|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
73.181.211.24
In Reply to: RE: SET Power supply in series with output trans posted by used-hifi on October 29, 2016 at 08:32:40
People have been experimenting with large and/or huge capacitors for decades, in the hopes that at some point they would have enough to function like regulators. I have not seen enough repeatability to say there is any consensus. In such experiments with regulators as I have done or heard, more capacitance is usually better but never good enough. And as noted by others, electrolytics especially have HF limitations requiring bypassing, one of the more arcane arts of audiophilia.
I speculate that frequency response is not the right criterion. My tentative candidate is envelope modulation - that is, the fluctuations in musical loudness. This is clearly dependent on the music as well as the amp time constants, which does muddy the waters. And if you are worrying about THAT, then the time constant of the interstage cap or choke or transformer should also be considered.
If you have access to simulations, it would be an advantage to look at the PSU/output stage together, allowing for the OPT inductance (and plate choke if parallel feed) to play its part. It is genuinely more complex than we think initially!
Follow Ups:
"I speculate that frequency response is not the right criterion. My tentative candidate is envelope modulation - that is, the fluctuations in musical loudness. This is clearly dependent on the music as well as the amp time constants, which does muddy the waters."
One needs to distinguish between class A and class AB amplifiers here. In class A, the average current draw from the power supply is essentially independent of the loudness of the music. The fluctuations in the current draw are on the timescale set by the audio frequencies in the musical signal. Essentially, the job of the final smoothing capacitor is to hold the supply voltage nearly constant during the period of the lowest frequencies that one wants the amplifier to reproduce without colouration. (Bypassed by smaller capacitors too, if the main capacitor has higher-frequency deficiencies.)
By contrast, in a class AB amplifier the average current draw from the power supply can increase very substantially when the music gets louder. In this case, sagging in the power supply voltage during extended loud passages could certainly be a problem in the case of a poorly designed power supply. It seems to me that it is in these class AB cases that the envelope modulation you are alluding to would be a problem.
Of course, if one actually wants a colouration in the sound of a class A amplifier, such as a low-frequency fall off, then a less than "ideal" power supply might give a more pleasing sound. But this, or any other colouration of the sound, might be better achieved in a more controllable fashion, using purpose built audio filters.
Chris
SETs in general are borderline AB - that is, while the output triode is never really cut off, its average current does increase with power. It's not as severe as typical PP amps, but it's not insignificant either.
For example, in the 1950 data sheet for the 300B (which is the one I have at hand this morning) there are two suggested operating points. One has 62mA no signal and 74mA at full power; the other is 60mA to 77mA.
"SETs in general are borderline AB - that is, while the output triode is never really cut off, its average current does increase with power."
Isn't that another way of saying that SETs generate a large amount of even-harmonic distortion? To the extent that the average current increases with the power output, this means that the current on the positive halves of the cycle increases disproportionately more then current decreases on the negative halves of the cycle.
That is, it seems to me, the essence of even-harmonic distortion.
If the distortion is so great that the average current draw is increasing significantly with loudness, then one is probably talking not so much about high fidelity reproduction as about sound effects.
Chris
You can exaggerate the effect by reducing the quiescent current without changing the plate voltage or transformer impedance. There are some on this forum who like that effect.
It's a legitimate question whether the brief against even-harmonic distortion is due to the distortion itself, or the effect on envelope modulation. It will be a long time, if ever, before measurements catch up with psycho-acoustics.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: