Thread: Valve preamp.
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Old 24th Jun 2019, 1:15 pm   #66
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Re: Valve preamp.

Hi Andy,
Further to my previous post and in answer to your question as to what I think of Fred's design.
The basic principles are sound, the use of an operational amplifier with the RIAA filter in the feedback is standard practice.
The use of valves to form an operational amplifier using a differential amplifier input followed by a common cathode amplifier is fine.
Where I have issues is with the compromises needed to cope with a lack of a negative supply. I set out the issues in a previous post but in summary:

1 Poor common mode rejection and the use of un-balanced anode resistors in the differential amplifier
2 Having to have the grids of the differential amp at +27V and the dc blocking capacitors needed in consequence.
3 the need to use neons as voltage dropping elements for the dc feedback loop.

Restoring the negative supply and removing the compromises makes it much better circuit.
I attach a revised schematic without the neon / zener element in the feedback.
If you do build it and find the anode voltages of the differential amplifier are un-balanced adjust the value of either the 560K (R4 in my Schematic) or the 270K (R2 in my schematic) resistors. Or change the 560K to a resistor plus pot and tweak for balance.

I also suggest adding the final 47K resistor and 100pF capacitor (R116 and C108 in the original circuit) to ensure HF stability.

Let me know how you get on if you build it.

Peter
Attached Files
File Type: pdf F Nachbaur Valve Pre-amp Schematic 2.pdf (41.6 KB, 137 views)
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