isolating the power would help most, the two monitors which make things worse, if possible isolate their power supply as well as the cables. There's only so much you can do with old computer analogue audio, it'll not be perfect even in the best case scenario, however going all digital where possible will help.

Provide physical separation between your cables, use coax cables with a grounded outer jacket, or both. Isolating the supply is really only critical if you're mitigating a ground loop.

What kind of mixer is it, and how do you have things connected?

I would start by methodically disconnecting/reconnecting and muting/unmuting things until you can identify what part of your setup is generating the noise. Noise pickup is usually due to poor shielding, typically due to bad or incorrect cables.

gdjacobs wrote on 2020-04-27, 03:30:

Provide physical separation between your cables, use coax cables with a grounded outer jacket, or both. Isolating the supply is really only critical if you're mitigating a ground loop.

Dom't think it's a ground loop. as I said it's not 50Hz, sounds a bit higher.

johnnycontrario wrote on 2020-04-27, 05:12:

What kind of mixer is it, and how do you have things connected?

Behringer RX1602 mixer with sound cards (3.5mm jack or IEC if available) connected to it.

I would start by methodically disconnecting/reconnecting and muting/unmuting things until you can identify what part of your setup is generating the noise. Noise pickup is usually due to poor shielding, typically due to bad or incorrect cables.

Mixer itself seems unaffected. If I mute everything, it's quiet - but once I open any channel, noise ensues.

dionb wrote:

Mixer itself seems unaffected. If I mute everything, it's quiet - but once I open any channel, noise ensues.

Time to make yourself some shielded cables.

I shared a lot of information here, and I apologize if you already know what I'm talking about.

dionb wrote on 2020-04-28, 00:25:

Mixer itself seems unaffected. If I mute everything, it's quiet - but once I open any channel, noise ensues.

If you haven't done so already, try this procedure:

1. Unmute all channels you normally use and set them to their normal level.
2. Disconnect all input cables from the mixer (not from the audio source). At this point, the only noise should be some faint white noise generated by the mixer and your power amplifier. Otherwise, you have a mixer or power amplifier problem.
3. Plug in and unplug each cable so that only one sound source is connected at a time, and listen for noise.

Is the noise present on a particular cable, or all?

If the audio cables are bundled together, maybe all cables are picking up the same noise. See if rerouting helps. Run a single cable along a different route and see if the noise changes on that cable. Avoid routing audio cables near power supplies or anything else that generates high voltage pulses, like CRT monitors. Running near mains cables should be avoided too. Theoretically, the cable shielding should prevent interference, but real life can vary somewhat.

Your system may be set up perfectly and the noise may be from another device behaving badly. You can determine the source of the noise by systematically turning things off. With the audio system running and noise present, systematically turn off all devices in the room not inputting or outputting audio. I really mean ANYTHING that uses electricity: lamps, lights, fans, laptops, humidifiers, dehumidifiers, monitors, chargers, etc. Keep in mind that some power supplies generate EMI as long as they're plugged into the mains, so turning them off may not be enough. Hopefully, the noise stops when you turn off the offending device. But, if you end up in a dark room with only speakers and audio sources running, and you still hear the noise, start powering off each audio source. If you still have the noise after turning audio sources off, start unplugging input cables at the mixer.

dionb wrote on 2020-04-28, 00:25:

Behringer RX1602 mixer with sound cards (3.5mm jack or IEC if available) connected to it.

What do you mean by IEC? The only options on that mixer are: unbalanced TS - Tip, shield, 1/4" phone, (2 conductors: 1 gnd, 1 signal); and balanced TRS - Tip, Ring, Shield, 1/4" phone, (3 conductors: 1 gnd, 1 signal, 1 signal of reversed polarity).

Cables & Cable Quality:
To connect a 3.5mm jack sound source to this mixer, I would use a Y splitter cable (as short as possible) that converts 1x 3.5mm stereo male to 2x 1/4" mono female. I would connect that to the L and R input of a single channel on your mixer using high quality patch cables, again as short as possible. Instrument cables (guitar cables) would give you the best shielding, but may not be necessary.

To connect a balanced sound source, the cable quality is slightly less important. You can usually use any XLR to TRS adapter cable, or TRS cable. Shielding is less of a concern because balanced signals will remove most noise picked up by the cable. It's the same principle as those twisted pair, LVDS SCSI cables from back in the day. Usually, my only concern with cables carrying balanced signals is mechanical durability.

Regarding cable quality, I don't know what brands are available to you in the Netherlands, but here in the US, Hosa brand cables are inexpensive and are reasonably good quality. They also make adapter cables, which could save you some money over buying separate patch and Y cables. If you're considering cables that cost as much as Monster brand cables, consider some more average-priced cables instead. There is nothing new or special in cable technology. Audiophiles are idiots. Cables are a commodity.

MIxer Settings:
That mixer expects all inputs to be line level, either the pro level of +4 dBu, or the consumer level of -10 dBV. You should take that with a grain of salt because consumer audio devices can vary wildly. I'm sure you already know this, but you want the output of your sound sources to be as high as possible without clipping the input of the next device (the mixer), this should make it so the EMI noise picked up by the cable is negligible compared to the signal. Based on the info in Behringer's manual, the "Level" knobs on each channel are actually input gain knobs. This is how I would adjust your mixer.

1. Turn the master volume down on the mixer.
   (To save your ears)
2. Set all channels to +4dBu and with the "Level" knobs all the way down.
3. Turn the volume all the way up on all your audio sources.
   (Some poor quality sound sources distort when turned up all the way. In this case, go as far as you can without distortion.)
4. While playing the loudest material possible on the audio source, adjust the "Level" knob on each channel until the "Clip" light turns on OR until the knob is at the 12:00 position.
   (The labels aren't clear, but based on context, I can only assume that 12:00 is the 0dB point)
5. If you were able to light the "Clip" light:
   Turn down the "Level" knob just enough to stop clipping. That channel is set as good as it can be. Mark the "Level" position with a sticker or marker: that is the 100% level on that channel.
   If you still have noise, troubleshoot that sound source or its cables.
6. If you were NOT able to light the "Clip" light, turn the "Level" back down on the channel, set that channel to -10 dBV, and repeat the process. If you have to go beyond the 12:00 position on the knob, your audio source is outputting a weak signal. BUT, if you don't have noise on that channel, it's not a real problem.

If Behringer's manual is incorrect, and the "Level" knobs are not for input gain, then they will have no effect on the "Clip" light. In this scenario, they are simply faders for each channel and you can use them freely (within reason) to add or remove signals. In this case, the procedure is somewhat reversed. Instead of increasing "Level" until you get clipping, you want to turn up the sound source until you get clipping. Then, of course, turn it down slightly to avoid clipping.

dionb wrote on 2020-04-27, 00:04:

What I was thinking was to separate analog audio from all the digital (KVM, MIDI) and power cables, running the latter under the back of the desk and the audio along a shelf above the desk. Also the worst noise seems related to two monitors (one CRT, one TFT) being powered on. I'll try to keep everything relating to audio as far away from these two as possible.

You know, I wrote you a novel with a bunch of troubleshooting steps, but I re-read your original post, and I think you were on the right track to begin with. Separating audio from power cables is a good idea. Digital signal cables (MIDI, VGA, etc.) are usually OK to run near audio cables. Keep audio cables as far away from monitors and power supplies as possible. And, make sure you have good quality cables.

johnnycontrario wrote on 2020-04-28, 21:48:

I shared a lot of information here, and I apologize if you already know what I'm talking about. […]
Show full quote

I shared a lot of information here, and I apologize if you already know what I'm talking about.

dionb wrote on 2020-04-28, 00:25:

Mixer itself seems unaffected. If I mute everything, it's quiet - but once I open any channel, noise ensues.

If you haven't done so already, try this procedure:

1. Unmute all channels you normally use and set them to their normal level.
2. Disconnect all input cables from the mixer (not from the audio source). At this point, the only noise should be some faint white noise generated by the mixer and your power amplifier. Otherwise, you have a mixer or power amplifier problem.
3. Plug in and unplug each cable so that only one sound source is connected at a time, and listen for noise.

Is the noise present on a particular cable, or all?

If the audio cables are bundled together, maybe all cables are picking up the same noise. See if rerouting helps. Run a single cable along a different route and see if the noise changes on that cable. Avoid routing audio cables near power supplies or anything else that generates high voltage pulses, like CRT monitors. Running near mains cables should be avoided too. Theoretically, the cable shielding should prevent interference, but real life can vary somewhat.

Your system may be set up perfectly and the noise may be from another device behaving badly. You can determine the source of the noise by systematically turning things off. With the audio system running and noise present, systematically turn off all devices in the room not inputting or outputting audio. I really mean ANYTHING that uses electricity: lamps, lights, fans, laptops, humidifiers, dehumidifiers, monitors, chargers, etc. Keep in mind that some power supplies generate EMI as long as they're plugged into the mains, so turning them off may not be enough. Hopefully, the noise stops when you turn off the offending device. But, if you end up in a dark room with only speakers and audio sources running, and you still hear the noise, start powering off each audio source. If you still have the noise after turning audio sources off, start unplugging input cables at the mixer.

dionb wrote on 2020-04-28, 00:25:

Behringer RX1602 mixer with sound cards (3.5mm jack or IEC if available) connected to it.

What do you mean by IEC? The only options on that mixer are: unbalanced TS - Tip, shield, 1/4" phone, (2 conductors: 1 gnd, 1 signal); and balanced TRS - Tip, Ring, Shield, 1/4" phone, (3 conductors: 1 gnd, 1 signal, 1 signal of reversed polarity).

Cables & Cable Quality:
To connect a 3.5mm jack sound source to this mixer, I would use a Y splitter cable (as short as possible) that converts 1x 3.5mm stereo male to 2x 1/4" mono female. I would connect that to the L and R input of a single channel on your mixer using high quality patch cables, again as short as possible. Instrument cables (guitar cables) would give you the best shielding, but may not be necessary.

To connect a balanced sound source, the cable quality is slightly less important. You can usually use any XLR to TRS adapter cable, or TRS cable. Shielding is less of a concern because balanced signals will remove most noise picked up by the cable. It's the same principle as those twisted pair, LVDS SCSI cables from back in the day. Usually, my only concern with cables carrying balanced signals is mechanical durability.

Regarding cable quality, I don't know what brands are available to you in the Netherlands, but here in the US, Hosa brand cables are inexpensive and are reasonably good quality. They also make adapter cables, which could save you some money over buying separate patch and Y cables. If you're considering cables that cost as much as Monster brand cables, consider some more average-priced cables instead. There is nothing new or special in cable technology. Audiophiles are idiots. Cables are a commodity.

MIxer Settings:
That mixer expects all inputs to be line level, either the pro level of +4 dBu, or the consumer level of -10 dBV. You should take that with a grain of salt because consumer audio devices can vary wildly. I'm sure you already know this, but you want the output of your sound sources to be as high as possible without clipping the input of the next device (the mixer), this should make it so the EMI noise picked up by the cable is negligible compared to the signal. Based on the info in Behringer's manual, the "Level" knobs on each channel are actually input gain knobs. This is how I would adjust your mixer.

1. Turn the master volume down on the mixer.
   (To save your ears)
2. Set all channels to +4dBu and with the "Level" knobs all the way down.
3. Turn the volume all the way up on all your audio sources.
   (Some poor quality sound sources distort when turned up all the way. In this case, go as far as you can without distortion.)
4. While playing the loudest material possible on the audio source, adjust the "Level" knob on each channel until the "Clip" light turns on OR until the knob is at the 12:00 position.
   (The labels aren't clear, but based on context, I can only assume that 12:00 is the 0dB point)
5. If you were able to light the "Clip" light:
   Turn down the "Level" knob just enough to stop clipping. That channel is set as good as it can be. Mark the "Level" position with a sticker or marker: that is the 100% level on that channel.
   If you still have noise, troubleshoot that sound source or its cables.
6. If you were NOT able to light the "Clip" light, turn the "Level" back down on the channel, set that channel to -10 dBV, and repeat the process. If you have to go beyond the 12:00 position on the knob, your audio source is outputting a weak signal. BUT, if you don't have noise on that channel, it's not a real problem.

If Behringer's manual is incorrect, and the "Level" knobs are not for input gain, then they will have no effect on the "Clip" light. In this scenario, they are simply faders for each channel and you can use them freely (within reason) to add or remove signals. In this case, the procedure is somewhat reversed. Instead of increasing "Level" until you get clipping, you want to turn up the sound source until you get clipping. Then, of course, turn it down slightly to avoid clipping.

Very thorough comment. Factory made shielded cables can be had for a reasonable price if needed.
https://www.monoprice.com/product?p_id=5346

johnnycontrario wrote on 2020-04-28, 21:48:

I shared a lot of information here, and I apologize if you already know what I'm talking about.

A bit of it, but better to be thorough than miss something stupid because 'of course he knew that' - thanks, this is the kind of in-depth reply I was hoping for.

If you haven't done so already, try this procedure: […]
Show full quote

dionb wrote on 2020-04-28, 00:25:

Mixer itself seems unaffected. If I mute everything, it's quiet - but once I open any channel, noise ensues.

If you haven't done so already, try this procedure:

1. Unmute all channels you normally use and set them to their normal level.
2. Disconnect all input cables from the mixer (not from the audio source). At this point, the only noise should be some faint white noise generated by the mixer and your power amplifier. Otherwise, you have a mixer or power amplifier problem.
3. Plug in and unplug each cable so that only one sound source is connected at a time, and listen for noise.

Is the noise present on a particular cable, or all?

If the audio cables are bundled together, maybe all cables are picking up the same noise. See if rerouting helps. Run a single cable along a different route and see if the noise changes on that cable. Avoid routing audio cables near power supplies or anything else that generates high voltage pulses, like CRT monitors. Running near mains cables should be avoided too. Theoretically, the cable shielding should prevent interference, but real life can vary somewhat.

Your system may be set up perfectly and the noise may be from another device behaving badly. You can determine the source of the noise by systematically turning things off. With the audio system running and noise present, systematically turn off all devices in the room not inputting or outputting audio. I really mean ANYTHING that uses electricity: lamps, lights, fans, laptops, humidifiers, dehumidifiers, monitors, chargers, etc. Keep in mind that some power supplies generate EMI as long as they're plugged into the mains, so turning them off may not be enough. Hopefully, the noise stops when you turn off the offending device. But, if you end up in a dark room with only speakers and audio sources running, and you still hear the noise, start powering off each audio source. If you still have the noise after turning audio sources off, start unplugging input cables at the mixer.

Not a particular cable, but most if not all of them. The inputs are currently in between KVM switch (below) and MIDI patchbay (above), and run close to the SGI Multilink adapter for my 1600SW monitor, which is a prime culprit.

dionb wrote on 2020-04-28, 00:25:

Behringer RX1602 mixer with sound cards (3.5mm jack or IEC if available) connected to it.

What do you mean by IEC? The only options on that mixer are: unbalanced TS - Tip, shield, 1/4" phone, (2 conductors: 1 gnd, 1 signal); and balanced TRS - Tip, Ring, Shield, 1/4" phone, (3 conductors: 1 gnd, 1 signal, 1 signal of reversed polarity).

Sorry, got my devices mixed up - 6.3mm mono jacks, probably the 2-conductor ones you refer to.

Cables & Cable Quality:
To connect a 3.5mm jack sound source to this mixer, I would use a Y splitter cable (as short as possible) that converts 1x 3.5mm stereo male to 2x 1/4" mono female. I would connect that to the L and R input of a single channel on your mixer using high quality patch cables, again as short as possible. Instrument cables (guitar cables) would give you the best shielding, but may not be necessary.

That's what I aim for, although at present it's a mixed bag.

Regarding cable quality, I don't know what brands are available to you in the Netherlands, but here in the US, Hosa brand cables are inexpensive and are reasonably good quality. They also make adapter cables, which could save you some money over buying separate patch and Y cables. If you're considering cables that cost as much as Monster brand cables, consider some more average-priced cables instead. There is nothing new or special in cable technology. Audiophiles are idiots. Cables are a commodity.

Definitely NOT considering Monster, they are pure con-artists. Decent shielding is my goal, but wires are currently a mixed bag. Will look into fixing that.

MIxer Settings: That mixer expects all inputs to be line level, either the pro level of +4 dBu, or the consumer level of -10 dBV […]
Show full quote

MIxer Settings:
That mixer expects all inputs to be line level, either the pro level of +4 dBu, or the consumer level of -10 dBV. You should take that with a grain of salt because consumer audio devices can vary wildly. I'm sure you already know this, but you want the output of your sound sources to be as high as possible without clipping the input of the next device (the mixer), this should make it so the EMI noise picked up by the cable is negligible compared to the signal. Based on the info in Behringer's manual, the "Level" knobs on each channel are actually input gain knobs. This is how I would adjust your mixer.

1. Turn the master volume down on the mixer.
   (To save your ears)
2. Set all channels to +4dBu and with the "Level" knobs all the way down.
3. Turn the volume all the way up on all your audio sources.
   (Some poor quality sound sources distort when turned up all the way. In this case, go as far as you can without distortion.)
4. While playing the loudest material possible on the audio source, adjust the "Level" knob on each channel until the "Clip" light turns on OR until the knob is at the 12:00 position.
   (The labels aren't clear, but based on context, I can only assume that 12:00 is the 0dB point)
5. If you were able to light the "Clip" light:
   Turn down the "Level" knob just enough to stop clipping. That channel is set as good as it can be. Mark the "Level" position with a sticker or marker: that is the 100% level on that channel.
   If you still have noise, troubleshoot that sound source or its cables.
6. If you were NOT able to light the "Clip" light, turn the "Level" back down on the channel, set that channel to -10 dBV, and repeat the process. If you have to go beyond the 12:00 position on the knob, your audio source is outputting a weak signal. BUT, if you don't have noise on that channel, it's not a real problem.

I have noise regardless of the input, but yes, this is good practice I will re-do.

If Behringer's manual is incorrect, and the "Level" knobs are not for input gain, then they will have no effect on the "Clip" light. In this scenario, they are simply faders for each channel and you can use them freely (within reason) to add or remove signals. In this case, the procedure is somewhat reversed. Instead of increasing "Level" until you get clipping, you want to turn up the sound source until you get clipping. Then, of course, turn it down slightly to avoid clipping.

They do seem to affect the 'clip' light.

johnnycontrario wrote on 2020-04-28, 22:01:

dionb wrote on 2020-04-27, 00:04:

What I was thinking was to separate analog audio from all the digital (KVM, MIDI) and power cables, running the latter under the back of the desk and the audio along a shelf above the desk. Also the worst noise seems related to two monitors (one CRT, one TFT) being powered on. I'll try to keep everything relating to audio as far away from these two as possible.

You know, I wrote you a novel with a bunch of troubleshooting steps, but I re-read your original post, and I think you were on the right track to begin with. Separating audio from power cables is a good idea. Digital signal cables (MIDI, VGA, etc.) are usually OK to run near audio cables. Keep audio cables as far away from monitors and power supplies as possible. And, make sure you have good quality cables.

Thanks, good to hear I'm on the right track. My SC-55ST arrived today so I could check it would physically fit, and it will. Left side of almost 2m wide desk will be for analog audio mixer and the sound modules, right side for the rest - and powering everything from the right side, so apart from the power cables for mixer and the sound modules (which I'll keep as separate as possible) there should be no power cables fouling the audio. Do you think a shielded cable duct (i.e. using some aluminium profile) would help get the audio cables past the monitor in the middle of the desk with less/no ingress?

Try not to make power cables and audio cables cross (and keeping them apart is always a good idea).

However, also make sure it's not an issue with one of your audio devices. I have an effect unit that when plugged directly into my mixer will cause excess noise on any other connected device, but when hooked up via an audio patchbay does not (the mixer uses TS connectors, the patchbay is balanced TRS, probably there's a bad solder joint somewhere in the effect unit...). So see if disconnecting the audio devices one by one (or different combinations/configurations) solves the issue - turn down the volume on and turn off the devices before disconnecting and reconnecting them!

Separating video monitor/computer power connections from those of audio devices can help, but in Dutch houses they will often still be on the same power circuit (to others: because our fuseboxes are centralized). So also check whether there's nothing else on the circuit that could induce noise, such as washing machines. You may also want to check if the power that comes from your wallplugs is actually stable, even under load (in older houses and certain areas this can be bad).

Edit: Stupid question, but where did you put your mixer and MIDI devices? Hopefully not right next to the monitors...

yawetaG wrote on 2020-05-01, 05:33:

Try not to make power cables and audio cables cross (and keeping them apart is always a good idea).

However, also make sure it's not an issue with one of your audio devices. I have an effect unit that when plugged directly into my mixer will cause excess noise on any other connected device, but when hooked up via an audio patchbay does not (the mixer uses TS connectors, the patchbay is balanced TRS, probably there's a bad solder joint somewhere in the effect unit...). So see if disconnecting the audio devices one by one (or different combinations/configurations) solves the issue - turn down the volume on and turn off the devices before disconnecting and reconnecting them!

Separating video monitor/computer power connections from those of audio devices can help, but in Dutch houses they will often still be on the same power circuit (to others: because our fuseboxes are centralized). So also check whether there's nothing else on the circuit that could induce noise, such as washing machines. You may also want to check if the power that comes from your wallplugs is actually stable, even under load (in older houses and certain areas this can be bad).

Fortunately it's relatively modern, the electrical installation is from 2004. 99% certain that washing machine is on a different group/circuit - and the noise is constant, even in the middle of the night when I'm 100% sure no washing machine or kitchen appliances (they are definitely on two separate circuits, had the second one installed myself in 2015) is running. Only thing it correlates with are the two monitors, and the CRT seems less of an issue (perhaps only when degaussing and/or warming up) than the TFT, its power supply and that Multilink adapter (and its power supply)

Edit: Stupid question, but where did you put your mixer and MIDI devices? Hopefully not right next to the monitors...

Closer than I'd like. This is the way the setup looked a year ago:

The mixer is the row of red lights on the right-hand side. The power supply for 1600SW and its Multilink adapter are basically behind the mixer close, but no audio cables actually pass them.

What I intend to do at present:
- remove the SparcStation 20 (I have a better place for that anyway, and despite this being a big desk, those keyboards are constantly in each other's way)
- move the mixer from right stack to left one
- make two separate paths for cables, one behind the table-top, one behind the upper plank
- when building the new setup, work step-by-step with the audio:
-- first make sure the connection from mixer to speakers is noise-free
-- add devices one-by-one, both to check for noise and get the levels right as johnnycontrario advised
-- if any cables prove dud/noisy, replace

I'm also contemplating getting a second mixer, as I underestimated the number of inputs I needed, and I'm not happy with the results of looping two sound cards together in a single system - getting levels right is hell and noise levels there are really bad (and specific to the output of the card being looped through the other one). I don't expect that to change the solution/work flow, just more of the same.

dionb wrote on 2020-05-01, 14:26:

The power supply for 1600SW and its Multilink adapter are basically behind the mixer close, but no audio cables actually pass them.

This might explain things. Noise from that power supply could be coupling directly into an audio circuit in the mixer or power amplifier. Try moving that power supply away from the mixer and power amplifier.

dionb wrote on 2020-05-01, 14:26:

What I intend to do at present: - remove the SparcStation 20 (I have a better place for that anyway, and despite this being a bi […]
Show full quote

What I intend to do at present:
- remove the SparcStation 20 (I have a better place for that anyway, and despite this being a big desk, those keyboards are constantly in each other's way)
- move the mixer from right stack to left one
- make two separate paths for cables, one behind the table-top, one behind the upper plank
- when building the new setup, work step-by-step with the audio:
-- first make sure the connection from mixer to speakers is noise-free
-- add devices one-by-one, both to check for noise and get the levels right as johnnycontrario advised
-- if any cables prove dud/noisy, replace

I'm also contemplating getting a second mixer, as I underestimated the number of inputs I needed, and I'm not happy with the results of looping two sound cards together in a single system - getting levels right is hell and noise levels there are really bad (and specific to the output of the card being looped through the other one). I don't expect that to change the solution/work flow, just more of the same.

I would say this is a solid plan. It doesn't look like any cable runs need to be excessively long, so you should be able to minimize noise by using good cables and managing the distance between devices that interfere with each other.
Getting a mixer for the sound cards will definitely make your life easier. It may also have the side-effect of improving the noise situation if you use very short cable lengths from sound cards to the first mixer. Connecting mixer to mixer with balanced 6.35mm TRS cables (6.35mm stereo cables) would be ideal, but not necessary.

johnnycontrario wrote on 2020-05-01, 17:31:

dionb wrote on 2020-05-01, 14:26:

The power supply for 1600SW and its Multilink adapter are basically behind the mixer close, but no audio cables actually pass them.

This might explain things. Noise from that power supply could be coupling directly into an audio circuit in the mixer or power amplifier. Try moving that power supply away from the mixer and power amplifier.

I'll do the opposite, move the audio stuff away from the power.

I would say this is a solid plan. It doesn't look like any cable runs need to be excessively long, so you should be able to minimize noise by using good cables and managing the distance between devices that interfere with each other.
Getting a mixer for the sound cards will definitely make your life easier. It may also have the side-effect of improving the noise situation if you use very short cable lengths from sound cards to the first mixer. Connecting mixer to mixer with balanced 6.35mm TRS cables (6.35mm stereo cables) would be ideal, but not necessary.

My powered speakers have two line inputs. At present one comes from the "modern" Phenon II X4 system I use for general web browsing and support - viewed through that epic but noisy 1600SW - and the other comes from the mixer. WIth a second mixer, I would hook it up to the second input on the speakers.

dionb wrote on 2020-05-01, 18:02:

My powered speakers have two line inputs. At present one comes from the "modern" Phenon II X4 system I use for general web browsing and support - viewed through that epic but noisy 1600SW - and the other comes from the mixer. WIth a second mixer, I would hook it up to the second input on the speakers.

I don't think there is necessarily a wrong way of doing it. I assumed you meant to use a second mixer to mix the outputs of the sound cards to a single stereo pair, and then run that pair into a channel on your line mixer.

Update here: fixed 😀

My big cable order arrived and I also upgraded my 'modern' system. So I disassembled everything and reassembled it all. No pics as up against wall/window, but overview:

- Analog audio is now on left side of desk, with 2x Behringer RX1602 mixers feeding into the 2x input on my speakers.
- Only non-analog stuff on left side is my three MIDI modules (MT32, SC55ST, MU50) and their power.
- Rest is on right side of desk, so MIDI patchbay and KVM switch.
- Only analog audio now needing to go from right to left is 2x well-shielded cables from digital piano (Roland, MIDI 😜 ), right speaker and audio in and out from my 'modern' system.
- All analog audio runs at 'high' level, parallel to top of my 19" racks. Routed it through Ikea steel panel intended for magnetic knives or similar for shielding.
- All power runs along a basket under desk, towards the rear.
- All other stuff (MIDI, VGA) runs in same basket towards the front.
- That noisy SGI Multilink adapter for the SW1600 monitor is now under the desk, on top of the modern system.

Somehow I now ended up with ground loop problems with the modern system - but only when the 1600SW monitor was active. Solved that with a filter. Since then, it's perfect, near-silent when listening to a connected but inactive input with volume turned way up.

Only issues:
- I forgot 6.3mm stereo TRS plugs for 'headphone' out to allow me to record all the goodness going over the mixers. ETA tomorrow.
- Turns out MIDI in #3 on my Edirol UM550 patchbay is dead. That's irritating as I already had too few inputs. So I'm now actively looking for a Roland A-880 or Edirol UM880 or similar - something with 8 MIDI-in. Not cheap, but "necessary" 😉

Edit:
Regarding that ground loop - I'm a bit puzzled. It's 50Hz, it is completely removed by the transformer/filter thingy so it walks and quacks like a ground loop. However I was under the impression ground loops only occurred on devices having two separate paths to ground. I've been very careful to connect all devices here to the same (earthed) power outlet, moreover the two devices associated with the sound (SGI Multilink and/or SGI 1600SW monitor - can't distinguish which is the cause as they cannot be run independently) run off two 12V bricks with ungrounded (2-pin) plugs. The signal cable does have an outer sheath connected to GND, but the only place that would connect to ground is via the computer power cable, which is plugged into the same extension cable as the mixer.

Here's a schematic diagram:

If there is output on the 1600SW, there is hum out of the speakers.
If no output on 1600SW, no hum, even if PC is powered on and otherwise active.
A transformer/ground loop filter connected between the PC audio out and the cable from PC to mixer solves the hum - so it's not ingress from where the power cables for those 12V adapters pass by the audio cable. I chose my best shielded cable for this.

This is the kind of ground loop filter I use:

It works, but I want to solve the problem as I also want to record sound from the PC and don't want to have a whole pile of these filters in between...

dionb wrote on 2020-07-08, 23:10:

If there is output on the 1600SW, there is hum out of the speakers.
If no output on 1600SW, no hum, even if PC is powered on and otherwise active.
A transformer/ground loop filter connected between the PC audio out and the cable from PC to mixer solves the hum - so it's not ingress from where the power cables for those 12V adapters pass by the audio cable. I chose my best shielded cable for this.

I can think of 2 possibilities.

1. There may be a ground loop between the PC and Mixer. The PC and Mixer see 2 paths to ground via the audio cable's ground conductor. This theoretically shouldn't happen with properly designed equipment, but here we are. Noise from the monitor could be conducting to the PC via the signal cable ground conductor and seeing 2 paths to ground: one via the mixer (audio cable) and the other via the PC power supply.
The easy way out is to disconnect the earth conductor on the power cord of either the PC or Mixer. I can't, in good conscience, call this a safe practice. I personally haven't had any problems and I carry one of these abominations in my gig bag just in case I come across a ground loop.

These are very common in the states, I'm sure there's an equally common version in the EU. I would note that, as your diagram indicates, the monitor is probably getting its earth connection via the PC, so un-earthing the mixer's power cord would make sense here (un-earthing the PC may cause other noises).

2. The SGI power supplies may be terrible. They may simply have poor EMC/EMI filtering. The solution would be to connect an EMC/EMI filter between the mains and the SGI supplies, or to replace the supplies with a decent-quality, modern equivalent and install SGI compatible connectors. This may or may not be worth the trouble.

Personally, I'm leaning towards #1.
Installing the ground loop filter in the signal path from PC to mixer eliminates the noise, so it's conducted noise, not electro-magnetic. As you said, it quacks like a ground loop. Honestly this doesn't surprise me at all. Even with pro-audio gear, ground loops happen. Basically any time 2 earthed devices are connected to each other, a ground loop is possible. Using the filter is a more proper solution than un-earthing one of the devices (it's safer too). In my opinion, this is a perfectly appropriate use of a ground loop filter, and you have solved your noise problems.

dionb wrote on 2020-07-08, 23:10:

It works, but I want to solve the problem as I also want to record sound from the PC and don't want to have a whole pile of these filters in between...

I overlooked the last sentence in your post. I would try it and see. I doubt that you will need a ground loop filter on every input, just 1 or 2 problem devices.

johnnycontrario wrote on 2020-07-11, 06:34:

dionb wrote on 2020-07-08, 23:10:

It works, but I want to solve the problem as I also want to record sound from the PC and don't want to have a whole pile of these filters in between...

I overlooked the last sentence in your post. I would try it and see. I doubt that you will need a ground loop filter on every input, just 1 or 2 problem devices.

Thing is, the PC is the problem device (at least, once the SGI monitor is on) and I want at least 3 maybe 4 connections into it: 1 out, 2 or 3 in (although the third 'in' would be digital, so using Toslink to avoid an electrical connection would keep that out of the equation). Also the audio in would come from two different mixers (I have two RX1602 and want to capture from both). Other devices only have a single audio out (or maybe 2 or 3 if I have a lot of sound cards- but always the same mixer) but no other device in the setup causes problems.

As for the SGI power supplies - yes, they are highly suspect, but I've replaced one already after it blew up (bad caps inside), and that didn't change this experience one bit. Also a bit lost as to how ungrounded devices could cause a ground loop, as it's definitely the two SGI things that are to blame - if I hook up a different monitor, the PC is fine.

Still, I'll try the various suggestions and see what works best. Thanks!

seeing as this is caused by the monitor, I'd rather call this interference from the monitor than an actual ground loop.

it feels like the monitor is somehow introducing 50Hz noise to the PC and that reflects in the audio signal.

also, please do not remove the earth connection from your PC psu.

Idk how to solve this, but you could maybe try powering the 12V devices from your PC directly?