AC coupling questions

[Mark Amundson]

media at mail1.nai.net wrote:
> As you may know, I'm working on the capacitor FAQ....  
> On this subject the FAQ says, "Use 0.47uf metallized polyester or
> polyproplyene.  Values less than this will attenuate bass frequencies."
> Mind you it makes no mention of impedances.  Regardless, a .47uF
> polypropylene cap is the size of a bumblebee.
> 
> Is there a capacitor value that is typically used for this in modulars??
> Is there a simple formula that one could use to calculate the best value??
> Further, is blocking DC always necessary??  I'm guessing typical coltrol
> voltages wouldn't hurt modules with op-amp imputs. MARK

Mark, I think Don's and Eric's advise was good. I did kind of bit my lip
when I contributed information to the FAQ and later read that universal
capacitor remark.Don's equation of:

    C = 1 / (2 * pi * f * Z)  where Z equals the impedance desired at a
certain                                       frequency

is a good reference equation. Note: remember impedance is resistance and
reactance lumped together.

The impedance for the synth input becomes the guessing game. Usually
impedances in series between the input jack and the inverting (-) input
of the op-amp set the total input impedance. Shunt Impedance between the
common non-inverting (+) input of the op-amp (also the input jack) also
would set the input impedance. The question is does the signal reach the
inverting or non-inverting inputs of the op-amp first?

Once the input impedance is judged, make an assumption for your lowest
highpass -6dB (half voltage) frequency. I usually assume 20 Hz and use
the equation to arrive at some capacitance value. Then I take that value
and choose the next higher standard value. When designing circuit with
several highpass values, you may have choose even lower frequencies to
keep the net -6dB cutoff frequency at a desired value. All those dB's
add up!

For example: 10k-ohm impedance computes to a -6dB value of .795uf at
20Hz. Choose a 1uf value or a 2.2uf value if several are used in the
signal chain.

Mark Amundson,