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

[rsdio at audiobanshee.com]

On Feb 6, 2016, at 1:45 AM, Gordonjcp <gordonjcp at gjcp.net> wrote:
> On Sat, Feb 06, 2016 at 03:37:06AM -0600, 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.
>> 
> 
> This is pretty much it.
> 
>> 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.
>> 
> 
> If you're sticking sufficient voltage and current backwards into the electrolytic capacitor to damage it, you're likely to zap other things as well.  Electrolytics are nowhere near as fragile as people think they are.

Electrolytic capacitors may be sturdy enough to survive extended reverse voltage, but the signal passing through will be distorted.

I recently worked on a high-end microphone preamp circuit which had polarized capacitors on the inputs. These were electrolytic, but the problem was due to the polarization, not the chemistry per se. Standard THD tests failed on these preamps during the prototype stage, and the problem was tracked down to the polarized caps. I'm not sure exactly what the solution was, but it did require a redesign to get THD into acceptable levels.

Brian