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In Reply to: RE: Pentode vs. Ultra Linear posted by Eli Duttman on April 19, 2015 at 10:58:26
Eli, I don't want to hijack this thread, but I notice in the 12AQ5 schematic that you're using a CCS at the splitter cathodes. Along those lines, have you considered also using something more "predictable" to bias the outputs? Not a CCS, but maybe an IC voltage source like the "enhanced bias" technique that's been discussed here previously? You know, the more ways I find to test push pull output stages that use the standard cathode bias technique, the less I like it. I think there might be significant advantages in most PP amps to nailing this down.
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Follow Ups:
The CCS in the LTP's tail is absolutely essential. Forced symmetry is what makes applying NFB to the non-inverting triode's grid work. Please remember the history of the project. The idea was to get something highly listenable at very low cost. We succeeded way beyond our expectations. I will not argue, 1 way or the other, about your biasing idea's technical merit. However, it could go against the cheapskate's nature of the project.
Eli D.
About technical merit, I haven't actually tried it myself. I've got the regulators the original author used (LM337), but the only amps I've modified since reading his work, I wanted more B+, so went with fixed grid bias instead. Any of this is more costly than simple self bias, but I do wonder if these inexpensive ICs render the latter obsolete. Even from a top-shelf supplier, they're less than a buck. That's an inexpensive way to insure the operating points don't change on the fly.
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Buy Chinese. Bury freedom.
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