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Old 18th September 2012, 04:02 PM   #1
teleman is offline teleman  Norway
diyAudio Member
Join Date: Jul 2008

I have just finished constructing a mic/piezo blender preamp circuit,but experiencing one persisting problem with it. The circuit works as it should but when I touch the metal mute switch ( DPDT-centre "off") or the aluminum plate on which all the controls ( all plastic "Omeg" controls) are mounted,then there is considerable hum. However, when the "mute switch which functions as a signal inverter for both signals, inverter is in off position or when I touch the metal part (signal ground) of either the input or output of the "Lumberg" type mini sockets the hum stops completely!

The "hum" is not as loud as a "broken,open circuit or bad signal ground hum & doesn't increse as much with the increase in volume.

Both the +/- 15v dc supply voltages are stable & the signal grounding seems ok!

I have used up all my logical deduction desperately need some one to tell me or give me some suggestions,as to how I could isolate the problem!

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Old 18th September 2012, 04:31 PM   #2
diyAudio Member
Join Date: May 2001
Location: Bangalore, India
Connect a braided wire (or a reasonably thick insulated wire) from Signal Ground to the metal plate.
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Old 18th September 2012, 10:16 PM   #3
teleman is offline teleman  Norway
diyAudio Member
Join Date: Jul 2008
Thanks, I tried your suggestion but it didn't help! The inputs are unbalanced, but I have grounded the balanced output " signal ground" to the chassis via a ground lift switch,but this seems to have no effect what so ever, as the hum still persists.

I then isolated the piezo side of the preamp & it works without any hum, so I presume the problem is with the mic preamp section. I will check this circuit later & see if I had overlooked something.

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