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Re: Ultrasonic AC for DHT Filament

Posted by Triode Kingdom on March 6, 2003 at 13:23:54

In Reply to: Re: Ultrasonic AC for DHT Filament posted by dave slagle on March 6, 2003 at 10:18:52:

> i often wonder if it has more to do with ac current paths rather than ac or dc...

I am very suspicious of the common mode current path back to the AC line. The fact that various regulators impart different sounds to DHT amplifiers strongly implies that the issue is not one of AC vs. DC, or of the gradient across the filament, but rather of the components in the common mode path.

The "AC vs. DC" effect is rather pronounced in my latest amplifier. I may take Ed's suggestion regarding a battery, just to test the sonics. I've been reluctant to do that until now, because the end result isn't practical. The ultrasonic AC circuit will also provide a test, perhaps with more practical and immediate potential in the search for a solution. I have already ordered one of the electronic transformers, claimed to have an isolated, ultrasonic, AC output of 12V RMS at up to five amps.

Regarding regulators, I suppose the common mode issue can be mitigated by grounding one side of the filament. However, this also eliminates the resistive artifical centertap (similar to a hum balancer) that is often retained, even when DC is used. That, in turn, removes the small amount of degenerative feedback produced by the resistors, and I consider that an undesireable sacrifice.

Another solution, although expensive, might be the addition of two audio quality chokes, one each in series with each leg of the filament. That seems unnecessary though, given that an ultrasonic transformer would serve much the same purpose, and in addition, like DC, residual energy would not manifest itself as audible hum. If an overall reasonable cost can be achieved, the ultrasonic route certainly seems to offer the best of both worlds.

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