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previous: Ben N My bad--I didn't mean Rk, I meant t... -- 1091079529 View Thread

Re: Separate NFB for BF/SF channels?

7/29/2004 11:32 AM
Wild Bill
Re: Separate NFB for BF/SF channels?
Ben, you've got a good idea but you should keep in mind that the nfb is fed from a voltage divider - the 820R and the 100R from the PI input to ground. If you use a drastically different value in place of the 820R you may not have any nfb at all if the 100R is not changed.  
The ratio usually is about 10:1, like 1k:100R or the Marshall 47K:4K7, which gives the same ratio in nfb voltage of the speaker tap level and what goes into the PI input.  
One trick I sometimes use is take that centre-off ground reverse switch that's doing nuthin' since you went to a 3-wire power cord and bring the speaker nfb wire to the centre tab. Then you take the 820R and maybe a 1K6 from either tab and put their ends in common and run it back to the nfb input at the PI.  
What this does is give you 3 positions of nfb - stock, none and "half". This allows some interesting tonal variation. You can play with the 1K6 for taste - one jazz/blues player liked a 470R to increase the nfb and make the amp even more damped.  
---Wild Bill

Don Symes Is that a setup you could switch 'h... -- 7/29/2004 5:35 PM