Passive Mastering Monitor Controller

[SmokeyJoe]

Hey folks. I've sketched up a rough idea for a basic passive mastering monitor controller, as I had a bit of a search around and couldn't find exactly what I need.

The idea is to use my one nice(ish) DAC for both monitoring and going out to my analogue mastering chain, with the ability to monitor the direct output from the DAC or the return from the rack.

It's loosely inspired by @bjosephs design and the discussion around it. I've followed Ian's advice to use a linear pot with a slug resistor for monitor volume control - is my implementation correct?

Most designs I've seen have 2k2 resistors or similar in series with the hot and cold lines, but I didn't want to blindly add this without fully understanding its purpose. Is this to present an input load? Is the best location between the monitor source switch and the pot?

I'd also like to add a dim switch - could this be implemented as a toggle between the 2k2 resistors and higher value? Bearing in mind I want to have as minimal effect on the feed to the mastering chain as possible.

Potential extra nice-to-haves are a secondary parallel/switchable monitor output, and a parallel output to my headphone amp. Anything to keep in mind there, or can I just parallel from the monitor output?

 

[bjosephs]

It's loosely inspired by @bjosephs design and the discussion around it. I've followed Ian's advice to use a linear pot with a slug resistor for monitor volume control - is my implementation correct?

I think so.

Most designs I've seen have 2k2 resistors or similar in series with the hot and cold lines, but I didn't want to blindly add this without fully understanding its purpose. Is this to present an input load? Is the best location between the monitor source switch and the pot?

You need them or at least a similar value. It keeps the attenuator pot from loading down the output of the device driving the input of the controller. It also forms the resistive divider that allows attenuation. As drawn you just have a variable load. And yes, between the switch and the pot is where looks best in your drawing.

I'd also like to add a dim switch - could this be implemented as a toggle between the 2k2 resistors and higher value? Bearing in mind I want to have as minimal effect on the feed to the mastering chain as possible.

That’s how I did it. You don’t really mix or monitor with dim enabled, it’s a convenience so you can chat without changing your the pot level. When it’s not active it has no effect; when it is active you don’t need the accuracy. The worst case scenario is some treble loss when active anyway.

Potential extra nice-to-haves are a secondary parallel/switchable monitor output, and a parallel output to my headphone amp. Anything to keep in mind there, or can I just parallel from the monitor output?

It is that simple if you only want one knob. I did a separate knob for each pair of monitors - a fancy one for mains and cheaper one for alts.

 

[SmokeyJoe]

You need them or at least a similar value. It keeps the attenuator pot from loading down the output of the device driving the input of the controller. It also forms the resistive divider that allows attenuation. As drawn you just have a variable load. And yes, between the switch and the pot is where looks best in your drawing.

Hey thanks for the tips. I've added in the load resistors, plus the dim resistors - you mentioned in your thread that you paralleled yours rather than toggled, is that to reduced DC pop? You also said you used rotary switches - do know if they were make before break?

The layout feels about there now. I would like to have incorporated the mono switch into S3 but couldn't figure out a way - really I want to engage mono along with Left Speaker Only for true mono monitoring, but I'll settle for it being two separate buttons.

Annoyingly I need to use 1/4 TRS rather than XLR in order to fit it in a Hammond 1456FE2 desktop enclosure, as the compact form factor is important for this project. Also I think I'm going to have ditch the output to headphone amp, as the amp is unbalanced.