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Ron, Switching PS for Filaments?

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I posed this question on the tube forum also. What do you think about using switching power supplies for filaments. Most 5v units can be cheated up to over 6v and the switching frequency is at least 50khz, well out of the audio band. 3.3v units can be cheated down to 2.5v for SE tubes. The output design of these supplies is basically a choke input filter network as the regulation is done on the primary side. They are cheap surplus, small, and will not produce a strong 60hz field like high current linear supplies do.

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Re: Ron, Switching PS for Filaments? Ron W. 11:20:28 01/23/01 (0)
A few audio companies are turning to switching power supplies. While they are generally not as good of a regulator in a dynamic environment, they are more efficent, thus their popularity in computers.
But since filament supplies are more of a static load it would seem that a switching supply would be perfect for this application. The only thing you have to watch out for is the noise they can induce into an audio signal. Even though they may be switching at 100khz or more, they can still induce noise. For example, they can induce switching noise on the ground leg of your supply and this will get into your system. Therefore I would probably steer clear of the "cheap" surplus units and look for one with good specs.
I have a friend that was using one in his amp design and it appeared to be working well until he tried to bias up the filament supply for his SRPP driver stage, at which point, it introduced all kinds of noise.
But give one a try and let us know if/how it works.
Ron

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