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RE: PP Amp Input Stage

Posted by KevinC (M) on October 19, 2015 at 16:34:44

In Reply to: PP Amp Input Stage posted by dhieber on October 18, 2015 at 11:56:55:

Thanks for your kind words about the ST-70 upgrade. I, too, think that it sounds pretty good, but the new input stage sounds better.

I made a comparison between resistor plate loads and current source plate loads and the gain in clarity with the current source loading was compelling. The frequency response was better in the high frequencies with the current sources, but that difference was well about 20KHz. This choice was sonically motivated.

I had a "free" negative voltage supply in the ST-70 using the original fixed bias voltage tap, but, in general, most tube amp power transformers lack that extra winding, so I settled for a "short tail" on the new input stage. Even so, it's an easy matter to add a negative supply, a simple current source, and a couple of 100K resistors to get a fully differential input stage.


Kevin Carter
K&K Audio
www.kandkaudio.com

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