Re: [ARSCLIST] RIAA phono EQ and Neumann time constant

[Goran Finnberg <mastering@xxxxxxxxx>]

Eric Jacobs:

> 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 µS time constant is calculated using the actual Neumann used R
and C to around 75.004 µS.

The stopper resistor limits the rise to around 1.65 µS or - 3 dB at 96
kHz.

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 we´ll 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.

-- 
Best,

Goran Finnberg
The Mastering Room AB
Goteborg
Sweden

E-mail: mastering@xxxxxxxxx

Learn from the mistakes of others, you can never live long enough to
make them all yourself.    -   John Luther