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Old 17th October 2016, 04:35 PM   #1
kwilder is offline kwilder  United States
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Default Input Jack = ground loop??

Hi All,

I am building my own tube guitar amplifier, this is for a Electrical Engineering Senior project at university. I have done the design, and am currently trying to construct the amplifier. I have asked a few questions on this board before, and I have one more:

How do you avoid ground loops with the input jack? The outer conductor of the jack (obviously the ground) touches the chassis, and therefore will likely create a ground loop if I tie the outer connector from the guitar to circuit ground. I already have a separate screw on the chassis that ties circuit ground and mains ground to the chassis at one point and one point only. I have been told to not use the input jack as the one place the circuit ground is tied to chassis.

Is it OK to have the ground from the guitar tied to chassis rather than circuit ground? How about the output jack? Can it's ground be chassis ground too, or do I need to insulate these jacks from the chassis and tie them to circuit ground directly.

Any input on this would be greatly appreciated. Thanks!
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Old 17th October 2016, 05:52 PM   #2
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Plastic (insulated) jacks can be used for both input and output.

If the signal ground to chassis is physically close to the metal input jack, the loop created is quite small and I think you can get away with it.

If the amp includes global negative feedback the common wire from the secondary of the output transformer should be grounded to the local signal ground at the point where the feedback signal is re-introduced (usually the PI ground).

If the amp does not include global negative feedback, a ground on the speaker connection is still a good idea for safety reasons, but it carries negligible current and can be made to the chassis.
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Old 17th October 2016, 09:55 PM   #3
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Quote:
Originally Posted by kwilder View Post
... I already have a separate screw on the chassis that ties circuit ground and mains ground to the chassis at one point and one point only. ....
It's better if the mains ground has its own dedicated bolted connection to the chassis (with star washers for a good long term contact). The signal ground connection to the chassis should be separate from that.

Have a look at Merlin's excellent article:

http://www.valvewizard.co.uk/Grounding.pdf
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Old 18th October 2016, 01:53 AM   #4
kwilder is offline kwilder  United States
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Quote:
Originally Posted by Malcolm Irving View Post
It's better if the mains ground has its own dedicated bolted connection to the chassis (with star washers for a good long term contact). The signal ground connection to the chassis should be separate from that.

Have a look at Merlin's excellent article:

http://www.valvewizard.co.uk/Grounding.pdf
Thanks for the article, seems really useful. And yes I was going to have separate screws for each, don't know why I worded it that way. I will put the signal ground to chassis screw close to the input jack.
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