[sdiy] DC blocking caps on inputs - or not?

[mskala at ansuz.sooke.bc.ca]

On Sat, 6 Feb 2016, Mattias Rickardsson wrote:
> Another capacitor question popped up:
> What's your view on having electrolytic caps right at the input of an audio
> device in order to block DC and/or protect the circuit?

I think it's not so much a question of "protection," as of allowing you to
choose the DC bias point of the first stage.  If you don't block DC, then
anything you connect to that input is going to affect the bias in a
hard-to-predict way.

> Does a cap really add any protection here? The DC issue can be taken care of
> later in the signal chain, and avoiding input caps saves cost, space, and

In *some* circuits - some op amp circuits in particular - you may be free
to remove the DC offset later.  In others, like the typical
discrete-transistor amplifier, that'll be difficult or impossible and you
really need to get rid of DC right from the start.

I'd avoid electrolytic caps in this application because they don't
tolerate reverse voltage, and so it'd become necessary to protect the cap
from that, one way or another.  Not so much an issue of signal quality.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/