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Old 3rd August 2014, 11:06 AM   #1
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Default hum in DIY amps, TD7293, PCB

Hi all,

I found this forum via this thread 50 Hz in home made power amp and read the linked thread as well.

Now, here is my troubles: the amplifiers hum a little bit with nothing connected, if I close their inputs to ground, the hum is gone.
The hum gets worse, if I connect the amplifiers to the crossover. When I disconnect the signal cables from the crossover, but leave them on the amps, it gets worse again. So far I twisted the signal cables (as I got good experiences noise wise with twisted pair cables in another application).

the power supply
Click the image to open in full size.

the crossover
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the amp
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amp schematic
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amp PCB
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could the Molex conns be the problem in the amp? Or where GND jumps at the jumper J1 and gets to pin 4 SGNGND of the TDA?

thanks for your help!
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Old 3rd August 2014, 11:53 AM   #2
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50 hz humm is usualy a ground loop issue.
examinate the amp and the crossover connected as a whole unit for possible ground loops.
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Old 3rd August 2014, 01:04 PM   #3
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I disconnected everything from the amps, still get the hum.

The ground meets on a reference point on the chassis, not on the power supply. There is 3 power supplies, one for the crossover, one for the amps and one for the delayed switch on of the amps, they have a green wire to that chassis point as well.
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Old 5th August 2014, 08:32 AM   #4
blu_glo is offline blu_glo  England
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How is your power supply configured I notice you have one transformer for the power amps how many windings, rectifiers etc etc are you using??
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Old 5th August 2014, 09:39 AM   #5
AndrewT is offline AndrewT  Scotland
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With the input disconnected you cannot hear any audio.
Hum in this situation is hardly a distraction.

You ONLY listen to audio with the input CONNECTED.
If there is no hum when the input is connected to the powered Source then you have no problem !
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Old 5th August 2014, 11:06 AM   #6
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the transformer windings I don't know, but its cast in and in a metal cup, 225VA, 22V secondary.
The power supply has 4 diodes TO-247 HFA, 4 22uF capacitors in parallel. Then there are 4 resistors before the main capacitors. The filter caps are 4x 10 000uF and 4x 470uF and if I remember right 4 x 2.2uF
Click the image to open in full size.

@AndrewT: so I don't have to worry about the amplifier PCBs, just get the Groundloops sorted? Small relief. Maybe I should make the supply cables to the crossover twisted and connect its ground to the star point next to the power in on the bottom right of the amp picture
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Old 8th August 2014, 10:35 PM   #7
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I drew it up, I suppose its the grounding, nothing else

Click the image to open in full size.
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Old 10th August 2014, 10:15 AM   #8
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thats how its should be, if someone is reading:

Click the image to open in full size.
remove some of the copper, so the power supply's star point is at the central tap of the transformer.

then connect everything to the star point.

Click the image to open in full size.

thanks, David!

will post when I am done rewiring
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Old 10th August 2014, 03:33 PM   #9
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The AC power SG/PE ground connects to the chassis at the power cord entrance.
The star-point Main Audio Ground connects to the chassis near the input signal jacks.
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Old 11th August 2014, 12:08 AM   #10
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they are pretty close to each other, would not make much difference. Anything will be better than it is now...
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