I stand corrected, Tiziano. When the X-mod bass pot in a PAS-3X is centered, C5 appears to be shorted, but C7 is in parallel with the 1.0uf cap. The key, I think, is that the pitchfork wiper in the x-mod bass pot shorts the three terminals of the pot when centered. If you envision the action of the pitchfork wiper, you can see that C5 is not only shorted when the pot is centered, but C5 is also shorted for one half of the pot’s rotation.
Your formula for C-out total might be heading in the right direction, but I think it might depend on the position of the bass and treble pot wipers at any given moment(?)
But, if you are talking about taking a minimalist approach to bypassing the tone controls, then, IMHO, I have always thought it would make sense to emulate the condition of the circuit when the two X-mod pots are centered – but without the parts. In particular, I suggest that the output from C4 (0.2uf) would go only to:
- the parallel combination of the 47K resistor and the 33pf capacitor
- the 1.0uf cap (to block DC from V1 pin 3) which then goes to the 62K resistor and then the 510K resistor if you keep it
I believe the method Roy describes will indeed bypass the tone controls, but, in my mind, it leaves a question of the necessity of C5 (considering in the original design C5 could be shorted-out much of the time). I think there might also be a question of the effect of C5 on the feedback parameters with the rest of the tone control circuit removed - I believe C5 was originally intended to only work as part of the bass control circuit.
With regard to the advantages of the 1.0uf cap, the PAS is known to be sensitive to loading on its output. I base my opinion on the content of the original Dynaco manuals. The original manuals state that the PAS-2X/3X is compatible without modification with amplifiers having a Z-in as low as 100K, where the non-X-mod PAS only works best with amplifiers having a Z-in of 250K with modification and 470K without modification. The 1.0uf cap is one of the features that distinguish an X-mod from a non-X-mod PAS.
Here’s an interesting thread that has some relation to the discussion: http://www.audioasylum.com/forums/dynaco/messages/1/14568.html
Of course, I say this in all deference to Roy or anyone else who disagrees. I could be wrong about all of this, but this is how I have understood it, FWIW. If anyone would care to explain it differently in detail, I’d be only happy to listen.
Last edited by PeterCapo on Thu Feb 07, 2013 2:41 pm; edited 2 times in total