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: Theres
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 is like an electronic bribe to the tapes magnetic particleswithout
it, analog tape would sound like a digital childrens toy. It is
basically a minimum distortion adjustmentsort 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) signalanywhere 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! Dont 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
headsErase and Record/Syncso 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
- Record Level (1
- HF EQ (level):
10 kHz and
- 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.
to Page 2
with permission from
Magazine, October, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved