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Double PCB LM1875 problem hum need help
Double PCB LM1875 problem hum need help
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Old 29th November 2020, 05:17 PM   #1
denbret is offline denbret  Canada
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Default Double PCB LM1875 problem hum need help

Hello everyone,

I have 2 purchased PCBs already assembled made by Zero Zone.

I have only one toroidal transformer of 100VA (18-0-18v), the transformer has no ground wire. If I plug in a mono PCB at a time, there is no hum, but as soon as I add the 2nd, the hum appears.

Screws on PCBs don't seem to have any grounds, so that suggests to me that PCBs aren't grounded. In addition there is a separate rectification on each PCB.

If you have any suggestions or ideas that can help me, that would be much appreciated. I also want to mention that I am quite new to electronics.

Thank you.
Denis.
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Last edited by denbret; 29th November 2020 at 05:29 PM.
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Old 30th November 2020, 11:49 PM   #2
denbret is offline denbret  Canada
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@ moderator can you move this thread to the "Solid State" section please??? And remove the word LM1875.
thank you
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Old 30th November 2020, 11:55 PM   #3
jwilhelm is offline jwilhelm  Canada
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Is the hum present with the inputs shorted? Is the audio ground isolated from earth ground and the chassis?
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Old 1st December 2020, 12:20 PM   #4
OldDIY is offline OldDIY
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Use only one PCB power supply (rectifier bridge). Connect the second with three wires (+, -, zero) in parallel after the rectifier bridge of the first directly to the large supply capacitors.
Capacitors to capacitors, is that clear?
You have to experiment with signal ground. Take isolated RCA connectors. Connect everyone to their PCB.

Last edited by OldDIY; 1st December 2020 at 12:23 PM.
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Old 1st December 2020, 04:09 PM   #5
OldDIY is offline OldDIY
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# 4 is wrong. Take a close look at the power, capacitor, and signal ground circuits. Select a grounding point to the chassis.
Shielded input wires. You have them intersect with the exit. Route the outlet along the bottom of the case.
I don't see where your diodes are? One or two bridges? Is there a podium under the torus?

Last edited by OldDIY; 1st December 2020 at 04:12 PM.
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Old 2nd December 2020, 12:28 AM   #6
Vovk Z is offline Vovk Z  Ukraine
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Amp PCBs are intended to use as 'dual mono'?
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Old 2nd December 2020, 03:35 AM   #7
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by denbret View Post
I also want to mention that I am quite new to electronics.

Thank you.
Denis.
I am VERY WORRIED at your mains wiring.

Power cable black and white immediately go to a screw terminal strip where they change colours, so you lose reference, then go to various places in a not too clear path, but worst is you show a plain LED connected straight to mains voltage , the other end is lost in a cable tangle.

Connections are floating, completely insecure.

No need to repeat that MAINS VOLTAGE IS DEADLY.
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Old 2nd December 2020, 04:30 PM   #8
Mark Whitney is offline Mark Whitney  Netherlands
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Two rectifier bridges connected to a single transformer is going to create hum.
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Old 4th December 2020, 05:17 PM   #9
denbret is offline denbret  Canada
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Hi everyone,

thanks for your wise responses.

I will read your messages carefully and get back to you as soon as possible.
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Old 6th December 2020, 07:42 PM   #10
denbret is offline denbret  Canada
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Quote:
Originally Posted by JMFahey View Post
I am VERY WORRIED at your mains wiring.

Power cable black and white immediately go to a screw terminal strip where they change colours, so you lose reference, then go to various places in a not too clear path, but worst is you show a plain LED connected straight to mains voltage , the other end is lost in a cable tangle.

Connections are floating, completely insecure.

No need to repeat that MAINS VOLTAGE IS DEADLY.
The led is not connected to the AC, we cannot see clearly on the photo, but it is connected to a DC transformer. This LED has worked for years without problems. Yes I agree, I put a terminal block and it is not standard and safe. It looks worse than it is. The wiring that goes from front to the switch is bent and above the wiring below. It's an old project that I got back. I've seen a lot worse in tube amps. But I will follow your advice and I will redo the wiring, remove the screw terminal block and solder the wires cleanly.
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