Manley TAPE HEAD PREAMPLIFIERS - Manual (Page 11)

There is a fair amount of confusion amongst engineers regarding record and repro eq
curves. The most important fact is that all repro heads have a frequency response
characteristic that rises at 6 dB per octave. The prime EQ curve of any head preamp is
the inverse of that rising response. A preamp “first” has to have a characteristic that
falls in a straight line at 6 dB per octave. This translates to over 50 dB more gain at 20
Hz than at 20 kHz (AES 30 IPS). This also “shapes” tape noise, distortion and preamp
noise.
Where it gets confusing is that different curves exist for different speeds and that there are
separate NAB and CCIR curves. Each specifies a variation on top of that 6 dB falling
line. In other words it makes that sloping line into a sloping curve. The numbers we see
associated with each standard, ie 17.5uS, 3180 & 50uS, 35 uS, refer to these where
these deviations from the straight line occur or ,basically, at what frequency. This part
of the curve is first applied one way to the record electronics and then the inverse added
to that 6 dB falling repro curve. All to help get the signal on and off moving rusty
mylar.
So what is the difference between 30 IPS and 15 IPS curves or CCIR curves? Lets start
with the NAB (USA) standard 30 IPS repro. It looks closest to that downward sloping
straight line but near the bottom at around 5 Khz it slightly straightens out.
15 IPS and 7.5 IPS NAB curves are the same curve. In the lows it starts as a shallow slope,
gets steeper around 100 Hz (that basic 6 dB per octave) and almost flattens out starting
around 3 kHz. The result is “only” 33 dB more gain at 20 Hz than 20 kHz. If one were
to only consider the variation from 6 dB per octave you would see an 8 dB cut at 20
and about 1dB cut at 100. The top end is boosted - 2 dB at 2K and 15 dB at 20K.
The European CCIR curves are the same for 15 and 30 IPS. They are pretty close to the
high freq boost of 15 IPS NAB but starting at a slightly higher frequency (35 uS instead
of 50 uS) and only boosting about +13 dB at 20K. Like 30 IPS NAB, the low
frequencies are not modified. You should use the 15 IPS setting on the preamp to be in
the best range for the HF EQ. While not “perfect” theoretically, once you align, the
error from the standard will be a fraction of a dB or less than machine to machine or
head variations.
CCIR 7.5 is similar to 15 / 30 but the HF boost begins an octave lower (70uS)
Once again, the 15 setting (50 uS) is close enough, after aligning, to be within a fraction of
a dB.
NAB 1.875 and 3.75 are the same curve and similar to 7.5 / 15 but the HF boost begins
around 500 Hz and ends with +20 dB at 20K. This is probably out of range of the
preamp but you woundn’t need this kind of a preamp for that kind of a tape.
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EQ