# Beginner's capacitor question

Dennis Boone drb at msu.edu
Tue Apr 15 16:06:46 CDT 2008

``` > I found some old directions on hooking up a PC speaker to a line input,
> and was confused by the use of a capacitor -- I would have thought that
> a resistor would have been more appropriate, to limit the signal
> perhaps?  In any case, here are the instructions, followed by my
> question:

> Parts required:
> - 6' to 12' shielded cable with RCA plug (male) on one end
> - Two alligator clips
> - One 4.7 uf capacitor

Others have pointed out that an electrical connection isn't going to
achieve what you want, but some background on the capacitor:

It will, of course, block DC.  If I'm not mistaken, mathematically, it's
a high pass filter, with a half-power (3dB down) cutoff frequency of
1/(2*pi*R*C), where R is the impedance of the load (speaker, mixer,
whatever) and C is the value of the capacitor in Farads.  You would have
to work out the impedance, select the desired 3db point, and turn the
crank to calculate the needed cap value.  You would want to keep in mind
that the graph of the filter effect has a shoulder, so that response
won't be flat toward the low end of the passband.  Their capacitor value
would make sense for an 8 ohm speaker and a 4 Hz cutoff.  My
recollection is that the basic interpreter didn't let you go below about
16 Hz, so that would give you two octaves of "footroom" if you will.

Since the signal is apparently 0-5V, based on other posts, you would
need to pad it down, but you'd probably want to try to more or less
match the impedance the driver circuit expects.  That sort of thing is
accomplished with a resistor voltage divider.  Line inputs are usually 1
V peak-to-peak.

Again, I think others have this right: put the machine in one room,
extend the speaker into a quiet recording space, mic it.  Or just mic
machine and all, since its noise was part of the original experience. ;)

De

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