Eden WT-800 Bridged Mono oscillation

[rf7]

I've got an Eden WT-800C that runs fine in stereo mode, but when run in Bridged Mono Mode there is an ultrasonic oscillation on the output. It's only 17mVac, but I'd like to get rid of it. Any ideas?WT800C schematic(1).pdf

[mozwell]

Do you need to run it in bridged mode ? i guess if a repair for a customer, then it should be fixed.
I would try adding some more compensation caps to both power amps. add 22pF in parallel with both C7 & C8 on both amps, OR add an 220pF in parallel with C12 and 470pF in parallel with C9
Try each of these one at a time and see which has any effect.

It looks like in bridge mode, the bridge audio signal comes from the output of the left power amp and feeds it into the inverting input of the power amps input differential amplifier, at R16/R17
Any oscillation on the output of the left power amp will then be present at the right power amp.
Fix the left power amp first, then do the same changes to the right power amp.

Others here who are more knowledgeable than me may offer other insights

[rf7]

The customer actually runs it in Bridged Mode. I installed the 22pF across C7 & C8 on both power amps and that fixed it. Just doing the Left power amp did not do the trick.

Thanks for your help!

[mozwell]

i wonder why the oscillation was there in the first place.
If you still have the amp, double check the Zobel network on the power amp output R36 & C13 is ok.

Happy to help

[rf7]

Yeah, I wondered that too. The Zobel network components check out OK with my meter. Thanks again.

[mozwell]

just an afterthought, the 22p added caps should be rated for >160 VDC

[rf7]

I used 1KV caps that I had on hand.

[nevetslab]

The Sanken MT200 packaged 200W power xstrs (2SA1295 & 2SC3264) in general were always a bit touchy, as their gain-bandwidth is so bloody high (35MHz). I used to use the 2SA1494's & 2SC3858's on BGW Systems power amps, a little slower (20MHz), but later changed over to the TO3P parts, using MJL21193 & MJL21194...slower and far more rugged parts than the Sanken's. And you're right...the output zobel network is an important ingredient on keeping these stable.

Eden doesn't use base resistors to de-Q the output stage, but would be real PITA to try and install...at least on the older 2-pair designs. There's a small value on the drivers, which could be increased....at least on the 3-pair design. I don't have the 2-pair schematic in front of me to see if they did add base resistors on the drivers of that version.

[rf7]

Interesting.

What do you mean by de-Q?

[nevetslab]

De-sensitize the tendency for the power transistors to become common base oscillators. If it was Tektronix doing this, you'd find appropriate valued' inductors in those locations, so there's not as much loss. We see this technique used all the time on grids of our power tube stages....called Grid stopper resistors. Doing the same thing.

[rf7]

Ah, I see. Thanks for the info!