0
\$\begingroup\$

I have a metal rack that contains a single item: A Samson S Patch Plus Patchbay. The patchbay is well isolated from the rack using nylon shoulder insulating washers, and I've thoroughly tested this with a multimeter. Setting it to continuity I've attached one probe to the patchbay and one to the rack and there is no continuity. I've done this in a large number of places and I'm confident in this.

I'm passing an instrument level signal through the patchbay via a TS cable and out the other side of the patchbay via another TS cable, into an audio interface. There is nothing else plugged in to the patchbay.

I've tested the continuity of both tip and sleeve of both cables individually and both cables when attached to the patchbay jacks and there is continuity for both tips, and for both sleeves, but not in between. I've also tested continuity between both cables and both the patchbay casing, and with the rack chassis, and there is no continuity.

I've noticed that there is significant noise on the audio that is received by the audio interface, and further I've noticed that attaching a ground wire to the either the patchbay casing, or to the rack chassis and running it to a metal radiator pipe massively reduced the noise. I'm monitoring the noise using a VST called Fab Filter Pro Q and I'm doing this by setting the gain to the center position on the D.I. input of the audio interface and measuring the Dbs with the guitar on a stand.

Measurements:

Nothing plugged into interface: -100Db (minimum value)

Guitar => interface: -88Db

Guitar => Patchbay => interface: -76Db

Patchbay => interface: -28Db (loads of noise)

If I run the ground between the patchbay casing and the rack, there is a small increase in the noise (~4 dB).

I'm in the UK. 3 Pin plug 240v. iMac is a desktop with original power supply. Interface power supply is 12V DC 2A.

My questions are:

  1. If the patchbay is isolated from the rack chassis, how can grounding the chassis make any difference to the noise on the audio?
  2. If the TR cables and path through the patchbay are isolated from the casing, how can grounding the casing make any difference to the noise on the audio?

Here is a diagram of the equipment involved.

enter image description here

\$\endgroup\$
9
  • 1
    \$\begingroup\$ My first guess would be the noise is coming from the power supply for your iMac. Is your iMac a laptop? Does it have a 2 prong, or a 3 prong connector? Are you in the US where plugs are polarized or somewhere where plugs may be reversed? Is you power supply made by Apple, or is it a replacement? Do you still get the noise if the guitar is not attached to your patch bay? Do you still get the noise when your Audient ID44 is disconnected from the patch bay? \$\endgroup\$ Mar 18, 2022 at 23:01
  • 1
    \$\begingroup\$ what does continuity mean on your multimeter? ... try measuring the resistance instead \$\endgroup\$
    – jsotola
    Mar 18, 2022 at 23:11
  • 1
    \$\begingroup\$ When you say "there is significant noise on the audio that is received by the audio interface," how exactly are you observing this? Looking at data on the iMac? Hearing it with headphones attached to the iMac? Or? I don't see your ear or anything that is driving your ear in the diagram. Everything starts to matter. Also, if you disconnect the guitar is the noise there? Or not? What happens when just tying the patch bay plus to the rack, but leaving the rack and patch bay plus otherwise floating and not grounded? And is the patch bay plus powered? (I don't see its power connection.) \$\endgroup\$
    – jonk
    Mar 18, 2022 at 23:24
  • \$\begingroup\$ @MathKeepsMeBusy I've updated my question with everything you suggested. Maybe the most important thing is that there is a lot of noise with just the patchbay attached to the interface. \$\endgroup\$ Mar 18, 2022 at 23:43
  • 2
    \$\begingroup\$ What is the dominant frequency of the noise? Is it clean like a hum or spread out like a buzz? Is it a hissing type noise? \$\endgroup\$ Mar 19, 2022 at 0:04

1 Answer 1

1
\$\begingroup\$

Most audio noise comes from high common mode noise with unbalanced impedance loads, so crosstalk occurs more in signal than ground.

Repair requires balancing the impedances or shunting the noise with an RF cap from the frame to AC PE ground.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.