Another passive monitor controller

[bjosephs]

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.

 

[bjosephs]

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.

 

[bjosephs]

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.

 

[bjosephs]

And the scary part is done. Just need to fit the Alps pot and I can start some wiring.

 

[ruffrecords]

Looking good.

Cheers

Ian

 

[bjosephs]

Lots of wires. Starting to wish I just made a pcb

 

[bjosephs]

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.

 

[corgan4321]

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?

 

[bjosephs]

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.

 

[bjosephs]

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.

 

[dmp]

How is the switching with the relays? Do you have pops?

 

[bjosephs]

How is the switching with the relays? Do you have pops?

Nope, not a single switching artifact with any function. I’m very happy.

 

[dmp]

Cool - thanks. Nice project!
Another question - what R values did you use and are you happy with the volume when switching to mono?

 

[bjosephs]

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.

 

[vgb]

Looking good.

Cheers

Ian

Hey Ian, I’ve read through your messages and was wondering if you could check over my design for a sub $50 version. I don’t wanna spend $120 CAD on a mackie big knob I’d rather get a radial SAT-2 if it’s not worth building my design. Is there’s anything you’d recommend changing?

Regarding the dim would it be better to switch resistors individually or parallel?

Im wondering about the 2.7 slug resistor you mentioned, would that go from wiper to the back of the pot and would that be beneficial?

For xlr pin 1 should I connect all of them together and ground to the case or just straight through from in to out?

Thank you for your time!

-Vern

 

[ruffrecords]

That design looks elegantly simple to me. Excellent.

For the DIM, wiring it wither way should work. I can see no obvious advantage of one over the other.

With the 5K pot connected from hot to cold, the 2K7 resistor goes from wiper to cold (assuming with the pot fully off the wiper is connected to cold)
Edit: the hot output is from the pot wiper and the cold is from the cold.

If it is in a metal box, pin1 of each XLR should go direct to the chassis at the connector. Since there are no active electronics in the box then you can also just wire all the pins 2 togther. Whichever way you do it it they all need to be connected to each other.

Cheers

Ian