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Effects of Source to Ground adjustment

Posted by jyourison (A) on March 30, 2015 at 14:32:43

In Reply to: RE: 6Y6GT/6922 Link to larger schematics and PDF file posted by Eli Duttman on March 30, 2015 at 13:05:43:

Eli (and Tom),

Here's what I experienced on the scope in adjusting the AC balance pot. I was using dual inputs to show both sides of the splitter. The images are for 1 KHz at 2Vrms with 10 V per division I believe (please pardon the dust!):

1. At equal resistances, one side of the sinus form had a flat spot. That disappeared upon raising the overall resistance of the Source to Ground leg slightly.

2. Further increase in the lower leg "evened out" the top and bottom of the sinus form, so that their amplitudes matched (well, as much as I could get it to match).

So I thought that I was reducing second order HD by making the form symmetrical. Is that the correct idea? It did require slightly mismatching the resistances.

There is still a minor mismatch in amplitude of top and bottom, but note that the scope had drifted slightly in vertical position, so it's a little better than the reticle grid shows.

What is causing the flat spot in the first image with equal resistances on both sides of the splitter? Is bias insufficient in the 6922? Peak voltage at 2Vrms probably exceeds my 6922 measured bias at idle. Or is this a cutoff at the MOSFET? Or a sag? If this flat spot symptom could be corrected at the cause, would I be able to match the resistances and not have much distortion?

Thanks,

Jeff

Edits: 03/30/15 03/30/15

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