FM3 4CM squealing - Input 2 question

L

Lavely

New Member
I have a relatively complicated setup - several amps connected to a switcher, effects rack, mixer, reamping, etc. I'm working to incorporate the FM3 into the rig. Setup is:

Input 1 - from my RJM Loop Switcher with regular guitar cable
Output 2 - Humbuster cable from FM3 back to RJM loop switcher

Input 2 - send from Mackie mixer Aux, with regular guitar cable
Output 1 - using XLR to TS cable (as recommended in the manual) to the inputs on my Mackie mixer

When I connect the Input 2 cable (the Aux send from the Mackie), I get regular, steady high pitched squealing. Pull the cable, squealing goes away. This happens when the unit is in the audio path or out (switched out). I'm assuming this is a ground loop of some sort, but my questions are:

1 - Am I using proper cabling, as described above? It seems to match the 4CM instructions on p36 of the FM3 manual, but I'm a little unclear as to whether Input 2 cable should be something different than a regular guitar cable?
2 - Is there a setting on the FM3 that would alleviate this issue?
3 - Should I purchase one of the ground lift units for $100 or so? Any recommendations? Seems crazy expensive...
 
I assume by "This happens when the unit is in the audio path or out (switched out)" you're referring to the RJM, which of course could also create a feedback loop if it routes an output into an input that is also connected to the same output - but let me know if that's not what you mean.

How are you routing within the presets? If you are sending the signal to the mixer input (from output 1), and then using the aux for that channel to send the signal back into the the FM3 (into Input 2), if Input 2 connects to output 1 at any point in the routing for a particular preset I would expect feedback in the form of a high pitched squealing.

Re ground loops, my experience is that they don't sound like 'high pitched squealing'. They sound more like a loud hum.

As for cabling, if the mixer has balanced inputs I'd be inclined to use an XLR-TRS cable to get the benefit of the balanced outputs. But unless you're running long distances and/or are in a super noisy room, it should be just fine as is.
 
I appreciate the help!

In/out of the audio path - I mean yes, either switched in or out of the RJM and/or using the feed to the mixer (though I suppose that's not a switch, just a volume)

If the squeal is there even when the unit is neither (a) switched in the RJM nor (b) using any volume through the Mackie mixer, would the preset routing in the software make any difference?

I tried the XLR-TRS cables first, found this squealing. So, I re-read the manual, made some XLR-TS cables which it recommends, still have the squealing.
 
Lavely said:
If the squeal is there even when the unit is neither (a) switched in the RJM nor (b) using any volume through the Mackie mixer, would the preset routing in the software make any difference?
Click to expand...

It could, in part because the loop can exist outside the RJM entirely and because having the volume down may not be the same as 'off' - i.e., it may be low, but may still be enough for a feedback loop.

Is there a mute button on the mixer channel? If so, does pressing it stop the squeal?

If you load a preset with on the FM3 with no ins or outs enabled does it stop the squeal?

Does the squeal show on the FM3's meters page?

In order to troubleshoot further I'd likely need to understand the entire rig routing (including the monitoring) to see where other loops may exist. But based solely on the facts you've provided the most likely culprit is what I describe above - i.e., since if you have a circuit from the input of a mixer channel through the aux send and back into into the same input, I would expect a feedback loop.
 
Nope. The unit screeches no matter what program is loaded, whether all inputs/outputs are on or off.

The squeal was only happening when I connected to the mixer. Now, I can connect to the mixer and use In2, Out1 without noise, but the RJM switcher connections (In1, Out2) cause the squealing whenever either Jack is connected to the RJM.

In this configuration, if I enable the In2, Out1 blocks, I can get an effect and fine sound if In1, Out2 cables are not connected to RJM switcher. Yes, squeal shows in Input 2 meter in this configuration (when either cable is connected to RJM).

Mute buttons on the mixer have no effect. The squeal is coming out of the speaker connected to In1, Out2, which is pre-mixer.

It is a very high pitched squeal, turning down amp volume lessens it. Volumes on FM3 have no effect.

Other ideas? What am I missing?
 
Everything powered by the same AC circuit?

Including computer and network router if wired, if FM3 connected via USB?

Complicated setup, many opportunities for ground issues...
 
No computer involved, no network connection. Yes, all plugged into one AC outlet.

Fractal support is suggesting an isolation transformer (not totally sure exactly what kind is needed).
 
Lavely said:
The squeal is coming out of the speaker connected to In1, Out2, which is pre-mixer.

It is a very high pitched squeal, turning down amp volume lessens it. Volumes on FM3 have no effect.
Click to expand...

Sorry .. where is the speaker and the amp in the signal chain?? Can you describe all the connections … because it’s not (yet) clear to me how your rig is connected, or even what the relevant components are.
 
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