Another passive monitor controller

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Thanks for all the input, everyone.

The prototype is working. I had some trouble with the L/R cut not working when in mono, turns out I had reversed in and out at the DB9 connector and was driving the circuit backwards.

With a 10k pot I can’t turn the volume up passed half way on mastered music so a 2-5k will probably be the right fit so I have some level when things aren’t yet pushed to the limit in a mix.
 

Attachments

  • 7FED6E86-5602-403E-BE04-0C01E755FFF8.jpeg
    7FED6E86-5602-403E-BE04-0C01E755FFF8.jpeg
    177 KB · Views: 96
  • EFC6ED6A-40EB-4A3C-96E8-F02589E388E5.jpeg
    EFC6ED6A-40EB-4A3C-96E8-F02589E388E5.jpeg
    182.4 KB · Views: 88
Last edited:
I put the prototype aside for a while to select parts and start thinking about the design. Attached is a redrawn schematic, a bit different from where I started. I moved the attenuator to the output (as I did on my prototype because I couldn't fit any more wires into the connectors where I had intended). This was somewhat advantageous because I had intended to put a DC control resistor across the outputs (say, 100K) to help control switching pops. The attenuator can fulfill this role as well for free.

I have some switch options coming from digikey (not a lot of options in stock these days), as well as a rack case, and I'll need to make some decisions about inputs, outputs, and switching so I don't lose too much momentum.

I'm thinking 3 ins and 3 outs simply because 4P3T rotary switches are cheap (or not, if you like) and are easy to wire. I kind of have this idea stuck in my brain about having an "aux out" that will route a passively split signal from those inputs back to an input on my computer. Kind of like a little patch bay but I may sideline that for simplicity.
 

Attachments

  • Schematic.png
    Schematic.png
    13.5 KB · Views: 81
Quick update, since I’m about to start drilling my front panel and start wiring this week:

I modeled the back panel so a friend with CNC capability could make the larger holes for me. Somehow I managed to reverse the mounting holes and the panels on these Hammond cases aren’t reversible so there was a little panick before I realized that I could rotate the connectors a bit. I think it may actually be easier to unplug cables in this configuration in a tightly packed rack. We’ll call this “innovation” rather than an error. 🙃

After demoing tons of switches I’m going with these super cheap latching push buttons from Adafruit.com. They feel nice and don’t take up much space. From left to right they will be: cut L, cut R, Mono, Phase, and Dim. I left some room for a 6th switch should I ever come up with a need.

I splurged and got some nice goldpoint parts for the “mains” attenuator and source/destination switches. I’ll use an Alps RV27 for my “alts” level (still need to build those), destination 3 won’t have an attenuator (will use terminating resistors at the jacks) as I plan to route it to a headphone amp with its own volume control built in. The values are higher than we discussed above but I didn’t want to make my own attenuators. I think it will be ok even if they never get turned up more than half way.
 

Attachments

  • 937CBAA8-F42C-439E-BB16-1E137E764772.jpeg
    937CBAA8-F42C-439E-BB16-1E137E764772.jpeg
    59.8 KB · Views: 55
  • AF1C62B5-4ACE-4E0F-B0F3-7A255F919C7B.jpeg
    AF1C62B5-4ACE-4E0F-B0F3-7A255F919C7B.jpeg
    128.5 KB · Views: 49
  • D90E4629-0371-473C-A163-755537D91C81.jpeg
    D90E4629-0371-473C-A163-755537D91C81.jpeg
    147.6 KB · Views: 52
Lots of wires. Starting to wish I just made a pcb 😂
 

Attachments

  • D06B0E64-ACC7-4A17-8164-0831F9FD86DF.jpeg
    D06B0E64-ACC7-4A17-8164-0831F9FD86DF.jpeg
    170 KB · Views: 34
  • 9E4D9CCE-D500-415C-BF73-7EE21CAE9F97.jpeg
    9E4D9CCE-D500-415C-BF73-7EE21CAE9F97.jpeg
    130.2 KB · Views: 37
Good news is everything worked first time (aside from having connected one input to an output and vice-versa again). The bad news is somehow, despite weeks of planning/sketching/ and discussing, I managed to put in 5 buttons when I needed 6. I had forgotten about the L/R flip feature I wanted. So, for now I sacrifice the DIM function and will hack it in later.

Unlike some other rack cases I've used this particular rack case uses the full 1.75" heights of the standard so I can't sneak pan head screws on the bottom for mounting the relay board. I'm not set up to countersink the bottom panel so this board will be held in with some double stick tape on a custom 3D printed standoff. I'm using an adhesive barrier strip as well where input power splits off to the buttons/board.
 
Last edited:
Good news is everything worked first time (aside from having connected one input to an output and vice-versa again). The bad news is somehow, despite weeks of planning/sketching/ and discussing, I managed to put in 5 buttons when I needed 6. I had forgotten about the L/R flip feature I wanted. So, for now I sacrifice the DIM function and will hack it in later.

Unlike some other rack cases I've used this particular rack case uses the full 1.75" heights of the standard so I can't sneak pan head screws on the bottom for mounting the relay board. I'm not set up to countersink the bottom panel so this board will be held in with some double stick tape on a custom 3D printed standoff. I'm using an adhesive barrier strip as well where input power splits off to the buttons/board.
Not sure if this is helpful at all for you, but in the past I tried experimenting a little bit with mounting a small tactile switch behind a potentiometer whose body was spring loaded against the inside of the front panel. With just a 1/32" of play between the nuts on the shaft, you have a kind of custom push/pull pot. Maybe that could be how you access your DIM function?
 
Not sure if this is helpful at all for you, but in the past I tried experimenting a little bit with mounting a small tactile switch behind a potentiometer whose body was spring loaded against the inside of the front panel. With just a 1/32" of play between the nuts on the shaft, you have a kind of custom push/pull pot. Maybe that could be how you access your DIM function?
That is quite clever, though I'm not sure it would be easier for me at this point. It will be scary but I can pull the front panel off and carefully remark and drill it. DIM isn't important enough to slow down now over.
 
Ok. I’m calling this version finished. I may make changes later (like adding that DIM switch) but for now it’s getting racked and used.

I did ultimately 3D print a set of simple brackets to double-stick foam to the chassis. I also replaced the DIM resistors with smaller wattage parts to save space and insulated their leads for reliability.
 

Attachments

  • E67AE815-564B-4030-9A36-654DCF5A7A1A.jpeg
    E67AE815-564B-4030-9A36-654DCF5A7A1A.jpeg
    105.3 KB · Views: 24
  • 7649BA75-4A08-4205-8796-6E6A7320BFCA.jpeg
    7649BA75-4A08-4205-8796-6E6A7320BFCA.jpeg
    89.7 KB · Views: 27
  • 08F3410B-2BDF-4151-B841-18DE20DA142A.jpeg
    08F3410B-2BDF-4151-B841-18DE20DA142A.jpeg
    243.3 KB · Views: 32
  • 28AEEF84-34B4-4F0F-A9C5-132DD8E93EF1.jpeg
    28AEEF84-34B4-4F0F-A9C5-132DD8E93EF1.jpeg
    182.6 KB · Views: 36
  • 071FD804-BD63-450B-8635-4C1C59CE76FB.jpeg
    071FD804-BD63-450B-8635-4C1C59CE76FB.jpeg
    169 KB · Views: 38
Cool - thanks. Nice project!
Another question - what R values did you use and are you happy with the volume when switching to mono?
 
Cool - thanks. Nice project!
Another question - what R values did you use and are you happy with the volume when switching to mono?
22k series resistors with 2k7 switched in parallel (switching them out constitutes my dim function) so essentially 2k2. Mono volume drop is fine, I don’t adjust monitor volume to compensate or anything.
 
Back
Top