LM3875 gainclone problem

cjv998 · 1st July 2007, 05:17 PM

Hi everyone, I just finished building my LM3875 gainclone (Peter Daniel's kit), and I have a problem with the left channel. First off, when I checked the voltages at each of the pins of the LM3875, when I got to pin 4 (the V- connection), I got an arc. Everything still works fine tough. At any rate, my voltages are fine at the points they connect to the amp PCB (+/-36V DC) , and all the other pins are fine. Also, when I turn up the volume (increase the power) too much, I get overheating and I believe the op amp's Spike protection kicks in (not sure, all I know is the sound gets very static-y (but you can still make out the music), and messes up the picture on my LCD monitor). Keep in mind, this is only the left channel; the right channel seems to work flawlessly. Can anyone clue me in on what's happening?

Oh, also, I looked over the solder connections on the PCB, and everything looks alright there, and it doesn't look like anything is contacting something it shouldn't be.

I was thinking it may be a heat dissipation issue, but then why would it only happen on one channel when both channels are constructed nearly identically, and also, then why is pin 4 arcing?

My othet thought was a faulty op amp, but then why does it work seemingly fine until I turn the volume past a certain point?

If it helps any, my heat sinks are two 3" by 4" finned aluminum pieces bolted to my 2mm thick aluminum chassis, and the op amp is then attached to the chassis on the other side of the heat sink, with a thin thermal conducting film (I believe it's called Thermasil) in between the op amp and the chassis.

impsick · 2nd July 2007, 12:22 AM

could be a bad chip but check for solder splashes/shorts. Post up some pics, close ups.
i dont think one channel would get hot more then the other. what kind of transformer are you using?

cjv998 · 2nd July 2007, 12:58 AM

The transformer is a 250VA Avel Lindberg with dual 25V secondaries. I'll try and get some pics later this evening.

Actually, it seems okay as long as I am playing something with it (the left channel still gets warmer than the right one though). However, when the sound stops, it starts warming up.

cjv998 · 2nd July 2007, 02:54 PM

I noticed this morning that the left channel has more gain as well (at least, it plays louder than the right one). Therefore, I thought I may have switched the resistors, but they seemed correct. Any thoughts?

impsick · 2nd July 2007, 03:18 PM

so you have the correct resisters in place? feedback resister? its either something very simple being over looked or the chip is defective. double check those resisters.

cjv998 · 2nd July 2007, 05:07 PM

I checked the resistors, and they match up (the left and right channels each have the same resistors in the same places), and I know that I got the right channel correct.

cjv998 · 3rd July 2007, 03:13 PM

In case anyone was wondering, my problem has been at least partially fixed. It turns out I had a short in the coax cable that connected the right channel's RCA input jack to the 10k pot, which explains why the right channel was quieter. I'm also going to remove the heat sinks and put thermal paste in between them and the chassis to aid heat conduction, and I'll probably replace the Thermalsil with thermal paste as well, since the Thermalsil pieces don't cover the whole surface of the chip. In addition to this, after looking closer, some of my solder connections are pretty bad, so I'm going to go over those again (that's what happens when you're up at 3am trying to solder with an iron where only one side of the tip heats up...yeah, figure that out

).

impsick · 3rd July 2007, 08:51 PM

Quote:

Originally posted by cjv998
(that's what happens when you're up at 3am ).

no kidding. good to hear you figured that out. have fun

cjv998 · 7th July 2007, 10:44 PM

Okay, I re-ran all my signal cables, and used thermal paste like I mentioned above (which helped heat transfer to the heat sinks a ton, by the way; I'm impressed), but the left channel op amp still gets very hot. A minute or two after being turned on, it's very hot to the touch, whereas the right channel will still be almost room temperature. I'm thinking I need a new op amp for the left channel (I probably messed this one up when I got the arcing by pin 4, which is definitely scorched black now), can anyone confirm this? Also, does anyone know where I can get a new feedback resistor (22k Phoenix SFR16S that comes with Peter Daniel's basic kit, I couldn't find them on Digikey or Mouser)? I don't need one, but it would be easier to solder on to the new chip if I have longer leads.

P.S.: I'll hopefully get pictures tonight, I have to wait for my batteries to charge.

gootee · 7th July 2007, 11:08 PM

Maybe you could cut off the chip pins that that resistor is soldered to, near the chip body, and then de-solder (from the board) the portions of the pins that have the resistor soldered to them, cut off the tips of the old pins, and solder all of that to the new chip's pins.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

1st July 2007, 05:17 PM	#1
cjv998 diyAudio Member Join Date: Jun 2007	LM3875 gainclone problem Hi everyone, I just finished building my LM3875 gainclone (Peter Daniel's kit), and I have a problem with the left channel. First off, when I checked the voltages at each of the pins of the LM3875, when I got to pin 4 (the V- connection), I got an arc. Everything still works fine tough. At any rate, my voltages are fine at the points they connect to the amp PCB (+/-36V DC) , and all the other pins are fine. Also, when I turn up the volume (increase the power) too much, I get overheating and I believe the op amp's Spike protection kicks in (not sure, all I know is the sound gets very static-y (but you can still make out the music), and messes up the picture on my LCD monitor). Keep in mind, this is only the left channel; the right channel seems to work flawlessly. Can anyone clue me in on what's happening? Oh, also, I looked over the solder connections on the PCB, and everything looks alright there, and it doesn't look like anything is contacting something it shouldn't be. I was thinking it may be a heat dissipation issue, but then why would it only happen on one channel when both channels are constructed nearly identically, and also, then why is pin 4 arcing? My othet thought was a faulty op amp, but then why does it work seemingly fine until I turn the volume past a certain point? If it helps any, my heat sinks are two 3" by 4" finned aluminum pieces bolted to my 2mm thick aluminum chassis, and the op amp is then attached to the chassis on the other side of the heat sink, with a thin thermal conducting film (I believe it's called Thermasil) in between the op amp and the chassis.

2nd July 2007, 12:22 AM	#2
impsick diyAudio Member Join Date: Dec 2006	could be a bad chip but check for solder splashes/shorts. Post up some pics, close ups. i dont think one channel would get hot more then the other. what kind of transformer are you using? __________________ "Nothing happens for a reason. There are just reasons why things happen." Quote me on that. http://stores.ebay.com/AudioGrade?_rdc=1

2nd July 2007, 03:18 PM	#5
impsick diyAudio Member Join Date: Dec 2006	so you have the correct resisters in place? feedback resister? its either something very simple being over looked or the chip is defective. double check those resisters. __________________ "Nothing happens for a reason. There are just reasons why things happen." Quote me on that. http://stores.ebay.com/AudioGrade?_rdc=1

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2nd July 2007, 12:58 AM	#3
cjv998 diyAudio Member Join Date: Jun 2007	The transformer is a 250VA Avel Lindberg with dual 25V secondaries. I'll try and get some pics later this evening. Actually, it seems okay as long as I am playing something with it (the left channel still gets warmer than the right one though). However, when the sound stops, it starts warming up.

2nd July 2007, 02:54 PM	#4
cjv998 diyAudio Member Join Date: Jun 2007	I noticed this morning that the left channel has more gain as well (at least, it plays louder than the right one). Therefore, I thought I may have switched the resistors, but they seemed correct. Any thoughts?

2nd July 2007, 05:07 PM	#6
cjv998 diyAudio Member Join Date: Jun 2007	I checked the resistors, and they match up (the left and right channels each have the same resistors in the same places), and I know that I got the right channel correct.

3rd July 2007, 03:13 PM	#7
cjv998 diyAudio Member Join Date: Jun 2007	In case anyone was wondering, my problem has been at least partially fixed. It turns out I had a short in the coax cable that connected the right channel's RCA input jack to the 10k pot, which explains why the right channel was quieter. I'm also going to remove the heat sinks and put thermal paste in between them and the chassis to aid heat conduction, and I'll probably replace the Thermalsil with thermal paste as well, since the Thermalsil pieces don't cover the whole surface of the chip. In addition to this, after looking closer, some of my solder connections are pretty bad, so I'm going to go over those again (that's what happens when you're up at 3am trying to solder with an iron where only one side of the tip heats up...yeah, figure that out ).

7th July 2007, 10:44 PM	#9
cjv998 diyAudio Member Join Date: Jun 2007	Okay, I re-ran all my signal cables, and used thermal paste like I mentioned above (which helped heat transfer to the heat sinks a ton, by the way; I'm impressed), but the left channel op amp still gets very hot. A minute or two after being turned on, it's very hot to the touch, whereas the right channel will still be almost room temperature. I'm thinking I need a new op amp for the left channel (I probably messed this one up when I got the arcing by pin 4, which is definitely scorched black now), can anyone confirm this? Also, does anyone know where I can get a new feedback resistor (22k Phoenix SFR16S that comes with Peter Daniel's basic kit, I couldn't find them on Digikey or Mouser)? I don't need one, but it would be easier to solder on to the new chip if I have longer leads. P.S.: I'll hopefully get pictures tonight, I have to wait for my batteries to charge.

7th July 2007, 11:08 PM	#10
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Maybe you could cut off the chip pins that that resistor is soldered to, near the chip body, and then de-solder (from the board) the portions of the pins that have the resistor soldered to them, cut off the tips of the old pins, and solder all of that to the new chip's pins. - Tom Gootee http://www.fullnet.com/~tomg/index.html

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