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Simple OP amp preamp hum only with both channels connected
Simple OP amp preamp hum only with both channels connected
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Old 12th March 2006, 10:49 PM   #1
Bing Yang is offline Bing Yang  United States
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Default Simple OP amp preamp hum only with both channels connected

I built a very simple preamp with input jacks, channel switches, a volume pot, an OPA2134, and output jacks. It is pretty simple, non-inverted, and built on a perf board. The power supply is very cleanly regulated.

The input jacks are connected the chasis ground. PSU ground is also conneted to the chasis ground. Volume pot body is connected to the input ground.

However, when I connect the preamp with my gainclone amplifier, it is absolutely quiet when one channel is connected. The moment the other channel is connected, the left channel started to hum. When I connect the output jacks also to the chasis ground, both channels hum when both channels connected. Again, remove one channel, no hum at all. Absolutely quiet with nice music.

Can someone give me a hint what might be the problem? Why does it hum only when both channels connected?

By the way, my gainclone does not have any hum when connected with my HK preamp.
Bing Yang
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Old 13th March 2006, 12:27 AM   #2
Leolabs is offline Leolabs  Malaysia
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ground loop problem
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Old 13th March 2006, 12:41 AM   #3
myhrrhleine is offline myhrrhleine  Belize
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Ground loop.
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Old 13th March 2006, 02:27 AM   #4
acenovelty is offline acenovelty  United States
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Star ground everything to one central point at the power supply.
Two people can keep a secret if one of them is dead.
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Old 13th March 2006, 02:42 AM   #5
rabbitz is offline rabbitz  Australia
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Don't connect signal grounds to the chassis.

Check the grounding scheme on your pre amp board so the power ground is separate to the signal ground and only come together at power ground input to the board. Sometimes a 10R resistor can help by placing it between the power ground and signal ground connection. I also make sure the grounding doesn't go all the way around the board and has a break in it.

Here's a pic of a little pre using a OPA2134 in a plastic case (powered by a 16VAC wall wart). You'll notice the power ground is at the opposite end to the signal grounds and those grounds all come back to the power ground input. There is a break in the signal ground track between the red signal input and output at the top of the board.

If all fails, add a 10R resistor between ground and chassis which tends to shut it up.
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Old 17th March 2006, 02:01 AM   #6
Bing Yang is offline Bing Yang  United States
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After spending a lot of time trying all kinds of combinations, hum persisted. When combined with other equipment, neither the Gainclone nor the preamp caused any hum.

This prompted me to search all Gainclone related articles. I finally came to the conclusion that it is the connection between the signal ground and power ground on BrianGT's board that causes the hum.

After I unsoldered the resistor right on top of the connection and cut the trace, the amp is now nice and quiet. There is still a very low trace of hum on the left channel. Since I understand how ground works now, I think I know the reason. The left channel has a longer wire from the board ground to Power Ground on the chassis. I am sure it will be reduced once I change that into a shorter and thicker wire.

It is nice to finally find out why!
Bing Yang
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Old 17th March 2006, 06:23 PM   #7
AndrewT is offline AndrewT  Scotland
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your solution to the hum elimination of cutting the signal to power ground interconnection resistor indicates that you have a second signal to power ground interconnection.

It is the second you should find and cut and then re-establish the resistor connection that Brian has designed in.

Power Ground on the chassis
that sounds suspicious.
regards Andrew T.
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