FAQ: High Frequency Noise on Auxes

[RoadRanger]

The conclusion of a six page thread on the old forum seems to be that using unbalanced connections (AKA TS or instrument cables) between the DL1608 aux outs and a powered monitor or amp will be noisy. One guy "solved" this by disconnecting the AC ground on the power supply but that's dangerous at best. Has anybody had any luck running unbalanced?

[sam.spoons]

I suggested disconnecting the earth on the PSU, IIRC it wasn't particularly successful in removing the noise (and as you say removing the safety earth is not recommended though I'm still not convinced it's an issue with a low voltage supply like the DL PSU, most PSU's of this type have only a two conductor mains cable with no safety earth anyway).

I did have success with a custom lead, I tried several variations and I'll check later but I'm pretty sure the one that worked was a TRS to TS with the ring on the TRS left unconnected (i.e. no 'cold' connection). It only works if you use a TRS with the ring unconnected at the DL end not if you strap the ring to earth (or use a TS to TS lead which amounts to the same thing).

[RoadRanger]

I did have success with a custom lead, I tried several variations and I'll check later but I'm pretty sure the one that worked was a TRS to TS with the ring on the TRS left unconnected (i.e. no 'cold' connection). It only works if you use a TRS with the ring unconnected at the DL end not if you strap the ring to earth (or use a TS to TS lead which amounts to the same thing).

I would have guessed that the "winning" combination would be:
TRS -> TS :
S not connected
R -> S
T -> T

Yes, please check what you did and post it here .

[sam.spoons]

Yes, that's what I thought and tried first but I've checked and the one that works is as above, TRS to TS
T > T
R > unconnected
S > S
Its number 9 on this chart https://dl.dropbox.com/u/15685857/Rane%20chart2.bmp

[RoadRanger]

Thanks!

[WK154]

The general rule on balanced to unbalanced is to leave the shield floating at the receiver (amp, instrument etc.) end and to tie return and shield together at the sender end (DL1808). A detailed explanation of grounding and tracing ground issues can be found here and is well worth reading.
http://www.jensen-transformers.com/an/generic%20seminar.pdf
Definitely do not lift AC ground.
TRS  end        ->           TS end
S connected -> not connected
R               ->         S
T                ->         T

Not sure if that's what you had in mind RR. This is with a standard 2 wire plus shield audio cable. If the ground currents are significant between equipments a isolator would be needed.

[sam.spoons]

Agreed but that doesn't solve the aux noise issue on the DL, see my post above.

[prosoundco guy]

The conclusion of a six page thread on the old forum seems to be that using unbalanced connections (AKA TS or instrument cables) between the DL1608 aux outs and a powered monitor or amp will be noisy. One guy "solved" this by disconnecting the AC ground on the power supply but that's dangerous at best. Has anybody had any luck running unbalanced?

I think that you'll find that using unbalanced cables a noisy option using any board. Best solution is to use a TRS to XlR short cable from the Aux outs into an XLR mic cable for the run to the monitor(s).

[sam.spoons]

The conclusion of a six page thread on the old forum seems to be that using unbalanced connections (AKA TS or instrument cables) between the DL1608 aux outs and a powered monitor or amp will be noisy. One guy "solved" this by disconnecting the AC ground on the power supply but that's dangerous at best. Has anybody had any luck running unbalanced?

I think that you'll find that using unbalanced cables a noisy option using any board. Best solution is to use a TRS to XlR short cable from the Aux outs into an XLR mic cable for the run to the monitor(s).

I suggested trying the DL with the safety earth disconnected, but strictly as a diagnostic (though I did surmise that a safety earth on a low voltage device may not be strictly necessary). In the event it did not solve the problem however the lead detailed above did solve the noise issue on my rig.

to reiterate at the DL end use a TRS plug, leave the Ring unconnected, tip = hot. sleeve = ground
at the unbalanced amp end use a TS plug, tip = hot. sleeve = ground

This works with my DL sending a line signal to my AER Compact 60 (used as a monitor), a TS-TS lead or any other connection scheme generates the high pitched noise.

The DL aux outputs are impedance balanced and an explanation of why this wiring works can be found here http://community.avid.com/forums/t/58612.aspx

[prosoundco guy]

The conclusion of a six page thread on the old forum seems to be that using unbalanced connections (AKA TS or instrument cables) between the DL1608 aux outs and a powered monitor or amp will be noisy. One guy "solved" this by disconnecting the AC ground on the power supply but that's dangerous at best. Has anybody had any luck running unbalanced?

I think that you'll find that using unbalanced cables a noisy option using any board. Best solution is to use a TRS to XlR short cable from the Aux outs into an XLR mic cable for the run to the monitor(s).

I suggested trying the DL with the safety earth disconnected, but strictly as a diagnostic (though I did surmise that a safety earth on a low voltage device may not be strictly necessary). In the event it did not solve the problem however the lead detailed above did solve the noise issue on my rig.

to reiterate at the DL end use a TRS plug, leave the Ring unconnected, tip = hot. sleeve = ground
at the unbalanced amp end use a TS plug, tip = hot. sleeve = ground

This works with my DL sending a line signal to my AER Compact 60 (used as a monitor), a TS-TS lead or any other connection scheme generates the high pitched noise.

The DL aux outputs are impedance balanced and an explanation of why this wiring works can be found here http://community.avid.com/forums/t/58612.aspx

Most powered monitors I've seen only have XLR inputs anyway. So TRS to XLR is the way to go.

[prosoundco guy]

Cable tie six TRS to XLR cables together for a mini
aux snake and you're done.

[sam.spoons]

TRS to XLR is undoubtedly the best way to go but for the few people who still have unbalanced inputs to serve (old/budget power amps/speakers or in my case my very expensive AER which, inexplicably, has an unbalanced line input) a simple (but not obvious in terms of wiring scheme, just lifting the earth does not work for example) custom lead solves the problem. 

[Greg C.]

TRS to XLR is undoubtedly the best way to go but for the few people who still have unbalanced inputs to serve (old/budget power amps/speakers or in my case my very expensive AER which, inexplicably, has an unbalanced line input) a simple (but not obvious in terms of wiring scheme, just lifting the earth does not work for example) custom lead solves the problem. 

It's silly that you might need such a lead. Mackie screwed up on their output design to be certain and is refusing to accept blame for such a "Mixing Boards 101" debacle. I mean that should have been one of the first things tested by their QA dept. when they were in the design stage. Really basic stuff. After all, they've probably produced more small format mixers than anyone on the planet. They should know better than to make such an amateur mistake. Can you tell I'm peeved?

[nottooloud]

I finally bought a DL1608 after watching it since release. The feature set and form factor are simply uneatable for some of my applications. First thing I did was plug an aux into a powered Hotspot using an unbalanced cable. Hey presto, high frequency noise, slowly cycling and changing. I'd guess I'm eavesdropping on the CPU. Replacing the unbalanced cable with a trs balanced cable eliminated it completely, as excpected.

I built a Jensen Transformer Pin 1 hummer and, much to my surprise, found no indication of a Pin 1 problem whatsoever. See http://pin1problem.com/ for lots of excellent info on this topic. The reported symptoms matched perfectly, but that doesn't seem to be what's wrong.

I carry a ProCo IT-4A 4 channel isolation transformer whenever I'm going into a school or similar venue where things are likely to be dodgy. Unbalanced into and out of that and flip the lift switch and the noise is still there, but very greatly reduced, and quite tolerable. So that's my basic solution. Balanced whenever possible, but keep the IT-4A packed with the DL1608 in case of trouble.

[Greg C.]

I don't think the issue is a traditional "pin 1" problem so much as a digital circuit RF infiltration issue. It seems mackie did a poor job keeping the digital RF hash out of the analog domain.