DC Blocking Cap use

[burningman]

I have read through some of the posts on DC blocking caps and can't seem to find answers to a couple of questions I still have.
The first being: are there conditions under which it is not necessary or even desirable to omit DC blocking caps on the input and output of a circuit?
Is there a reason why some circuits show a metal film cap on the input as opposed to a electrolytic capacitors? Would it be at all related to capacitance values and also noise?
Thanks.

[R.G.]

The first being: are there conditions under which it is not necessary or even desirable to omit DC blocking caps on the input and output of a circuit?

In general, it is never desirable to omit DC blocking caps on the input and output of the circuit, except in special circumstances. The special circumstances include:
- circuit is powered by bipolar (i.e. both positive and negative power supplies, with ground)
AND
- circuit has to have DC amplification for some reason (and there are a few of these; again, special conditions)
OR
- circuit has to have AC frequency response or input impedance so low that capacitors, even electrolytic caps, are impractically large and/or expensive
OR
- circuit has to squeeze every last penny out and someone figured out a way to get away without DC blocking

These notes are for audio effects circuits. There are other situations (like inside integrated circuits) where it's impractical to fabricate a capacitor, so you come up with special ways to do without.

Is there a reason why some circuits show a metal film cap on the input as opposed to a electrolytic capacitors? Would it be at all related to capacitance values and also noise?

Yes, yes, and no. Yes, there is a reason; in general, never use an electrolytic if you have a feasible and affordable alternative. Electros have generally poorer performance as capacitors than other types, as well as having to be specially inserted because of polarization. Electros are what you do when you can't use a film or ceramic (and therefore nonpolarized) cap. Electros leak more, and wear out. Yes, it's related to capacitance values, because the capacitor size needed for low input impedance circuits is sometimes bigger than you can afford or fit; then you use an electro. No, it's not related to noise.

[burningman]

Thanks again R.G. - best regards.