polarity of C27 in offset circuit

[psupine]

Hi all,

I'm a newbie with a typical kind of newbie question - I have to start somewhere I guess I searched the forums but couldn't find it asked before.

In the RevD1 HH schematic, C27 is part of a low pass network nulling the offset current into the first gain stage (unless I have missed it entirely). The voltage divider centred on R18 is symmetric across the rails, so it looks like it allows the centre tap to be pulled either side of ground. How then, do we know the correct polarity to install C27?

I've only lurked here for a few days so far. This is a fantastic site and a credit to the people who get involved. I expect to feel like a newbie for some time.

[mschmahl]

look here on silkscreen

http://www.geotech1.com/forums/attac...1&d=1298991057

[psupine]

Thanks for your reply, but I didn't ask the question I really meant.

I know the "+" is marked on the silkscreen, and it's clear on the schematic too, and I'm not saying it's wrong either. My question is why is it that can we use that polarised cap, when the circuit can put the +ve leg below the -ve leg? Why don't we need a non-polar cap?

Is it because the voltage when the R18 pot is set correctly, is so close to GND that it doesn't matter?