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 Post subject: 60 Cycle Hum
PostPosted: Fri Oct 27, 2017 2:51 am 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
I appear to have a slight 60 hz hum issue in my rack. I never really noticed it until I tested a fuzz face pedal in the GCX and it amplified the buzzing noise.

Everything is plugged into a Samson Power conditioner

Then the signal flow is like this:

Wireless Rear Panel output -> korg tuner rear panel input
Korg Tuner Rear panel output -> GCX rear panel loop 5 in (I think that's right... I'm at work at the moment so will check when I get home)
GCX rear panel loop 8 out -> Area 51 Wah input
Area 51 Wah output -> Triaxis rear panel input
Triaxis rear panel L & R outputs -> Randal power amp L & R Inputs

The G-Force is wired directly into the Triaxis FX loop so not in series

From the rear patch panel I have a power cable going to a smaller second rack case that houses the Randall power amp. Effectively everything in both racks is going through the power conditioner.... I have only one plug going into the wall.

I remember reading in another post that you can safely lift the ground in the GCX, from inside the unit. I need to conduct some trouble shooting, but am a loss as to where to start. I tried lifting the ground on the Triaxis and that didn't help.

Although I'm not using HumFrees, I have put plastic washers behind all rack ears so they don't make contact with the rack nut bar.

Any ideas??

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"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Fri Oct 27, 2017 10:00 am 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6760
Location: The Lab
If you have isolated the GCX rack ears from the rack rails, then there is certainly no need to lift the internal chassis ground connection. Also, if you are not using the Guitar In/Out or Feedthru jacks, then neither method will do anything anyway.

I think your first step would be to figure out where the problem is that is causing this noise. It could be a ground loop or it could be a noisy power supply, or even a generally quiet supply with a failing audio cable near it. It could even be that you need to make sure something ground lifted actually needs a connection to ground. The first step would be to start moving where your guitar is plugged in down thru the signal path. Here is what I would do to hopefully get an idea where to start looking...

1. Pull the cable from wherever the wireless output is going (GCX Loop 5 In?) and connect the guitar with a cable directly to this loop on the GCX. Test for any change.
If no change...
2. Connect the guitar directly to the wah Input. Test for any change.
If no change...
3. Connect the guitar directly to the Triaxis Input. Test for any change.

If the noise is still there then you have at least eliminated a bunch of things that are not the problem. ;)

If the noise goes away somewhere in there then you have a base idea where to start focusing.

Let us know how it goes!


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 Post subject: Re: 60 Cycle Hum
PostPosted: Sat Oct 28, 2017 12:40 am 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
Thanks John, the thing is I thought this was a quiet rig, and generally it is...... Until i put the fuzz face into the mix, then it sounds awful. When i turn the fuzz face off it's just drawn my attention to the noise that was originally there, albeit quite low....

I'll try all the steps you've suggested, and report back later. What I did fail to mention is I have the option of using either the wireless, or a cable. The Korg tuner has 2 separate inputs with a selector button on the front. So i have it wired in the rack with the wireless going into input 1, and a jack plug input socket going into input 2. All through these tests i'll use a cable as suggested. i don't use the wireless at home or in the studio as there's no need so effectively I can turn that off which will be one thing out of the equation.

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"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Sun Nov 05, 2017 11:34 am 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
Right... Where do I start.

I turned off the radio receiver which took that out of the equation, I also bypassed the wah totally so that's not in the loop. so the audio path is as follows:

Korg Tuner Rear panel output -> GCX rear panel loop 5 in
GCX rear panel loop 8 out -> Triaxis rear panel input
Triaxis rear panel L & R outputs -> Randal power amp L & R Inputs

I plugged the guitar into GCX rear panel loop 5 in, no change

I plugged the guitar directly into the input of the Triaxis the noise disappeared... silent, the G-Force was still going through the Triaxis FX loop and sounded great, so no issues there.

it would appear there's an issue with the GCX?

Bear in mind this is my "new" stereo GCX, although it came from a pro rig, and was in good working order. I don't know who did the upgrade to stereo whether it was Voodoo lab or the guy that sold me it. I'm assuming it would have been Voodoo Lab, as both boards have been replaced. i'm not suggesting that the upgrade caused an issue... i'm just putting that on the table for info.

Also i noticed that when I pull the midi cable from the pedal in of the GCX rear panel the sound reduces significantly, but not entirely. however when I pull the midi cable from the ground control there's no real change, maybe just a small drop.

Any ideas John??

_________________
"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Mon Nov 06, 2017 11:09 am 
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Joined: Tue May 05, 2009 2:33 pm
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Location: The Lab
Cool, that eliminates a lot of stuff!

As I recall, your GCX is isolated from the rack rails, but is the internal chassis ground still left in place?
If so, try running the GCX Feedthru between the Tuner and Loop 5 In. If the internal chassis ground is not still in place, please re attach it and test again.

Separately, assuming this test is with all the loops bypassed, try powering down the GCX and removing its power supply. If the sound still exists, try pulling power from anything else that is not in the current signal path.

Let us know what you find!


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 Post subject: Re: 60 Cycle Hum
PostPosted: Mon Nov 06, 2017 12:16 pm 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
Hi John

Yeah, the GCX is isolated, to a fashion. I've placed plastic washers between the GCX and the rails, however there is still a chance that contact is being made via the rack screw through the rack ear into the rail, if the screw is making contact on the inside of the rack ear hole.

I have no idea if the chassis ground is in place or not..... I'll try you're suggestions and report back.

Can you please explain what you mean by running the GCX feed thru between the tuner and loop 5 in... I'm having a thick moment currently. Can you explain the wiring?

Cheers

Andy

_________________
"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Mon Nov 06, 2017 2:03 pm 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6760
Location: The Lab
Sorry to not detail it out... I am suggesting plugging the Output of the Tuner to the GCX Feedthru (Front or Back), then running the other end of the Feedthru (Back or Front) to the Input of GCX Loop 5.

On a GCX, the audio signal is only tied to Chassis Ground via the Feedthru and Guitar In/Out jacks. Because you are going directly to the Input of a Loop, you are never tying your audio to the GCX Chassis Ground. The test is to identify if tying your signal to the Chassis Ground will solve the current noise issue you have.


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 Post subject: Re: 60 Cycle Hum
PostPosted: Mon Nov 06, 2017 4:15 pm 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
Understood.....

I'll do that tomorrow, as it's now 23:14 here now and i'm off to bed. As an alternative can I connect it as follows to achieve the same result?

Tuner -> Rear Panel Guitar Out
Rear Panel Feed Thru -> Loop 5 In

I'll let you know how it goes

Cheers

Andy

_________________
"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Tue Nov 07, 2017 12:57 pm 
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Joined: Fri Jun 11, 2010 12:13 pm
Posts: 186
Location: Darkest Suffolk UK
Right john, I have an update.... And quite interesting it is too.

First I wired it as follows:

Tuner -> Rear Panel Guitar Out
Rear Panel Feed Thru -> Loop 5 In

The result was the noise disappeared, but the sound coming out was very woolly, with no top end. Also the volume had dropped considerably.

I then wired it like this:

Tuner -> Front panel feed through
Rear Panel Feed Thru -> Loop 5 In

The result was the noise had disappeared, and the sound of the guitar was not as woolly as previously, but still not as it should be.

I finally wired it as follows:

Tuner -> Front panel guitar in
Rear Panel Feed Thru -> Loop 5 In

This was brilliant.... no noise, and the guitar sounded as it should, dynamic with top and bottom end.

So it looks like I'm going to have to compromise, and wire it into the Guitar In on the front of the GCX... not ideal, as i really wanted to wire it totally round the back. but hey ho..... At least I've sorted the hum.

Any ideas why the sound is so different using the feed thru's??

_________________
"My technique is laughable at times. I have developed a style of my own, I suppose, which creeps around. I'll never be a very fast guitar player."

David Gilmour


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 Post subject: Re: 60 Cycle Hum
PostPosted: Tue Nov 07, 2017 1:47 pm 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6760
Location: The Lab
So it looks like aside from the noise issue, you were likely experiencing some pickup loading that the buffer is solving... at least when using a cable rather than the wireless.

To cover some of what I see here...

1. "Tuner -> Rear Panel Guitar Out
Rear Panel Feed Thru -> Loop 5 In"

These jacks were never intended to be used this way. The Guitar Out is the output of a buffer circuit and would only ever be used as such when also using the Guitar In. the way you have it here is introducing a 100k Ω load across an unbuffered instrument level signal... so loss of highs is completely expected here. However, this did successfully tie your guitar signal to the GCX Chassis Ground which is why the original noise issue went away.

2. "Tuner -> Front panel feed through
Rear Panel Feed Thru -> Loop 5 In"

This too successfully tied your guitar signal to the GCX Chassis Ground and eliminated the original noise issue. Otherwise this should be the same as running a cable direct as there is no circuitry in the Feedthru path. That things didn't sound quite right here is where I suspect that the overall unbuffered cable length and connections were probably loading down the pickups. The buffer on a GCX is there specifically to eliminate this sort of problem. The Output of your wireless is itself buffered by default in that it is a driven signal and the transmitter provides a consistent load on your guitar pickups regardless of cables and connections after the receiver.

3. "Tuner -> Front panel guitar in
Rear Panel Feed Thru -> Loop 5 In"

This one eliminates the original noise and sounds good to your ears... then this is the one to keep! To solve the front, back wiring issue should be quite simple. if you wire as follows...
-Wireless Out to GCX Rear Panel Feedthru.
-GCX Front Panel feedthru to GCX Guitar In. (To bypass the Wireless and use a Guitar Cable, pull this short patch from the Guitar In and connect the cable from the guitar in its place.)
-GCX Guitar Out to Tuner Input.
-Tuner Output to GCX Loop 5 In.


This should provide all the benefits of the signal tied to ground as well as provide a good way to bounce between the wireless or a guitar cable.

Try it out and let us know if it suits your wiring dilemmas as well as keeps the noise away.


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