Low Impedance "side" of caps for dual Input PI

[ChrisM]

Looking at this Marshall 18W schematic
http://www.18watt.com/storage/18w%20Schematic.gif

We see that both inputs of the PI are being used. Each input of the PI has a coupling cap and before that a 500K pot.

Now the input of the PI has an input impedance of ~1M (the two 470K grid leads summed) I believe.

I believe the 500K pots provide an impedance of 500K prior to the cap (the output impedance of the preamps).

Now that being said how should those coupling caps be orientated? I think the outside foil should be on the 500K pot side. Correct?

[katopan]

Yes the cap outer foil should face the volume pot. But due to the balanced nature of the PI tail those 470k resistors are effectively bootstrapped in a fashion that makes the PI input impedance much higher than 1M.