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Gen 1 KRK Rokit 8 XLR/TRS Input Issue
Old 14th June 2018
  #1
Here for the gear
Gen 1 KRK Rokit 8 XLR/TRS Input Issue

hey guys, sorry for making this my first post, but i'm looking for some insight before i attempt to take this unit to a repair shop or replace it completely.

I just picked up a pair of gen 1 8s. RCA/unbalanced inputs work fine on both units. XLR gives me no sound at all, TRS gives me *enough* sound if i pull the plug like 1/4 of the way out. i have the monitor completely apart right now on my work bench, and i'm looking everything over... i don't see any glaring issues, the connector looks to be positioned correctly inside the TRS input, still not sure why XLR doesn't work at all

this is making me think that it's an issue further down the line, on the next PCB. i did some googling and can't really find anyone having this specific issue. i also cannot find the xlr/tps pcb as a replacement part, it seems like i can only fine full amp board replacements for the gen2 and up.

does anyone have any insight? i can provide pictures, i have a multimeter and soldering setup. i'm just not really sure where else to start and the internet seems flooded with various other issues with these, everything but the problem i am having.

and for the record, i did triple check to make sure it wasn't an output issue with my mixer, or the cable itself.
Old 14th June 2018
  #2
Lives for gear
The problem may simply be that you are not sending a correct polarity balanced signal to the XLR/TRS inputs. In a fully balanced circuit, the 2 and 3 wires in the cable carry the exact same mono signal but each has opposite polarity. Electronics in the speaker flip the polarity in one of the legs, which boosts the signal and cancels noise picked up in the cable run.
It looks from your description that you are sending a “balanced” signal which has the same polarity on the 2 and 3 wires. The electronics in the speaker flip polarity on one of the legs and you get a perfect cancellation of the signal (no signal).
The proof of this is your description of what happens when you partially insert the TRS plug into the speaker connector. The tip of your plug touches the ring part of your connector, which connects one of your signal legs, so you get sound. The other leg hasn’t connected yet, so the “flip and add” circuit in the speaker has not cancelled the tip signal by reversing the ring signal, because it hasn’t touched its leg of the speaker circuit yet. When you fully insert the plug, the circuit cancels and you get no signal.
There is nothing wrong with your balanced inputs on your KRKs. You aren’t sending them a correct, fully balanced input.
A second proof would be to take your soldering iron and disconnect either the tip or ring wire in your cable (at either end of the cable). Voila! You will have signal.
You aren’t sending a properly balanced signal to a balanced input.
Old 14th June 2018
  #3
Lives for gear
I hope you haven’t done anything to the speaker electronics or connectors in the meantime.
Old 14th June 2018
  #4
Lives for gear
While the unit is open, try cobbling together a test rig with 2 clip leads, one coming from the center conductor and one from the shield of your working RCA signal. Obviously you don't want to let them short.

Connect one clip to pin 2 of the XLR jack, and one to pin 3. Leave pin 1 unconnected for now.

Do you hear a signal? Possibly at about half the volume you'd expect from a proper XLR or TRS, but otherwise undistorted?

If so, the inputs are fine. You need to look at your balanced source.
Old 14th June 2018
  #5
Here for the gear
thank you all for the replies.

I don't see how anything could be wrong with my sources. we tried multiple mixers, verified that the right monitor was receiving a correct signal and then used the same TRS on the left monitor to no avail. XLR is also a complete no go and these two inputs are on the same PCB/3 pin connector wire

I will try what Jay recommended later today.
Old 14th June 2018
  #6
Here for the gear
Quote:
Originally Posted by Bushman View Post
The problem may simply be that you are not sending a correct polarity balanced signal to the XLR/TRS inputs. In a fully balanced circuit, the 2 and 3 wires in the cable carry the exact same mono signal but each has opposite polarity. Electronics in the speaker flip the polarity in one of the legs, which boosts the signal and cancels noise picked up in the cable run.
It looks from your description that you are sending a “balanced” signal which has the same polarity on the 2 and 3 wires. The electronics in the speaker flip polarity on one of the legs and you get a perfect cancellation of the signal (no signal).
The proof of this is your description of what happens when you partially insert the TRS plug into the speaker connector. The tip of your plug touches the ring part of your connector, which connects one of your signal legs, so you get sound. The other leg hasn’t connected yet, so the “flip and add” circuit in the speaker has not cancelled the tip signal by reversing the ring signal, because it hasn’t touched its leg of the speaker circuit yet. When you fully insert the plug, the circuit cancels and you get no signal.
There is nothing wrong with your balanced inputs on your KRKs. You aren’t sending them a correct, fully balanced input.
A second proof would be to take your soldering iron and disconnect either the tip or ring wire in your cable (at either end of the cable). Voila! You will have signal.
You aren’t sending a properly balanced signal to a balanced input.
this makes perfect sense, and thus makes the issue even more perplexing lol.

if the circuit was failing to flip the polarity, that would result in simply a reduced volume level, correct?
Old 14th June 2018
  #7
Lives for gear
When you're doing my test, don't let the RCA source and your speaker share a common ground... not from the case, and not from an AC ground pin. You might hear a bit extra hum and it may defeat any safety ground, but it's just for the test.
Old 14th June 2018
  #8
Lives for gear
Quote:
Originally Posted by carbon14c View Post
this makes perfect sense, and thus makes the issue even more perplexing lol.

if the circuit was failing to flip the polarity, that would result in simply a reduced volume level, correct?
No, if the circuit was failing to flip one leg of the balanced pair, and the send was correct (that is, the two legs are opposite polarity), when the two legs were combined you would get perfect cancellation and no signal to the speaker at all. If you have just one leg working and the other leg not connected, you would get six db less signal than in a truly balanced circuit with both legs connected. There is also another semi-balanced configuration where only one leg carries an active signal, but the no-signal leg is flipped in phase and combined with the signal leg. The signal level is not boosted, but common noise picked up by both legs is cancelled. It gives the noise reduction of fully balanced but doesn’t boost the signal as two active opposite polarity legs do.
Old 15th June 2018
  #9
Lives for gear
Quote:
Originally Posted by Bushman View Post
... There is also another semi-balanced configuration where only one leg carries an active signal, but the no-signal leg is flipped in phase and combined with the signal leg. ...
If the 'no signal' leg is unterminated at the far end of the cable, this could reject CM noise on the cable.

But where does the 'active signal' leg close its circuit? To ground? This would tie the system ground into the audio circuit, with potential for ground loops. All you're doing is running unbalanced with an extra line to cancel out cable noise... and how is this different from telescoping grounds in a well designed unbalanced room?

Or if it returns to something that is isolated but still goes to the input amp, then you've got a classic balanced circuit.

I've never come across this circuit. What am I missing? Can you draw a schematic, showing the current flow?
Old 15th June 2018
  #10
Lives for gear
No, I can’t. I have worked in audio for (too) many decades, but I have no formal electronics education.
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