My gainclone is humming

[Jimmy154]

I know there's lots of these "my gainclone is humming threads" but I don't know much about amps (maybe I shouldn't of tried to build one, but it's too late now) and I could not make sense of any of the other threads.

I removed the signal input wires so I can focus on what's making this humming noise.

I think I might have fired the caps also, I had something wrong with the soldering and/or cleaning of the PCB's and the caps started to smoke and also the power supply and also the soldering joints that go to the terminals and probably other stuff too. So if it's the caps, I don't know if I will ever solve this problem. I don't know if the amp can play music with damaged caps cause it does play music, only with that hum.

I have grounded the plug, and the one channel I'm testing at different places on the chassis. It still hums. I have resoldered everything and replaced all the wires. All wires are 18 gauge.

Also I think the heatsinks are grounded but I'm not sure. My chassis is aluminium, could this be the cause of my grouding/humming issues.

Should I plug in the RCA imputs and ground the shield to the chassis. Could this solve my problem? I haven't tried it cause I figure I should try to get rid of the hum before I mess around with the RCA's.

The RF/EMI interference I don't know how to fix. Should I use twisted CAT5 network cable wire pairs inside for all the wiring? This would take some time cause there 22 or 24 AWG I would have to use 2 wire pairs for to joints.

Do you (any one) think that I damaged the caps? The caps didn't start smoking I think it was V+ soldering joint under the cap that was making the smoke.

Here's a picture of it maybe some one can tell what wrong.

[mrskinny]

If you are using the non-isolated version of the chip (LM3875T ) which it looks as if you are,you MUST use isolating spacers between the chips and the case as the metal tabs on the back of the chips are live. Your input rca connectors should not be grounded to the case,use the insulating washers that they came with.Ground them,and the wire shielding to the signal ground.Its possible the hum is caused by a ground loop.I haven't found that shielding the amp or wires to matter at all- my GC is built on an open piece of wood and there are no problems with hum.

[jaycee]

Are you using the insulated version of the IC's ? If not, have you insulated the tab from your chassis ? If not, you are shorting -ve to ground if your chassis is earthed!

If your capacitors smoked and look like they are bulging, or the rubber seal at the bottom looks pushed out, they are knackered and need replacing. The hum may very well be supply ripple that should be being smoothed out by those capacitors, which isn't because the electrolyte has evaporated and the capacitors are doing nothing.

ALL ground wires should connect to one point, called the "star point", usually this is where your transformer centre tap connects, the ground from the pcb's connects, the speaker ground connects, and the ground from your RCA inputs should also return here. The RCA sockets should be ISOLATED from the chassis or it will cause a hum loop, and possible injected RFI.

If you are going to ground the case to mains earth, then you should isolate the star earth point from the chassis, earth the chassis to mains ground, and then connect star ground to mains ground via two diodes in opposing parallel, a 10R 1W resistor, and a 100nF capacitor in parallel with the diodes to prevent a ground loop via the mains earth.

Read http://sound.westhost.com/psu-wiring.htm and http://sound.westhost.com/project04.htm for more info on power supply wiring and avoiding ground loops that cause hum.

[Jimmy154]

Originally posted by jaycee

If you are going to ground the case to mains earth, then you should isolate the star earth point from the chassis, earth the chassis to mains ground, and then connect star ground to mains ground via two diodes in opposing parallel, a 10R 1W resistor, and a 100nF capacitor in parallel with the diodes to prevent a ground loop via the mains earth.

Didn't understand this part too well. But if I ground nothing; the AC plug ("earth" is the word for this?) and the PCB's, which is all that I have to ground as far as I know; they should not hum because of a ground loop, correct? Do they have a earth hole in wall sockets or plugs in England? Most of europe has 2 holes per plug, no?

I have the insulated version.

I don't just have a hum when I turn the power on. I have a loud pop and then the woofer (this is what I use for tests) either moves all the way in or out and stays in it's choosen position while it hums loudly and then when I turn the power off it pops again and goes back to normal position.

I will hook up the other channel to see if it does the same thing. Then unsolder the caps to see if they're good and switch them around. Cause only 2 of them on 2 different channels were smoking. And then if that fails I will read or vice versa maybe.

[dtm1962]

Just a question:

From your photo, it is hard to see but : do you have spacer (where you have the nut/bolt secure the board to the bottom of the case) to raise the bottom of the boards away from the case?

If I am off base, let me know.

Dan

[cjd]

Originally posted by Jimmy154
I have a loud pop and then the woofer (this is what I use for tests) either moves all the way in or out and stays in it's choosen position while it hums loudly and then when I turn the power off it pops again and goes back to normal position.

er... this sure sounds like you've got DC at output, since that's what a speaker driver does when you apply a DC current to it. Careful, 'cause you can melt a voice coil in no time that way.

C

[netgeek]

Originally posted by Jimmy154


I don't just have a hum when I turn the power on. I have a loud pop and then the woofer (this is what I use for tests) either moves all the way in or out and stays in it's choosen position while it hums loudly and then when I turn the power off it pops again and goes back to normal position.

Sounds like one of the power rails is on the output and no wonder there's smoke. Perhaps you should replace all the caps, then get a multimeter and check ALL connections and wiring before powering up again. Something is drastically wrong here - and should become obvious if you check carefully.

Good luck!

[Microchip_Usa]

Originally posted by netgeek



Sounds like one of the power rails is on the output and no wonder there's smoke. Perhaps you should replace all the caps, then get a multimeter and check ALL connections and wiring before powering up again. Something is drastically wrong here - and should become obvious if you check carefully.

Good luck!

From looking at your posted picture I can tell you a few things for a fact #1 that amp won’t be playing any time soon.
#2 your transformer is Blown “See black spot in front were insulation is melted off” #3 your speakers may also be blown from the DC that the amp is dropping on them.
#4 you need to improve your skills before burning down your parent’s home .As point of fact your power supply caps are also blown as you said there solder melted off the PCB .Not knowing that pure DC is what you have been hearing is very troubling indeed. Goes to show how even with a simple chip amp if you’re really a Newbie you can still blow stuff up! I thought in fact No one could do this bad…..I stand corrected Ill bet this cost you a bit huh?

Hey GT how about a refund for this guy?

[jaycee]

We call ground "earth" here in the UK, all UK plugs are 3 pin, no exceptions!

I think it is madness the places in the world that do not have an earth/ground terminal.

[BrianGT]

Jimmy,

Can you e-mail me some higher resolution pictures of your amplifier, or just a zoomed picture of the rectifier pcb, and amplifier pcb, so I can try to see how you have it wired up? It seems like something isn't right. I will also send you some replacement parts if some are damaged, so that you can get this thing working correctly.

Please be very careful testing your amplifier and follow the proper precautions, such as wearing safety glasses.

Drop me an e-mail and I will try to see if I can figure out where you are having problems.

--
Brian