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12.10.199.12
In Reply to: RE: "Low frequency ringing"... posted by Steve O on March 18, 2015 at 11:18:21
It may not be possible to use small caps. at the I/Ps of the "finals", due to grid to ground resistance limits. This is especially true, when "fixed" bias is employed.
As for blocking, I recall reading where 1 fellow was going to implement "PowerDrive" using ZVN0545As, even though positive grid current was not permitted. A rather high valued grid stopper prevented significant g1 current from flowing.
Eli D.
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...Ex: Heath W-3AM Original circuit uses 0.01uF/470K at input to driver (6SN7) for fc~34Hz and 0.25uF/100K at input to finals (5881 w/1K "stoppers") for fc~6.4Hz. Ratio of tc~5. It bounced at LF. Revised circuit uses 0.47uF/470K at input to driver for fc~0.72Hz and 0.1uf/100K/1K at finals for fc~16Hz. Ratio of tc~22. No bounce and nearly identical LF distortion @-3dB (~10W) of max rating with 20Hz THD easily meeting original spec of ~0.3%.
Interestingly, the revised values are exactly those of the Craftsmen C500A, an inherently stable triode Williamson design and they're similar to Dave Gilespie's stability enhancing revisions to the Heath W4, an ultralinear Williamson using 6SN7s and 5881s but with a lower quality OPT than the W3.
"0.1uf/100K/1K at finals for fc~16Hz"
This seems preferable to me also, but it does raise the issue of low frequency phase shift. I'm skeptical as to whether such shifts are audible, but many DIY audiophiles do whatever they can to minimize the effect. There's definitely a conflict of interest in resolving the problem.
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Buy Chinese. Bury freedom.
...can be significantly reduced thru judicious use of negative feedback. See attached plot from D.T.N Williamson's article in Wireless World. I'd also predict LF "ringing" @ 1-2Hz based on this plot. The circuit this plot is derived from has a tc ratio of about 1:1, much less than the 10:1 rule of thumb.
I love the way it goes through a 180 deg shift at about 2 Hz. I can practically hear the OPT screaming for mercy... :)
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Buy Chinese. Bury freedom.
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