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In Reply to: RE: Or ETF? posted by 91derlust on June 11, 2017 at 22:40:00
Don didn't think so.
"why is d.c. crazy?"
That isn't the question.
Here is the question:
WHY would anyone use anything else?
Here are the problems:
(1) transformer coupling reduces bandwidth,
compromises rhythm, pace and timing, and worst of all,
requires extra power to drive the transformer. Any power losses
in a signal chain translate directly into lost dynamics
and lost transparency. Tube choices for driving the
transformer directly also become more limited.
(2) There is NOTHING that can UNDERperform capacitor coupling.
Again, driving the devices gobbles up power. Again, driving the
device leaves out the best tube choices because they can't drive
both the capacitor and the Miller capacitance of the tube to be
driven, both at the same time without bandwidth loss.
(3) so-called "instability" in a D.C. couple can arise from
design and layout problems, and especially from grounding systems
issues, and the idiotic wish to run tubes "hot" instead of in the
exact middle of their performance curves. There is NO excuse
for instability in ANY amplifier. Power supply isolation between each
stage in ANY amplifier is paramount, in a D.C. couple, it is
(4) Good design and execution makes any amplifier into what it
can be at its best. Failure to address all of the issues in a
D.C. amp is a good way to waste the entire effort.
(5) D.C. amps can be designed into a 2-stage amp instead of
using more gain stages. The musical advantages of this simple
approach are profound-- that is an understatement.
(6) The advantages of the simple 2-stage D.C. amp are
lost if the driver tube isn't high-gain. Amp factors of
70 to 100 are appropriate here-- lesser values of gain
simply erase all of the sonic advantages and render the
entire effort ordinary in fun and jump-factor.
(7) Plate current in the driver stage is very critical.
Once it rises over a certain low value, all of the advantages
go out the window rather quickly.
(8) This kind of amplifier, if used in a Single-Ended
approach, is therefore going to be limited in power output.
(9) Any speaker that requires over about 1 watt to drive
at your favorite listening level, will negate the D.C.-couple
advantages-- you'll get better results with a more powerful
cap-coupled or transformer coupled, or Solid-State amp.
The low-power, 2-stage, high-gain driver S.E. amp has NO
sonic equal in all of audio. It can sound EXTREMELY dynamic
and powerful, clean, fast, and dead-accurate as far as
musician's intentions go. It requires that it be designed
and built correctly, and that it drive speakers of over 98 db/watt
efficiency. Speaker cables are a big issue here. You can
easily lose the whole thing in those cables alone.
Efforts to obtain more power out of these designs, while not
--at best-- too much worse than cap or transformer coupling, does
severely limit the D.C. amp's possible performance-- especially
in musical dynamics. Good dynamics are a function of
amplifier response time and power delivery speed, and ARE NOT
a function of amplifier power unless the speaker requires more power.
THAT would be an inferior speaker.
They are not the easiest to implement; they can be more challenging than to dial in, stability can be an issue, and the output tube is left exposed if the driver fails. There are ways of mitigating these practical challenges. I reckon the sonic advantages of direct-coupling outweigh its disadvantages.
The amp I'm currently designing (I've just finished designing the sub-chassis and mounts) will be a 6C6 direct-coupled to a 2A3 running at low dissipation. Don't know what it will sound like - hopefully different (maybe "better"?) than my present amp.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Don worked alone. Too bad his work can't be carried on commercially, although he may have left some hand-drawn circuits behind. He sent me one for my 2A3 monos, that he said, was "pretty close". I'll have to dig to find the email.
Incidentally, Stereophile posted an obituary a few days ago.
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