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Local power regulators
Local power regulators
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Old 16th May 2003, 07:38 AM   #1
jan.didden is offline jan.didden  Europe
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Default Local power regulators

Hi all,

I am working on a kind of universal channel amplifier. One of the issues is local power regulation for each active element. I thought about using an opamp with a reference voltage at its input and the output serving as the local supply voltage. Kind of like a super regulator without the output transistor. Performance would be somewhere between a super regulator and a standard 3-terminal regulator, for a low price, especially using dual opamps.

My unknown is the capacitive load on the opamp. Normally, this is a no-no. But using the opamp output to supply power to an active element that includes supply decoupling will lead to cap load. Using a small resistor in series with the opamp output can help, but will increase output impedance of the "regulator".

I think that "overloading" the opamp output with large (> 100nF) cap would probably give a low enough pole to restore stable operation, but I don't know for sure.

Anybody has an opinion, experience, or comment on this idea?

Jan Didden
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Old 16th May 2003, 02:00 PM   #2
ThorstenL is offline ThorstenL  Germany
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Default Re: Local power regulators

Koinichiwa,

Quote:
Originally posted by janneman

I thought about using an opamp with a reference voltage at its input and the output serving as the local supply voltage. Kind of like a super regulator without the output transistor. Performance would be somewhere between a super regulator and a standard 3-terminal regulator, for a low price, especially using dual opamps.

My unknown is the capacitive load on the opamp. Normally, this is a no-no. But using the opamp output to supply power to an active element that includes supply decoupling will lead to cap load. Using a small resistor in series with the opamp output can help, but will increase output impedance of the "regulator".

I think that "overloading" the opamp output with large (> 100nF) cap would probably give a low enough pole to restore stable operation, but I don't know for sure.
Well, some notes first. The "idea" of using the same Op-Amp (power or not) as regulator came to me from Dennis Morecroft (aka DNM), who, for his most prestigeous Amp (if can call these cute little plastic boxes in any way "prestigeous") used tow more of the same Amp's as external regulated supply.

On experiemntation I found this to sound very good and to be stable as long as the the circuit was suitably optimised. I used it only with CFB Op-Amp's (aka AD811/LM6181/LM6182) and took care to set the bandwidth of the supplied Op-Amp to more than two octaves less than the the regulator, in effect making the regulator faster than the main Amplifier (15 to 20MHz bandwidth is still plenty fast for Audio anyway).

I figured, with tight low capacitance and low inductance layout (Amplifier on top of PCB, Regs below so that the reg out & and chip +/-V pins almost touched) the shooting match was stable and so it was.

Now to capacitive loading. Please remember that ANY integrated regulator as well as such circuits as Jung/Sulzer (and Didden ) regulators all contain an Op-Amp for the AC loop.

The insertion of an emitter follower does not do all that much to the phase margin for stability (except making it worse). So as long as your Op-Amp will supply sufficient current and the capacitive load is either below or above the critical range the circuit will be stable.

My main argument for a "capacitor less" solution is philosphical. The Capacitor over certain ranges will dominate the PSU's behaviour, while across others theOp-Amp dominates. The cap often dominates in the Audio range, if we do something like that, what is the point of investing in a "good regulator".

If we start playing with capacitors we may as well do our homework and use correct capacitive decoupling with a choke to decouple the whole schebang from the "bad" regulators at higher frequencies (I like using that approach in existing gear being pooge'd), preventing the reg from doing anything "bad". Just watch the Q of the LC filter.

Sayonara

PS, attached a rendering of an experimental linestage from a few years ago, drawn by Carlos M. from a description by me, no guarantee for accuracy, I never drew a schema of this, all this is from memory and my memory is notoriously bad.

I think I had a 1k resistor (maybe 10k?) between reference Voltage and Op-Amp input for the reg.

It used LM317/337 Pre-regulators with very low resistance voltage set resistors and around 20V output, 10R - 1W & 150R or 180R - 5W. Sounded jolly decent, but I drifted over to Valves about that time, better sound.
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Old 19th May 2003, 02:33 PM   #3
jan.didden is offline jan.didden  Europe
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Default local regs

Sayonara,

Thanks for the info. I was looking at the AD8019 as the reg, which has very good PSRR and a low Zout (10mOhm I think) up to 100kHz or so. It can also source/sink a couple of 100 mAs (it's a DSL line driver really). And you are right of course on the emitter follower as worsening the stability margins. But you need it to get enough current. Since this opamp has enough current capacity, I'll like to try it without the follower.

So I should be able to run that without any additional local bypass cap on the load opamp. I can lower the bandwidth of the load opamp if necessary.

I'm not sure I agree on the cap issue. A good cap makes life easier for the reg, especially at the higher freqs. The LC filter you discuss would make it easier for the reg, but worse for the load.
Maybe the best way is to use a good quality cap with a value as low as possible for stability, say 10 or 100nF film. What is your experience with that?

The idea of low-value voltage set resistors is a good one to bias the prereg, I will use that also. It may also be a good idea for the opamp regs, to bias them in class A, although the static load (about 50mA) might already be enough.

Jan Didden
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Old 19th May 2003, 03:16 PM   #4
audiofan is offline audiofan  Canada
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You are about to build an overkill I tink, but if you care about part count an expanse you may check LM723 or equivalent you will find a voltage reference, an OP amp an a power transistor in a chip.
About the capacitor at the output of regulator I think it is the way to go and the most important about this one is the layout around the OP amp for groung loops and current loop. Analog Device had on is site an interresting text on the subject but they moved with TEXAS and I dont know if it is still on the web
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Old 19th May 2003, 03:46 PM   #5
jan.didden is offline jan.didden  Europe
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Default local power

Yes, thanks, I know about the 723 (in fact used it 20 years or more ago), but for me it is too slow to give a good wideband low noise and low dynamic output impedance.

It is overkill, in the sense that the improvement is small compared to the extra cost. But it will be fun, so I can absorb the cost (book it under "entertainment", mama).

Jan Didden
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Old 19th May 2003, 04:04 PM   #6
ALW is offline ALW  United Kingdom
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Quote:
My main argument for a "capacitor less" solution is philosphical. The Capacitor over certain ranges will dominate the PSU's behaviour, while across others theOp-Amp dominates. The cap often dominates in the Audio range, if we do something like that, what is the point of investing in a "good regulator".
I find that puzzling, looking at any frequency over the audio band the regulator (assuming it is 'good) will dominate impedance.

Xc for a 100u cap at 1k = 1.6R, at 10k = 0.16R

Zout for an decent reg with reasonable open loop gain will be microOhms at 1k, maybe several hundred uOhms at 10k.

The cap brings benefits at HF as open-loop response falls, where it then dominates output Z.

Other parameters vary along similar lines.

Or are you talking subjectively dominant - there's no doubt the caps around the reg affect the sound, not sure I'd consider it dominant though, although it may be with less 'good' regulators?

Andy.
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Old 19th May 2003, 05:13 PM   #7
audiofan is offline audiofan  Canada
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If you are havig fun and you know what you are about to do it allrigt with me I will not blame you, amateur audio is for entertainement listening to it or building it!
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Old 19th May 2003, 05:49 PM   #8
Fred Dieckmann is offline Fred Dieckmann  United States
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Default Puzzling Evidence*

'I find that puzzling, looking at any frequency over the audio band the regulator (assuming it is 'good) will dominate impedance."


If you look at the scenario with a few 1000 uF on the output of three terminal regulator and the capacitor will be the lower impedance down into the audio band. Three terminal regulators and followers with large output caps are very common approaches and the capacitor will dominate. Good is a little bit vague a term for the regulator, "very low impedance" might be more descriptive. There are cases where it is desired for the cap to be dominate over the regulator in the audio band. When using good caps they the may sound better than the regulator at certain frequencies. The reason for choosing a certain value cap for the regulator becomes clearer in light the relative capacitor and regulator impedance at a given frequency range.

You do point out a very good reason why some caps can sound very different in a regulator application than as a coupling cap. Their low frequency characteristic are not being swamped by lower impedance in parallel when used as a coupling cap.


*I hope you get ev'rything you need
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Old 19th May 2003, 06:53 PM   #9
jan.didden is offline jan.didden  Europe
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Default local regs

Fred,

The large electrolytics you quote will have so much ESR (and ESL) that they don't really lower the output Z compared to a 3-pin regulator. Most 3-pin regulators rise in output Z above a few kHz, and the way to battle that is with a good film cap of several uF (10-20 preferably), not electrolytics! You may need the electrolytic for stability, but not for performance as such.
If you have a regulator with very low Z like ALW's, I don't know of any physical cap that can better that this side of 1MHz.

Jan Didden
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Old 19th May 2003, 08:09 PM   #10
ALW is offline ALW  United Kingdom
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Default I have a feeling

Quote:
If you look at the scenario with a few 1000 uF on the output of three terminal regulator and the capacitor will be the lower impedance down into the audio band. Three terminal regulators and followers with large output caps are very common approaches and the capacitor will dominate. Good is a little bit vague a term for the regulator, "very low impedance" might be more descriptive. There are cases where it is desired for the cap to be dominate over the regulator in the audio band. When using good caps they the may sound better than the regulator at certain frequencies. The reason for choosing a certain value cap for the regulator becomes clearer in light the relative capacitor and regulator impedance at a given frequency range.
I must admit to not considering 3 terminal reg's when I made my post. They are frankly anathema to good quality analogue stages, IME. Not that they cannot sound good (I've plenty of things that sound great using them) but simply that they are far, far away from being optimal IME. I use them simply because the circuits haven't been updated with newer developments I've made since.

Digital stuff may be different though.

There was a good reason for putting 'good' in quotes and hence being a little vague, since if I define what I know to be 'good' I'd give away more than I'm prepared to at present.

You as always though make good points, and as with everything in life, a system is usually as good as it's worst bit, not it's best. A balanced outlook is always sensible.

That's my...

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anyway.

Andy.
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