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In Reply to: Re: Sorry??? posted by Steve O on April 9, 2004 at 20:42:58:
steve writes:
::::1. I did want to point out that I'm more-or-less dismissing the original compact amp design, not the basic concept.::::
that was my only real sticking point with ya!!!!
to appreciate the context of the orginal circuit shown on Ned's website... recall that the author is pretty explicit about pointing out that his circuit is tailored for the "cheapstake"....
and I only wanted to point out that this is equivalent in a sense to reading an article done by Eric Barbour in glass audio... on how to build a $49 (or was it $99) singe ended amp... yeah... give it a try... but if it doesn't 'ring your musical chimes' don't conclude anyhing universal about single ended amps from it.... or take it a step further.... get some purpose built single ended outputs and optimize the published circuit till ya get it to sing....
that's what the COMPACT offers... it is a starting point....
:::However, to my way of thinking, it must be implemented with a CCS (more active components) or a good quality inductor (more expense) in the common cathode circuit.:::I suspect your right here... it does need to be refined a bit... but... hey.... what a great starting point....
a simple circuit... with a ton of flexibility.... again, see yeo's site.... you can UL the critter, run it as a pentode, strap it as a triode....
or if you take heed of sector7g's post.... you can use a pair of triodes in the output stage....
gotta run it (as Paul Joppa points out in his contribution in this thread) pure class A.... but what the hell...
it's still an architecture with considerable flexibility.... and simplicity...
::::In either case the thing then deviates from the raison d’etre of the original design.:::::
I don't see this as being the case any moreso than using a plate choke on top of 45 tube in a SE amp makes the amp any less "single ended"... these are simply circuit refinements and enhancements that if they make the amp funciton better and sound better... then they are worth doing... and should be done....
::::2. At a couple of places in your discussion you reference the phase inverter of the MC240 as another implementation of the same basic circuit. If you look at the circuit you'll notice that Mac used an 18K resistor for the common cathode tail (much larger than ~100 ohm) AND unequal plate resistors.::::I do remember seeing the 18K resitor in the cathode circuit...yep....
and that's the point... author A (compact design author) used a different resistor value due to the biasing requirement of his circuit.... the small value resistor doesn't optimize the phase splitter function of that circuit....
mac... on the other hand.. used a larger value resistor in the cahtode circuit of their phase splitter... and apparently it worked reasonably well... or so I assume
but I don't recall seeing the unequal plate load resistors in the mac circuit.... I will loook again at the circuit on monday when I am back in the shop....
but my reference to mac was not about all the nitty-gritty details and choices of component values.... nor that's it's implementation was exactly like the compact.... it was to simply show that the concept has been used by others with some degree of success...
::::The point is that Mac realized that a simple implementation of a long tailed phase inverter was inherently unbalanced and they took special measures to balance it.::::
if by simple you mean not optimized.... this seems to me to be getting to bee a semantical argument.....
you say that a "proper" or a "refined" or an "optimized" circuit is proof of something being fundamentally askew.... that the need to "optimize" is itself proof of some shortcoming.....
as a designer of tranneys.... I know that optimizing is the whole secret to success.... I imagine that circuits respond likewise to wise refinements and enhancements...
:::their implementation bears only a passing resemblance to the compact amp (IMO of course).:::::well.... let's see.... the top tube is driven by the grid... the cathode of the top tube is tied to the cathode of the bottom tube.... the grid of the bottom tube is tied to ground.... the output of each tube comes off the plate and is the source for an external load...
geez.... those points look exactly the same in the mac phase splitter and the compact's phase splitter...
guess it's all in the eye of the beholder....
best of,
Mike
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- Re: Sorry??? - MQracing 21:27:01 04/09/04 (0)
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