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Original Message
RE: Determining the size of inter-stage coupling cap
Posted by Tre' on January 23, 2017 at 08:59:22:
A little more,
the "AC impedance" (known as reactance) of the choke falls with frequency. A 1000Hy choke has a reactance of 6.3 meg ohms at 1000Hz but only 125k ohms at 20Hz.
The other issue is this, when you have a cap feeding a choke you have a 'tank circuit' and that LC tank circuit will have a low frequency resonance.
Depending on the DC resistance of the choke, to keep the resonance frequency below the audio band (and low in amplitude) it takes a rather large coupling cap (3uf to 10uf just as a ballpark number).
The BCP-16 GC has 3850 ohms of resistance and that's not enough to allow a "normal" sized coupling cap (.1uf to 1uf just as a ballpark number).
When the coupling cap is too small you can get an audible increase in the level of the low frequencies. Some use this to "tune" a system.
Tre'