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ground problems

10/19/2004 3:52 AM
RichH ground problems
In my homebrew talkbox I have an ADG412 switching system that switches between talkbox amp or direct through to amp jack or both. It all works fine except for a ground problem. The power supply is a simple bridge rectifier type to +/- 12V regulators. The neutral wire goes to a transformer lug. The secondary PS circuit ground goes to another transformer lug. Now, the adg412 switching circuit operates on the +/- 12V supply with all grounds connected and then going to the secondary lug. The input and output jacks are isolated with their grounds going to the same lug.  
The problem is, I get a ground hum. When I move the in/out jack ground wires around, the hum gets more and less. When these wires are brough close to the chassis, it hums bad. I reduced the circuit to just guitar into adg412 input and from adg412 output to output jack. Pretty simple.  
If I disconnect the jack grounds from the lug groung (keeping the in/out grounds tied together), the hum goes away.  
Can someone please help with this problem as it is driving me nuts. As far as I can tell there are no ground loops. Why would the signal (guitar) ground hum so bad when connected to the power supply ground?  
10/19/2004 5:11 PM
Just an update on this problem.  
When all the grounds are tied together as normal and then tied to the chassis, there is a hum. When the chassis ground is lifted, the hum goes away. There is nothing else connected to the chassis except at this ground point so why would this create hum? Any ideas?
10/19/2004 5:48 PM
Another update.  
When all the grounds are tied together and then to the chassis BUT the 120VAC green ground wire is removed from the chassis the hum goes away. For some reason, the circuit ground don't like being connected to the 120AC ground. Any ideas?
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