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

[Tom Wiltshire]

I agree with Matthew. Input caps are necessary, but they don't usually have to be electrolytic. In anything with a reasonable input impedance, you can use film caps of a few hundred nFs and have plenty of low-end. So I don't see a need to use electrolytics for DC blocking.

Tom

On 6 Feb 2016, at 09:37, mskala at ansuz.sooke.bc.ca wrote:

> 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/
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