high voltage (>>37 V) negative regulator?

[capslock]

In a power amp, it is usually a good idea to run the input and VAS stages at a higher voltage than the ouput stage and to regulate if possible.

Unfortunately, the standard 317/337 regulators are only protected for input voltages up to 37 V. As they float, you can run them at higher voltage but then even a 100 µF cap on the output may kill them on power on.

I have seen and tried other approaches but so far they were either too complex or did not work satisfactorily.

The LM317HV will work up to 47 V, which is only a little better. There is the TL783 that will work up to 125 V which is great.

Unfortunately, there seem to be no negative regulators. The LM337HV will only work to -47 V and it has proved to be impossible to obtain in small quantities in Germany.

I could use two positive regulators, but than I can no longer use a small 2x 9V transformer to add onto the main transformer's voltages. And low current, 2x40 V transformers are also virtually unobtainable.

- Is there a negative HV regulator that I might have overlooked?
- Is there a simple, protected circuit that offers similar performance to a 337?

Thanks in advance,

Eric

[mirlo]

There were some other threads on reasonable discrete active regulator topologies, but for the low current demands of the VAS and input diffpair, I think you can probably get away with something really simple:

R-C filter a reference made from a current source and a string of zener diodes, feed it to a PNP emitter follower, put a reasonably large cap at the output.

I don't know of anything simpler.

[capslock]

Yeah, that's what has been sitting in the amp I built a couple of years ago. Problem is, the buffer transistor is not protected so it has to be really big so it can survive a discharge from the filter caps. I was looking for something smaller...

Eric

[carlmart]

How can you use a 317/337 on higher voltages and not have it blown as you describe?

I have seen applications using those regulators straight on, only being careful on the input/output being less than 37v, and others using zeners, that apparently protect it more.



Carlos

[e96mlo]

Isn't it possible to put a zener diode across the 317 to take care of the caps charge/discharge?

/Marcus

[vbd]

You can use a LM 337 in a regulator with higher than 37V output, if you protect it for max input-output voltage at startup and in fault conditions, the steady state being safe.. Good examples in the LM317L datasheet from N.S. (and not in others models... why...?...) The positive designs are easily converted to a negative one with a LM337L for instance...

http://www.national.com/ds/LM/LM317L.pdf

[AudioFreak]

You can use the LM317/LM337 to regulate very high voltages indeed so long as the regulator never sees more than the rated 37V across it from input to output... In fact, thanks to the adjust pin, these regulators can be very versatile indeed. Once you know the regulated output voltage that you want, just subtract 1.25V and then make a reference (perhaps a zener stack with a constant current source) for this voltage ... Then just make sure that Vin will never exceed Vout +/- (whichever is relevant) 37V.

[ftorres]

Some time ago, Elektor published a high power amp (Titan 2000 I think) that used discrete HV (about 80V if memory serves) regulators for VAS and driver stages. I'll try to dig the schematic out and scan it if you're interested.

[jackinnj]

so that the external transistor used as a PASS element doesn't exceed its breakdown voltage.

I cringe at the notion of publishing my HV regulator with the zener string since there were so many complaints about noise (even though it's very much similar to that used in a couple Audio Research pre-amps). The published circuit was a positive regulator, but the same principal can be applied for a negative regulator. Search under HV Regulator and you will get the idea.

Also, consider regulating the ground return. If you can live with a little (some tens of microvolts) switching noise, you can use an opto-coupled feedback loop with a low noise switcher like the LT1533. (Unfortunately these chips are only available in SMT).

[ftorres]

Here are the scans for the positive and negative high voltage regs published by Elektor (02/99 French edition), and used in the Titan 2000 Amp. They take 85V unreg, and output 78V. Quite classical design, opamp is replaced with a diff pair, and a zener is used as a reference, itself being fed by a FET CCS (note the 30V zener used in the CCS, to help the Fet to run at such high voltages). Output voltage can be adjusted to your needs by changing the ref. Zener (39V Here) and the voltage divider at the output. Higher currents with another pass bjt.