fail of a rail in a regulated PS

ErikdeBest · 1st March 2006, 08:05 PM

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

gmphadte · 3rd March 2006, 07:52 AM

Build a voltage supervisory ckt and use it to connect/disconnect supply to the amp.

Gajanan Phadte

ErikdeBest · 3rd March 2006, 08:20 AM

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

gmphadte · 3rd March 2006, 08:40 AM

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

pinkmouse · 3rd March 2006, 09:35 AM

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

Eva · 3rd March 2006, 10:54 AM

If you aren't already, use diodes to avoid the negative rail being pulled above ground and the positive one being pulled below ground. This usually reduces the damage in case of failure.

ErikdeBest · 3rd March 2006, 12:52 PM

Quote:

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

gmphadte · 4th March 2006, 09:12 AM

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

gmphadte · 4th March 2006, 09:25 AM

AN easy solution

gmphadte · 6th March 2006, 08:06 AM

Resistor 8K2 at collector of top sensing pnp is wrong and should nearly be double in value.
Just wrongly assumed the voltage to be around 30 instead of about 60volts.

Gajanan Phadte

1st March 2006, 08:05 PM	#1
ErikdeBest diyAudio Member Join Date: Mar 2003 Location: Itoculo	fail of a rail in a regulated PS 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

3rd March 2006, 09:35 AM	#5
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	If a rail fails in any amp, not just a gainclone, it is likely to let the smoke out of something... __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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3rd March 2006, 07:52 AM	#2
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	Build a voltage supervisory ckt and use it to connect/disconnect supply to the amp. Gajanan Phadte

3rd March 2006, 08:20 AM	#3
ErikdeBest diyAudio Member Join Date: Mar 2003 Location: Itoculo	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

3rd March 2006, 08:40 AM	#4
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	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

3rd March 2006, 10:54 AM	#6
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	If you aren't already, use diodes to avoid the negative rail being pulled above ground and the positive one being pulled below ground. This usually reduces the damage in case of failure.

6th March 2006, 08:06 AM	#10
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	Resistor 8K2 at collector of top sensing pnp is wrong and should nearly be double in value. Just wrongly assumed the voltage to be around 30 instead of about 60volts. Gajanan Phadte

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