Flat Phono Preamp Based on John's P10 and 2SK389

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Gold
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Mon Jul 01, 2019 2:34 pm

I have about a dozen of these PV26b amps. It occurred to me these might make a good front end for the PTS. I thought I'd post a schematic and see if I'm totally off base or close.


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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Jul 01, 2019 2:45 pm

Paul -

What an amazing find! If I read the date correctly it's a 1972-era fully-balanced flat phono preamp with a current-feedback "Demrow/Cohen" topology.
Was this module used as a feedback amplifier input?

Common mode rejection is realized in the output transformer.

If you can send me jpg scans and I'll stitch them together.

If its quiet it should make a nice front end for the EQ board.

Thank you!

EDIT: Looks like you re-attached jpgs - even better.
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Gold
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Mon Jul 01, 2019 2:55 pm

Wayne,

It's just a flat amplifier with an available 26dB of gain. It was used as a generic amplifier in Neumann consoles of the era. There is a PV15, PV26 and PV46. They are not all the same circuit but they are all from the same design era.

I actually thought it had an unbalanced front end from memory and thought it might make a good unbalanced flat front end amplifier to feed the PTS base station. When I looked at it I realized it had a differential front end and thought it may work as a balanced front end like your preamplifier PCB's. The PV15 has an unbalanced input and the PV46 has a transformers balanced input. I know the front end would need a few values changed if it would work at all for an MM cart.

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Jul 01, 2019 3:03 pm

It's pretty much a transformer-based instrumentation amp.

I could see it used as a mic preamp, mix amp, cutterhead feedback amp or flat phono front end.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Mon Jul 01, 2019 3:22 pm

mediatechnology wrote:
Mon Jul 01, 2019 3:03 pm
It's pretty much a transformer-based instrumentation amp.

I could see it used as a mic preamp, mix amp, cutterhead feedback amp or flat phono front end.
To use as a flat phono front end for an MM cart would you change C1 and C2 to be 23k5Ω resistors?

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Jul 01, 2019 4:37 pm

Keep C1 and C2 in circuit for AC-coupling.
The bases are at +10V DC.

As it's drawn the differential input impedance is 54KΩ||50pF formed by R3+R4 and C4 in series with C5.
It's almost perfect for MM (with some added C) as-is.
That's why I saw it as a phono preamp.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Mon Jul 01, 2019 4:41 pm

Ah, cool. Thanks!

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Jul 01, 2019 4:53 pm

Image
Neumann PV26B Schematic

Large format: https://proaudiodesignforum.com/images/ ... ematic.jpg
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Tue Jul 02, 2019 11:28 am

Here is the PV46SD line amp. This is my favorite era of Neumann design. The next era using the OA10 opamp doesn't sound as good and neither does the later IC based designs. The SP66 console I have uses them. The signal flow of the SP66 is PEV EQ-RV66 Attenuators-EE66-HT66 passive 12dB/oct HPF/LPF-PV46-Fader. All Neumann mastering consoles were offered in a rack mount or a console version. I used to have a console version with a Telefunken M10A transport. That used Neumann designed playback electronics. It used PV26's as line drivers for the tape machine. I now have a rack mount version with no tape machine.

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