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Re: Torroids as audio

Posted by Paul Barker on February 5, 2002 at 05:39:43

In Reply to: Re: Torroids as audio posted by Scholl on February 5, 2002 at 04:45:43:

Hi, the ratio is a little worse than 2 : 1 on the input. I paralleled the primary.

My preamp is Nuance by Famco. It has a little gain, but not much. The combinatuion I settled for of gain is right for my application. Were I in need of gain I would use a 6sl7 voltage amp and 6sn7 driver, that would be plenty.

Of interest I am achieving in the real world a gain of 43 and 5 out of my 6em7.

This is a far cry from Lynn's stuff, it is deliberately budget conscious, I have not allowed myself any excesses, therefore no PP chokes for lc coupling that I'd otherwise have used, or better still interstage transformers. The intermediate stages are difficult to inductively couple on the cheap.

I would like to actively load the two intermediate stages, but again I am about simplicity and cheapness for the time being while I explore the bounds of possibility within a budget.

There is no need for an HT transformer, but I have isolated the HT with a pair of 240 to 30v transformers placed back to back, and for good measure I balanced the power within them. Because they were surplus torroids I had to wind a common mode choke for the filtering between the two isolating transformers, had they been E.I. I could have avoided the choke. The amp (largely due to this filtered supply) is absolutely silent, you could hear a pin drop.

You have to decide where to stop. I can see progress in employing this input stage and driver topology in about any push pull amp. What isn't flexible is the torroidal output. This is the achiles heel of the whole set up. Purchase a good pair of push pull outputs and this becomes a very versatile arrangement.

Were I building an amp of this nature from scratch with no holds barred I would simply build Lynn's, and leave all this hard development work in his capable hands.

But this cheap amp has earned a full build job. I am off to punch holes tonight.

Regards

Paul Barker

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