AC-Coupled module inputs

[jh]

>I was pondering something last night about module design philosophy. Why
>would you put ac-coupling caps on module inputs, or have them be >switchable?
>Wouldn't it make more sense to have all inputs uncoupled, and just make sure
>that any module that is supposed to produce or process audio ac signals
>doesn't have a dc offset on it's output?
>
>..and for external sounds have a separate ac-coupled input mixer /
>amplifier...
>
>- Comments?
>
>- CList

I do this where ever I can, but there are some arguments against it, too.

(1) Offset currents. 
Balancing the input of a VCA (OTA) or Ringmod  will slightly depend on
the source impedance of your input signal. You can add input buffers
with low offset type opamps, of course, but there are also arguments to
not add more amps than necessary (see below).

(2) The special sound of single ended circuits and /or circuits with low
amount of negative feedback.
Many people agree (and I have joined them during the years on this
mailing list) that our standard opamp circuits aren't always the best choice
for high level signals as they occur in synths. They so such nasty things
when overdriven, for instance.
You can build much better (sound-wise, that is) circuits without feedback
loops over several transistor stages. A little bit of local feedback in each stage
will give enough stability to compensate for component tolerance and
to make sure the circuit works as intended, but its hard, if not impossible, to
get zero DC offset over a chain of modules with such circuits.

(3) There are circuits that will introduce considerable amount of varying
offset voltage (unless you go for ultra expensive components), such as
VCFs. Ok, I take that partly back, there are VCFs that suffer from this effect
less than others, but aren't these the "boring" ones at the same time ?

JH.