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In Reply to: RE: is it really that awful ... maybe in spice? posted by deathtube 667 on October 12, 2016 at 20:13:45
I'm not sure what resistive values you used for the components in the two-stage. Also, the input termination IMO needs to be larger. 50 Ohms isn't likely to accurately represent the resistive component of the primary and secondary winding of the PT, plus chassis wiring and house wiring. I've been using 150 Ohms, but even that might be optimistic.I wouldn't consider 300uf to be a practical value for the last cap. I've changed it to 100uF in the plot below. Note that Z under 10 Hz rises to a value that eventually equals the cumulative DCR of the series components. There's a resonant peak hidden in there (the one that's visible if the termination is changed to 10 Ohms), but it's pretty well swamped by the resistance.
PS filters also need to be tested for ripple content, of course, and for frequency response looking into the filter from the input side. The frequency response test helps to determine whether the filter will be susceptible to undesired frequencies entering from the line.
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Buy Chinese. Bury freedom.
Edits: 10/12/16Follow Ups:
I guess the question is... "What does it mean?"
In looking at DDT667's two peaks I'll agree they do not look pretty but how does that actually relate to the actual measured frequency response and the sonic character of the circuit?
I have seen the "ps resonant blips" in measured frequecy response plots but they only seem to show up outside of the passband. In the sim below I would expect to see the peak at 10hz show up as a dip in the rolloff of the low frequencies but assuming a full bandwidth amp, will the 60hz bump also show up?
dave
Variations in output Z don't generally correlate well with the filter's "forward" frequency response. That's why these filters need to be analyzed from both directions. What does it mean? Difficult to quantify precisely. Most amplifier designs are susceptible to modulation of their B+ with various symptoms; distortion, attenuation, degradation of dampening, etc. It's not clear to me how high PS output Z can become before such symptoms are audible, so I simply design supplies to be as low and flat over frequency as possible.
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Buy Chinese. Bury freedom.
I put in the L resistive values , adjusted R1 to 150 and added ESR for caps C1 & C2 (just a guess at this point) so it would look closer to TK's graph.
L1 is the Triad C56U and L2 is one of drlowmu's designed chokes.
That looks much better, but I need to question whether the impedance rise below 100 Hz might be audible. It would be interesting to SPICE an entire SE amp, feed it with this supply, and look to see what's happening at both the output of the supply and output of the amp. Alternately, one could build this supply, drive an SE amplifier, and then probe the B+ line with a scope while delivering audio power to a load. The question is how much that low frequency rise will allow the B+ to wiggle around...
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Buy Chinese. Bury freedom.
This is what I have as a filter with my RH84 project amp.
3 stage LC flywheel to finals.
Add an UF4007 isolation diode + 1K ohm R dropper + C4 20uF//200K bleeder (to driver stage) for the complete filter. The spice Z vs. Hz graph looks quite different.
As a full range amp, perhaps there might be some "psychoacoustic" effect as Chip647 would suggest, but who knows?
I did see some sidebands coming up an octave or two beyond the 120Hz ripple with a different flywheel filter on my spectrum/distortion graph.
Might not even hear anything unusual if you are using bass-shy bookshelf speakers or in a multi-amp system driving some horns, so who knows?.
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