Super Regulator, collecting the facts

[WaltJ]

In figure 4 of the AE article from 4/00 there is no bypass on R102 -- 825R from adjust to the output of the SR -- should this also be a.c. coupled to the output?

Not necessarily, as it works fine just as it is shown. The proposed AC bypass you mention could be part of your stability problem re the 337. I can't see where it should be necessary at all.

wj

[peranders]

Originally posted by jackinnj
In figure 4 of the AE article from 4/00 there is no bypass on R102 -- 825R from adjust to the output of the SR -- should this also be a.c. coupled to the output?

The gain of LM337 is set to slightly under 2 so a cap will only reduce the gain < 6 dB, not much. It's more important if the gain would have been 5-10 or more.

[janneman]

I think it INCREASES the gain by 6dB as it increases the feedback for the error signal. I'm with Walt here, this extra cap could cause the 337 instability.

Jan Didden

[jackinnj]

Originally posted by peranders
Can you show us how you have made the grounding for all parts. This is very sensitive.

"Totae viae ad Romae duc*unt"

Edit: The positive regulator doesn't oscillate, the negative one with the pre-reg did -- both circuits are mirror images of each other with one central ground.

The pic I showed has connectors for ease of experimenting -- these have mucho milliohms of impedance.

I've re-read the 1995 articles and will incorporate Gary Gallo's current sensing scheme on the regulator board --

[peranders]

You could try to insert 47 ohms ( or near) in series with the power connection (pin 4) and the output. Add also 10-100 nF across pin 7 and 4. It might help.

[janneman]

Originally posted by jackinnj
[snip]Edit: The positive regulator doesn't oscillate, the negative one with the pre-reg did -- both circuits are mirror images of each other [snip]

Makes sense though - neg regs generally have more stability problems due to the (slower) PNP's used as series pass transistors. That's why some people use two pos regs with one pos output conneted to the gnd of the other to get bipolar supplies. But you need fully separated secondaries on the xformer for that trick.

Jan Didden

[jackinnj]

I can't make the elegant noise measurements of the 2/1995 TAA articles -- if you are going to do any measurements on this thing don't use a scope probe -- solder the measuring cable directly to the output pins.

Using my HP3581 (which closely corresponds to the Boonton 1120 and Tektronix AA501) I measure 600 nanoVolts of random energy at 10kHz using the 300 Hz bandwidth.

I use an Oreo cookie tin...wish I had an AP.

[peranders]

Originally posted by janneman
Makes sense though - neg regs generally have more stability problems due to the (slower) PNP's used as series pass transistors.

One other possible explanation is that the opamp is worse with you switch polarity.

[janneman]

Originally posted by peranders

One other possible explanation is that the opamp is worse with you switch polarity.

I wouldn't think so. You still use the opamp in inverting configuration with the same gain/ same feedback factor. I can't see any difference in that respect that could change the frequency behaviour.

Jan Didden

[peranders]

The PSRR is different at the positive vs. negative supply pin.