I use this Mackie ProFx6v3 mostly as an interface when playing live with friends. But it doesn’t do a really simple thing I want it to do:
- Have instruments coming in so I can live modify modify them on my computer
- Listening on my headphones so I can monitor what I’m going to go next
- Send my main output to the room mixer
What happens by default with this device is that the main output is sent back to the USB input. It is fine if you use that thing only to record everything that gets on the main output but in my case, it would send back my music on the input that I am processing
This is the initial configuration of the mixer. USB Input means that the computer is receiving audio from the mixer.
flowchart LR
PI[Physical input] --> MB[Main bus]
MB --> UI[USB Input]
MB --> PO[Physical output]
UO1[USB Output 1,2] --> HP[Headphones]
UO2[USB Output 3,4] --> MB[Main bus]
What I want is:
flowchart LR
PI[Physical input] --> UI[USB Input]
PI --> Volume
Volume -->MB
MB --> PO[Physical output]
UO1[USB Output 1,2] --> HP[Headphones]
UO2[USB Output 3,4] --> MB[Main bus]
Thankfully, Mackie gives a descent schematic in their manual (some brands could learn from them). I just wish they would give a real schematic that doesn’t cost them much more and that would save a lot of time for users and repair people…
I want to move the red connection in the schema below to the green position. That way I am always receiving the full signal in the inputs and I can still send the inputs to the main if I want to use it as a normal mixer.
The USB board itself it is connected to the main mixer board by the header on the left (8x2, we only see the pins).
First step was to find out where the audio signals were getting in and out of the USB board.
I made a table like so measuring voltages was easy and the only pins left had to be the audio I/O problem was to find which is which.
| A | B | |
|---|---|---|
| 1 | GND | ? |
| 2 | GND | ? |
| 3 | GND | ? |
| 4 | GND | ? |
| 5 | GND | ? |
| 6 | +5V | ? |
| 7 | +15V | -15V |
I sent a sine from the USB on the left and the right side and figured that looking at an oscilloscope trace (in FFT mode so I get a nice sharp peak)
| A | B | |
|---|---|---|
| 1 | GND | USB 1 In or 3 Out |
| 2 | GND | USB 2 In or 4 Out |
| 3 | GND | USB 1 In or 3 Out |
| 4 | GND | USB 2 In or 4 Out |
| 5 | GND | USB 1 or 2 Out |
| 6 | +5V | USB 1 or 2 Out |
| 7 | +15V | -15V |
Sending audio (tip: use another frequency so that’s easy to see what is where) on the inputs 1/2 showed where the input was:
| A | B | |
|---|---|---|
| 1 | GND | USB 1 In |
| 2 | GND | USB 2 In |
| 3 | GND | USB 3 Out |
| 4 | GND | USB 4 Out |
| 5 | GND | USB 1 Out |
| 6 | +5V | USB 2 Out |
| 7 | +15V | -15V |
Snipping the header pins of the USB board this can always be reverted with some wires:
Wires connecting to the pot to get the signal PFL (before attenuation) that way we can decide to send the audio to master anyway:
And the final product with a sane amount of hot-glue.
While I was at it, I figured out the pinout of their custom Op amps:
┌───────────────────┐
│ (dot) │
Out/1 ○ 1 8 ○ +15V
│ │
In-1 ○ 2 7 ○ Out/2
│ │
In+1 ○ 3 6 ○ In-2
│ │
-15V ○ 4 5 ○ In+2
│ │
└───────────────────┘
My understanding is that they are derivatives of the NJM4580
Next steps
Deciding if I want to put switches to go back to normal behavior, for now I am ok with it.
Updates
2025-03-23: Moved the wires from the FX button to the pot so I can at least choose if I send to the Main bus or not.