LM3875 gainclone help!!!

DavidWardlaw · 15th March 2007, 07:54 PM

Hello all,

I recently put together a gainclone amplifier from Peter Daniel's website. Obviously it doesn't work. I did everything exactly as instructed, except the chassis ground is not connected (CHG). There is no sound whatsoever coming from the amp and the 4 inner diodes in the power supply are extremely hot compared to the other diodes. When I disconnected the PS from the amp, 35 Volts were passing between V+ and V-. I have attachted (high) quality pictures of the front of the amp. If anyone knows what is wrong with it, please help!!!

DavidWardlaw · 15th March 2007, 07:56 PM

The back picture is attachted...

SpittinLLama · 15th March 2007, 10:23 PM

If you have a meter, check the supply rails are where they should be first. Then we can move forward from there.

-SL

DavidWardlaw · 15th March 2007, 11:30 PM

Quote:

If you have a meter, check the supply rails are where they should be first.

SpittinLLama, how do i do that?

Tristanc1 · 16th March 2007, 12:15 AM

Quote:

how do i do that?

Make sure you're getting +35 or so V on V+ and -35 or so V on V-.

Does your transformer have dual secondaries? Is your amp set up as a monoblock or stereo? I went through some problems with my second gainclone aswell so I might be able to help you out...

Tristan

DavidWardlaw · 16th March 2007, 12:30 AM

I checked and made sure that there is about +35 Volts on V+ and -35 Volts on V-. I don't know what you mean by monoblock...see pictures...they may help...As far as the transformer, all I know is that there are 2 wires for the input ~120 VAC and 2 wires for the output ~27-28 VAC.

Tristanc1 · 16th March 2007, 12:50 AM

Carefully measure pin 1 on the lm3875 from ground and you should get +35, then measure pin 4 you should get -35 Volts there, careful not to slip and short anything though, it's easy to slip as I found out....

Also, do you have a heatsink connected? That was my problem everything was hooked up correctly, but the heatsink I had was not up to the task, so all i'd get was very quiet distortion, that would cut in and out, and no sound whatsoever if I had no heatsink hooked up, which was evidently the chips protection kicking in. Might be the problem, I didn't see a heatsink in the pics.

Good luck!
Tristan

kuldeepsingh · 16th March 2007, 01:34 AM

Kindly clean your PCB track side with a brush, in your picture i can see small wire burrs/ pices lying here and there..These may cause short circuit or heavy leakage currents.

DavidWardlaw · 16th March 2007, 01:43 AM

Here is what I did: I checked the Voltage between pin 1 and ground and got 1 Volt. Then I disconnected PG+ and PG- and got 35 Volts. What is PG??? Does the amplifier need PG???

I will clean PCB when I get a chance...

Greg Erskine · 16th March 2007, 04:17 AM

Hi DavidWardlaw,

Your transformer should have 4 wires on the output side.

PG = Power Ground, there are 2, PG+ and PG-

Did the fuses blow?

You might want to have a look here at pictures of the BrianGT version of this amp for wiring examples. http://www.briangt.com/gallery/nigc

regards

16th March 2007, 12:50 AM	#7
Tristanc1 diyAudio Member Join Date: Jan 2006 Location: edmonton	Carefully measure pin 1 on the lm3875 from ground and you should get +35, then measure pin 4 you should get -35 Volts there, careful not to slip and short anything though, it's easy to slip as I found out.... Also, do you have a heatsink connected? That was my problem everything was hooked up correctly, but the heatsink I had was not up to the task, so all i'd get was very quiet distortion, that would cut in and out, and no sound whatsoever if I had no heatsink hooked up, which was evidently the chips protection kicking in. Might be the problem, I didn't see a heatsink in the pics. Good luck! Tristan __________________ When I was young I really wanted a bicycle, I used to pray every night for god to bring me one. But as I got older I realized god didn't work that way so I stole one and begged for forgiveness.

16th March 2007, 04:17 AM	#10
Greg Erskine diyAudio Member Join Date: Jan 2002 Location: Sydney/Australia	Hi DavidWardlaw, Your transformer should have 4 wires on the output side. PG = Power Ground, there are 2, PG+ and PG- Did the fuses blow? You might want to have a look here at pictures of the BrianGT version of this amp for wiring examples. http://www.briangt.com/gallery/nigc regards __________________ Greg Erskine

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15th March 2007, 10:23 PM	#3
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	If you have a meter, check the supply rails are where they should be first. Then we can move forward from there. -SL

16th March 2007, 12:30 AM	#6
DavidWardlaw diyAudio Member Join Date: Jul 2006	I checked and made sure that there is about +35 Volts on V+ and -35 Volts on V-. I don't know what you mean by monoblock...see pictures...they may help...As far as the transformer, all I know is that there are 2 wires for the input ~120 VAC and 2 wires for the output ~27-28 VAC.

16th March 2007, 01:34 AM	#8
kuldeepsingh diyAudio Member Join Date: Aug 2006 Location: Chennai, India	Kindly clean your PCB track side with a brush, in your picture i can see small wire burrs/ pices lying here and there..These may cause short circuit or heavy leakage currents.

16th March 2007, 01:43 AM	#9
DavidWardlaw diyAudio Member Join Date: Jul 2006	Here is what I did: I checked the Voltage between pin 1 and ground and got 1 Volt. Then I disconnected PG+ and PG- and got 35 Volts. What is PG??? Does the amplifier need PG??? I will clean PCB when I get a chance...

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