Problems with Parellel LM 4780 GC, get -21V on outputs.

akunec · 25th February 2005, 02:18 AM

Im having problems getting my gainclone to work. As soon as I turn it on, the chip gets really hot, and i get -21V on the outputs. So I figured there was a short on my negative voltage rail. I can't find anything. There is no heatsink yet, and the back isnt touching anything so thats not the problem. My capacitors are of the right polarity. My rectifier bridge is working properly so thats not the problem.

I disconnected the wire connecting to the neg voltage rail, and now when i turn it on, i get like 3volts on the outputs, and the chip doesn't get burning hot in a second. (A side note, im not using any input capacitor because i forgot to buy them, and i heard they are optional anyways. ) When i touch the neg voltage rail to the disconnected wire, i get a little arc so there must be alot of current running through it.

Does anyone have any suggestions how to fix this?

skyraider · 25th February 2005, 02:53 AM

I had similiar problem before but not with the same chip. My circuit is inverting(part of the BPA200 circuit) and it shorted to negative rail if the input is left open. Connecting the input to a source will bring the dc offset to a few mA.
I guess a resistor from input to ground will solve this. But I used a buffer at the input because a buffer is required for the design anyway..

akunec · 25th February 2005, 03:04 AM

I think that is a different problem then mine as I do have a 47kohm from input to ground.

soundNERD · 25th February 2005, 03:08 AM

try something closer to 500 ohm or 1k

akunec · 25th February 2005, 03:47 AM

Wouldn't 500 or 1kohm affect the input impedance? National recommends between 10k and 47kohm.

akunec · 25th February 2005, 03:49 AM

Also, I noticed that there are several V+ and V- inputs on the 4780. I only have to use one of each right?

Mr Teal · 25th February 2005, 04:14 AM

You should connect them all. Different pins may go to different parts of the chip, and the LM4780 is really two 3886 dies in one package, so each need their own power connectors.

25th February 2005, 02:18 AM	#1
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Problems with Parellel LM 4780 GC, get -21V on outputs. Im having problems getting my gainclone to work. As soon as I turn it on, the chip gets really hot, and i get -21V on the outputs. So I figured there was a short on my negative voltage rail. I can't find anything. There is no heatsink yet, and the back isnt touching anything so thats not the problem. My capacitors are of the right polarity. My rectifier bridge is working properly so thats not the problem. I disconnected the wire connecting to the neg voltage rail, and now when i turn it on, i get like 3volts on the outputs, and the chip doesn't get burning hot in a second. (A side note, im not using any input capacitor because i forgot to buy them, and i heard they are optional anyways. ) When i touch the neg voltage rail to the disconnected wire, i get a little arc so there must be alot of current running through it. Does anyone have any suggestions how to fix this?

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25th February 2005, 02:53 AM	#2
skyraider diyAudio Member Join Date: Jan 2004 Location: malaysia	I had similiar problem before but not with the same chip. My circuit is inverting(part of the BPA200 circuit) and it shorted to negative rail if the input is left open. Connecting the input to a source will bring the dc offset to a few mA. I guess a resistor from input to ground will solve this. But I used a buffer at the input because a buffer is required for the design anyway..

25th February 2005, 03:04 AM	#3
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	I think that is a different problem then mine as I do have a 47kohm from input to ground.

25th February 2005, 03:08 AM	#4
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	try something closer to 500 ohm or 1k

25th February 2005, 03:47 AM	#5
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Wouldn't 500 or 1kohm affect the input impedance? National recommends between 10k and 47kohm.

25th February 2005, 03:49 AM	#6
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Also, I noticed that there are several V+ and V- inputs on the 4780. I only have to use one of each right?

25th February 2005, 04:14 AM	#7
Mr Teal diyAudio Member Join Date: Nov 2003 Location: Saskatoon	You should connect them all. Different pins may go to different parts of the chip, and the LM4780 is really two 3886 dies in one package, so each need their own power connectors.

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