New Low Cost High Quality Monitor Controller Project

[nielsk]

I had a request to build a monitor controller, and looked around at the possibilities. What I found was either very expensive, awkward, lacking features, or lacking quality. I wanted a rackmount unit for the audio portion, and a small desk unit for controls and talkback, with one cable in between. So I began to explore the various ways of doing this, and built a prototype of a VCA based volume control w/ relay switching for source / destination selection. The VCA circuit works excellently, sounds transparent, tracks perfectly, and has extremely low distortion. The ability to use a low cost single gang pot makes it cheap & easy to obtain continuously variable, very smooth level control.
I realize there are some with prejudice against VCAs, but I suspect that their opinions are not based on actual experience with extremely high quality low distortion modern components.
I am sending the prototype around to some engineers for testing, and thought I’d toss this out to the forum to see if there is any interest in this as a project, and get some input on features. If there is interest I would consider offering it as a board set or possibly a kit.

Here is the main feature set:
Relay selected balanced stereo source select.
Balanced stereo inputs, feeding a VU meter buffer / driver, the headphone / cue section, and the VCA speaker level control.
Relay selected balanced speaker feeds.
One stereo headphone amp will be fed with the selected source; another will be fed with the selected source & the talkback mic signal. This CUE signal will also feed a balanced line level output.
A dedicated mic pre chip (THAT 1512?) talkback preamp in the desk unit with gain control and press to talk switch, wired to an XLR for user mic.
Some initial areas for feedback:
Should I use the tried and true 5534 based balanced input and output stage (what is in the prototype), or a dedicated line receiver and transmitter? The dedicated does cost more, but could offer higher performance, and a lower parts count.
The meter will follow the selected source, what about a peak circuit?
What about using something like the RNK80 DtoA board for a digital source input?
A –10 unbalanced input option?

With the idea of making aspects modular so one can build to suit, I contacted the EXPAT guys to see if they could program their switch board to interlocking, and they have obliged. Their current version supports 3 lighted switches, this would accommodate 3 input sources and 3 speaker selects. They are working on a 4 switch version. Use of their modular relay board could allow as many or as few of each as desired. They are going to send some samples to me for testing

 

[Ptownkid]

Interested indeed. Cheers

 

[Darholm]

Also interested, any pictures?

Best Regards

Nikolaj

 

[Rochey]

I would consider using a PGA2310 for your volume control section, controlled remotely from your desktop unit.

The PGA2310 is already used in a LOT of Pro Audio equipment. With a DNR of 120dB+, it should fit your requirements nicely.

Stay tuned

 

[nielsk]

PGA2310 sounds like it is worth checking out... got a hint on how to get samples?

 

[3nity]

Hey Niels.
I tought about the idea of the VCA volume control.
i was reading the That Corp http://thatcorp.com/datashts/dn116.pdf
In your opinion it sounds good? Thanks.

 

[MikoKensington]

Nothing technical to contribute here, but I have some ideas for features I'd love to see in a unit like this.

 

[nielsk]

Very interested in input on features!
My proto is using the THAT circuit, and it sounds great. I am working on getting some of the PGA2310 to compare...

 

[grantlack]

I would consider using a PGA2310 for your volume control section, controlled remotely from your desktop unit.

might also be read as

I would consider using a PGA2310 for your volume control section, controlled remotely from your desktop

which, along with controlling the relays, would make the implementation possibilities far broader.
how are you planning on doing the volume control for the headphone channels?

 

[Rochey]

Hello Grant,

Baby steps! I'd suggest starting with the basics and growing the system. At least that way you get incremental verification that the system is working!

I'd be tempted to use multiple PGA23xx or PGA43xx for multichannel volume controls.

You'd then have a 19" rack unit, and a small desktop pod, connected via Ethernet Cat5cable (cable, not protocol).

With that, you could easily send serial data, basic power and maybe even some talkback/headphone.

Cheers

/R

Roadmapped would be midi integration, so that you could connect it to a PC/MAC.

 

[grantlack]

Hello Grant,

Baby steps! I'd suggest starting with the basics and growing the system. At least that way you get incremental verification that the system is working!

haha oh i'm wholeheartedly with you on laying the foundation, just making an argument for expansion possibilities as it runs headlong into things i'm exploring.  can't build up something that already exploits the full extent of its feature set.

I'd be tempted to use multiple PGA23xx or PGA43xx for multichannel volume controls.

i'd be inclined to agree, once the first implementation is sorted the others would likely become more straightforward.  the incorporated mute is handy.

 

[desun]

Did you really have to start this thread just after i've finished working out the design for my own monitor controller?! Would like to see this

 

[Rochey]

I'll be prototyping a PCb this week (gonna diy it with the new Cnc)
Then it's a software game - eeeeeek!

 

[0dbfs]

Some feature ideas:

Buttons / lamps /pots for:
Mute Left
Mute Right
Mono
Invert L
Invert R
Swap L/R
Dim btn.
Dim pot.
Output trims for each output.
Input trimmers.
SLS outputs with tb enable and volume pot, dim, mute.
Remote TB.
Momentary / Latch TB
TB/listen Lamp relays (yellow/green light in CR/studio)
Red light button/relays
Line level tb input for summed mic's.
Listen input.

Just sayin....

Best,
Jonathan

 

[jwhmca]

I have the same request! Let's get cracking team! GO-GO!

 

[mitsos]

Hey Niels, will definitely watch this developing... need something economical and reliable.

I'd like to see the various mute, invert, and swap functions 0dbfs mentioned, talkback you already said, and whatever else will fit.

I'd also say stick with the 5534 for input/output. Nothing wrong with it and it is way cheaper and available all over the world, unlike some drivers/receiver chips.  Also, it's already working in your prototype...

good luck.. keep us posted!

 

[abbey road d enfer]

Should I use the tried and true 5534 based balanced input and output stage (what is in the prototype), or a dedicated line receiver and transmitter? The dedicated does cost more, but could offer higher performance, and a lower parts count.

Really depends if you want to make it a "commercial" product; I used to do balanced ins and EBOS with TL0's and 5532's, and I changed for THAT chips: I'll never go back. But I understand, for a guy who doesn't have a clock ticking, the savings can be a serious incentive.

The meter will follow the selected source, what about a peak circuit?

It depends so much on what facilities the rest of the equipment offers, but I would think most DAW include or allow using a meter plug-in that would be much more versatile and accurate than whatever hardware you could reasonably build in a small desk pod.

A –10 unbalanced input option?

Seems to be a requisite. In my set-up, I have 3: one for CD player, one for soundcard output and one for i-pod (all adjustable).

With the idea of making aspects modular so one can build to suit, I contacted the EXPAT guys to see if they could program their switch board to interlocking, and they have obliged. Their current version supports 3 lighted switches, this would accommodate 3 input sources and 3 speaker selects.

IMNSHO, 3 ins is not enough. Just a starting point.

 

[0dbfs]

I would also consider adding the ability to set the main volume pot offset via a trim-pot or something such that the you can set it so your most desired volume is at sat 12:00 on the volume pot (with dim as dim/boost-settable?). I've got a presonus central station now which is passive and I normally need to keep the pot way down towards the bottom of the throw. I'm thinking of building a pad into it somehow so I can get a more useable range. I mean, I never really need to crank it so there is probably generally room for padding the output somewhere before the output relays. Thought i'd mention it since it's "an issue" with my current setup. Albeit not the end of the world.

Best,
j

 

[nielsk]

All good suggestions, thanks for the input.
Although I see the merit of a programmed device digital control and surface mount components, it takes this out of the hands of a significant portion of DIY. Even though they do work excellently, I just don't like the zipper noise of DCAs.
An overall cut & separate L & R cuts should be easy to implement.
Mono summing is very useful, along with DIM, I'll put thought into a clever way to implement it
Momentary / on-off TB is easily handled with a spring one way toggle switch.
Trims are easily (and possibly better) handled by the speaker / amp.
SLS? Hmm... unfortunately seldom used these days. Although more expensive, it may be better to take a more "SSL" approach and have 2 level controls, Main & Mini, and and ALT switch for an outboard relay. A way to latch Main & Mini both on and a "SLS" alt switch to feed one to the studio could be a "build it in if you want it " option (?) or a relay to feed the CUE to an SLS output
Pretty much the same deal with Red Light. Everyone wants one in their studio because it looks cool, I have yet to see one used in a studio I have built...and that is more than a handful. Add to that any people who would pay attention to it are the ones who are aware enough to not walk in on a session in the 1st place! The only way to get this to actually work is to interface it with the recorder so the light turns on automatically when you press Record an electronic door lock may be more effective ;D
I'd like to have a "calibrated level" set for a detent position on the level control (more of the film mixing type approach), this should satisfy the level offset concern
Talkback switch signal could be routed to a connector to interface an outboard wireless remote
Headphone and CUE level will be controlled with an analog pot on the rack unit (K.I.S.S.)
Listen mic? I can see with a small workstation & no console, one may not have an extra mic input to use for this, and it is handy. I'm not sure at this point if it tips the scales form easy & low cost to difficult & expensive
Part of it comes down to if I want to try and make this a commercial product for the world as well as something for us. There are many monitor controllers on the market, are there any that are not very expensive that are very good? I do not like the "Big Knob" approach, too many cables (not to mention audio quality), I hear reports of other inexpensive units failing early in their life...

 

[abbey road d enfer]

The only way to get this to actually work is to interface it with the recorder so the light turns on automatically when you press Record an electronic door lock may be more effective ;D

Then instead of having someone creeping in the session, you'll have them banging on the door!