fail of a rail in a regulated PS

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Recently I got hold of a defect Harman Kardon PM665Vxi amplifier. Very nice piece, completely mono built, but I don't have the skills and money to repair it. So I decided to use it's components to build a gainclone with some OPA chips I got as samples - a long time ago.

This amplifier has two identical trafo's, one for each channel. The lowest voltage is a 74VAC CT, so i went for a full wave rectifier (two diodes) that gives me 50VDC. Off course too much, but a regulator would do fine here. I used the OPA549 as regulator and two lm317 to get 5V at the input of the OPA. Gain was set to 7, so the output is 35VDC. I've build this twice, to get +/-35V. This is used to feed two OPA541 opamps.

Now my question is...what to do if one of those rails fails? I already simulated it, disconnecting the upper rail, leaving the OPA541 being fed by just the 0/-35V. The result was -2,5VDC at the output. Luckily the chip didn't die: with the +/-35V it played fine again. But what to do if this happens sometimes when it is connected to my real speakers!

I apologize if this has already been asked, but it's quite impossible to find something through the search option. And I am sure some of you can help me, as the designs with the lm338 as regulator (from CarlosFM) would have a similar effect if one of the regulators failed. Or are the LM chips protected to one rail falling out? With protected I mean that they will not put DC at the output. I can imagine very complicated designs, but would like something simple...maybe I am just overlooking something!?

I thank you very much for reading and for the attention

Erik
 
HI gmphadte

Thank you for the reply! I was already thinking about a circuit that allows me to cut the PS, but saw some drawbacks. I am using some large capacitors, and it take some seconds to fully discharge them, so even when I cut main supply there is a chance for DC at the output. Eventually I decided to go for relays at the output...I know relays aren't the ideal solution, but I picked up some large ones and I think they will do. I am no electrical engineer, but I'll try to figure something out...or maybe you have an idea or link?

The fact I posted this question here is more to see what others have done. I think it is risky to post schematics about regulated PS for use with gainclones without warning for the risks of one rail failing...or maybe the LM chip won't put DC out, so it is not an issue. I don't know, I was waiting for a reaction of carlosfm or nuuk, but nothing until now.

Erik
 
U have to use this arrangement after the capacitors preferably at the regulated voltage. That is what your amp will be supplied with.

The simple idea could be two low voltage detectors, may be readily available ics, one for each supply. Go for the one which lights the LED when OK. If the LED lights when voltage is low, it will go off with no supply, so not good. This LED then could be inside an optocoupler, and then it is upto your imagination...

Gajanan Phadte
 
If a rail fails in any amp, not just a gainclone, it is likely to let the smoke out of something...

You are absolutely right here. It is just that with a simple non regulated PS, fed by a center tapped trafo, there are much less possibilities for something to go wrong. If it goes, probably the fuse at the primary will blow. In my case I have a trafo, diodes, capacitors, 2 regulators, 1 power opamp, one more capacitor - per rail, much more possibilities for a problem - and thinking about Murphy's law, I would like some protection! But I am gathering some ideas, and hope more will come.

Erik
 
Here is 1 possible solution

The contact on left is of the left relay and the c/o contact is of the mains relay.

Gajanan Phadte
 

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Yes, but if u don't have regulators i.e. more components that increases chances of rail failure, then u may not need it. It is upto u to decide.
But do it after the regulators if there are any. The idea is to prevent power amp damage on PSU failure.

I forgot to mention that the second portion of the ckt should have its own supply, in the first solution.

Gajanan Phadte
 
Hi Gajanan Phadte

I like the 'easy' solution very much - it sure cuts power to the speakers when one rail (or both) fails.

I thought of one circuit myself, but don't know if it would work that well...see the attached picture. The two 2k2 resistors connecting positive and negative rail create a 0V which is compared to the ground. When one rail lowers in value, there will be a difference between inputs at the opamp, difference that will be amplified and will open the relay. This circuit will (preferably) need it's own PS, with the ground connected do the ground of the amp. What I don't know is:
- clipping of the input - a solution would be to add a potential divider so the input of the opamp will not exceed about 15V.
- output of the opamp: it can be both positive or negative, depending on which rail fails. I don't really see a problem here, but maybe in practice...

But here again, lot's of components, apart PS...and if the protection circuit fails, the relay (which is a NC) will be held close. For the moment I will use your 'easy' solution!

Erik
 

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Since you have comparatively high rail voltage, instead of a chipamp you should consider using a the National LM4702 -- in this way you can take full advantage of the high potential across the power supply rails -- and you only need a few parts.

using a regulator only burns energy that would otherwise be filling your ears -- in the case of the Overture series of amplifier chips the PSRR is pretty good and regulation is pretty superfluous.
 

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