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Old 16th May 2016, 06:36 AM   #1
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Default buzz from amp when connected to speaker

I have made an amp from an old circuit, but when i am connecting it to the speaker( 8 ohm) as per the amp specification, i am getting a loud noise, independent of the pots. Is the circuit wrong? Or what can be the probable cause?
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Old 16th May 2016, 11:37 AM   #2
Mrcloc is offline Mrcloc  South Africa
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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.
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Old 16th May 2016, 12:34 PM   #3
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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?
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Old 16th May 2016, 12:46 PM   #4
JMFahey is offline JMFahey  Argentina
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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.
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Old 16th May 2016, 12:48 PM   #5
Mrcloc is offline Mrcloc  South Africa
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What JMFahey said, first and foremost!

You could post a pic or two.... I'm sure it would be eye-candy anyway...
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Old 17th May 2016, 04:38 AM   #6
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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...
Attached Files
File Type: pdf roland-jc120-jc160-amp-circuit-diagram.pdf (818.4 KB, 18 views)
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Old 17th May 2016, 10:07 AM   #7
Mrcloc is offline Mrcloc  South Africa
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I will have a look at this. I'm assuming you only built the power amplifier bit to the top right? And your board?
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Old 17th May 2016, 10:08 AM   #8
Mrcloc is offline Mrcloc  South Africa
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Looks like it's spec'd to have an offset at the output. Put a 1000uF capacitor between output and speaker, and see what happens. Positive of capacitor must be at the amplifier side.
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Old 17th May 2016, 10:45 AM   #9
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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?
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Old 17th May 2016, 01:02 PM   #10
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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....
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