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In Reply to: RE: I wasn't discussing what you hear posted by Tre' on February 10, 2016 at 19:17:15
A larger shunt capacitor will always create greater peak current in the diodes and transformer secondary. However, the larger cap does not always create a smaller pulse width. In addition to capacitor value, pulse width and shape are affected by load resistance, series resistance and the slope of the wave over the period of conduction. The latter is particularly important, because the speed with which the "inrush" of current can take place when a diode first conducts is strictly limited by the slope of the wave. In the end, when all the factors are accounted for, there is no fixed relationship between capacitor size and pulse width or shape, even if the load remains unchanged.
This is evident when the circuit is analyzed using SPICE. In the circuit above, the 20,000 uF shunt capacitor creates current pulse widths in the diodes of approximately 4.3 mS. Contrary to the theory promoted in this discussion, reducing the capacitor to 20 uF creates a smaller pulse width of about 2.8 mS. However, when the capacitor is made even smaller, say 2 uF, pulse width increases again to about 5.3 mS. Lack of a definitive relationship between capacitor size and high frequency content is further emphasized when FFT plots are created using these values. The power supply with a 20,000 uF capacitor simply does not contain increased high frequency energy.
Finally, there's the fact that at 20 kHz, a 20 uF shunt cap exhibits 0.4 ohms to ground, whereas a 20,000 uF cap presents 0.0004 ohms. In this context, the idea that the larger cap would pass more high frequency energy into the amplifier looks really silly.
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Buy Chinese. Bury freedom.
Follow Ups:
"However, the larger cap does not always create a smaller pulse width. In addition to capacitor value, pulse width and shape are affected by load resistance, series resistance and the slope of the wave over the period of conduction. The latter is particularly important, because the speed with which the "inrush" of current can take place when a diode first conducts is strictly limited by the slope of the wave."
I can see that. Doesn't R1 have the largest affect?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think R1, R2, and R4 have a similar effect. They're all in series with the current into the capacitor.
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Buy Chinese. Bury freedom.
OK, so is the following true?Theoretically, in the absence of R, the larger the cap value the more HF/RF.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/11/16
"Theoretically, in the absence of R, the larger the cap value the more HF/RF."With no R in the circuit (except a fraction of an ohm at the 60 Hz source so SPICE will run, and the load resistor), increasing capacitance appears to consistently create narrower pulse widths in transformer and diode current. However, even then, HF energy does not increase at the output of the supply. FFT analysis indicates decreased HF energy with larger value capacitors. My hypothesis is that the reduced impedance from larger capacitors is much more effective at attenuating HF energy than the narrowing of the pulses is at generating it.
Also, note that the series resistance after (and in) the secondary of the transformer has to be absolutely zero for the conduction angle to consistently decrease. Even five or 10 ohms in the winding defeats the theory. We can't get even close to that in real world amplifiers.
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Buy Chinese. Bury freedom.
Edits: 02/11/16
Thank you for that analysis.
The last thing I want to understand is whether or not the RF can be broadcast through the air to the circuit even though it's being attenuating on the line by the cap.
This is something Lynn Olsen has spoken of.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre', an FFT plot of diode current pulses does not reveal any RF. Energy even at 20 kHz is 70 dB below the 60 Hz spike. I really think this issue has been blown way out of proportion by audiophiles looking for a way to explain what they hear. With a 20,000 uF capacitor in the circuit I posted, current pulses through the diodes are still nearly 1mS wide. In addition, their corners aren't sharp, probably due to the slope I mentioned previously. This just isn't RF territory.
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Buy Chinese. Bury freedom.
OK, I wonder what Lynn was getting at?
Scroll down to "Power Supplies and Noise Spectra"
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
If I were to re-title that section, I would call it "Deficiencies of Poorly Designed Power Supplies." In my opinion - and I say this with considerable respect for much of Lynn's work - this particular lecture borders on fear-mongering. Nearly every caveat and doomsday scenario is based on one small area of the piece, specifically the following:
"...both spikes [wide and narrow] yield a comb spectra going out to at least 100 kHz or more, depending on the residual inductance of the first power-supply capacitor."
Just to be clear, Lynn is saying that undesired energy of consequence will extend to 100 kHz even if the first capacitor exhibits zero inductance. That is categorically untrue. I'm sure it's possible to create a supply that performs so poorly in this regard, but modern parts and general audio design principles make it highly unlikely that anyone would. FWIW, I've also analyzed many power supplies, and I've done so with Lynn's favorite instrument - spectrum analyzers. You can believe that if any of the typical analog supplies behaved in this manner, I'd be blowing the horn on this topic myself.
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Buy Chinese. Bury freedom.
TK, Thanks for all your time on this.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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