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Old 20th January 2007, 06:12 AM   #31
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Default Series Mosfet regulator

I'm a little late to this thread, but thought you would be interested in what we do in the Artemis Labs preamps. Attached is a fragment of the schematic of the LA-1 line amp. It is the B+ regulator, for one channel. The zener diode reference string is shared between channels. The exact same circuit is used in the PH-1 and PL-1 phono preamps.

Click the image to open in full size.

The zener diode D100 and resistors R100 and R102 are necessary for stability and protection. R104 and C103 provide both noise filtering and a slow turn-on. Q106 is driven by a time delay circuit that kicks in about 45 seconds after main power is applied.

This circuit is what I call a "low-gain" regulator - it depends on just the transconductance of the MOSFET for its performance. Since it is low-gain, it should not be shared between channels or between high and low-level stages. However, it sounds pretty good! I've found that adding a lot of gain in this type of regulator, especially an op-amp tends to make the overall amplifier sound "solid-state", i.e. hard and cold.

This is not an original circuit - I got the idea for this from the old MFA preamps, where this circuit was called a "buffer". It works well, and with the protection devices shown, is quite reliable. If the output is accidently shorted, resistor R102 blows. It should be a metal oxide "flame-proof" type.

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Old 20th January 2007, 06:33 AM   #32
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John,

Thanks, what you have posted is exactly what I have in mind (apart from Q106). I plan to get my delay by making R104 about 2.2Meg and C103 around 1uF, high quality cap, What I was describing to SY and Carlos is replacing D104 and D105 with a resistor and R106 with a CCS.
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Old 20th January 2007, 12:23 PM   #33
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John,

Any idea what the IRF820's transconductance is at 14mA? Or, asked another way, what is the power supply's output impedance? The data sheet shows only large currents, of course, and the curves appear almost cut-off where 14mA would be.
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Old 20th January 2007, 06:25 PM   #34
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Default MOSFET regulator impedance

Brian -

I don't have output impedance numbers at hand for the IRF820 regulator. However, in the next day or two I will be testing a new Artemis Labs power amp (push-pull 2A3s) that uses this same regulator circuit for the driver stage. While it is on the test bench, I'll run a sweep of regulator output impedance vs frequency, and post the results here.

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Old 20th January 2007, 06:56 PM   #35
sajti is offline sajti  Hungary
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Quote:
Originally posted by Brian Beck
John,

Any idea what the IRF820's transconductance is at 14mA? Or, asked another way, what is the power supply's output impedance? The data sheet shows only large currents, of course, and the curves appear almost cut-off where 14mA would be.

My MOSFET follower power supply has the output impedance about 20ohms with 20-25mA load. Measured with fixed 20mA, and modulated 5mA load, with scope. I used IRF 720 in the regulator.


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Old 20th January 2007, 07:01 PM   #36
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Default Re: Series Mosfet regulator

Quote:
Originally posted by JohnAtwood


This circuit is what I call a "low-gain" regulator

John;

I would call it "A miracle", since it is a meaningless mess made of details.
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Old 20th January 2007, 07:03 PM   #37
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Quote:
Originally posted by Brian Beck
John,

Any idea what the IRF820's transconductance is at 14mA?
Why do you think that HEXFET of N-type with negative voltage on drain will conduct 14mA?
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Old 20th January 2007, 07:06 PM   #38
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Sorry guys, I did not look on voltages drawn. Usually input is drawn on left side.

John, my apologies. If to look right to left (as in Arab countries people draw and write) it is a source follower that definitely will work Ok.
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Old 21st January 2007, 12:11 AM   #39
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LOL! Yes, it had me fooled, too, at first glance.

It does seems to be a workman-like solution, if you're not after high precision. At least it's nice and simple!
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Old 27th January 2007, 04:33 AM   #40
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Default MOSFET source-follower results

Here are the results from some tests I did on the Artemis Labs DP-2 MOSFET regulator. This circuit is identical to the earlier schematic diagram for the LA-1, except that the input voltage is about 345V, the output voltage (and associated zener diode string) is 245V and R106 is 150K.

I first put a constant-current load on the regulator (disconnecting the regulator from the amplifier load), then modulated this current with an AC sine wave. I can do this through a special voltage-controlled current sink that I built for testing power supplies. I tried three cases: 10, 20, and 40mA. The AC modulating signal was constant at 2.5mA rms. The Audio Precision was connected to the regulator output and the AC voltage measured as the frequency was swept from 10Hz to 100KHz. At 1KHz, the following AC voltages were measured, and the implied regulator output resistance given:

10mA - 54.5mV --> 21.8 ohms
20mA - 32.7mV --> 13.1 ohms
40mA - 21.2mV --> 8.5 ohms

As expected, the regulation gets better with higher DC current, since the transconductance of the MOSFET increases. The good news is that the output impedance is virtually flat from 20Hz to around 10KHz, where it starts to drop, likely due to the 0.47uF capacitor at the output. I have found that power supplies that have a flat output impedance across the audio band sound the best.

Then, I decided to test line regulation. I did this by supplying the regulator from a bench supply with a 1K ahead of the regulator. I used my voltage-controlled current sink at the output side of the 1K resistor to make a 5Vrms signal riding on top of the 345VDC input to the regulator. Here is the resulting AC output voltages for two different DC current loads:

10mA - 2.63mV --> -65.5dB
20mA - 2.84mV --> -65.0dB

The dB numbers are the amount of filtering. In this case, the current doesn't matter too much. The filtering is constant over the audio band, although it gets worse below about 25Hz - perhaps due to the time constant of the 470K and 0.22uF capacitor.

The filtering isn't perfect and neither is the regulation, but they are decent and constant over frequency.

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