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diyAudio Member
Join Date: Nov 2003
Location: flyover country
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my DC coupled stereo OTL amplifier
This is a webcam type pic, so sorry about the sorry quality. I thought this amp would be a natural in a fireplace.....you get the heat, plus, with a mirror in back, even an orange glow 
I've used this amp almost continuously since 1988, although I've rebuilt and upgraded the power supply - you wouldn't want to know how many corners I cut in the original version -for instance, I originally used a 1400VA toroid kit without winding any secondary, instead taking one of the primaries as a half wave rectified source for the output stages, but for the last ten years, it's got a 700VA toroid with proper homebrew secondaries for the sixteen output 6AS7G supply rails and their filaments.
It's a fully balanced all triode design (except for the DC level shifters that use pentodes as current sources and as part of the output stage bias circuit, but the signal doesn't pass through them, so they 'don't' count  )
This amp does respond down to DC, although I don't hate my speakers enough to not at least roll it off to unity gain down there. There is no servo offset control and no silicon devices anywhere in this amp except for supply rectifiers. Only negative feedback provides offset correction. I recently took it off the shelf where it had been sitting for two years & powered it up. One channel was sitting at 25mV dc and the other at about 100mV dc - hadn't been adjusted for at least four or five years & this with three fresh 6AS7G's (some had been mechanically damaged during storage) and snapped this pic after playing it for a few hours this evening.
I think this amp probably still has more than half the original tubes in it 
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4th December 2003, 07:52 AM
