Local Feedback in a Mu Amp Stage Question

[Vivek]

Another schematic with somewhat similar design ideology

http://www.redcircuits.com/Page120.htm

[PRR]

> Anyone tested the mu-Valve with very different R1 and R2?

Why? To a first approximation it makes no difference; the devices show high impedance to this node's voltage. It does affect clipping symmetry, if you swing that way.

But the best answer: cost you 12 cents to try it.

[MG]

OADS of gain, a bit of Local Negative Feedback seemed to be in order

Hi KC,  Not referring specifically to your circuit, but... In general, negative feedback is not a good matchup with 'gradual distortion.'   If the feedback is able to correct for nonlinearities, it will do that right up to the point when it can't.  Hence, sharp transition into distortion.  If I have more time, I'll look through this thread and check your circuit.  But for now, you could see if you can get a look on an oscilloscope with, and without, the NFB.

[Fancy Lime]

Hi Andreas,

sorry to be so very late to this party! I only now saw this thread but I had almost the same exact "problem" some time ago that you had at the beginning of the thread. Namely too much gain and harsh transition into clipping from mu-amp stages when used to dirtify bass a little.

I solved that, at least to my satisfaction, by instructing a gain pot in series with C4 (in the schematic in the first post). Try this (if this is still relevant almost two years later): take your original schematic, take out R5 and C5, and put a 1M log pot wired as a variable resistor in series with C4. Increasing the resistance via turning the pot should decrease the gain. At maximum resistance, much less AC signal from Q1 reaches the gate of Q2 and therefore Q2 cannot act as a fully complementary push-pull element to Q1 anymore but (mostly) provides a constant current to the drain of Q1. You can essentially dial the "mu-ampiness" in and out without loosing the non-linearity, as you would with negative feedback (see MG's comment). Also, as Ron said in #10, do make sure to present the circuit with a well-defined load. An inverting opamp buffer is an easy way to choose the loading freely and doubles as a volume control. Try a 10k or 100k input resistor and use the same value pot in the feedback of the opamp and see where it goes.

Cheers,
Andy