205 Dual Matrix Mixer

I was sent this 205 to address the bleed issue. The mixer is organized as two independent 5x4 mixers where each of the five inputs can be assigned a level to any of the four outputs. The switch also sends that signal to a pair of left and right monitor outputs. Although they are designated left and right, the monitor channel is mono. A separate set of four outputs in the center combines the left and right outputs.

 

The issue with this mixer was bleed to other channels. I measured the bleed as -51 dB, a low number but still audible. In looking at the schematics, Don used a 2R2 resistor between the CCW terminal of the potentiometers and ground. I have no idea why he did this but this injects some signal into the remaining channels. I replaced R26, the 2R2 resistor, with a jumper and I could no longer measure any bleed.

 

The mixer is inverting and the All Out jacks are again inverted so are in phase with the input. The monitor outputs are also inverted.

The V1.0 BOM specifies a TL071 for IC4 but the part number is for a TL072. The MTA 100 connectors and 15 mm FF standoffs are missing from the BOM.

205 Corrected BOM

 

The jumper can be seen in this rear image to the left of IC4 in the center.

 

 205 Build

I built this 205 with Davies knobs.

 

I knew trying to align all these potentiometers through the panel would be a challenge. I chose to bus the grounds for the output jacks and wire the output pairs together to minimize the number of wires to feed through the PCB. I would have liked to bus the grounds on the input jacks but the switches are a bit in the way so I ran individual ground wires.

 

There are a lot of potentiometers to align with the panel. I found this process worked best.

  1. Solder just the center pin of the potentiometer with about 1/32" of the pin extending through the PCB.
  2. Bend all the wires to the top and bottom so to not deal with them during assembly.
  3. Start with the switches extending through the panel.
  4. Work the bottom row of potentiometers through the panel. You can use a small tool to help align the shafts to the holes working down from the top. You only need to get a bit of the shaft through the panel.
  5. Use some masking tape on the sides of the bottom row from the panel front to the PCB rear to keep the panel tight.
  6. Repeat for the next row up and apply tape to hold.
  7. Repeat again.
  8. Repeat again. Now the potentiometer shafts are all slightly through the panel.
  9. Turn the module panel face down and now work each of the jack wires through their respective holes.
  10. Start to work the panel now close to the PCB. Once the standoffs get close to the PCB then insert a screw to hold it in position. I find that a 12 mm MF standoff works good for this as a type of a thumb screw.
  11. Continue working the panel down until the standoffs touch the PCB. Then use a regular screw to mount the PCB.
  12. Install all the washers and nuts and hand tighten.
  13. Reflow the center potentiometer pin to relieve any stress.
  14. Solder the remaining potentiometer pins.
  15. Tighten all the front panel nuts.
  16. Solder the wires. They are not silk screened but you can see the heat relief pads on the ground pads.

 

 

The Left and Right Monitor outputs are in parallel with individual 15K series resistors. There is direct monitor output with the power cable pads.

 

 

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