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Old 3rd June 2012, 05:23 PM   #11
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by Exrabies View Post
KSTR,

Thanx for your suggestions. Still baffles me why earlier measurements didn't show anything wrong.

Will check 3) first since osc is indeed ~200kHz
1) Do you mean a 100nF cap across Vs-/pin 15 and Vs+/pin 13?
Any idea why the osc only shows at the sine bottom?

Adam.
Yes, pins 15/8 and 13/7 (assuming that they are connected)

The internal block schematic of the TDA7294 shows a local power buffer where the loop is closed around the low side power MOSFET. This is probably why the neg swing is more prone to oscillation when the MOSFET increases it gate-to-drain capacitance when voltage accross the MOSFET is low, decreasing overall stability.

Please post exact schematic and wiring (or PCB) diagram/photo for further help.

I have seen batches of TDA7294 that were more prone to oscillation than others. ST doesn't even specify a unity gain bandwith, let alone a full bode plot for the part.

From the fact your oscillation is at ~200kHz (that is about the intersect of the a gain of 20x and the estimated bode plot gain magnitude curve -- I also roughly measured that estimated bode plot), I would think your problem is one of global feedback and/or load isolation issues like you seem to have found out now. OTOH, the TDA7294 can easily handle capacitive load of about 1nF.

Last edited by KSTR; 3rd June 2012 at 05:26 PM.
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Old 3rd June 2012, 06:37 PM   #12
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@KSTR

Will start fiddling with it tomorrow evening. Thanx for all the feedback, much appreciated. Steep learning curve for a Sunday evening however. Bode plots are deeply buried in memory (technical school some 20 years ago). Time for a refresh methinks.

Adam.
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Old 3rd June 2012, 08:20 PM   #13
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Yo all,

Problem solved.

Solution: zobel (2R7ohm + 100nF) across TDA7294 Out and V- and small inductor + 15ohm on negative speaker out. Sine wave is now as thin as a hair.

Thanx for all your input. I'm extremely impressed by the willingness to help out, and the knowledge available.

Regs to all.

Adam.
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