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Topic: Chassis ground impedance question |
Steve Waltz
From:
USA
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Posted 19 May 2020 11:47 am |
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I have an amp with chassis ground but they also ran a bus wire from each tube socket, jack or pot back to the main location on the amp.
How does this not create ground loops? I think it's because the wire is lower impedance and so the ground chooses the wire over the chassis and ends up being like a ground bus system? In effect it ignores the ground to chassis?
Just curious how this system works.
Also, in the picture you can see the ground wire bus on the tone control pots but that doesn't connect up to the preamp tubes which have their own ground bus. The two don't meet and I think the common ground for them is chassis. Any reason not to connect the two? The preamp ground bus is on the left of the picture near the 100K resistor.
Thanks,
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 19 May 2020 12:29 pm
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| "Star grounding" maybe? Picture doesn't tell enough. |
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Steve Waltz
From:
USA
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Posted 19 May 2020 12:47 pm
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That is just the preamp section. I can't find the photo of the power amp portion. I'm calling it ground Bus because the wire that I'm talking about starts at one point and then goes through the amp from one location to the next. It doesn't go from one part directly to a single ground point for each part and I think that is what Star ground is.
The preamp may seem like that since the preamp tubes have one ground bus and the tone controls have their own but I really think that was a mistake. The wire on the tone controls continues out of the preamp section through a umbilical cord back down to the amp and connects the ground bus on the power amp section.
In one picture you can see the yellow wire of the umbilical connects to where the ground bus from tone controls connect. Ground bus is on top of the tone controls.
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 19 May 2020 1:38 pm
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| Steve Waltz wrote: |
| It doesn't go from one part directly to a single ground point for each part and I think that is what Star ground is. |
Correct, when done by the book. If done by trial-and-error (listening) it may end up a bit different, mainly because how power-sections may pull asymmetrically on the ground in badly distributed layouts.
Measuring it may tell if that's the case, or if they have just gone overboard and grounded wherever they liked. I have fixed some bad cases in my time…  |
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Steve Waltz
From:
USA
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Posted 19 May 2020 1:49 pm
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Georg,
Can you explain what to measure? I assume you mean to check for voltage on the chassis in different areas? I feel like I have done that and never found any voltage on the chassis but it's been a while.
I'm going to experiment with attaching the preamp tube bus to the volume control bus and see if there is any change. I assume there is no down side to that? Right now if I jumper the preamp board to the power amp chassis I get a decrease in noise. That's why I feel the current ground set up is lacking. |
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 19 May 2020 1:58 pm
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With a live amp, measure microVolts deviations at all ground-points you can reach, relative to one, chosen, ground-point – power supply GND should do fine but instrument GND should do also.
BE AWARE of HIGH VOLTAGES near those points. |
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Steve Waltz
From:
USA
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Posted 19 May 2020 2:08 pm
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| Got it. I'll do that. |
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Bill A. Moore
From:
Silver City, New Mexico, USA
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Posted 19 May 2020 3:12 pm
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| Steve, are you getting a hum or noise? |
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Steve Waltz
From:
USA
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Posted 19 May 2020 3:54 pm
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I am getting a hum but I know that can come from a lot of different places. Taking it step by step. I'm actually interested in this type of ground system in this amp and was curious about the details of it such as why the preamp tube socket ground was not tied to the tone control pot ground.
BTW, the pots, jacks and tube sockets are not isolated from the chassis. |
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Jeff Highland
From:
New South Wales, Australia
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Posted 19 May 2020 7:02 pm
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Grounding is a bit of a black art, but often pots jacks etc are still separately grounded as well as contacting the chassis because a loose nut or corrosion could easily break the earth connection.
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