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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

RC filter component selection
RC filter component selection
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Old 9th November 2017, 12:49 PM   #61
Mark Whitney is online now Mark Whitney  Netherlands
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Quote:
Originally Posted by robinlawrie View Post
for example if i added a 500nF capacitor in parallel with that 100pf one ( easier than swapping, its a 603 smt component) i should (if my studying has paid off) get a lowpass of 3.18khz there, instead of the current 15mhz one.
A problem with using 100 Ohm / 500nF is the low impedance at higher frequencies. This could be a problem for the opamp connected as the source.
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Old 9th November 2017, 03:21 PM   #62
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by Mark Whitney View Post
A problem with using 100 Ohm / 500nF is the low impedance at higher frequencies. This could be a problem for the opamp connected as the source.
Yeah i thought that might be the case.

I assume adding a resistor in series with the capacitor to raise impedance at the high frequencies, without affecting impedance at lower ones is a "thing" probably well known and with a name.

I assume it would reduce effectiveness of filter, and depend on ratio between resistor chosen, and input impedance of next stage of amp...?
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Old 10th November 2017, 06:14 AM   #63
PRR is offline PRR  United States
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Quote:
Originally Posted by robinlawrie View Post
for example if i added a 500nF capacitor in parallel with that 100pf one......
I say try it.

Cap loading is not an issue here.
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