Acoustic ringing?


2000-12-31 9:31 am
I am testing this ss amp I built from the schematics and I something is mechanically resonating at what appears to be the input frequency. The noise is audible from 1kHz to about 18 khz (the limits of my ears). Testing is being done with scope, signal generator and the amp is hooked up to a resistive load.
This is really annoying. The amp is working fine and it's now in its permanent container the only things with juice are the PS and the amp board that is mounted onto an heatsink I think the noise may be coming from the rectifier as early testing with a different PS did not produce these weird "sounds". However, I can't pin-point the location of this noise by touching things around or by listening as the noise is high in freq and as such appears to be coming from everywhere. Has anyone seen this before?
It's not all that uncommon.
My first suspicion would be caps. Think of a cap as a miniature electrostatic speaker. If the cap isn't wound tightly, or sealed well, the plates may vibrate. This is not to say that other items aren't at fault...I'll give it some more thought.
Try the old trick that car-repairmen use. Take something like a plastic pen, hold one end to your ear and touch the other to various parts. The vibrations will conduct up the pen and be louder, comparatively. I have had mixed success with this technique over the years, but it's cheap and quick. Needless to say, the plastic (i.e. non-conductive) aspect is crucial, else you may conduct more than sound into your ear...
I won't say that it's impossible that it's the rectifier, but unlikely, unless the power supply you're using now is much smaller than what you were using for testing. You shouldn't be getting too much backlash through the rails and into the early part of the power supply. Particularly as higher frequencies (you mentioned 18kHz) should shunt to ground through the main power supply caps.