patchbay hum problem

cjcayea

cool as a cucumber in a bowl of hot sauce
Silver Supporting Member
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4,042
so i ordered all the parts and with some help from TGP i made my very own patchbay to go with my interface. 4 XLR and 4 TRS. they're wired directly: cable soldered directly to the PB terminal with an XLR or TRS male end inserted into the interface. the XLR inputs work as expected but the 4 TRS inputs all have a ground hum that disappears when i plug something into it. it's obviously a ground issue but i have no idea how to fix it. what am i missing? thanks
 

Motterpaul

Tone is in the Ears
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13,921
If the patch bay is grounded disconnect it. The cables are grounded to your guitar & amp, and a difference in potential can cause hum.
 

cjcayea

cool as a cucumber in a bowl of hot sauce
Silver Supporting Member
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4,042
the patchbay isn't grounded as far as i'm aware. it's simply a faceplate with input terminals that have cables soldered to their back sides and plugged into the interface, trs xlr. the weird thing is that the hum flies away when i either plug something into it (i.e. an unbalanced synth) or change the input source on the interface from "inst" to any other one.
 

Rex Anderson

Member
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5,524
Are you absolutely sure you wired things correctly? I had a guy wire cables for me once and he switched pin one and two on XLR cables. They passed signal but ground wire was on hot and hot wire was on ground. The cables had ground hum like you describe.

Another thought. Now you have something similar to a guitar cable plugged into an amp but not plugged into the guitar. There is nothing terminating the instrument input, you have an open cable plugged in to it and it is acting as an antennae. Are you plugging an unbalanced 1/4" guitar cable into a balanced TRS input jack? Maybe you should have used TS jacks for unbalanced instrument inputs.
 

cjcayea

cool as a cucumber in a bowl of hot sauce
Silver Supporting Member
Messages
4,042
yes, i'm using balanced inputs on the bay and the instruments are unbalanced. it's exactly like you describe, a guitar cable plugged into an amp without the guitar on the other end.

i really hate that the interface inputs are all on the back. so far there has been no simple solution where i can have convenience and flexibility. i might sell the apogee interface and buy an RME instead.
 




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