> When a disc cut on a Neumann lathe is played back on an RIAA phono
> preamp, the frequencies between 10 kHz and 20 kHz are more rolled
> off than they should be. Discs cut on a Neumann lathe will play
> -0.2 dB at 10.9 kHz and -0.64 dB at 20 kHz. If your average phono
> preamp is -0.5 dB at 20 kHz, that means that you could be off by as
> much as -1 dB or more.
I have the schematic of the Neumann SAL74 & 74 B version which were the
last before the penultimate copper cutter.
The input stage stage contains a two pole Sallen & Key active filter set
to 50 kHz cutoff - 3 dB at 12 dB/Octave.
This filter is down no more than 0.1 dB at 20 kHz.
After that there is a further active stage with a 6 dB/octave roll off
at 482 kHz - 3dB.
The 75 ”S time constant is calculated using the actual Neumann used R
and C to around 75.004 ”S.
The stopper resistor limits the rise to around 1.65 ”S or - 3 dB at 96
The total error at 20 Khz is around -0.15 dB.
This is the actual frequency response errors of the cutter electronics
up to the moving coil cutter itself and its 600 W power amplifier.
Now we have a moving coil driver that actuates the cutting needle which
have a hugh mechanical resonance in the midrange which is being
flattened by feedback from a feedback coil on the cutting head.
The feedback is user adjustable to provide as flat reponse as possible.
I doubt weŽll see better than +- 0,5 dB in normal working practice from
this source alone.
> The Neumann lathes have a fourth time constant of 3.18
> microseconds (50 kHz turnover) which was introduced around
> 1968 to increase cutter life.
This is nowhere to be seen in the SAL74 & 74 B electronics.
I really wonder were all the myths originate?
Forget the whole thing its a dead alley going nowhere.
The Mastering Room AB
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Learn from the mistakes of others, you can never live long enough to
make them all yourself. - John Luther