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Old 27th January 2007, 01:42 PM   #41
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OK, now try this -- put a 1 ohm resistor in series with C100.

Then, in addition, put a 4.7uF cap in parallel with the output.

Sweep to 10MHz, take two aspirins and call me in the morning.
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Old 27th January 2007, 02:00 PM   #42
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Interesting John. Sorry to put you to a task (again).

Your output resistance results are better (lower) than I might have guessed from squinting at the IRF820 data sheet. And, as you say, the fact that output resistance is nearly constant across the audio band bodes well for its good sound. As you well know, this is in stark contrast to most high-loop feedback regulators that wimp out in the upper octaves of the audio band, even though they may achieve lower output resistance figures at low frequencies (similar to the behavior of most solid-state amps, not coincidently). Signal-current transients will cause overshoot or damped ringing, and the output Z takes on reactive qualities at upper frequencies in these high-feedback designs. That shouldn’t be an issue with your simpler series-pass design.

I’ve used variants of the LM317 Maida design a lot, and these tend to sound better to my ears when lightly decoupled and then shunted by serious composite capacitors at the load, where the effective power supply Z at high frequencies becomes capacitor dependent, for better or for worse.

I had shunned the simple pass MOSFET at these lower currents for fear that transconductance would be so low that the output resistance would be quite high. It isn’t just that it could be high, but that it is bound to be very non-linear (as your data bears out). I seek to keep any non-linear resistance as low a ratio of the plate resistance values as possible, as a rule of thumb, so that non-linear current-induced voltages will factor very little in the output voltage. In your case, a nominal 14mA draw might give an output resistance of roughly 18 ohms. I don’t know your Artemis circuits, but that must be at least 1000 times lower than any plate resistance, which is probably a safe-enough ratio. If the Artemis both “pushes and pulls”, keeping power supply current draw fairly constant, so much the better. Have you attempted to measure distortion on the B+ line with your Audio Precision? No, please don’t do it on my account

I just bought some IRF820s to repair a Tek scope and have leftovers, so maybe I will try this series-pass design next time. Also, while wasteful of power and more demanding on the raw supply upstream, a current loading power resistor across the B+ would both lower the output resistance, and lower the ratio of signal current to quiescent current. Probably not a great idea for commercial designs, but perhaps this would be an easy and temporary test to see how much difference it makes, and whether there is further “gold to be mined” in power supply improvements.
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Old 27th January 2007, 03:02 PM   #43
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Here's a sim -- with and without a 1R resistor in series with C100 and an additonal 5uF on the output -- i think I ran it with a 40ma current draw:

Click the image to open in full size.
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Old 27th January 2007, 03:44 PM   #44
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Quote:
Originally posted by jackinnj
Here's a sim -- with and without a 1R resistor in series with C100 and an additonal 5uF on the output.
Jackinnj,

This looks like it could be an improvement. What did you plug into the simulator for the ESR and ESL of the 5uF?
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Old 27th January 2007, 06:11 PM   #45
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Quote:
Originally posted by Brian Beck


Jackinnj,

This looks like it could be an improvement. What did you plug into the simulator for the ESR and ESL of the 5uF?

Ooops -- I was thinking of low voltage electrolytics -- the 250VDC and higher rated ones have ESR tens to hundreds higher. i.e. the low value, high voltage Mallory's have ESR around 2.4 ohms.

The resistor on the capacitor is going to lower the Q.

Here's the phase-gain plot of the Maida regulator -- simmed exactly per the Linear Brief 47 parts:

Click the image to open in full size.
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Old 27th January 2007, 06:47 PM   #46
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I'm fairly sure I have some IRF820. If so, I could put them on my curve tracer to measure the transconductance if someone wants to give me an operating point (voltage drop and current drawn - 14mA?).
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Old 27th January 2007, 07:10 PM   #47
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Default Regulator distortion

The DP-2 is back in its box, getting ready to be shipped back to Sean Ta in Simi Valley. However, I remember this distortion on the regulator output being around 4-6% with a 10mA load - nearly entirely 2nd order. It would be nice if this was lower. In the LA-1, the load is a choke-loaded triode (and is mostly choke-loaded triode in the phono preamps and power amps), so the current fluctuations seen by the regulator are minimized.

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Old 27th January 2007, 07:50 PM   #48
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Jackinnj,

When we think of regulator output impedance, we expect to see a rise in the magnitude at higher frequencies, and the phase therefore would go inductive. I'm not sure that I know what you're plotting in the magnitude plots from your simulations, particularly in the Maida case. What is the Y axis variable? I know it's log-encoded in dB, but, dB of what divided by what? The X axis is clearly log frequency from 1 Hz to 20MHz.
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Old 28th January 2007, 12:03 PM   #49
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Quote:
Originally posted by Brian Beck
Jackinnj,

When we think of regulator output impedance, we expect to see a rise in the magnitude at higher frequencies, and the phase therefore would go inductive. I'm not sure that I know what you're plotting in the magnitude plots from your simulations, particularly in the Maida case. What is the Y axis variable? I know it's log-encoded in dB, but, dB of what divided by what? The X axis is clearly log frequency from 1 Hz to 20MHz.
The plot shows the magnitude of the a.c. voltage at the output compared to the input.

the 5uF capacitor could be a polypropylene type as used in the LastPAS regulator -- a 250 Volt electrolytic of this value would have an ESR of (and I am guessing) 4 ohms.
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Old 28th January 2007, 01:08 PM   #50
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Quote:
Originally posted by jackinnj
The plot shows the magnitude of the a.c. voltage at the output compared to the input.
OK, thanks. You were looking at line rejection then. Always good to label those graphs, as my old professors would say.

Another interesting sim would be output voltage divided by output current, across the frequency band. That gets to output impedance.
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