Problem with parallell LM3886

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2010-01-20 11:01 am
Hi I'm currently building my first chipamp and i have finally come to the end.. at least i thought so. I followed this schematics But instead of doing everything on one pcb i built it kind of modular just to understand everything. But the schematics are the same in the end anyways. I have made 4 single pcb's with one LM3886 chip on each one. I have tried them all one by one and they all work fine! The problem is when im going to wire them up parallell. When i wire them up according to the schematics it just becomes a terrible noise and the chips become very warm fast. Anyone have any idea of what the problem could be? Best regards
to set up your power amps, you must measure and adjust.

For output offset measurement and adjustment you must fit a source resistance that, matches the only source you will ever fit, or set Rs=0r0 for test and then recheck when you connect the actual source and also check what happens when you turn ON & OFF the source while the amp is powered up.

In a parallel arrangement, DC output offset is critical. To minimise the risk of this biting you I think the set up must be AC coupled for each of the paralleled amplifiers. Set the DC output offset to be equal when ALL THE CHIPS ARE COLD.
Then add a DC servo to each of the chips to ensure the DC offsets remain equal while the chips warm up to operating temperature.
If the input offset currents and input offset voltages are compromised by choosing different resistances to feed the +IN & -IN inputs for ALL the chips then you will have further problems with keeping output offset low enough to avoid the problem all beginners have when trying to copy bad schematics of parallel implementations before they have learned what the chipamps are designed to amplify.

Build single AC coupled chipamps and learn the subject before progressing to more complicated arrangements.

you have hit precisely this beginners' quandary.
Build a single AC coupled chipamp. Build 4 of them. Have you learned why and how they work?
Now the big jump, let's join them together using a bad schematic before one has learned to read and understand what is bad about that schematic.
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It is sad to see how many people get problem with the parallel LM3886 setup. Please listen to Andrew. Even professional commercial companies seem to have trouble with it. (Event). However I truly believe and experience myself if one follows the National Application sheet, it can work very well. Especially the version with the DC servo's. Sure it is more complicated but one can just blindly follow the application sheet.

I never liked the variable source resistance method to balance DC offset. In many cases it destroys other things.

With kind regards,
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