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coupling capacitor.

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Gaston

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in the isd1100 spec sheet it recomends a coupling cap between the output and the input of the amp. my question is, will this filter out any of the frequncies? i know a high pass filter uses a cap and a resistor. and being that this doesn't have a resistor i don't know if it will act as a filter. my next question is, how do you calculate the size of the capacitor?
 
As a first approximation you compute the capacitative reactance over the frequency range of interest.
Code:
Xc = 1 / (2*pi*f*C)
What you want is a reactance that is small with respect to the input impedance of the next stage over the frequency range of interest.
 
so Xc = the input impedance of the amp? and would you use the highest frequency you plan on passing or the lowest?

oh i see, Xc needs to be small in comparison to the input impedance.
 
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The frequency where the Xc equals the input impedance is the cutoff frequency. The level at that frequency is attenuated by 0.707 times which is 3dB, half power, a small drop. For a highpass filter, lower frequencies are attenuated more at the rate of 6dB (half the level) per octave. The response is flat at about 5 times the cutoff frequency.
 
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