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101.113.2.63
In Reply to: RE: I don't direct couple because I use filament bias. posted by drlowmu on August 07, 2014 at 12:10:40
Hi Jeff,
I'd like to.. I had it running with the 2A3 supply stacked on the input/driver B supply without the IT, it made me nervous enough with the OPT not being sufficiently rated for the voltage WRT 0V (UBT-1), had the core floating at 255V to make it sort of acceptable. I had TS jack for measuring driver filament voltage and it shorted, output stage ran away.. I don't know how it didnt melt down the OPT but I'm just not needing that sort of stress. It sounds good enough, perhaps not as good, but eh, you know.
Follow Ups:
Hi Shane,
From my listening experiments, surprisingly, a two-stage DC amp sounds better with one supply, not two, and not two "stacked".
This single supply result is the opposite of all other amps I have built and listened to, where separate ( low DCR ) B+ supplies always sounded best.
The UBT-1 trannie is likely wound with 1,500 VDC insulated wire.
Jeff Medwin
Hi Jeff,
Just for reference, it was Mr Atwood who stated that the UBT1 was very marginal at ~510V and then only on condition that the secondary wasn't grounded and that the core was floated up to the voltage that the output stage was referenced to. He's the One Electron guy, so I take his word.
I'm sure there was probably some margin to his comment, and I guess it comes down to what you're comfortable with. In the end.. I'm happy with compromise.
I usually don't ground my secondaries.
Jeff
I like the idea of a fault current path in the event of a 3 stage DC meltdown !
MCB's here will shut it down in <0.3s, without the 0V reference the coil melts down in concert with the OPT.
Just not worth it.
Shane
There are poor PP amps and poor SET amps.
There are top of the heap PP amps and top of the heap
SET amps.
My experience says the very best SET amps, used with a speaker that does not tax them heavily, are cleaner, clearer, more dynamic, and sound more like music than the very best PP. And that's not because they are "thick, tubey, colored, etc." I hear less of those colorations, just utter clarity, with the best SETs.
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