AC coupling and things

Tony Allgood oakley at
Sun May 10 19:55:56 CEST 1998

Hi folks,

Some days ago there was a post about ac coupling an input into a synth's
internal VCF. There then followed various comments about the value of the
capacitor to use. However, there was some confusion how the cut-off
frequency affected the signal. Perhaps, this was just me... but decibels are
confusing things. So I checked it again....

The cut-off frequency is the -3dB point, not -6dB. This corresponds to an
input/output voltage ratio of about 0.7. This gives the half POWER point.
ie, the output pd produces half the power in a fixed load compared to what
it did during the pass band. That is, if the input was 10V rms, the output
of said RC network would be 7.07V rms at the cut-off frequency. The phase
shift is 45deg at the -3dB point.

I think... :-)

Tony Allgood, Cumbria, UK

e-mail: oakley at

My latest project; a rack mounted VCF module with warmth. PCB available

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