Chipamp working OK, then bad distortion

Gecko123 · 23rd April 2009, 08:30 PM

Well I constructed my lm3875 chip amp the other day, had some slight hum issues which I still need to figure out, but when you play music you can barely hear the hum. So I suppose I was driving it fairly hard, but I wanted to see how hot the heatsinks would get, and the sound was not distorting, then about an hour into playing tunes, the speakers just belched out really loud and bad distortion, I thought my speakers were toast, but they were fine after I checked em. So what could have caused the issue? The heatsinks were barely warmer than my hand at the time, I measured the secondary voltages and DC offset and all was good, exactly as specced in the chipamp building tutorial, very curious what the issue was, I will post pics soon so you can see my setup.

Atilla · 23rd April 2009, 09:12 PM

If DC is fine then it could be problems with the PSU - bad connections, bad caps, or it could be thermal protection kicking in.

Have you tried touching the chips, are they mounted on those heatsinks nice and tight, using paste?

Can you tell us more about your setup - transformer rating, capacitance, rail voltage, etc.. What are the voltage limits of your PSU caps?

Check for bad solder connections, lose wires, or anything that might cause a component to disconnect or and/or make a short circuit.

Finally, pics will help people identify potential problems as well.

Gecko123 · 24th April 2009, 04:29 AM

Here's some pics

http://i524.photobucket.com/albums/c...8/IMG_0361.jpg
http://i524.photobucket.com/albums/c...8/IMG_0362.jpg
http://i524.photobucket.com/albums/c...8/IMG_0363.jpg
http://i524.photobucket.com/albums/c...8/IMG_0364.jpg
http://i524.photobucket.com/albums/c...8/IMG_0365.jpg

So the transformer is Plitron 300VA, 2x25V, the kit is the classic kit from audiosector.com
http://www.audiosector.com/lm3875.shtml
So i guess the capacitance would be 3000uf per channel. Secondary voltages came out at right around 30V or just over, cant remember exactly, since I have the 10uf smoothing caps installed on rectifier board, and I dont have a volume pot installed so the voltages at the speaker terminals were around 69-70mV.

Gecko123 · 24th April 2009, 04:29 AM

Ok heres some more recent data.
Right channel at speaker terminals - 72.6mV
Left channel at speaker terminals - 86.9 mV

V- to PG- 32.2mV
V+ to PG+ 32.2mV

And the solid copper wire you see there is connected from one channel board OG to the other channel board OG, and I have left speaker(-), Right speaker(-), PG-, PG+ all connected to that, and then that is connected to the chassis grd. Why do I still have hum as well? and do those speaker terminal voltages look right? seem high. thanks for anyones help.

Redshift187 · 24th April 2009, 04:42 AM

You mean 32.2 V, not mV right?

pacificblue · 24th April 2009, 08:09 AM

Sounds like the protection system kicking in. Either too hot or clipping. Probably too hot. A 2x25 V transformer gives quite high rails for the isolated package even with the enormous heatink you have. The IC simply cannot transfer the heat fast enough to that heatsink.
You have three options.
- Replace the LM3875TF with the LM3875T.
- Add a fan.
- Reduce the volume.

Atilla · 24th April 2009, 08:27 AM

What impedance are your speakers? Are you trying to drive 4ohm speakers on 32V rails? That can get the chip very hot, very fast.

I've a LM3886 setup with much smaller heatsinks though, and with those and slightly voltages I don't seem to have any problems. I'm using the TF packages as well. Is lm3875 worse at handling heat?

martinbls · 24th April 2009, 09:04 AM

Quote: "And the solid copper wire you see there is connected from one channel board OG to the other channel board OG, and I have left speaker(-), Right speaker(-), PG-, PG+ all connected to that, and then that is connected to the chassis grd."

Are you sure that this is the right way to do it? From what I read I guess only CHG of both amp boards have to be connected to chassis ground (earth), perhaps with a 10R resistor. OG is for speaker minus rail only, and PG- and PG+ have to be connected only to PG- and PG+ at rectifier board. Perhaps it's worth a try?

I know that there is already a connection between OG of speaker and CHG on amp boards, but the exact way you connect all this might be importand, especially when it comes to hum problems.

Regards - martin

martinbls · 24th April 2009, 10:02 AM

Hi,

I'm sorry, but perhaps I'm wrong! Just found this post

http://www.diyaudio.com/forums/show...7773#post787773

and I think this is exactly the way you did it and it's supposed to work well. But this concept of grounding is new to me, I thought I read somewhere that only CHG of both boards have to be connected to chassis ground...

Well, now I'm asking myself how I am going to connect all this on MY amp project! Funny...

Regards - martin

martinbls · 24th April 2009, 10:04 AM

Sorry again, link doesn't work.

http://www.diyaudio.com/forums/showt...773#post787773

23rd April 2009, 08:30 PM	#1
Gecko123 diyAudio Member Join Date: Jan 2009	Chipamp working OK, then bad distortion Well I constructed my lm3875 chip amp the other day, had some slight hum issues which I still need to figure out, but when you play music you can barely hear the hum. So I suppose I was driving it fairly hard, but I wanted to see how hot the heatsinks would get, and the sound was not distorting, then about an hour into playing tunes, the speakers just belched out really loud and bad distortion, I thought my speakers were toast, but they were fine after I checked em. So what could have caused the issue? The heatsinks were barely warmer than my hand at the time, I measured the secondary voltages and DC offset and all was good, exactly as specced in the chipamp building tutorial, very curious what the issue was, I will post pics soon so you can see my setup.

24th April 2009, 08:09 AM	#6
pacificblue diyAudio Member Join Date: May 2008	Sounds like the protection system kicking in. Either too hot or clipping. Probably too hot. A 2x25 V transformer gives quite high rails for the isolated package even with the enormous heatink you have. The IC simply cannot transfer the heat fast enough to that heatsink. You have three options. - Replace the LM3875TF with the LM3875T. - Add a fan. - Reduce the volume. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

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23rd April 2009, 09:12 PM	#2
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	If DC is fine then it could be problems with the PSU - bad connections, bad caps, or it could be thermal protection kicking in. Have you tried touching the chips, are they mounted on those heatsinks nice and tight, using paste? Can you tell us more about your setup - transformer rating, capacitance, rail voltage, etc.. What are the voltage limits of your PSU caps? Check for bad solder connections, lose wires, or anything that might cause a component to disconnect or and/or make a short circuit. Finally, pics will help people identify potential problems as well.

24th April 2009, 04:29 AM	#3
Gecko123 diyAudio Member Join Date: Jan 2009	Here's some pics http://i524.photobucket.com/albums/c...8/IMG_0361.jpg http://i524.photobucket.com/albums/c...8/IMG_0362.jpg http://i524.photobucket.com/albums/c...8/IMG_0363.jpg http://i524.photobucket.com/albums/c...8/IMG_0364.jpg http://i524.photobucket.com/albums/c...8/IMG_0365.jpg So the transformer is Plitron 300VA, 2x25V, the kit is the classic kit from audiosector.com http://www.audiosector.com/lm3875.shtml So i guess the capacitance would be 3000uf per channel. Secondary voltages came out at right around 30V or just over, cant remember exactly, since I have the 10uf smoothing caps installed on rectifier board, and I dont have a volume pot installed so the voltages at the speaker terminals were around 69-70mV.

24th April 2009, 04:42 AM	#5
Redshift187 diyAudio Member Join Date: Aug 2008	You mean 32.2 V, not mV right?

24th April 2009, 08:27 AM	#7
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	What impedance are your speakers? Are you trying to drive 4ohm speakers on 32V rails? That can get the chip very hot, very fast. I've a LM3886 setup with much smaller heatsinks though, and with those and slightly voltages I don't seem to have any problems. I'm using the TF packages as well. Is lm3875 worse at handling heat?

24th April 2009, 09:04 AM	#8
martinbls diyAudio Member Join Date: Feb 2009	Quote: "And the solid copper wire you see there is connected from one channel board OG to the other channel board OG, and I have left speaker(-), Right speaker(-), PG-, PG+ all connected to that, and then that is connected to the chassis grd." Are you sure that this is the right way to do it? From what I read I guess only CHG of both amp boards have to be connected to chassis ground (earth), perhaps with a 10R resistor. OG is for speaker minus rail only, and PG- and PG+ have to be connected only to PG- and PG+ at rectifier board. Perhaps it's worth a try? I know that there is already a connection between OG of speaker and CHG on amp boards, but the exact way you connect all this might be importand, especially when it comes to hum problems. Regards - martin

24th April 2009, 10:02 AM	#9
martinbls diyAudio Member Join Date: Feb 2009	Hi, I'm sorry, but perhaps I'm wrong! Just found this post http://www.diyaudio.com/forums/show...7773#post787773 and I think this is exactly the way you did it and it's supposed to work well. But this concept of grounding is new to me, I thought I read somewhere that only CHG of both boards have to be connected to chassis ground... Well, now I'm asking myself how I am going to connect all this on MY amp project! Funny... Regards - martin

24th April 2009, 10:04 AM	#10
martinbls diyAudio Member Join Date: Feb 2009	Sorry again, link doesn't work. http://www.diyaudio.com/forums/showt...773#post787773

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