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back in the saddle again......

Posted by vinnie2 (A) on August 10, 2015 at 17:13:59

Ok. I stripped the 26 preamp all the way down and started over again. I used a separate breadboard for each channel, the only thing they share is the B+ supply. Both channels are set up per the attached schem. I went through all the various suggestions I got and I have settled on Palustris's operating points for now. I had to make do with the parts I had but will change those out later. I think these will prove to be very good operating points as per deafbykhorns analysis.
Now we get to the interesting part...... I have the 26 pre feeding into my 91a clone. Both the 26 and the 91a have volume controls on them. When I have both volume controls on the 91a open about half way and bring up the sound with the volume controls on the 26, it starts making funny noises like buzzing or silibence or some sort of distortion, just like before. I turned one 26 volume control all the way down thinking that would cut the sound off completely, but it didn't. So I turned the right channel volume on the 91a all the way off. Well all of a sudden the left channel is playing beautifully at any volume, nice and clear with no distortion of any sort. I then proceeded to switch channels and got the same result. As long as I turn the volume all the way down on one 91A channel, the other channel is fine.
What the heck would cause this? It doesn't happen with my 76 preamp when it is hooked up to the 91a. I am really puzzled.
On the good side, if I can get this straightened out I think the 26 is going to be a really great preamp.
Hoping someone has some ideas what the problem might be.

Edits: 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/10/15 08/11/15 08/11/15

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