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not if you don't refine it...

152.163.252.5

Hi Steve:

you wrote:

::::I built this amp (original compact as on Neds site) over a year ago to resolve whether or not it was really a super elegant implementation of a single ended design using a more commonly available PP OPT.::::

the "original" as published on Ned's site.... is a starting point only... it shows, imo, the 'architecture' of the concept... the 'black boxes' that then must be filled in and optimized...

::::The "speculation" was that the undriven 6BQ5 was actually a current source designed to null out the DC from the driven 6BQ5 thus resulting in zero net flux in the OPT under idle conditions.::::

and the speculation proffered was worth not much more than speculation, imo. very little insight and understanding into how the circuit actually works.... heck.... if I remember right... some folks were saying it was a single ended amp....

::::Testing indicates that the output stage is nothing more than a phase inverter capable of delivering power.:::::


OK.... so now we have a phase splitter (necessary for PP operation) and a stage capable of delivering power.... two necessary ingredients or functions electrically for a PP amp....

:::It is very poorly balanced WRT drive on each phase due in large part to the relatively miniscule "tail".::::

ok... so now you know what needs to be 'fixed'..... are there any solutions/optimizations available?

:::::Power output is also a bit low by 6BQ5 standards at about 7W @ 1KHz with very obvious clipping...this using a regulated PS for optimum measured performance.::::::

regulated or not.... all amps will clip at some point... again, you need to optimize the design for the performance requirements/goals that you set...

there is nothing inherent or necessary in using an EL84 output stage.. you could use a 6550, 845, 2A3, 807, etc.... and vary the power output accdg to needs and tastes.....

::::THD was not measured but I expect it to be high for a PP design because the "undriven" 6BQ5 operates at about 50% of the drive of the "driven" 6BQ5.::::

second harmonic distortion should be largely eliminated in the output stage as per normal PP operation if your balance is good....

:::::One interesting improvement should be replacement of the common cathode resistor with a CCS but I never got around to trying it this way.::::

so you need to improve the balance..... look at yeo's site for some ideas and insight into how this can be accomplished.....

and there are actually many or several approaches... for example you could use a 'plate choke' in the tail... and get substantially higher ac impedance than the 130 resistor provided as in the original schematic.

get creative.. put on that thinkin' cap... beauty is there...


::::The amp sounded just OK but after a few weeks was dismantled because I needed the OPT for another project.::::

in other words it was never fully optimized.....

:::::Overall, I don't believe this amp will satisfy the discriminating listener over the long term. However, it's simple and low enough cost that nothing is lost if one wants to try it. IOW, YMMV.::::::

sorry... but your conclusion is a laugher...

one of the biggest, best and well known recording engineers in the states commissioned an amplfier that was built on the basic architecture of the "compact"..... and I would guess his ears have some pedigree....

the designer of the studio's commmissioned amp... is also a seasoned builder.... and it more than satisfied his ears....

of course, he went beyond... the circuit as published on Ned's site.... and refined, refined, refined it... and used some pretty advanced current sources and shunt regs and etc...

but... it's much to early and counter to the evidence that this puppy.... is incapable of being anything less than a stunning performer....


MSL



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