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Old 30th January 2006, 04:02 AM   #1
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Default Con someone look at this buffer/mixer for me?

I am making a little 4 channel PA out of an few LM3886 chips, and I need to rig up a simple buffer, so I threw this idea down. I only included the buffers... they will be fed from 4 discrete preamps with gain controlls, and then sent out to an "insert" for processing... then into these buffers.

Is there a better way to do this? Do I even need the buffers? I figured they would keep from loss from input impedance and maybe kill any ground loops a bit.
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Old 30th January 2006, 05:57 AM   #2
lineup is offline lineup  Sweden
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In same way as when paralleling OP-amps or LM3886,
you need to use a series resistor from output of each of 4 transistors.
They are all connected to the output capacitor.

This way each input can be different and voltage difference will be across these resistors.
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Old 30th January 2006, 07:03 PM   #3
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Quote:
Originally posted by lineup
In same way as when paralleling OP-amps or LM3886,
you need to use a series resistor from output of each of 4 transistors.
They are all connected to the output capacitor.

This way each input can be different and voltage difference will be across these resistors.

Crap... I forgot those... thanx. Is it OK other than that? Pretty simple.. I just wanted to bounce it off someone else.
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Old 31st January 2006, 12:13 AM   #4
clem_o is offline clem_o  Philippines
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It would seem simpler to use one op-amp configured as a summing amplifier to do the job. If you use high values for the input resistors to the summing amp, the source impedance(s) of the previous stages should become pretty much negligible. A FET-input opamp should be able to work with those impedances properly...

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Old 31st January 2006, 12:16 AM   #5
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Yup. Google "virtual earth mixer" for the schematic.
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Old 31st January 2006, 02:28 AM   #6
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Quote:
Originally posted by clem_o
It would seem simpler to use one op-amp configured as a summing amplifier to do the job. If you use high values for the input resistors to the summing amp, the source impedance(s) of the previous stages should become pretty much negligible. A FET-input opamp should be able to work with those impedances properly...

Cheers!

That's "plan B" in case the fets did not work out. I really wanted to use discreets, and I thought that I might avoid ground lops that way, which might be a problem for a virtual earth setup... meybe not in this context though.. I dunno...
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Old 31st January 2006, 04:05 AM   #7
clem_o is offline clem_o  Philippines
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Howdy - don't see how using discrete FETs can avoid ground loops. Each FET still has a ground, which will get common'd through the power supply...

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