Humming from Amp with/without input - diyAudio
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Old 16th October 2011, 01:09 AM   #1
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Default Humming from Amp with/without input

My amp sounds fine but it recently started humming while the input is unplugged from it. It only does this on channel 1, channel 2 is dead silent the way channel 1 was. Both channels sound fine while being played, and the hum doesn't increase or lessen when the input is plugged in.

The amp is a AD30HTC, I have changed the tubes and same results. Where should I check? I assume channel 1 is somehow not grounding or its recognizing an input even when there is none, so a leakage.

any help would be appreciated!
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Old 16th October 2011, 02:54 AM   #2
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Quote:
Originally Posted by Mantua404 View Post
My amp sounds fine but it recently started humming while the input is unplugged from it. It only does this on channel 1, channel 2 is dead silent the way channel 1 was. Both channels sound fine while being played, and the hum doesn't increase or lessen when the input is plugged in.

The amp is a AD30HTC, I have changed the tubes and same results. Where should I check? I assume channel 1 is somehow not grounding or its recognizing an input even when there is none, so a leakage.

any help would be appreciated!
I misworded the last part... I believe its not recognizing when the input is removed on channel 1.
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Old 17th October 2011, 08:20 AM   #3
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Input jacks have a shorting switch on them, when you unplug the input it shorts across, it's this short that makes it quiet.

It may be that the switch isn't working?.
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Old 18th October 2011, 11:00 PM   #4
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it would be unlikely as it goes silent on channel 2 but not channel 1. I was thinking perhaps a filter cap but figured i'd ask here where the most intelligent place to start would be.
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Old 19th October 2011, 02:10 AM   #5
Enzo is offline Enzo  United States
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FIlter cap wouldn;t know what channel you were in, it would just hum all the time.

Since plugging into the jack stops the hum, I like the jack explanation. If one channel has higher gain than the other, an unterminated input would hum more when the higher gain channel is slected. Ther are not extra mute contacts on this one.
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Old 19th October 2011, 02:28 AM   #6
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The humming doesnt stop with the jack being plugged in... ok according to the schematic the AD30 is a 2 channel amp completely separate. When a jack isnt plugged in the amp should be silent... EXCEPT this amp has malfunctioned and only on channel 1 it hums as if a jack is plugged in and when you do plug one in it doesnt get louder it just hums the exact same.

When the jack is unplugged channel 2 is silent but channel 1 the amp hums as if it had a source plugged in.
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Old 19th October 2011, 02:45 AM   #7
Enzo is offline Enzo  United States
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I am looking at the AD30VT, is that different from yours?
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Old 19th October 2011, 09:36 AM   #8
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http://users.telenet.be/orangefg/OFG...C_mainpcb1.pdf

thats the schem.
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Old 19th October 2011, 10:20 AM   #9
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Quote:
Originally Posted by Mantua404 View Post
The humming doesnt stop with the jack being plugged in... ok according to the schematic the AD30 is a 2 channel amp completely separate. When a jack isnt plugged in the amp should be silent... EXCEPT this amp has malfunctioned and only on channel 1 it hums as if a jack is plugged in and when you do plug one in it doesnt get louder it just hums the exact same.
This isn't what you said originally - and you're still not making it very clear?.

Does it hum when you actually use it! - plugging an unterminated jack lead WILL make it hum, as it defeats the muting switch.

If it's OK in use, the it can only be the muting switch.
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Old 19th October 2011, 10:28 AM   #10
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OK - I see you've posted the schematic now (which is quite bizarre? - why have two duplicated identical preamps and phase-splitters?).

But assuming that's the correct circuit, and it only has one input socket? (at the bottom left) then I would suggest telling us what effect the various controls have, the Gain and EQ ones. These controls isolate the various sections.
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