Cure for HF oscillation on positive cycles only

Last night I hooked my oscilliscope to the outputs of my power follower. There looked like there was an oscillation (>100Khz) on the positive increasing part of the sine-waves.

I decided to increase the cap I have across the base and collector of the NPN transistor used to sense voltage on my CCS. Essentially the same as the Pass CCS using the MPSA18. My CCS is connected to -12v using a IRFP140N.

This cleared the oscillation.

After listening to this though... it seemed that the highs were either A) more detailed or B) more "etched".

Is this the correct way to cure this oscillation? Or would it be better to use a 5uh inductor and a 1 ohm resistor in parallel on the outputs?


p.s. my circuit is a IRFP140N connected to +12 then another IRFP140N used as a CCS to -12. Gain is ~1. DC coupled output.
Before you start playing with caps, try dressing the power leads. For instance, when I was building the prototype Mini-A, the placement of the negative power lead was critical for getting rid of a little glitch very much like what I think you're describing. The positive power lead had no effect whatsoever.
Also try moving around the input leads, etc. as your circuit may have other sensitivities.
I had something else in mind, but it fluttered away before I could pin it to the Riker mount. I'll get out the butterfly net and see if I can recapture it.

power leads

dressing power leads???

I haven't heard that one.

I did try physically moving them around... also I have a 100uf cap across each +12/ground and -12/ground at the amp (not at batteries) that should take care of that issue.

What is your take on paralleling batteries with caps? Is this a non-issue since the amp IS pure class A?


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I think increasing the caps on the NPN reference for the
current source is probably the ticket. Also, you always
need 100 to 300 ohms in series with the Mosfet Gates.

It is common for parasitic oscillation to muddy the sound,
and removing it will sharpen up the top end.