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

[Mattias Rickardsson]

Are they necessary now? That's not what Matthew said as I understood it. :-)

And about high input impedance - it can invite unwanted signals in the
cables because of the low currents involved, so people often seem to prefer
a moderate input impedance somewhere between 10k and 100k.

/mr
Den 6 feb. 2016 11:04 fm skrev "Tom Wiltshire" <tom at electricdruid.net>:

> 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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>
>
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