Monitor Controller Project (help)

[andia]

Dear DIYers,

during covid I started this monitorcontroller project. I am pretty far into it right now but there are some little problems you maybe can help me with.

First I want to describe the project. It was inspired by igors monitor controller, that unfortunately is not available anymore.

the monitor controller consists of
- a mainframe with all the audio inputs and outputs and the relay matrices and a talkback input/output.
- a remote control connected with ethernet cable to the mainframe that communicates over i2c with the mainframe, it also has a headphone amplifier built into it and houses a metering circuit.

the monitor controller has the following functions:
- switching between sources (4 stereo inputs) and destinations (4 stereo outputs)
- level control over relay attenuator (63steps/1dB)
- mono (with compensation)
- dim (dim level can be user programmed 0 -20dB)
- cut (LR, L or R)
- polarity inverse L and/or R
- LR swap
- Talkback (latching or momentary, user programmable TB-dim)
- 3 user programmabel preset levels for fast access
- 3 digit display
- rotary encoder for level setting and programming
- outputs can be programmed for a certain dimlevel in 0.25dB steps (up to 15dB)
- inputs can be programmed with offset of +-10dB


Problems:

On the picture you can see the remote control module. the lowest 2 bars of the meter leds are iluminated even with no audio signal present. is that a digital crosstalk from the i2c communication? why only on one side?
and I also hear some very low volume clicks in the headphone part (which takes only the + part of the balanced source to the control unit) when I am changing parameters on the monitor controller. What could that be? would it be better to buffer these sends to the remote on the mainframe?


frontside of remote control pcb
some more pics:

rear side of remote control pcb


mainframe pcb in temporary case

 

[ccaudle]

the lowest 2 bars of the meter leds are iluminated even with no audio signal present. is that a digital crosstalk from the i2c communication? why only on one side?

My first guess would be DC offset in the meter circuit somewhere, but since you did not provide even a brief description of how the metering works, much less a full schematic, I could no begin to guess if that is likely or where it would occur.
Digital crosstalk could be possible, but why would the I2C be running continuously and not only when there was a command to make a change? If it is not running continuously then why would you think it would make a continuous offset on the meter?

I also hear some very low volume clicks in the headphone part (which takes only the + part of the balanced source to the control unit) when I am changing parameters on the monitor controller.

That could be I2C crosstalk, or it could be relay coil pulses or other control signals getting into the audio. Hard to say from an unlabeled picture of one layer of a PCB.
I will say that it is a bad idea to just take the hot leg of the balanced source. Is that buffered in some way? If not you are unbalancing your inputs right at the beginning of the chain. Again, without a schematic it is hard to guess what you have done, but best practice would be to have a diff amp driving the headphones, or if you are converting the inputs to single ended for processing in the controller take the headphone feed there.

 

[shot]

I hope you solve this problem and offer some pcbs for us to enjoy! I would surely like to build it and upgrade my studio a bit! Your project looks marvelous!
I know it does not help at all with your build, but just recently I've had an issue with two leds staying always lit on a GR meter for a device I was building. Found out that one LM339N had an unsoldered pin. Easy mistake but very hard to spot.



Luka

 

[mhelin]

Too bad many monitors (even sub $200 ones like the RCF Ayra) these day are digital and have a digitally controlled attenuator somewhere between the DAC and the power amp (hopefully have, unless they are cheap and just simply bring down the program data to lower levels in digital domain). Problem is that there is no common protocol to adjust the gain remotely. Some folks that though reverse-engineered something like Genelec's GLM (GitHub - markbergsma/genlc: Unofficial Python module for managing Genelec SAM monitors), at least partially. Using analog (or digital before the DAC) attenuator before the ADC does work but also reduces the resolution when not all bits get used in the process. Nice work anyway!

 

[boji]

Have you tried out different resistor values for i2c bus?

 

[andia]

I hope you solve this problem and offer some pcbs for us to enjoy! I would surely like to build it and upgrade my studio a bit! Your project looks marvelous!
I know it does not help at all with your build, but just recently I've had an issue with two leds staying always lit on a GR meter for a device I was building. Found out that one LM339N had an unsoldered pin. Easy mistake but very hard to spot.



Luka

The lit meter leds were in fact a faulty LM317. Thank you very much, this problem is solved.

 

[Di_Sousa]

Congrats for the work you have done already, the unit looks awesome.

If this ends up being a project in the future I would do it for sure

 

[Di_Sousa]

It was inspired by igors monitor controller, that unfortunately is not available anymore.

Igor's project is still available,
Purusha is selling it.

I don't know if Purusha did ever got any consent from Igor to sell his project,
but it's being sold in his website:

https://www.diy-racked.com/product/dr-smc/

 

[Quantum J Audio]

If this ends up being a project in the future I would do it for sure

Me too, I have the presonus central station + and I really want to upgrade that. LR reverse It's something I like while mixing.

I'm tempted by sound sculptors controller but, I don't think it has this function. seems seems a shame considering the great feedback it has.

 

[Di_Sousa]

Me too, I have the presonus central station + and I really want to upgrade that. LR reverse It's something I like while mixing.

I don't personally need any option for Mono, LR reverse or Dim, all of those are completely irrelevant to me and I find them useless, but I really would like a monitor controller that has the simplest path possible because I'm a bit paranoid by the sound that reaches my speakers to go through extra Opamps, transistors or circuitry.

Most of the commercial options in the market have so much circuitry before the sound reaches the speakers that i don't believe that that doesn't modify or affect the decisions that you make by what leaves the monitors.

Passive and simple path with high quality relays seems a great solution for me

I'm tempted by sound sculptors controller but, I don't think it has this function. seems seems a shame considering the great feedback it has.

The Sound Skulptor unit does looks very nice for sure, but it's a rackmount unit without any convinient remote control box to have on top of desk. That's a bummer, they did a great unit and forgot one of the most important interface details about a monitor controller.

 

[Quantum J Audio]

I don't personally need any option for Mono, LR reverse or Dim, all of those are completely irrelevant to me and I find them useless.

I use these functions a lot when working. Here are some examples of how a lot of other engineers and producers find them useful.

Mono

• To check mono compatibility. Digital radio and streaming will go mono if signal/bandwidth is not strong enough.
• For hearing phase cancellation between stereo sources.
• Useful when tracking drums to central kick and snare on the OH mics. Hitting mono to stereo quickly lets you hear how close the kick and snare are to centre.
• When you want to pretend you are working before 1960. I also wear a white coat when I do this.

Dim

• It's a good way to remove the inevitable issues of a stereo pot being out over L&R at low levels.
• It's a quick way to cut the volume to hear someones input or have a discussion and then return to the same level.
• Press it when making a request to an assistant then return to same level.
• Dim normally activates when using TB and stops them hearing the control room source.

LR reverse

• Great for checking sounds stage. Phantom centre may sound good with kick & snare vocal etc. However other stuff in the image can be wonky due to comb filtering, subtle speaker or amp issue. L&R reverse shows this up any differences pretty quickly.
• Useful if you get a bit of ear fatigue.

 

[Shroomystic]

How hard would it be to add the functionality to send out the Master Signal onto a Output that can be used to feed a set of normal hifi speakers through a amp, and add 2-4 Headphone Outputs to the Controller ? That would push it into absolute "want to have and need to build" territory...