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NFB - PI Question

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7/19/2003 5:03 PM

NFB - PI Question

I'm working on an amp that has two channels and in order to use the channels together without phase cancelling each other, I have to come into both coupling caps in the PI - just like a DC30. I have noticed that I get less bass and gain when I use this type of setup. If I just use one channel into a traditional Marshall style PI, everything sounds great for that one channel but obviously I can't use two channels without phase cancelling the signal.

Obviously there are two solutions.

1 - remove one gain stage from the second channel and that will bring the two signals together correctly (using 2x 220k mixer resistors)

2 - Use the DC30 PI and lose the tone.

I'm thinking I will just remove one gain stage (I'm running two triodes in parallel, so I think I can remove this and be in good shape).

Also I want NFB on the dirty channel but not on the clean channel and I've yet to be able to find a solution for this. I tried Bruce's recommendation but I did not hear any effect. If any of you have any ideas for what I'm trying to achieve, please let me know.

Thanks for all your help.

7/19/2003 5:35 PM
Bruce /Mission Amps	http://www.firebottle.com/fireforum/fireBB.cgi?cfg=ga&forum=gadc&read=250238-250090.msg

7/19/2003 5:45 PM
Scroggins	Thanks Bruce!

7/21/2003 12:07 AM
jbrew73	are you sure that your signals are out of phase? you said one stage was a parallel triode. that would make the output the same as if it was a single triode. add up your stages again maybe you made a mistake. just a guess from the info provided

7/21/2003 12:30 AM
Scroggins	Here are the stages. Channel One: EF86 (1 gain stage) Channel Two: Two Triodes in Parallel - One Triode - One Triode (3 maybe 4 gain stages depending on if paralleled triode counts as an additional gain stage)

7/21/2003 12:57 AM
jbrew73	looks to me they are the same phase and should be connected to the same side of the PI with two mixing resistors. someone please correct me if i'm wrong.

7/21/2003 1:28 AM

quote:
"if paralleled triode counts as an additional gain stage"

It doesn't. It won't invert the signal, which is what you need to know.

So the EF86 channel has one gain stage and the other has three gain stages. They should be mostly in phase.

I say "mostly" because the two additional blocking caps in the 3-stage channel are going to add in some phase shift compared to the EF86 channel.

Shea

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