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Should I use 100 ohm resistor to "lift ground"
Should I use 100 ohm resistor to "lift ground"
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Old 6th February 2018, 05:15 PM   #1
garybdmd is offline garybdmd  United States
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Default Should I use 100 ohm resistor to "lift ground"

(Below I am talking about putting a 100 ohm resistor between the preamp output jack and ground, "lifting the ground" of the signal to prevent ground loop hum. I am not referring to taking the ground off the power cable.)

Hi all thanks for looking. I was building a preamp, and I was considering putting an optional ground switch on my output. In my study of schematics, I looked at different guitar amplifier schematics. For example, I was reading the Marshall JCM 900 schematic to see what they did.

Rather than putting a ground lift switch on the effects send, the direct out, and the "recording compensated, (ie speaker simulator/voiced/filtered), they simply placed a 100 ohm resistor on all grounds preemptively.

My question is, should I put no ground lifts, ground lift switch, or preemptively place a ground lift resistor?

Can having a 100 ohm ground lift resistor that is not needed cause problems?

Thanks anyone for your experience and expertise.
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Old 6th February 2018, 05:54 PM   #2
Keruskerfuerst is offline Keruskerfuerst  Germany
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I donīt think this will help to prevent loop hum.

It is possible to prevent loop hum by using a transformer.
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Old 6th February 2018, 06:13 PM   #3
scottjoplin is offline scottjoplin  Wales
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Sometimes a resistor is put in the signal return (0V) is that what you mean? It's usually more around 5 to 10 ohms
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Old 6th February 2018, 10:56 PM   #4
garybdmd is offline garybdmd  United States
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Default 100 ohm resistor on fx send and preamp outputs

One time i had hum between preamp and amp and it was suggested i add a small resistor between the grounds and it worked like a charm. Yhis was suggested by a saavy member of this forum. Here is a link to an article what Im talking about. Avoiding Ground Loop Hum

I have attached a small portion of a schematic to illustrate what im talking about. It looks to me like they are placing 100ohm resistors on jack output grounds. I have assumed this is to prevent hum. Maybe there is some other purpose.
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File Type: jpeg 1820B65F-D6C2-4B64-AF26-3A1BC490004B.jpeg (121.4 KB, 51 views)
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Old 6th February 2018, 11:12 PM   #5
scottjoplin is offline scottjoplin  Wales
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Yes, it can help, in that article it says 10 to 100 ohms, I'd use the lowest resistance that does the job, in a well designed and laid out system it shouldn't be necessary
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