buzz from amp when connected to speaker

Hi. This can be a difficult problem to fix. I'm assuming the amplifier works otherwise.

1. Make sure every ground path is connected to a single star ground.
2. Make sure your bias current is correct.
3. Check that your power supply capacitors are still in good order.

You can also leave the earth wire loose (make sure it does not touch anything), and touch it to ground or leave it disconnected, and see what difference you get. A ground loop can also be caused by external devices.

You can also check your input. Sometimes a floating input can cause buzzing, but in my experience you either get more or less buzzing due to the input.

My computer caused a lot of buzzing when it was my source, so try another source too.
 

sb8801

Member
2016-05-16 6:25 am
Well i would go by your guidelines, but have one question....I have just made the amp circuit on a veroboard and don`t have a chassis...there is only one ground line in my veroboard where i have connected all the ground points...and with a wire connected that line to the power supply earth....so is that a problem?
 
Disconnect that speaker NOW and check that you do not have DC at the speaker terminals.

Only after discarding that possibility you can continue testing, otherwise you risk killing the speaker.

Post whatever you measure.

And please be a little more specific about what "an old amp" means, how can anybody help you otherwise?

At least, post the schematic.
 

sb8801

Member
2016-05-16 6:25 am
Its a circuit of JC 120 amp....I have worked with only the first channel of the amplifier...Hope you guys have seen this schematic before....I have attached the PDF file with this text...
 

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sb8801

Member
2016-05-16 6:25 am
Thanks man, I would definitely try that...n I have worked with CH-1 only...starting from the guitar input to the speaker...and regarding the power supply, I made 3 outputs from the transformer....30-0 , 37-0 , 37-0....and used bridge rectifier to get +30v, +37v, -37v...Is that approach correct?
 
One more thing is i have not connected the input female jacks yet...can that cause the problem? And as i measured the voltage using a digital multi meter in A.c its 14.5 v and D.c its showing -36v....
Your amplifier needs to be connected.
Otherwise the input operates as a sensitive radio receiver with an aerial attached to it's input.

You need to add a dummy zero ohms loading plug to the input RCA.
Then you can test for this input condition.

You can replace the zero ohms dummy load with other values to see what happens @ 100r, 1k0, 10k, 100k, 1M, to see how the resistor loading affects output noise and hum and offsets.