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Coupling cap question

Hi fellas!

I'm having a little problem with my pre-amp (seen on the photos thread): it lacks bass.
the input cap is a 22nF and the output one is 68nF(or is it 680? it appears marked .68...)
Could i ommit them or if not, what value should they be?

Thank you very much,



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The input as it is in the diagram has a -3db point at 20hz, this is too high in my opinion, tripple size of the cap and and the -3db point will be at 5hz.

The output cap depends on the amplifiers input impeadance, lets say bad case senario of a 10k input power amp, the the output cap will have a -3db point at 234hz really bad.
If you do'nt have a power amp hooked up the output cap sees the 330k resistor only then the -3db point will be 7hz which is good but as soon as you hook up a power amp the 330k changes to what ever the the parralel of the 330k and the power amps input impeadace is and the bad case that i calculated was the 10k this is not acceptable.

So the cap for 10k will have to be 4.7uf which will be good for nearly 100% of the poweramps out there.

Cheers George


2004-06-21 8:04 am
big smoke
Another point often neglected is that the output impedance rises dramatically with small caps. In georgehifi's example it's literally 10 kohm at 234 Hz. Un- or poorly shielded interconnects need not apply. I found very small caps resulted in increased susceptibility to RF interference even when used as input caps in a shielded chassis.
4,7uF on the input and 10+4,7uF on the output.
Had it connected today in my car's subwoofer amp (low-pass at around 200Hz) playing a compiled CD with 5 to 400 Hz sine wave tracks, and it goes down to 10Hz (well it plays the 5Hz signal, but 10Hz seems to have a bit more "strenght")!
Too bad it hums a bit, but it's only audible very close to the speaker ...

Thank you!!!!:D