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Old 7th May 2008, 09:15 AM   #1
CJ900RR is offline CJ900RR  Sweden
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Default 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: http://www.leetmaa.se/misc/q1.pdf

But there is them who claims that this one, http://www.leetmaa.se/misc/q2.pdf is the way to go.

Witch should it be?
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Old 7th May 2008, 09:40 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
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.
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Old 7th May 2008, 12:03 PM   #3
sreten is offline sreten  United Kingdom
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http://www.national.com/an/AN/AN-1192.pdf

Hi, what is wrong with the arrangement on page 8 ? , /sreten.
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Old 7th May 2008, 06:05 PM   #4
CJ900RR is offline CJ900RR  Sweden
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Ok i get the point. This schematic then? Any thoughts about that?

Click the image to open in full size.
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Old 8th May 2008, 07:20 AM   #5
AndrewT is offline AndrewT  Scotland
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hi,
r8 can go anywhere before the filtering cap, even before the input cap.

[r8 + Rsource] define the RC time constant of the RF filter.
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Old 9th May 2008, 08:24 AM   #6
CJ900RR is offline CJ900RR  Sweden
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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: http://www.leetmaa.se/misc/qp.pdf

Many thanks...
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Old 9th May 2008, 08:36 AM   #7
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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.
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Old 9th May 2008, 08:40 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
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: http://www.leetmaa.se/misc/qp.pdf
that works. Whether it's more correct or better is debatable.
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