Qu-16 : NOISE using mix as send/return for external effects

[cisa]

I have a problem with my QU-16:

I use OUT MIX 5-6 for using external effects ( Strymon BigSky reverb) as inserts or send/return for my synths.

The routing is:

QU-16 (OUT MIX 5-6) -> (INPUTS L-R) Strymon BigSky (OUTPUT L-R) -> (INPUT 5-6) QU-16.

In QU-16 monitor I see (and hear from main mix and other mixes) that there is some noise continuous (like white noise, I am not an expert in mixers). I can “silence” the noise using the QU-16 GATE, but the problem is that everytime I play a note with my synths the noise is still here (until the gate cut off the sound).

If I connect only:
SYNTH (OUT L R) -> (INPUTS L-R) Strymon BigSky (OUTPUT L-R) -> (INPUT 5-6) QU-16 (so, without using Qu-16 as insert/send return), the noise disappear.

I also tried to change BIG Sky with a DBX266XS compressor connected in same way and same thing:
if the routing is

QU-16 (OUT MIX 5-6) -> (INPUTS L-R) Compressor (OUTPUT L-R) -> (INPUT 5-6) QU-16. the noise is here

if the routing is

SYNTH (OUT L R) -> (INPUTS L-R) Compressor (OUTPUT L-R) -> (INPUT 5-6) QU-16. the noise disappear

What can it be?

Thanks in advance to everyone for help….
Cesare

[Andreas]

I guess your Mix output carries the noise from some other input (maybe the test-tone generator?) and noise goes away when lowering the Mix5/6 master fader, right? I’d start checking that the return from your Fx is not fed back into the mix…

[MarkPAman]

Does the Strymon BigSky have balanced inputs & outputs? Given that their website does not seem to mention this, I’m guessing they’re unbalanced.

What is your cabling to convert from/to the desk’s balanced outputs/inputs? I suspect that something that gives an earth lift in at least one part of the signal path would sort this out.

Can you try a DI box (or two) between the BigSky output and the desk?

[cisa]

I’ll check tonight, one question: where I have to go to turn-off the test-tone generator? if it’s the one where you can select white noise/pink noise/square… and below there is a volume in DB, I already tried to turn it off (not red light) and set db to -infinite, but the noise is still here.

I have MIX 5-6 OUT fader volume to 0 DB (tonight I’ll try to lower it and see if noise reduces)

I have only input channel 1-2 with some volume fader (the channel where I connect OUT LR of my Synth)
All the inputs (1-16, st1, st2) in mix 5-6 are at -infinite, all channels are assigned. (tonight I’ll check if there is noise even with all faders channels to -infinite).

Thanks in advance.
Cesare

[cisa]

Strymon big sky outputs are not balanced (I assume, in manual it’s not specified).

I connect QU-16 (OUT MIX 5-6) -> 3mt cable XLR FEMALE to TRS male (unbalanced) (INPUTS L-R) Strymon BigSky (OUTPUT L-R) -> 3m cable trs male to trs male unbalanced (INPUT 5-6) QU-16. -> tonight I’ll try with balanced ones or using a stereo DI box.

But If I connect Elektron Analog 4 (my synth) (OUT LR) -> cable trs male to trs male (INPUTS L-R) Strymon BigSky (OUTPUT L-R) -> 3m cable trs male to trs male unbalanced (INPUT 5-6) QU-16 the noise disappear.

Same thing with my compressor DBX 266XS:
QU-16 (OUT MIX 5-6) -> 1mt cable XLR FEMALE to TRS male (unbalanced) (INPUTS L-R) DBX 266XS (OUTPUT L-R) -> 1m cable trs male to XLR male (balanced) (INPUT 5-6) QU-16 -> NOISE

QU-16 (OUT MIX 5-6) -> 1mt cable XLR FEMALE to XLR male (INPUTS L-R) DBX 266XS (OUTPUT L-R) -> 1m cable trs male to XLR male (balanced) (INPUT 5-6) QU-16 -> NOISE

So it seems that the noise is generated by MIX OUT 5-6 I think the problem is not the cables (I tried many ones, same problems).

[Andreas]

I’d second what Mark said and check the cabling. Particularly when going from balanced XLR to unbalanced TRS, that’s where a reverse DI comes into play. Without access to a reverse DI, check cabling for Pin 3 from the XLR. If that’s connected to ground, try to cut that wire.
Dick Rees posted a nice link some time ago: https://www.rane.com/note110.html

[Andreas]

Slight correction, after reading The Manual, which states in Section 4.2:

Professional equipment provides ‘balanced’ connections for maximum interference rejection over long cable runs. If you are connecting to ‘unbalanced’ equipment then make sure the XLR pin 3 (signal cold) is linked to pin 1 (signal ground).

So: Do not cut Pin 3 wire but ensure this is connected to ground (probably servo driven output, so that unterminated end induces bad noise on the positive side).

[MarkPAman]

I find that following Rane’s “Sound System Interconnection” guide, (as already linked to by Andreas c/o Dick Rees) provides trouble free results at least 99% of the time – been using it for many years. I have a printed copy on my workshop wall as reference for whenever I have to do something a little unusual.

https://www.rane.com/note110.html

From this I think you need cable #5 for the send {Desk —> FX unit}, and cable #14 for the return {FX unit —> Desk} leads. Time to get the soldering iron out I suspect.

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