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In Reply to: RE: RH84 test amp: driver stage wiring change = lower THD% posted by deathtube 667 on September 15, 2016 at 16:46:39
Is this the circuit you're using?
Why would both triodes of the 12at7 be wired together?
Shouldn't there be a plate resistor (one for each triode) between the B+ and the plates?
Doesn't each plate get a FB resistor, each from it's own channels el84 plate?
I guess I'm confused.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
My circuit is based on RH84 but not exactly the same.
I posted the schematic in an earlier thread. Here it is again.
Each 12AT7 triode section has a plate resistor which was connected with a short piece of wire, then a single wire going to B+.
I cut the connection wire and used separate wires from each plate resistor to the B+ node.
Each channel has its own feedback resistor.
OK, now I understand what you meant.Shouldn't each driver stage have it's own drop resistor (labeled 1K) and decoupling cap (labeled 50uf)?
On second thought maybe not. The drop resistor is so small there wouldn't be much isolation anyway.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/15/16
I should have noted on the schematic that I used a common PS for both channels.
There is the 1K R dropper and 50uF cap that provides driver stage B+ for both channels.
Sorry for the confusion, this circuit is still a work in progress.
My guess is what you are measuring is actually less crosstalk. Crosstalk could manifest it's self as distortion on your analyzer.This is not a case of doubling wires in milliamp circuits making improvements. What you are now doing is providing a separate voltage drop circuit for each stage rather than sharing it.
That has some merit as you have measured.
Edits: 09/15/16
I wondered about crosstalk as well.
Thanks for the comments.
It could also be less noise with your modification as well.
Hard to say without seeing and working with your layout.
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