AC coupling questions

[Mark Amundson]

Mark Amundson wrote:
> 
> 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,