Balanced outputs - how to find out?

[Mesmerised]

You're referring to the K2500 keyboard version, I assume? I've got a K2500S keyboard and it's really quiet concerning noise (Yamaha 01v desk here). Note that the K2500 outputs are not symmetric even though they use TRS jacks! (it's a tricky send/return thing they did there) Just use TS cables to connect them.

[ʝɵʝɵ]

Mysterious. I can't even picture where the issue might be coming from in the system. have you tried unscrewing a lifting the K2500 out of the rack? The loop might be through the casing and rack rails?

The K2500 isn't a rack synth, so none of that applies unfortunately. The only physical connection it has to the rest of the system is through audio, MIDI and power cables (well, and through the carpet on the floor... ).

You're referring to the K2500 keyboard version, I assume? I've got a K2500S keyboard and it's really quiet concerning noise (Yamaha 01v desk here). Note that the K2500 outputs are not symmetric even though they use TRS jacks! (it's a tricky send/return thing they did there) Just use TS cables to connect them.

Indeed they're unbalanced, and even in the K2500 manual they essentially recommend the usage of a DI box or an AC isolation transformer (the latter would be a bit of an overkill for my situation, and I'm not even sure that would be safe to use with all the other gear not being isolated). The noise introduced by the synth itself is perfectly acceptable - but it's picking up EMI, most likely from my graphics card, somewhere on the way. I should also say that this EMI noise is at an extremely low level that won't hurt recordings (certainly if quantized to 16 bits) - but I take this as an interesting exercise to solve and to better understand my audio setup, especially since none of the other synths expose the problem, no matter if rack-mounted or keyboards.

Regarding what I earlier said about the power being plugged in making a difference - it still does make a difference. The EMI doesn't completely disappear when the K2500's power cord isn't plugged in, but it is definitely attenuated.

[Mesmerised]

Hmmm - tricky!
Did you try connecting the K2500 to a different power outlet? (not the same multi as the PC)
I have had good experiences generally with "star-formation" power cabling. All my audio gear is fed from one wall outlet. From a sturdy main mult it branches out - yes, like branches of a tree. So all parts have one common ground reference.
You CAN try to put an extra ground cable to the K2500 shell - but that might be a bit extreme (back then in an old house and with different equipment, I needed to have extra grounding cables that met at the mixing desk's ground)
IF you can get an AC isolation transformer (quite heavy things!) that would be the golden way to go. And don't worry - it's absolutely safe! (in fact, I used to at a big radio station and it was the golden rule to always connect unknown guest musicians' equipment to AC iso transformers. If any device should fail you don't die. )

[madtheory]

...even in the K2500 manual they essentially recommend the usage of a DI box or an AC isolation transformer...

Cheeky! There are quite a few major Kurzweil fans at the Sound On Sound forum. Actual professionals. Might be worth asking there?

[ʝɵʝɵ]

Did you try connecting the K2500 to a different power outlet? (not the same multi as the PC)

So as mentioned above, whether the K2500 itself is connected to power at all doesn't change anything but the volume of the EMI (though by how much it differs seems to depend on the time of day or the K2500's mood) - the only thing I need to hear the EMI noise is the K2500 being connected to my mixer. I did try plugging the mixer both into the same outlet as the PC and another one, but that didn't seem to make any difference.

I'll rearrange some parts of the studio on the weekend as I will be getting a new keyboard stand for the K2500 (for a completely unrelated reason, of course ), so I will be moving stuff around and see if that makes any difference to the intensity of the EMI.

[ʝɵʝɵ]

Update after setting up the new keyboard stand, which gave me the opportunity to rewire the whole setup.
When I just connected audio cables to the K2500, the noise floor was pure white noise, without EMI. As soon as I connected either the SCSI cable going to my Triton-Rack or the power cable, the EMI came back. This is interesting because I am sure I have disconnected the SCSI cable before and it didn't make the difference - probably because the power cable was still connected. The SCSI cable is pretty long and passing by the PC (though not that much closer than the K2500 itself, really), but isn't connected to the PC really (apart from the MIDI connection going from the Triton to the MIDI interface).

So... not much changed really. I gave the ART DTI another try, this time with balanced cables going to the mixer's input, and it's still performing poorly. Re-reading the reviews on Thomann about this unit, I don't seem to be the only person running into this problem. Here's an automated translation of the top German review on Thomann:

I ordered four of these devices to use them for balancing/unbalancing and ground separation in difficult cases (grounded mixer to PC, amplifier (grounded) to other circuit than mixer/rack (grounded), etc.). I already had good experiences with a self-built box with Palmer transformers, but due to time constraints I now wanted something ready-made. Unfortunately, this box did not meet my requirements - everywhere where hum threatened, it was reduced, but, in contrast to the Palmer transducers PMT-08 and PMT-11, a residual hum was always audible.

Since I had four boxes with the same behavior, scattering can be ruled out. Measurements showed that this unit probably has bad transformers and/or bad signal routing - it separates floating, but has a parasitic coupling capacitance in each channel! Therefore, although the 50 Hz hum is well blocked, the also known higher sounding capacitive "creak" (like an open input on a guitar amp) remains. Sold all four and now bought Palmer PLI-02 - these contain the PMT-11 transformers, alternatively the PLI-03 with the PMT-08 transformers will do.

These devices meet studio requirements and solve all cases - the ART DTI I can only recommend as a simple plug system adapter and for matching balanced <--> unbalanced, where NO hum interference is to be expected. The DTI does not get rid of them.

There's a few other reviewers observing the same issue. So... I'll put the Behringer HD400 on the outputs of the K2500, as it will get rid of the EMI very effectively. That's two times in a single thread that I was totally ready to buy something more expensive than Behringer, but the more reputable competition failed in one way or another in both cases.

[ʝɵʝɵ]

The original issue is solved, but with my new setup I can find all kinds of new noise to eliminate... Soon I will spend more time eliminating noise than actually using my gear

I got an Ensoniq SQ-R this weekend to fill the last slot in my rack. Out of pure practicality (stacking smaller rack gear on top of larger gear), I placed it below my RX1602 mixer. I've now noticed that as soon as I turn on the SQ-R, without even being being connected to the mixer, it induces some 50Hz hum into the mixer's signal. So I guess the power supply must be badly shielded? Are there any things to try that could reduce the issue? Again it's mostly an academic problem because the hum isn't very audible (adds +3dB to the noise floor level), but noise sources do add up and it means I have to remember to turn if off and not just mute it if I want the cleanest possible recording from another synth.. So if there's an easy way to fix it, I'm all ears.

[madtheory]

I'm sure you've thought of this. The easy way is to move it further away. Which will mess up the rack aesthetics unfortunately. But it might induce hum in another device....

[ʝɵʝɵ]

Aesthetics are important. I mean, just look at how perfect the combination of SQ-R and the Roland U-220 looks! https://sagagames.de/ithumb/show/ensoniq_sq-r.jpg The blue decorations of the two devices just match perfectly. Can't change that. I could probably move them below the Triton and hope that the Triton doesn't pick up anything from the SQ-R.
I also have ordered updated ROMs and a battery holder for the SQ-R, so I will have to open it later this week anyway, and I guess I will see if there's anything suspicious about the power supply.

[ʝɵʝɵ]

I rearranged the three synths in various ways, but in the end the hum never went fully away (but was reduced in volume), and since the original order was the most practical (with regards to the depth of the synths and and thus how easily their MIDI and audio connections would be reachable), I went back to the original order (as seen in the picture). Gotta live with a bit of hum.