Bridged amp lowpass question

chrisi · 18th June 2011, 07:35 PM

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?

sofaspud · 18th June 2011, 09:55 PM

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.

chrisi · 18th June 2011, 10:07 PM

That's what I thought. New design then.

chrisi · 20th June 2011, 05:55 PM

Have a solution now: Caps from the inputs to the non-inverting amp inputs and from there maching resistors to ground. Very good results.

18th June 2011, 09:55 PM	#2
sofaspud diyAudio Member Join Date: Nov 2005 Location: San Antonio TX	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. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

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18th June 2011, 10:07 PM	#3
chrisi diyAudio Member Join Date: Jun 2011	That's what I thought. New design then.

20th June 2011, 05:55 PM	#4
chrisi diyAudio Member Join Date: Jun 2011	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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