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Old 25th March 2012, 11:12 PM   #41
tomchr is offline tomchr  United States
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Quote:
Originally Posted by tubelab.com View Post
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.
Yep. I do that as well. It doesn't give you a nice number for the output impedance, but it does tell you if the supply acts up under certain loads. It also gives you the transient response, thus, an idea of the phase margin or stability of the circuit.

Quote:
Originally Posted by tubelab.com View Post
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.
Been there, done that -- at least as far as the semiconductor load goes. I find turn-on transients will destroy a MOSFET in a tiny fraction of a second. I actually used an op-amp to create a current source with a MOSFET driver. But after exploding a few expensive MOSFETs I decided that maybe this circuit wasn't optimal...

It didn't dawn on me to use a tube, but that's a really good idea. That should be a fairly rugged solution. I have some Sovtek 6L6'es that would be up to that task.

~Tom
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Old 26th March 2012, 02:18 AM   #42
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I've used a Jensen wideband transformer 600:600 JT-123-BLCF for impedance and PSRR measurements. Its good to 200kHz.
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Old 26th March 2012, 05:04 AM   #43
tomchr is offline tomchr  United States
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I used George's method for figuring the output impedance (thanks George!). I rigged the regulator to my 300B SET and cranked the signal generator. I measured the AC component of the B+ and the AC current drawn by the amp using a 10 ohm resistor in series with the OPT primary (that I have in there for easy bias point check anyway). Result's attached. Around 60 mOhm across most of the audio band. Not shabby...

I question the validity of the data below 100 Hz, though. I find it odd that the output impedance would be higher at DC than at mid band. But until I get a better test rig, this will have to do.

~Tom
Attached Images
File Type: png MaidaReg_1P0_OutputImpedance.png (25.3 KB, 835 views)
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 26th March 2012, 01:57 PM   #44
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Quote:
Originally Posted by tomchr View Post
I question the validity of the data below 100 Hz, though. I find it odd that the output impedance would be higher at DC than at mid band. But until I get a better test rig, this will have to do.
~Tom
Try lowering the value of C5.

If you want to measure values below a a milliOhm, Kelvin Clips are almost a necessity, and you want to measure Zout as close to the regulator as possible.

The simulated values I get with the "other LT LDO" are quite close to what I observe on the bench. Y-Axis is Ohms (milliOhms):
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File Type: gif LT1963_Zout.gif (15.4 KB, 790 views)
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Old 26th March 2012, 09:40 PM   #45
tomchr is offline tomchr  United States
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Quote:
Originally Posted by jackinnj View Post
Try lowering the value of C5.
I beg to differ. Please stare at attached simulation results.
Graphs show the effects of varying C5 from 10 nF to 1 uF. I included C5 = 1 Farad as well to show the theoretical limit (ain't simulation wonderful).

It must have been a while since I looked at the output impedance in the simulation as it correlates very well with the behavior I'm measuring in the lab. The simulation predicts a mid-band output impedance in the low 10's of mOhm. I measure 60's of mOhm. I bet the difference is the routing resistance on the PCB as I measure at the output connector, not at the output of the regulator IC.

I'm considering increasing C5 to 220 nF, but I'll try it in the lab to see if it makes that much of a difference. I may just add a footprint for the bigger cap if it'll fit nicely on the board. I don't see a point of going much beyond that as the resistance of connectors and wires from the regulator to the load will negate the (marginal) improvement in Zout caused by a higher value of C5. Remote sensing (as jackinnj pointed out earlier) would be nice, but that's a project for a different day.

BTW, the Q of the complex pole pair at 570-ish kHz comes in at around 0.75~0.8 implying a phase margin around 60~65 degrees.

~Tom
Attached Images
File Type: png Zout_Sim_C5_10nF.png (23.2 KB, 753 views)
File Type: png Zout_Sim_C5_100nF.png (23.8 KB, 735 views)
File Type: png Zout_Sim_C5_220nF.png (24.4 KB, 726 views)
File Type: png Zout_Sim_C5_1uF.png (23.9 KB, 121 views)
File Type: png Zout_Sim_C5_1F.png (25.0 KB, 130 views)
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 26th March 2012, 10:27 PM   #46
tomchr is offline tomchr  United States
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Lab measurement of Zout vs C5.
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File Type: png MaidaReg_1P0_OutputImpedance_vs_C5.png (33.1 KB, 286 views)
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 26th March 2012, 10:51 PM   #47
grufti is offline grufti  United States
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Make room for the new king, the 200 nF capacitor takes the crown. And everybody now runs out to buy heavy duty silver wire for the hookup. All kidding aside, this is a great result combined with stability and phase margin and ripple. I look forward to using it.
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Old 26th March 2012, 11:06 PM   #48
tomchr is offline tomchr  United States
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PCB layout. I did increase the size of the footprint for C5 to accommodate a 220 nF, 630 V cap (Epcos EF6224).

~Tom
Attached Images
File Type: png MaidaReg_1p0_PCB.png (53.1 KB, 383 views)
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Last edited by tomchr; 26th March 2012 at 11:12 PM. Reason: PCB routing optimization
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Old 26th March 2012, 11:44 PM   #49
tomchr is offline tomchr  United States
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Default PCB is in fab. Pre-order available on my website.

I made a couple of additional tweaks to the board and sent it out for fabrication. The lead time is normally 8~10 days, so I expect to have boards available by the end of next week.

The boards are available for pre-order on my website (see my signature). I will ship as soon as they arrive.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 27th March 2012, 01:05 AM   #50
tomchr is offline tomchr  United States
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Default Schematics & BOM

Schematics and Bill Of Materials attached.
Attached Files
File Type: pdf MaidaReg_1p0_SchematicBOM.pdf (77.4 KB, 334 views)
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