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jimmy audio 16th April 2011 06:38 PM

low pass filter issues
I built 3 stk amps 2 for mains and 1 for sub amp also built a low pass filter. I am splitting the input signal between the main amps and the sub amp problem is with everything hooked up the sub amp seems to have low output when used alone output is fine but when i plug in the main amps output drops. am i loading down the op amp in the low pass filter or what?any suggestions? ps main amps seem unaffected.

Shoog 16th April 2011 07:47 PM

Is there a buffer on the input to the LPF ?


jimmy audio 16th April 2011 11:52 PM

no 2 39k resistors combine the input signals. circuit is a standard unity gain sellen key filter using a tlo82 (had a few handy)figured a opa2134 would be overkill.not sure whats happening!

Shoog 17th April 2011 09:27 AM

I think you need a unity gain buffer on the input.

Br Cornelius

AndrewT 17th April 2011 01:06 PM

the low pass filter to the Bass only amp/speaker can be designed to sum the LF of both channels and act as a buffer and not load down the stereo signal and not interfere with the stereo crosstalk.

Use an MFB filter with a pair of summing resistors on it's Inverting Input.
All your problems go away. Just one opamp or discrete opamp and the right circuit wrapped around it is all you need.

This filter can have as many summing channels on the input as you need.
A 5channel surround sound system could have 5 summing resistors and the MFB would sum the bass signal from all 5 channels and send that filtered signal to the bass amp and speaker.

jimmy audio 18th April 2011 02:36 AM

Thanks for the input guys sounds like exactly what I need anyone have a appropriate circuit or link to one.

AndrewT 18th April 2011 08:57 AM

look up MFB filter.

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