Hi Bob, My main suggestion is to characterize the behavior by providing various input signals like impulses, steps, white noise, swept sine, etc. and looking at the output on a scope. You might also vary the output load. This gives you a good idea that you are not close to going unstable and you aren't experiencing "peaking" around one frequency band or a null in one band. I've had problems in the past with circuits like this where the output stage has a nonlinearity about zero (like crossover distortion) which can make the gain be reduced substantially near zero. This can result in instability by causing integrator wind-up while the output is in the low-gain region, followed by massive overshoot when it exits the low-gain region, followed by a compensatory wind-up in the other direction due to the overshoot, etc. This problem can be dealt with by making sure that the transistors in the output are always biased with some minimum current so their input-output gain never goes below a certain value. Note, too, that the op-amp can be treated as it's own finite gain stage, where you close the loop locally around it and then close the loop again around the whole system. This can make analysis of the entire loop easier because once you guarantee that the op-amp gain stage is itself stable, you can then treat it as a pure gain block or as a simple compensator (like an integrator or integrator plus proportional gain or a lead compensator or lead-lag compensator, etc.) You are sorta doing that already in that the compensation cap is turning the op-amp into a fast integrator. I have developed a "theorem" of simple control design (which I am sure has been stated before but I've never seen it put this way) - given any stable system P, one can always close the loop around P with an integrator H such that the closed-loop system will have zero DC error and be stable, for some value of integrator gain K in H, and the system will continue to be stable and exhibit zero DC error for any integrator gain Kprime < K. To put it more simply, you can always control a stable plant using a slow integrator and obtain at least the improvement of zero DC error, but also usually some amount of improvement in servo tracking and regulation against disturbances. Sean On Thu, Aug 24, 2017 at 12:32 PM, Bob Blick wrote: > Hi y'all, > > I'm trying to build some audio power amplifiers in order to use up some > parts I have left over from a previous life. > > In the past I've always built the input stages from discrete parts but > this one uses a conventional 4558-style opamp. Then I follow it with > voltage and current boosting parts and finally enclose the whole mess in > negative feedback. > > In this aforementioned past life I designed a lot of power amps so I know > to design each stage to have as linear and balanced response as reasonabl= e, > keep the gain low around the slowest components etc. > > Of course with the added gain and relatively low speed of the output > stage, the internally compensated opamp is not able to keep things stable= , > so I've added an external compensation capacitor. It does work fine, and > I'm not looking for super high fidelity, but it's always nice to make > simple changes that reap big benefits. > > So my question is, any ideas about compensation or changes I could make? > > I've attached a block diagram. Note that my output stage is the right-han= d > amplifier block, it contains many discrete components, and most > importantly, it is inverting. Therefore I am using the non-inverting inpu= t > of the opamp for the negative feedback. > > Thank you! > > Bob > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .