Ground Loops In PC Board Interconnects

[TomCarlos]

Friends....

On another thread, I was able to utilize your help to fix up a Traynor YVC80 Tube Amp. Thanks again! Now, I am turning my attention to the grounding of this amp.

As you can see from Pic 1, the amp uses green wires (connected to the chassis) to route a ground connection to the 3 PCs boards. The main board is the Power Supply and Output Stage. The second board is the preamp board. In this case, the ground wire connects to the input jack. The jack then sends a signal and ground wire to the preamp board. The third PC board is used for the Reverb Send/Return, Effects Loops, Headphone out, etc. That 3rd board also has a dedicated ground wire that returns to the chassis. This appears to be your typical Star Ground model.

Here is the potential problem. If you look at Pic 2, you will see that the PC boards are connected with ribbon cables. And those cables carry signal and ground connections through them. So I am wondering, do we have a potential for ground loops here or is the signal so minimal that you would never notice?

Ground wires in the ribbon cables are used for the plus and minus 15vdc supplies for the op-amps. We have ground wires for the Reverb Send/Return, another ground wire for the Effects loop.

Anyway... just wondering if you have seen this in other amps and if it is potential for audible hum.

Thanks, Tom

[Enzo]

Potential is for designing. You have the amp in front of you already made. it hums or it doesn't, no potential involved.


A ground loop is a condition, not a circuit, so if you don't have one going on, then one will not likely appear if the amp remains as it is. Chopping at the designed grounding arrangement is a good way to ADD ground loops.

[TomCarlos]

Point well taken. It's hard to say if one can make an already assembled amp "more quite." I did find one thing to fix, not sure if it made a difference. There was a single green ground that came from the chassis point to the Preamp board. From there, I found a "jumper" wire (green ground) that attached to the 3rd PC board. I disconnected that and ran a separate ground from board 3 back to the common.

The amp sounds ok- except for the horrific Celestion Seventy/80's which are blown. A test cab tells me how that I can get a nice sound out of this amp. So I'll replace the speakers.

There are some cleaver design ideas in this amp that I will use when building my "true project" amp. So I though I'd study the Star Grounding techniques.

Thanks Enzo.

Tom

[Alan0354]

I work with a lot of grounding in system. I can't tell exactly how you route the ground. I need to look at it close up and look at how the partition of the circuit. In general rule, I avoid this kind of one point star at all cost. This is dangerous as you know you can't really get a true star ground. You have to have a ground to accompany the signal from the preamp board to the power amp board. You have to have a return from power supply to power board and preamp board. That can easily become a ground loop if you are not careful.

I use ground plane ( the chassis in this case). It is a big mistake that people think that all the current is running all over in the ground plane and cross talk to each other. That's not true at all. This is where the field of signal integrity and electromagnetic theory comes in. Ground current FOLLOWS THE PATH OF LEASE IMPEDANCE, not the path of least resistance. The ground return current follow right under the signal trace on the ground plane at high frequency. Even at audio frequency, it is localized. In a non perfect star situation, actually ground current is forced to travel on the stared lines and can form ground loop if you are not careful. At higher frequencies, I can make the ground current winding and twisting every which way I want on the ground plane.

You don't have to look further than SF Fenders how they do the grounding and improve on it. You know the biggest ground current signal is the cathode of the power tube. You tie them all together at one point onto the chassis away from the preamp. You tie the low side of the reservoir cap right onto that star. You tie the low side of the screen grid filter cap on the same star as the screen current return on the cathode also. Doing like this, no ground current from the power tube will even get onto the chassis. You do the same for the PI stage at it's own star. The only improvement I did on the Fender SF amp is I lift the cathode grounding points and put it onto one screw on the chassis close to the power tubes. AND I tide the ground wire of the reservoir cap onto that point. That's all I do.

For the preamp, you put the low side of the filter cap close to the corresponding preamp tube section, those are small current, it's not that critical. You look at how Fender progress from input on one end of the chassis to the power tubes on the other end. There is no crossing of ground current. I helped one person here long time ago in trouble shoot humming. He did a few stars and he mistakenly put the ground return of one preamp stage to the star for the PI. Took me going through the pictures for a while to catch and fixed that. Star work ONLY if you really really know what you are doing. One little mistake and you'll be pulling hair.


I know this is radical and against all the convention believes. But it works and it's a lot easier. I never run into problems and I work on big system that we detect way into uV. You don't even know how many system I fixed follow this. I am not very experienced in guitar amp, but I sure work on grounding as I was in charge of passing CE test for our systems and almost half of the test is about noise and emission that mostly due to grounding problems. Want to learn more about this, look for books on Signal Integrity, these are the easy interpretation of electromagnetic theory.