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 Post subject: (Another) grounding adventure!
PostPosted: Mon Mar 05, 2018 6:50 am 
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Joined: Sun Aug 13, 2017 2:00 am
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Hi guys,

Here we have another post with some grounding questions. Hopefully you can share your ideas with me.
I’m building a pretty big rack, with three GCX’s, a lot of pedals, a Kemper and a pedalboard. And with that comes a lot of grounding questions. There is a small hum, but it’s workable the way it’s now, but I want to make it as quiet as possible.
I’ll start with the first on the path of the guitar.
The pedalboard has a midi controller (Liquid Foot Pro), an expression pedal, volume pedal, way pedal, pedal power 2 plus and a tuner. On the side of the pedalboard is an aluminum plate with the guitar input. That plate needs to be grounded, but does that need to be signal ground of the ‘real’ ground?

On the back side of the pedalboard is another aluminum plate with a powercon, 7pin XLR for the midi, and an XLR output for the guitar. I use the radial SGI to convert the guitar to XLR.
That backpanel also needs grounding. Again, signal ground or real ground/earth?

In the rack are the 3 GCX’s, 3 drawers for the pedals, a furman powerconditioner and a 19” plate for the connections.
The GCX’s, do they need to be grounded by the 19”rails or a jack in the feed through connected to the ground? And for the jack option: signal or real ground?

I can hear a difference when I touch the panels on the pedalboard, so they really need grounding, but, the 19” doesn’t seem to have a grounding thing. Does it need it? I’ve used humfrees on that one.

Thank you for reading and you thoughts and ideas.


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Mon Mar 05, 2018 1:20 pm 
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Joined: Tue May 05, 2009 2:33 pm
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Location: The Lab
I suppose I'd first eliminate the pedalboard, maybe just Guitar to SGI, SGI to amp, to be certain where the noise is coming from.

As for the interface plates on the pedalboard, I guess I don't quite know what to say. Are these custom made or are they a manufactured product? Are the audio jacks grounded to or isolated from the plates (basically are the jacks all metal or plastic) ?

If custom, can you maybe share a picture here or to info@voodolab.com?


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Wed Mar 14, 2018 10:04 am 
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Joined: Sun Aug 13, 2017 2:00 am
Posts: 12
Thank you for your reply.
I’ve eliminated all the gear from the board down to this:
Guitar - Radial SGI input - Radial SGI output - GCX loop 1 in - GCX loop 1 out- Kemper input.
This already has a slight hum. Do I need to use the guitar in, on the front of the GCX?

I’ve tried to use the humfrees, and also without. Can you give an explaination on then to disconnect the internal ground?
I’ve opened up all three GCX’s to see if something was damaged. And I’ve spotted an extra connection in one of the three. The ring of return of loop 2, is connected to the ground of the output, this is the same for loop 6. It’s not there in the other loopers.

Are the other two missing this, or isn’t this supposed to be there?


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Wed Mar 14, 2018 10:06 am 
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PS: I’ve bought all thee second hand.


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Wed Mar 14, 2018 10:58 am 
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Location: The Lab
roybeukers wrote:
Thank you for your reply.
I’ve eliminated all the gear from the board down to this:
Guitar - Radial SGI input - Radial SGI output - GCX loop 1 in - GCX loop 1 out- Kemper input.
This already has a slight hum. Do I need to use the guitar in, on the front of the GCX?

In this case, yes, I would suggest running thru the Guitar In or Feedthru before going to Loop 1.
What you have at this point is certainly not a ground loop, but it is likely the lack of a ground connection thru the GCX since the loops are isolated from chassis ground. Connecting thru the Guitar In/Out or Feedthru will tie the audio ground to the GCX Chassis.


roybeukers wrote:
I’ve tried to use the humfrees, and also without. Can you give an explaination on then to disconnect the internal ground?

Hummfrees accomplish pretty much the same thing as lifting the internal ground. So if Humfrees don't change anything, lifting the internal ground won't do anything either.


roybeukers wrote:
I’ve opened up all three GCX’s to see if something was damaged. And I’ve spotted an extra connection in one of the three. The ring of return of loop 2, is connected to the ground of the output, this is the same for loop 6. It’s not there in the other loopers.
Are the other two missing this, or isn’t this supposed to be there?

That was a fix on a specific run of boards that somehow got made missing a trace. I'd bet that it isn't needed on the ones that don't have this fix.


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Fri Apr 06, 2018 10:20 am 
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Joined: Sun Aug 13, 2017 2:00 am
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Hi John,

Thank you for your replies and help.
I've decided to change some stuff and redo the rack and was hoping you would want to take a look at the schematic I've made. Because I'm not a 100% sure it's possible to do what I'm hoping to do.

It's hard to make a very clean and clear schematic, but I've made the input for the guitar red, and on the left side is the pedalboard, the middle is are the GCX's and the bottom right are the amps and FX loop.

As you can see I'm trying to choose between a Kemper and 2 amp's, but also I'm trying to choose whether the FX are driven by the Kemper or amps. The same goes for the return. This way, I can use the Kemper to drive the power-amps, and the other way around.

On loop 4 & 5 on the third GCX i've connected the sleeves for the grounding. This is the way to do it right?
Do I have to do the same on loop 7 & 8?
The inputs of loop 4 & 5 are connected to the P-split. This is because the Kemper only has a mono FX send, so that needs to be 'stereo' before the first pedal in the FX-loop. Is it possible to do it like this?

I've seen a picture of a GCX mod which takes the internal grounding and connects that with a switch. I've done that, do you think it's best to turn the chassis grounding off?
Besides the stuff in the schematic, there is also a Furman in the rack, so the rack rails are grounded.
The Kemper is not in the rack, but it's in the same flightcase but in a different section in foam, so there is no weird grounding there.

The schematic file was to big to attach so I'm adding this link (and hope it works):
SCHEMATIC LINK


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Tue Apr 10, 2018 10:57 am 
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Joined: Tue May 05, 2009 2:33 pm
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Location: The Lab
Looks good to me... that is quite a setup!

Grounding the Sleeves as you have done is unnecessary since the amps' Return jacks will likely be lifted anyway, but it shouldn't cause any harm that you have done it already. Definitely not needed for GCX #3 Loops 7 and 8 where you are not using the Return jacks anyway.

I can't say for sure about which way you should have your modded chassis ground switch, but since its a switch now it should be quite easy to flip it one way or the other if there is noise that you suspect is caused by the chassis ground state.

Take some pics of this rig and post em!


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Thu Apr 12, 2018 3:02 pm 
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I'll definitely post some pictures when it's finished :D
I have one more question. On the GCX page on the Voodoo Lab website I've seen a schematic on using dual GCX's.
In that schematic they take the output of GCX 1, use the feedthru and then use the guitar input on the second GCX. Do you think that's something I have to do too? I've noticed that in that particular schematic, the first GCX's guitar input doesn't get used.

Here is a link to the schematic (page 2):
http://www.voodoolab.com/gcxdiagrams/08-Dual_GCX_(2of2).pdf


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 Post subject: Re: (Another) grounding adventure!
PostPosted: Fri Apr 13, 2018 9:37 am 
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Joined: Tue May 05, 2009 2:33 pm
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Location: The Lab
roybeukers wrote:
I'll definitely post some pictures when it's finished :D
I have one more question. On the GCX page on the Voodoo Lab website I've seen a schematic on using dual GCX's.
In that schematic they take the output of GCX 1, use the feedthru and then use the guitar input on the second GCX. Do you think that's something I have to do too? I've noticed that in that particular schematic, the first GCX's guitar input doesn't get used.

Here is a link to the schematic (page 2):
http://www.voodoolab.com/gcxdiagrams/08-Dual_GCX_(2of2).pdf


In that diagram the Loops of the first GCX were fed from a Wireless Receiver. We used the Feedthru to bring the signal back out to the front of the rack so we could run thru the Guitar In buffer as an easy way to split the signal to feed the inputs of the two amps at once.

Your rig is doing this with the P-Split at Loop 3 of GCX #3 and is likely a better way to do it since you have an isolation transformer on one of the amps.

I hope this helps!


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