"Virtual earth" mixing amp with tubes

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[quote author="NewYorkDave"]It has nothing to do with U.L. The screen grid is decoupled at AC, so the feedback is DC-only. It is clever, though, since it would tend to compensate for tube aging to some degree. Taking the follower's cathode return through the pentode shield connection is pretty slick, too--it prevents damage to the CF (or overheating of its cathode resistor) in the event the pentode dies or is removed with the mixer powered up. And then there's the method of coupling mixers together via mix amp cathodes... Yes, it's a clever circuit.[/quote]

Huh -- I'd forgotten it was decoupled. Wonder what would happen if you un-decoupled it?

Paul, that Boegli article appeared in Audio in December, 1960. It describes an active mixing circuit, but there's no illustration. It does include an illustration of an "amplifier of extremely low output resistance" that features a common-cathode direct-coupled to a CF with feedback to the grid of the first stage.

Boegli also wrote an article about a phono/control preamp using that circuit, I think in Electronics World[/i/] It got borrowed a few times; Audio Research used it in several products, including preamps and their electronic crossover. And I took out the feedback and used the idea (with different tubes) in a mic preamp design.

Peace,
Paul
 

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