ANALOG TAPE 101, Bias Magic

ANALOG TAPE 101
Part Three, Bias Magic

by Eddie Ciletti

Welcome to the final part of this series on analog tape recorder maintenance. Wrapping up each section has been a bit like making yogurt or sourdough bread: There’s always a bit left over to start another batch of tips. This time, bias is scrutinized in greater detail because it is the most critical parameter and, second only to input level calibration, the most inconsistently implemented on semipro (narrow format) machines.

Bias Magic
Bias is like an electronic bribe to the tape’s magnetic particles—without it, analog tape would sound like a digital children’s toy. It is basically a minimum distortion adjustment—sort of like tuning an old-fashioned AM radio; too little bias makes the sound bright on top and fuzzy on the bottom. Excessive bias results in dull and dirty recordings. Bias is a Radio Frequency (RF) signal—anywhere from 60 kHz (for cassette decks) to 432 kHz (for the Ampex ATR-100 Series)—that is mixed with the audio signal. As with digital sample rates, the higher the bias frequency, the better (for lower high-frequency distortion).

The correct amount of bias current is determined by the size of the gap in the Record head. The “complications” are tape oxide type, tape speed and the type of distortion being minimized. Magnetic tape does not have the same sensitivity at all frequencies, but when adjusting bias current, high-frequency output is most affected, hence its use in achieving the most precise adjustment.

If you followed the first two parts of the series, by now you should have aligned repro and/or sync (Playback) with the alignment tape. Now, put up a blank tape, set the oscillator to 10 kHz and press Record on all tracks. On a three-head machine, if the high-frequency tones waver more than 1 dB, do not pass Go! Don’t even waste your time until the problem is solved, either by thorough scrutiny of the tape path or by having the heads relapped. Remember that many narrow-format machines have only two heads—Erase and Record/Sync—so when you see “PAR,” it means “Playback After Record,” or PITA (Pain in the Ass).

The adjustments, in order, will be:

Bias (using 5 kHz, 10 kHz or 20 kHz)
Record Level (1 kHz)
HF EQ (level): 10 kHz and 15 kHz
Low Frequency Adjustment: Bass Sweep from 250 Hz down…

Note: On a three-head deck, record bass sweep while monitoring via Playback head. Align the low-frequency EQ until the last peak and dip fall on equal sides of “0 VU,” then select a low frequency that falls on “0 VU.” Print that tone on tape and note it on the box. Include the bass sweep if the tape becomes a mix master.

On a two-head deck, record bass sweep, “PAR,” noting the peaks and dips, then select a low frequency that falls on “0 VU.” Print that tone (on tape) and note it on the box.

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