effectiveness should be different between two-head and three-head decks. However, the chance of azimuth misalignment is greater for a three-head deck. Obviously, misalignment cannot occur if one records and plays with the same two-head deck.

But with a three-head deck, owing to use or misadventure, the record and play heads can become misaligned with respect to each other.

Radio Interference in Tape Decks

Q. I have a problem with background noise on tapes recorded with my deck. Specifically, I am picking up a radio station. This can't be bleeding from a tuner because I don't have one hooked up to my system. Do you have a solution to this? I have heard of a component from Adcom that is supposed to clean up a.c. power, but I don't know if this will help.

-David Cooper, Rochester, N.Y.

A. Radio frequency interference (r.f.i.) can come into your deck in various ways. One is through the power cord of the deck or the input cables to the deck; another is through the power line. Perhaps the radio station is being picked up by the first stage of the deck's own record electronics, or the preamp stage of your amplifier may be doing so, particularly if you are using its phono stage, which has high gain and is therefore sensitive to extraneous signals. You can probably figure out where the interference is originating by disconnecting components that feed into the deck.

Dressing (re-arranging the path of) power cords might help if r.f.i. is picked up by one of them. Reversing a plug's position in the wall outlet is also worth trying. Dressing cables leading to the deck--or the cables between your signal source and amplifier--may help. If r.f.i. is entering the deck directly, re-orient the deck and/or place it somewhere else.

An a.c.-line interference filter, avail able at audio dealers and elsewhere, may be useful. Line conditioners made by Adcom, Tice, and other companies are basically intended to suppress the effects of spikes, hash, surges, and other anomalies in the power line. Such devices are not cheap, and I think that the chance of one solving your problem is small. Therefore, if you do decide to try one, I suggest buying it on a trial basis.

(Source: Audio magazine, Dec. 1991, HERMAN BURSTEIN)

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