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previous: Gil Ayan q{Gil:  &... -- 7/12/2000 5:08 PM View Thread

Re: Trip + Presence + Why Dumble's Way?

7/12/2000 6:10 PM
Ray Ivers
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Re: Trip + Presence + Why Dumble's Way?
Gil,  
 
Sounds like you had a blast!  
 
About the 'scaling ratio' of the Dumble circuit; the actual effective Marshall pot value is roughly 4K to 5K, Gil, not 22K. The older Marshalls used the 'scratchy' circuit with the 5K pot passing the PI DC to ground, while the newer ones use a 22K variable resistance/cap series combination in parallel with a 4.7K resistor, to eliminate the DC thru the pot. So the Dumble circuit is using a lot more feedback, and the frequency breakpoint is much lower, than the Marshall 100K/5K/.1 uF circuit.  
 
I think the scaling used depends a lot on the choice of the resistance from the PI 'tail' to ground (whether pot, resistor, or combination of both), which affects PI bias conditions. I would think that you would set your PI static conditions and then adjust feedback loop values depending on desired presence control action/damping factor/etc. I would personally prefer to keep feedback loop resistances as low as possible, to minimize the chance of oscillation in the output stage.  
 
Ray

 
Replies:
Gil Ayan q{Gil,Sounds like y... -- 7/12/2000 8:50 PM