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Re: Hum and buzz - they are totally screwing up your sound.

I designed an oscillator tuned to 30 KHz that was supposed to be a sinewave oscillator with little distortion. It had two drawbacks - an unreliable start-up (sometimes power up just produced a flat line) and excess heat generated by a class B push-pull transistor amp stage. So I cranked up the gain a little more and clipped the sinewave which helped both problems (saturated transistors produce little power dissipation). Now I had the problem of figuring out just how much true RMS power I was delivering. I solved that graphically by splitting it up in segments of a curve - much like discrete integral calculus.

So the whole ultrasonic heater was built and placed in an aluminum case. For two output tubes driven by one PP amp output I wound small toroids with a few turns for two isolation transformers. You can go small because frequency is high and you are optimized for using ferrite cores for these. I got several watts of power passed across 20 turns on a 1/2" diameter toroid. Play with the amplitude of the amplifier and the turns ratio to get the proper power for the tubes in use. I was able to use this for anything from a 45 to a 10 filament (and a 10 filament is harder to power than a 300B).

TK below used a square wave generator at even higher frequencies. This is more efficient and is thus much easier to go to even higher powers, but it also needs higher frequency. That may not impact sound much at all but it does run the risk of getting that HF energy up into the front end of the low output MC step-up amps for phono if you have that.

If you do make these, it's probably important to think about power supply isolation since some switching supplies are right on the mains and I don't feel comfortable with that. That's why mine is not right on the mains. It has its own PS with filters and RFI filtering back to the mains. Also, mine is not totally switching, it's a "sinewave" oscillator that starts it off (albeit clipped so it sends more HF energy up there).

The schematic and project description for what I did is too much for this discussion. I would have to go into great depth as to what to do to make that work, something you might see in a magazine article.

Kurt

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