Yes, true and well described @atmasphere... When the tone in the treble turns from brightness to harshness, that's probably the most unpleasant, unforgivable flaw from an amp - at least to my ears !
That's why my little U-OTL doesn't have a general NFB loop, but solely the Futterman "speaker output to cathodyne phase shifter" balancing loop :
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At 10WRMS output @16R load - which is a modest power, but finally enough for me in everyday use - the results are excellent, given the compactness and the simplicity of the circuit :
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2020-2024, four years later : no deviation of the performances, no instability, no maintenance, no setup correction needed... Still operational like in the first day.
I tested random power tubes (I mean : in good condition, but not matched) 6080, 6080WA, 6080WB with graphite plates, and 6AS7G, 6AS7GT, without significant alteration in performance of the amps.
Thanks again Mr Futterman !
The waveforms are a lession for us designing using transformers. Well done. Excellent slew rates......but what about THD ?...........................Bench baron
DC Offset or bias instability isn't something I've ever encountered with the Circlotron. Its been very common to find that after 6 months of operation, the bias and DC Offset haven't drifted (unless a tube has failed). So I suspect this issue you encountered was not related to the Circlotron itself.
Possibly, yes.
It was a first and quick evaluation test that proved by and large inconclusive compared to the Futterman original patent circuit, that's why I went no further on the Circlotron path at that time.
But I should return the Circlotron way now - and more seriously ! - to check if I can meet the same performance - or maybe better ? - than my U-OTL, and I include simplicity, reliability and compactness in that performance evaluation.
T
For the moment @benchbaron, I have no setup available to measure THD, but given the look of the square waves at 10WRMS output 16R, the THD should be low, possibly less than 1% by my experience...
Below the sine waves at 10WRMS/16R, that show no visible deformations, another clue of a low THD :
I thought I had taken screens of the triangle waves tests, but no
... I should ! T
OTLs have always been very good at waveform tests. THD is a matter of load impedance (lower with higher impedances), how well the tubes are set up and whether there's feedback and how much. Futterman used up to 60dB in some of his designs, but at a price: it was very easy to exceed the phase margin of the amp, and distortion did rise with frequency because the Gain Bandwidth Product was lacking. But he did get some impressive THD values.
I'm of the opinion that getting a flat distortion vs frequency curve over the audio band is more important than the actual THD. Any zero feedback amp will have the flat part down no worries since there's no issue with the GBP. But we've been able to get THD down to 1% at full power by careful setup as well.
I'm of the opinion that getting a flat distortion vs frequency curve over the audio band is more important than the actual THD. Any zero feedback amp will have the flat part down no worries since there's no issue with the GBP. But we've been able to get THD down to 1% at full power by careful setup as well.
A Focusrite Solo USB audio interface is about $100 (3rd gen) and resolves to 0.0003%/-110dB THD.
IVX's COSMOS is at least an order of magnitude better at 2-3x the cost.
I see no good reason not to use any of these.
I do have all what is required : modern softwares and soundcards ! Just my DIY interface box needs to be modded from my old softs and PCs, I think. But I am lazy...
Yes, indeed. That's my point of view too. An even-order distortion, with a regular harmonic amplitude decay, reaching 2-3% is easily accepted...
Now I remember that a member at diyaudio.com here (I think it's @Brice, but maybe I'm confusing) who made a simulation of my circuit and found a THD inferior to 0.8% at 10WRMS/16R on the 5Hz-100kHz bandwidth, IIRC - Oh, I could not retrieve his messages...
T
One of the attractions of the Circlotron output is you can have a fully differential balanced circuit throughout. One of the advantages of this is its inherently lower distortion owing to even ordered cancellation at every stage throughout the amp; distortion is compounded less from stage to stage. This results in a dominant 3rd harmonic (cubic non-linearity as opposed to quadratic of SETs) which is at slightly less than the percentage seen in an SET, were both amps at full power (so an order of magnitude lower distortion in many cases, but a lot more power too). The 3rd is treated much the same as the 2nd by the ear (and is the only odd ordered harmonic that is musical) and is easily able to mask higher orders (FWIW, properly functioning tape machines also make a 3rd harmonic).