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My amplifier hums whenever no input is connected

rinchan6

Aug 27, 2013
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The title says it all.

My amplifier is about 40W mono amp, sitting on a good breadboard properly grounded (no chassis though). It sounds really good but whenever I remove the input jacks the hum kicks in. With jacks connected, my output voltage is just about 10mV DC. My amp has no volume control.

Where should I start looking at?
 

Arouse1973

Adam
Dec 18, 2013
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Can you supply a circuit diagram and maybe a couple of good pictures.
Adam
 

rinchan6

Aug 27, 2013
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sorry very late reply
 

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BobK

Jan 5, 2010
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If C7/C8 and C9/C10 are the only filtering on the + and - 24V supplies, then they need to be a lot bigger. Can you show the power supply circuit?

Bob
 

rinchan6

Aug 27, 2013
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oh my PS is straightforward it has 4700uF caps on positive and negative nothing else, almost looks like this just bigger caps
power_supply.gif
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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whenever I remove the input jacks the hum kicks in.

You could teach the amplifier the words... :D

Where should I start looking at?

With the input open circuit the relatively high input impedance will pick up noise. If you have jacks with a switch on them, you might be able to use it to short the input when you remove the jack.

Alternatively, proper construction (a breadboard is unlikely to be very good) with layout to reduce hum and shielded cable to the inputs will help.
 

rinchan6

Aug 27, 2013
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Your suggestion is useful and it gets rid of the hum, however, it doesn't feel like a user friendly amplifier since I built it as home audio system in which not just me uses it. I just found out placing a potentiometer as volume control helps remove this hum at low volumes and I think this is what I lack. Hopefully I would transfer it to a PCB but I'm still learning DIY PCB stuff. Thanks for the replies. :)
 
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