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Old 18th June 2011, 07:35 PM   #1
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Default Bridged amp lowpass question

I'm building an LM4781 Biamp for active speakers. My crossover uses an quad opamp and the 3 amps of the lm4781. The highpass works just fine, but because my LF is bridged the lowpass filters on the lm4781 don't work (see attachment). If I loose C2 and C3 everything works perfectly, but if I put them back in I get an horrible offset. I'd really like to have this lowpass there. How can I get it to work?
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File Type: jpg bridged.jpg (33.4 KB, 73 views)
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Old 18th June 2011, 09:55 PM   #2
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To be honest your post is a bit confusing for me, but I think I can safely say you need to redesign your circuit. Don't build the filters into your power amp (especially one with bridged outputs), build them into the opamp pre. Style points aren't awarded if the output is hammered jello.
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Old 18th June 2011, 10:07 PM   #3
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That's what I thought. New design then.
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Old 20th June 2011, 05:55 PM   #4
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Have a solution now: Caps from the inputs to the non-inverting amp inputs and from there maching resistors to ground. Very good results.
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