No-global-loop amplification

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#702
jwilhelm said:
I've been playing for a few years with the AX3 version of the amp whenever time permitted, but was never really happy with the way it operated. It seemed to work okay but something always bothered me about it, but I couldn't figure out what it was. Another builder assembled a set of boards and was seeing some odd behavior in his build, so I've dug back into this one.

I had noticed some very peculiar operation of the ODNF circuit, but by this time Valery had passed away so I need to figure this one out on my own. A few weeks ago I discovered the output stage itself seemed to be unstable. I think this was due to the way it was layed out. I'm in the process of designing a new layout with a few enhancements that Valery wanted to try. If it works out the design should be possible to scale back to more civilized power output levels. We had more input stage designs in the works as well.
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Hi jeff monitor the ODNF circuit see if there are any stability issues with this compensation or any values or layout which are affecting. There could be a chance of OPS instability as well but see what is triggering it? is it ODNF? coz even valery was telling about the THS4081 is very hard to stabilize if used as its highspeed opamp and there could be some challenges in the ODNF stability once loaded even with opa2134.
 
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#703
Most of the lower side of the board is a big output plane. The driver, pre-diver and bias circuits are directly above this huge plane. The pre-drivers are SMT devices with large planes on their collectors. I don't like the layout at all and am going to just bit the bullet and redo it. Valery was planning to revise the layout to accept a plug in input board, with a plug in ODNF board on it. I'm going to test this scheme.

The amp worked okay with the ODNF circuit disconnected, but the ODNF circuit was oscillating when I tried to connect it. I tried adjusting compenstion but it just kept getting worse. I actually noticed the instability of the output stage when I connected a different input stage to it for testing. The other input stage looks perfectly compatible in simulation. Gain and phase margin looked fine. The amp oscillated instantly when I tried to power it up though. That's what got me looking at the layout.
 
#704
New boards are in. Hopefully I have some time to do some baking this weekend.
PXL_20240830_230402116.jpg
 
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