Manley TAPE HEAD PREAMPLIFIERS - Manual (Page 12)

MORE EQ
We have these variable trims on all tape machine repro cards to further modify those “standard”
curves to align a tape machine to another machine’s reality. Different manufacturers have
different ways of accomplishing this. Some vary the repro characteristic curves by making some
of the components involved variable. Others have fixed standard curves and add a basic high and
low shelving EQ stage after the first network. The former is simpler (and better sounding) while
the latter gives a great deal of EQ range.
We use three different techniques. The VHF adjusts the impedance that the repro head is looking
into. This quite effectively varies the extreme hi frequency response of the head itself and as a
side benefit that allows different heads to be used with one preamp without component changes.
We adjust the values of the components in the 6 dB falling network to adjust HF and LF and 15/
30 IPS selection. We found that adding an extra stage just to vary EQ to be counter to purist
philosophy in this design. Less is more. All of this is in the negative feedback network that is part
of the initial preamp stage. This ensures that the signal is kept as intact as possible. For the VLF
we use a simple equalizer without adding a stage or passing the signal thru extra circuitry. Here
again we modify, slightly, a negative feedback network. This time it is in the line driver section.
By keeping this trim in a different stage from the main network the interaction is minimized. The
“turn-over” frequency of this “EQ” could be considered to be 40 Hz. Each step is 2 dB at 20 Hz
or 1 dB at 50 Hz and has no real effect at 100 Hz. The LF control begins its “knee” around 300
Hz but in EQ terms would be considered 150 Hz with progessively more effect at lower
frequencies. At 100 Hz the range is admittedly a small 5 dB but at 20 Hz it is about 20 dB.The
HF “knee” is at 2K and would be considered a 5 Khz EQ (3 dB point) with progressively more
effect with higher frequencies (12 dB @ 20K). At 10 kHz the range is typically 3 dB below the
AES curve to 7 dB above it. The VHF control has a tiny effect at 10 kHz but mostly controls 15
kHz and higher.
The above paragraph describes the 30 IPS setting. With the 15 IPS setting the highs are not cut as
much or effectively boosted to follow the standard NAB 15 and 7.5 IPS curve. The range of the
HF control is the same. The low frequency cut of the NAB curve is at the same frequency as the
LF control and well within its range.
In our own studios we usually record (and playback) to CCIR standards because we prefer the
increase in HF headroom and not for that few dB more tape hiss. We have no problem with the
preamps HF EQ point being slightly lower than the 35uS standard. Compared to new and vintage
machines these preamps reproduce “flatter” than the theoretically perfect 35uS cards. Part of
this, is being able to fine tune the extreme highs and lows. Other reasons can be attributed to real
world heads both in record and repro and thier slight imperfections and “head bumps” and
probably some variation in component tolerance (10% caps) and component aging. However if
you are a European customer and still not convinced we can either show you the curves or
change a few caps.
EQ being exhausted, don’t forget that azimuth, dirty heads, tension adjustments, worn heads, bad
record alignments (especially bias) and old tapes “kill” frequency response and are the most
likely culprits to mess up your day.
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