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Old 20th March 2012, 06:47 PM   #31
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I haven't run this with an LT1963A in HV, but I'm willing to give it a try. I just have to finish up with another project. I've been itching to do HV phase-gain plots anyway.

With a hat-tip to Mr. Yaniger -- I think that the Impasse and Master's Noise preamps sound so great is that he uses a depletion MOSFET pair as the current source, almost immediately adjacent to the business end, and after the Maida regulator which is on an umbelical anyway. walt Jung described the use of these in AX and the articles are archived on his website.
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Old 20th March 2012, 09:05 PM   #32
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Arent we drifting a little off-topic?
I liked the schematic Tom proposed, and all his design parameters seem still valid to me.
Low current to minimise dissipation,excellent ripple reduction, low enough output impedance and so on...
Short circuit proof isnt very important to me, a normal c l c supply isnt short circuit proof either.
Looks to me like all he needs is a way to ensure stability and proper startup on a somewhat larger buffer cap. I didnt like the 30k parallel resistor to ensure startup, its eating 5 watts all the time on 400v. isnt there anything you can do with a series resistor, or another path to load that capacitor?
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Old 20th March 2012, 10:44 PM   #33
tomchr is offline tomchr  United States
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Quote:
Originally Posted by pauldune View Post
Arent we drifting a little off-topic?
Yeah....... Back to the regular scheduled program. Every once in a while, the sidebar conversations are quite fruitful, though. But it's a balance.

Quote:
Originally Posted by pauldune View Post
I liked the schematic Tom proposed, and all his design parameters seem still valid to me.
Thank you. I like it as well. If I can make it start up into a capacitive load (I think that's possible) and the output impedance is reasonable, then I think we have a winner.

Quote:
Originally Posted by pauldune View Post
Looks to me like all he needs is a way to ensure stability and proper startup on a somewhat larger buffer cap.
Agreed.

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Originally Posted by pauldune View Post
isnt there anything you can do with a series resistor, or another path to load that capacitor?
That's what I need to find out. My simulation says it should be possible to start up into a 47 uF capacitor with 5 mOhm ESR. I have parts arriving in the mail on Friday so I can test it out in the lab over the weekend. I should also have my output impedance test rig ready by then. Hopefully, it'll work (sim says it should).

I think it'll definitely be possible to start up with an RC on the output of the regulator. I.e. cap with series resistor. But that defeats part of the purpose of having the regulator in the first place as it increases the output impedance.

~Tom
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Last edited by tomchr; 20th March 2012 at 10:50 PM.
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Old 22nd March 2012, 06:32 PM   #34
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This is what I mean by remote sense for the LT1963:

If you disconnect R4,5 from Vout, and instead connect them to the sensitive B+ node at the amplifier you'll effect the same change. When you do your boards you can just leave a connection to Vout which may be jumpered (for "local sense") or "remote sensed" by the user.

I think C5 @100nF may damp the response of the LT3080 too much.
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Old 23rd March 2012, 07:23 PM   #35
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Quote:
Originally Posted by jackinnj View Post
If you disconnect R4,5 from Vout, and instead connect them to the sensitive B+ node at the amplifier you'll effect the same change. When you do your boards you can just leave a connection to Vout which may be jumpered (for "local sense") or "remote sensed" by the user.
I think you'll end up shooting yourself in the foot if you route R4, R5 all the way to the load. Here's why:

The LT3080 is a floating regulator. It develops its voltage reference by sourcing 10 uA through the SET pin. This develops 1.00 V across R7. Vout is servoed by the error amp of the regulator, hence, the voltage on the output pin is the same as that of the SET pin. This means, 1.00 V will be across the combination of R5||(R4+R6). With the values in the schematic, 1 mA will flow in the R4~R6 combo, resulting in 1.01 mA (1 mA + 10 uA) flowing in R9. This sets the output voltage of the regulator.

If you route R4, R5 to the load as you suggest, all you will end up doing is increasing the resistance of the R5||(R4+R6) combo, thus, decreasing the output voltage by some amount. And as the voltage drop across the routing will be load current dependent, you've effectively shot yourself in the foot. It will result in higher output impedance (and possibility for instability and/or noise/EMI injection).

I won't go into a discussion of LT1963 vs LT3080. I made my justifications for choosing the LT3080 in Post #1. Feel free to disagree or comment, but I think my justifications are pretty reasonable.

I've attached the schematic to this post so readers don't have to flip back to Post #1 for the parts designators.

Quote:
Originally Posted by jackinnj View Post
I think C5 @100nF may damp the response of the LT3080 too much.
I arrived at the values for R8, C5 through extensive simulation in LTspice. Those values turned out to be a good compromise between stability and fast transient response. I may play with it in the lab, but right now I'm taking the "if it ain't broken, don't fix it" approach.

~Tom
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File Type: png MaidaReg_1p0_Schematic.png (48.8 KB, 879 views)
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Last edited by tomchr; 23rd March 2012 at 07:29 PM.
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Old 24th March 2012, 02:33 AM   #36
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You can count on me to get a couple boards when they're done!

My affinity for the LT1963 owes to its performance in a recent listening test with a bunch of folks (we used the LT3015 on the negative rail). I wouldn't sacrifice PSRR or output impedance to get a low noise figure -- just my 2 centimes. I have a bunch of the LT3080 and just haven't deployed it yet.

btw, my HP3577 does nice phase-gain plots but it is really unhappy with almost any DC on its inputs!
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Old 24th March 2012, 08:24 PM   #37
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Quote:
Originally Posted by jackinnj View Post
You can count on me to get a couple boards when they're done!
Sweet! Thanks.

Quote:
Originally Posted by jackinnj View Post
My affinity for the LT1963 owes to its performance in a recent listening test with a bunch of folks (we used the LT3015 on the negative rail).
This in a low voltage application?

I think you'll like the LT3080 as well. At least I found my 300B amp sounding cleaner than it did with my previous regulator. I think it may have to do with less hum on the amp outputs, but I have no measurements to confirm this. I was never able to pick up any 60/120 Hz IMD products on my spec an. But it is as if there's more quiet space in the sound stage. It was really quite a pleasant surprise.

Quote:
Originally Posted by jackinnj View Post
btw, my HP3577 does nice phase-gain plots but it is really unhappy with almost any DC on its inputs!
Yeah... RF gear is notorious for disliking DC. External protection diodes and DC coupling are definitely called for.

I've done extensive op-amp characterization at work using network analyzers as well. I'd love to get an HP 3577 for my home lab... But right now, my HP 3562A does the trick (at least up to 100 kHz).

~Tom
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Old 25th March 2012, 04:28 AM   #38
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Good news:

I made some tweaks. The regulator now starts up into a 47 uF capacitive load with no external resistive load. That's a 47 uF polypropylene cap with about 5 mOhm of ESR. So practically start-up into a dead short. I tried this several times with 560 V DC on the regulator input. The regulator starts up to 400 V (my target output voltage) every time.

I also tried my "loose tube socket" test. I use light bulbs for the test. Four 25 W bulbs in series draw almost exactly 200 mA. By partially unscrewing one of the bulbs and jiggling it, I'm able to simulate a load with high dV/dt - just like an output tube that's a little loose in the socket. Again, the regulator survives. It actually nicely goes into soft start and smoothly regulates back to 400 V DC.

As promised, I built up an output impedance test rig, but I'm not getting results that make sense. It seems to work fine with a low voltage supply, but on high-voltage supplies - like the present regulator - I'm getting weird results. In fact I'm measuring the exact same output impedance for my previous regulator as I do for this regulator and my HP 6209B. This makes no sense at all. So I'm afraid the AC measurements of the output impedance will have to wait.

That said, it is, of course, always possible to measure the output impedance at DC by varying the load. By measuring the voltage drop as I apply my light bulb load, I arrive at the following: For 0 mA output current, I measure an output voltage of 400.00 V. At 188 mA load current, the output voltage is 399.81 V. This translates to 1.01 ohm of output impedance. This includes my hook-up wires (about 120 cm worth total length). So not bad...

Another test I tried was to let the regulator startup without load and then apply my lightbulb load. Recall, that the cold filaments of the light bulbs represent nearly a dead short circuit. In reality, they measure about 180 ohm, so they should result in an inrush current of roughly 2.2 A. Again, the regulator takes this abuse without issue.

So let's summarize:
  • 400 V, 200 mA design target met.
  • Start-up into up to at least 47 uF purely capacitive load
  • Start-up into harsh load (2.2 A inrush light bulb load)
  • Soft start (10 second start-up with resistive load, 1 second with no load)
  • Handles high dV/dt current pulses
  • Well-tamed transient response
  • Output voltage adjustment range: ~15 % of nominal output voltage

I'm liking it...

~Tom
Attached Images
File Type: png MaidaReg_1p0_Schematic.png (40.6 KB, 815 views)
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Last edited by tomchr; 25th March 2012 at 04:31 AM.
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Old 25th March 2012, 05:00 AM   #39
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Grrr... I meant "high di/dt" not "high dV/dt" in above. Brain fart. Sorry...
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Old 25th March 2012, 01:37 PM   #40
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I have two methods to evaluate power supplies.

First, a very simple method of observing the output impedance of a power supply as the load "frequency" is varried. Since square waves are used there is no single frequency load.

I place a resistive load on the supply equal to the minimum desired load. I use a big mosfet to switch in another resistor with the desired maximum load. (source on supply negative, drain to big resistor the other end of which is connected to supply positive, function generator wired between source and gate.) You can then vary the frequency and watch the supply output with a scope. This one will catch weird stuff and frequency dependent transient anomalies.

The second started out as a variation of the old "alternator whine test" we used with mobile police radios at work. The original test used a large filament transformer (to avoid core saturation) with the secondary wired in series with the radio under test. The power source was a car battery. The primary was driven with an old tube type HP audio oscillator (model 200AB). This setup could put about 1 volt P-P of sine wave ripple on the 12 volt supply lines. It was used to evaluate the PSRR of the two way radio since the car battery was assumed to be a short at AC. The big filament transformer limited the test to frequencies below about 1500Hz but that was good enough.

I decided to build a similar setup to torture power supplies and test PSRR of amplifier circuits, but I used an SE OPT driven by a audio power amp. For testing power supplies place a motor run cap across the load. for testing PSRR of a circuit, the cap goes across the power supply. After blowing up a chip amp I found that the turn on transient stuffs a spike through the OPT, back up the speaker output of the amp used to generate the test signal, so I switched to a tube amp, one of my SSE's.

After a bit of head scratching it dawned on me that the two OPT's were redundant and weren't needed at all. My latest power supply / circuit tester is simply an KT88 triode SE amp without the OPT. I connect the plate of the output tube through a variable resistor to the power supply being tested. One scope probe goes on either side of the resistor. Drive the amp with sine waves, triangle waves, square waves, and even music. Adjust the slider on the variable resistor until the AC voltage on the plate is twice the AC voltage on the power supply output and the resistor value will be a decent approximation of the output impedance of the supply. For testing the PSRR of a circuit, connect it directly to the plate of the output tube.
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