Switching supplies for DHT?

[vetmed]

I'm trying to convert my S.E.X. amp to use VT-25s. It all works except with AC on the heaters its very noisy, unlistenable with headphones. I came across a wallwart switching supply on the Digikey site that provides 7.5 volts @2 amps, the vt25 needs 7.5V @1.25A. It would be a neat solution as in all truth there is very little space left under the hood. I may have to break down and use a bigger chassis but I am not overly fond of cutting metal! Thanks



Robert Lees

[Grainger49]

Robert,

My knee jerk reaction is no switching supplies on a DHT.  But what do you have to lose?  Bread board it up off the SEX amp and see.  If not I have a garbage bag of wall warts.  I might have one that would work.  I'll put it on my scope and see if there is any noise before I say it is right.

You know that most WW power supplies produce higher voltages if you don't have them fully loaded, right?

[Paul Birkeland]

It's certainly worth a shot!  What is the switching frequency of the supply?

[vetmed]

My gut instinct says it shouldn't work, but I think I'll give it a try. If it doesn't I may take you up on your offer Grainger. Precious little about the supply except that it has 75mv ripple on it, and mentions prominently several time that it is Class V efficiency, whatever that means. Certainly nothing about switching frequency in the data sheet.




Robert Lees

[Jim R.]

Hi Robert,

Interesting project -- still have it in mind that someday I'll build a type 10 amp of some kind.

What transformer are you using -- I always thought these tubes needed something like a 12 or 15k primary?

As for switching supplies, they are not created equal and in some situations they an be very nice -- very low output impedance.  The old miniwatt S1 had a decent switcher and that tiny little amp had bass slam and dynamics of a far larger, more powerful amp -- mostly due to the low impedance of the switching power supply as the circuit and transformers were nothing special -- lots of SMD parts and transformers with puny cores.

Will stay tuned to see how this project progresses.

-- Jim