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CJ900RR 7th May 2008 09:15 AM

Please help us with this schematic, parallel LM3886
Me and some other guys at a swedish forum have a question regarding a schematic for 3 LM3886. I think that this schematic is correct:

But there is them who claims that this one, is the way to go.

Witch should it be?

AndrewT 7th May 2008 09:40 AM

both those amp schematics are based on inverting topology.
Zin for inverting is the series resistor/s feeding the inverting input pin.
For one amp this is 1k+9k=10k
For three in parallel this is 10k//10k/10k=3k3.
The DC blocking cap (4u7F) sees a load of 47k//3k3=3k1.
The F-1dB of that filter is ~22Hz.
The inverting topology has killed the bass response.
4u7F should be increased to around 56uF for extended bass response.

But, can the pre-amp or source feed a Zin=3k1 load impedance?

My answer is neither.

sreten 7th May 2008 12:03 PM

Hi, what is wrong with the arrangement on page 8 ? , :)/sreten.

CJ900RR 7th May 2008 06:05 PM

Ok i get the point. This schematic then? Any thoughts about that?

AndrewT 8th May 2008 07:20 AM

r8 can go anywhere before the filtering cap, even before the input cap.

[r8 + Rsource] define the RC time constant of the RF filter.

CJ900RR 9th May 2008 08:24 AM

Hi Andrew and thank you for the time you put in to answering al my silly questions :)

I have made some thinking and please correct me if im wrong, but wouldnt this be a more correct way to aply the RF-filter:

Many thanks...

peranders 9th May 2008 08:36 AM

Carl, have you studied the AN-1192 and also understood it? Personally I have used a buffer in front of these LM3886 and you have both space and the supply voltage which makes it easy for you.

I would also make a complete bridge connected circuit on the same pcb. It's a clear advantage to minimize the wiring, both when it comes to making errors in building and also picking up noise, RFI etc.

AndrewT 9th May 2008 08:40 AM


Originally posted by CJ900RR
I have made some thinking and please correct me if I'm wrong, but wouldn't this be a more correct way to apply the RF-filter:
that works. Whether it's more correct or better is debatable.

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