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 Post subject: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Sat May 13, 2017 8:04 am 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
Dear All,

Recently bought a GCX Audio Switcher rack unit (new) to enable me to use two pre-amps as shown in the attached schematic. I am also using ground control pro (also new).

Problem is that whenever I use loop 6, i.e. Mesa Throttle Box pedal enabled, there is a loud humming noise, especially with loop 1 on, i.e. the ADA Preamp.

Noise is gone when I turn down pedal volume to 0. Pedal is powered with a Boss power transformer that is connected to the Furmann. Tried several cables and a Dunlop power transformer with no success. Noise is always there.

Second issue, when switching loop 1 and 6, there is always a popping noise. Very unpleasant and undesired when playing.

Ideas, comments, suggestions from around the world on hoe to solve this 2 noise issues?

Greetings from Portugal,
Joao


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Mon May 15, 2017 10:51 am 
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Joined: Tue May 05, 2009 2:33 pm
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Location: The Lab
Are you saying you only get this problem with the Throttle Box on Loop 6?

For instance, if you move it to Loop 5 is there the same noise?


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Mon May 15, 2017 3:22 pm 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
Hi John,
I have tried both preamps on loop 2. Same result, lots of hum (especially with the ADA MP1) and that loud pop sound when switching loops 1 (or 2) and 6. I do not think that loop 5 Will change anything, but Will try it tomorrow.

The ADA In overdrive alone (whithout the throttle box) makes also quite some hum.

The Marshall JMP 1 seems to be quite silent, no hum, only popping when switching between preamps.

João


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Mon May 15, 2017 4:03 pm 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6751
Location: The Lab
JPRS wrote:
Hi John,
I have tried both preamps on loop 2. Same result, lots of hum (especially with the ADA MP1) and that loud pop sound when switching loops 1 (or 2) and 6. I do not think that loop 5 Will change anything, but Will try it tomorrow.

The ADA In overdrive alone (whithout the throttle box) makes also quite some hum.

The Marshall JMP 1 seems to be quite silent, no hum, only popping when switching between preamps.

João


Oh, so without the Throttlebox you still have most of the same problems? What happens if you connect your guitar directly to the Guitar In on the front of the GCX and you swap the preamp connections, JMP-1 to loop Return and MP-1 to loop In? Does the hum stay with the MP-1?


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Tue May 16, 2017 2:31 pm 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
Hi John,

I have completely disconnected the throttle box and made the connections you have suggested.


First Trial:

GCX Rear Panel Guitar out to ADA Input & Feed Through to JMP Input

Clean sound: Loud Popping sound when switching loop 1
Hum is there but I think you can live with it

Overdrive: Loud Popping when switching loop 1. Hum is there but with noise gate I think you can live with it


Second Trial:

GCX Rear Panel Guitar out to JMP Input & Feed Through to ADA Input

Clean sound: Loud Popping sound when switching loop 1. Both preamps with hum, hearable also when playing

Overdrive: Loud Popping when switching loop 1. ADA with lots of Hum e JMP almost no hum


In view of the above, I think option 1 (Rear Panel Guitar out to JMP Input & Feed Through to ADA Input) is the better choice. Popping sound when switching is not acceptable, its to loud.

Additional Info: You might have noticed that I have 2 rack cases. I found out, that when the metallic frames from both cases are touching, the hum reduces. So all tests and results from today are with both frames aligned and touching.

Any Thoughts?


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Tue May 16, 2017 3:03 pm 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6751
Location: The Lab
The tests you did aren't the same as what I was asking you to try. The rear panel Guitar Out and Feedthru should be the same signal. However, maybe you have stumbled on to something by doing what you did. :o
Can you plug a 1/4" end into and out of the front panel Feedthru a few times to try to "clean" the normalling contacts of this jack to find if that makes any difference to what you have for your "First Trial" ? So with the rig turned off just plug a guitar cord into and out of this jack a few times, then remove the cord so it is just like your drawing then power up and test again.


What I wanted you to try in my previous post was to swap where the Outputs of your preamps are going. Your drawing shows Loop 1 with the JMP-1 going to the In and the MP-1 going to the Return. What I want you to try is this:

1. Output of the MP-1 to the In of Loop 1.
2. Output of the JMP-1 to the Return of Loop 1.


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Tue May 16, 2017 4:12 pm 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
Sorry, forgot to mention:

JMP1 to Return & Ada to In of loop 1 has way less humming. My previous post already considered this.

After connecting and disconnecting plug into front feed thru several times as requested, I notice that results are still different with less noise, when routing guitar out to Ada and feed thru to jmp1

Due to the late hour, I am now playing low volume, and must say that the humming is to loud, you can hear it even playing with overdriven sound. It s really annoying, so is the popping when switching


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Sun May 21, 2017 11:38 am 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
Hi,
I decided to completely rebuild the rig this weekend, first only one pre amp with the power amp, then the other, then both with the GCX switcher.

Until here OK, no popping sound when switching loop 1.

Then added the Gforce, and the popping sound is there. Tried to connect the GForce to another outlet, no improvement.
The Rocktron is completely disconnected now.

Any Ideas?


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Mon May 22, 2017 10:00 am 
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Joined: Tue May 05, 2009 2:33 pm
Posts: 6751
Location: The Lab
Interesting! Is this G-Force one of the early ones from the late 90's?


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 Post subject: Re: GCX with 2 Pre-amps - Hum & Pop Sounds
PostPosted: Mon May 22, 2017 12:09 pm 
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Joined: Sat May 13, 2017 7:53 am
Posts: 7
I bought the GForce 2 years ago, new in Germany...I believe it was one of the last units they manufactured. Do you have some more test I can do? It's getting really frustrating :oops:


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