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Modern Ground Schemes

Posted: Tue 05/07/19 7:48 pm
by JMPGuitars
Because this topic comes up so often, here you go.

Ideal Ground Scheme:
I use a partial star ground. You can see an example of this ground scheme in the drawing I did for the lite 2b: files/JMPGuitars_18_Watt_Lite_2b_Layout.pdf

You can apply the same principles to your amp regardless of the 18 watt circuit version:

Note: all ground points on the chassis should serve no other purpose than providing a connection to earth. No transformer bolts, or board screws or anything like that.

1 ground near the Input jack covers input, pots, preamp. Ideally the preamp ground bus connects to the input jack(s) and the input jacks connects to an independent ground spot as close to the jacks as possible.

Input jacks and gain pots should have shielded signal wire. Only connect one end of the shielding to ground.

A star ground near the PT or cap can. This covers PT center taps, power amp/PI, filter caps, and output transformer (if you earth the OT).

Do NOT use a ground bus connecting the back of the pots.

IEC socket or power plug ALWAYS gets its own independent connection to the chassis.

Re: Modern Ground Schemes

Posted: Wed 05/08/19 3:20 pm
by geoff 1965
here's a good pdf with "in depth" explanations of ground loops,star and bus grounding etc.

Re: Modern Ground Schemes

Posted: Wed 05/08/19 5:39 pm
by JMPGuitars
geoff 1965 wrote:
Wed 05/08/19 3:20 pm
here's a good pdf with "in depth" explanations of ground loops,star and bus grounding etc.
There's a lot of good information in there. However, some of the advice is dated, or not appropriate for these amps. For example, his disliking of cap cans isn't important here. Most 18 watt builds that use cap cans have them placed near the PT, which automatically puts them in a good location for a short lead to the partial star ground.

Maybe I missed it, but I think he left out the part of full star grounds potentially causing squealing on a dimed amp.

I would also prefer that he expanded on the use of bus grounds to include the separation of preamp and power amp sections. Though I did like that he was clear about connecting the bus at the input jack.

I also liked his mention of where to connect / which input jack to connect to earth in the case of multi-channel amps. I never give that much thought as I usually build single-channel amps.

Re: Modern Ground Schemes

Posted: Sun 06/02/19 8:27 pm
by JMPGuitars
Here's another one of our admins talking about improved ground schemes, including some reasons: viewtopic.php?p=178575#p178575

Re: Modern Ground Schemes

Posted: Fri 04/10/20 6:20 pm
by JMPGuitars
I learned something today/yesterday thanks to Zaphod_Phil. There was an inherent error in most of the layouts, including the ones I've made (which have now been corrected as of 4/9 and 4/10). Oops. 🙃

The missing rule: The preamp filter caps should not be sharing a ground/ground buss with the power amp. As Phil explained: "The PA/PSU stages have some big ground currents which could cause havoc with the higher gain of the stages which use 12AX7s."

Unfortunately this was easy to overlook as the common ~22μF preamp filter cap was historically placed, and copied, and copied, and copied, always in the power amp side of the boards, and usually sharing the same ground.

As Phil has mentioned in various threads, ideally, the filter cap should be placed next to that stage's cathode resistor/cap.

I've updated my layouts to reflect these best practices, and if you're having any noise issues, or you're bored, try it out.