Quote:
Originally Posted by Diabolical Artificer
Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.
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Do plots of the open loop gain and phase from the point where the NFB is applied to the point where the input to the NFB signal is taken from. (Without your circuit I can't say exactly, but that would usually be from the cathode circuit of a front end valve to the loudspeaker output.)
Once you have that, you can fairly easily see where (in the frequency domain) problems will arise if too much simple feedback is applied. It'll be somewhere near where the open loop phase shift reaches 180 degrees.
After that, the fun starts if you want to apply more feedback at lower frequencies and back it off as the phase shift increases. You need to cook up a suitable R-C network in the feedback path to do this.......