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ARSCLIST  April 2007

ARSCLIST April 2007

Subject:

Re: RIAA phono EQ and Neumann time constant

From:

Eric Jacobs <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Mon, 2 Apr 2007 18:18:28 -0700

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (86 lines)

Goran,

Thank you very much for providing this detailed information.

Do you have an approximate date for the SAL74 & 74B versions?
I'm wondering if the 50 kHz filter has always been 12dB/octave,
or if it may have been 6dB/Octave in some earlier versions.

At this point I'm trying to carefully document what is known.

Thanks again for this excellent contribution!

Eric


-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]]On Behalf Of Goran Finnberg
Sent: Monday, April 02, 2007 4:07 PM
To: [log in to unmask]
Subject: Re: [ARSCLIST] RIAA phono EQ and Neumann time constant


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 ”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
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 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.

--
Best,

Goran Finnberg
The Mastering Room AB
Goteborg
Sweden

E-mail: [log in to unmask]

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

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