Designing a modern mixing console - Part 1 (and introduction): Channel Input

[Nishmaster]

Greetings gurus,

I've decided to at least go through the paper exercise of designing a large-ish format for the modern Pro Tools based recording studio. While I am indeed aware of the gargantuan undertaking something like this involves, I am interested in taking it as far as my money and interest allows. There have been a number of console feature specifications tossed around amongst the various "I'm going to build a console; no seriously, I am" threads, but I think I'm shooting for something a bit simpler. My overall design goal is kinda for a pared down SSL 4/6/9k type desk for mostly mixdown. Automation, while quite nice, is something that can be done on the Pro Tools side of things with ease, and I'm not really terribly desirous of trying to tap into it somehow and interfacing the console with it. The time and expense on that piece alone would exceed the scope (and budget) of this project.

A secondary (in order, not in importance) goal of the project is to gather a working set of designs to be available to the community. I'm not so concerned with China stealing my designs, as I'll never have the working capital or foolhardiness to enter into a market that is near extinction. Therefore, all the schematics will be posted to the threads from start to finish. These schematics are for non-commercial use only; if any of this advances to the point where people want PCBs for themselves, then that bridge can be crossed. But truly, think of it as a community open-source type project.

The basic feature-set is likely as follows:

• Line input only. In this age of countless rack mic-pre options, if they are needed, they can be procured.
• Per Channel Inserts
• 4 Aux Sends
• 8(?) Stereo Buses. I'm not quite sure on this yet. I would like a flexible routing design where any channel strip can become a bus strip, but I haven't worked my head around a good approach yet. It could be something as simple as a summer in each channel strip, but that might not be too great from a performance standpoint, or economics standpoint. Ideas here are welcome.
• Hi-Cut/Lo-Cut per channel
• 4-Band EQ. Maybe Sallen Key or Bi-Quad/State Variable. The goal here is not to have to use silly 3-4 gang pots with center taps if possible, while still retaining decent filter qualities.
• No dynamics section. While I do dig having that many compressors/gates at my fingertips, in all reality I only ever end up using the board compressor on 3-4 tracks. Not having them saves on heat and expense.

I'm no monster designer. I am at best, an amateur. There will be gaffes that will make veterans eyes roll. But, the way I figure it, at this point in my life I have a great, good paying IT gig. I can either spend the next year gathering the cash for a ready-made console, or I can spend the next year coming up with one that I really want, learning in the process, and helping everyone here out. Who knows, perhaps I'll end up getting the Trident I've had my eye on anyway. But, this is bound to be a learning experience for everyone, so I'll do my best to keep my interest alive. As JR is fond of saying, consoles are the most complex simple circuits. I am certainly not forgetting that.

All revisions to the schematics will be added to this post. I will be breaking up posts based on the module involved. So, without further ado, the channel input module.

Simple and basic. A THAT 1200 front-end to avoid having to multiply transformer costs by any significant number of channels. Channel trim into a buffer into a 12-ish dB gain stage. Could use a clipping indicator perhaps, but I'm not sure if you could actually clip any of the amps coming straight out of modern convertors.

-Matt

 

[volker]

Hi Matt,

nice project, let's see where this goes this time .

You should put a cap between the trim and IC1A. Otherwise it will get scratchy over time due to the input current of the NE5532. Maybe between the receiver and the trim too, check the datasheet on the maximum offset to be sure. Also, R6 should have a cap in parallel, something like 33p. Any specific reason to make this stage inverting? The buffer wouldn't be needed as well if you make it non-inverting.


Volker

 

[abbey road d enfer]

Automation, while quite nice, is something that can be done on the Pro Tools side of things with ease, and I'm not really terribly desirous of trying to tap into it somehow and interfacing the console with it. The time and expense on that piece alone would exceed the scope (and budget) of this project.

I agree, although some may point out that it has some limitations; one that comes to mind is that if you insert a compressor in your hardware mixer, the DAW fades will be compromised by the post location of the comp. Not a big deal for me (I do automation in Samplitude and use the compressors in my hardware mixer) but something you must be aware of.

• 4 Aux Sends

[/quote] This maybe a tad restrictive. I have 12 Auxes and use them all, 6 for the cue system and 6 for my main reverbs, which stay patched permanently, because I'm a lazy old geezer. Now I know this is excessive for a hardware mixer, but think about a typical session where the singer wants to have reverb in his cans, you need at least 6 sends. Don't forget to make them switchable pre/post.

• 8(?) Stereo Buses. I'm not quite sure on this yet.

• It all depends on your tracking habits; I track drums on 8 tracks, with only the toms going through bus (pardon me PRR & JR, I don't remember the correct spelling), so technically only two of them would suit me, although I have 24 to play with.

  I would like a flexible routing design where any channel strip can become a bus strip, but I haven't worked my head around a good approach yet. It could be something as simple as a summer in each channel strip, but that might not be too great from a performance standpoint, or economics standpoint. Ideas here are welcome.

That is the standard Jeep Harned /Dave Harrison concept; it's time-proven, but with a restricted number of bus, the separate group concept is quite valid.

• Hi-Cut/Lo-Cut per channel

I never use a LPF when tracking, but always an HPF. Most of the times, LPF's are too steep and I go for a high shelf EQ if needed. And most of the times, I wish the HPF would be steeper than the usual 12dB/octave. For me the most used EQ when tracking is an HPF.

• 4-Band EQ. Maybe Sallen Key or Bi-Quad/State Variable.

• I don't know what you think a Sallen & Key EQ is... I know S&K filters.

  The goal here is not to have to use silly 3-4 gang pots with center taps if possible, while still retaining decent filter qualities.

You choose SVF parametrics for their ability to make "surgical" cuts and you choose inductor-based Neve-ish EQ's for their musicality or you choose Wien-bridge "british" EQ's for their convenience. I think the "surgical" thing is done much more efficiently in the DAW. Some designers have opted for a very simple, but thoroughly refined LF & HF shelving EQ for basic smoothing and tone-shaping. I would tend to do that too. Dual pots in sweepable midrange EQ's are a PITA today.

• No dynamics section. While I do dig having that many compressors/gates at my fingertips, in all reality I only ever end up using the board compressor on 3-4 tracks. Not having them saves on heat and expense.

I agree 100%. Having dynamics in a mixer is a major undertaking. Isolating the local high-current loops is a nightmare.

A THAT 1200 front-end to avoid having to multiply transformer costs by any significant number of channels. Channel trim into a buffer into a 12-ish dB gain stage. 

Why not? The first things you need to do are:
[list type=decimal]
[*]A block dgm
[*]A level dgm
[/list]
Phase is reversed because of the inverter stage. Is it deliberate? You'll need to correct that some place.
It is DC-coupled. Do you intend to leave it that way?

Could use a clipping indicator perhaps, but I'm not sure if you could actually clip any of the amps coming straight out of modern convertors.

I agree with that, but I must say that a level indicator of some kind is, for me, a very useful accessory. No need for a high-res thing, just a few LED's.

 

[tmuikku]

Hi all!

yeah, nice, there hasn't been that many "concept" threads lately! I'll be following this eagerly!

I'm also lacking a mixer and it is my favourite brain twister to try to figure out what kind of topology would be most suitable, cheap and still good sounding (read, doesn't sound bad). Many times while thinking I get back to square one: basic 8x2x2 or similar mixer with proper master monitor section. However I believe there is still a lot of possibilities to think "out of the box".

I agree, although some may point out that it has some limitations; one that comes to mind is that if you insert a compressor in your hardware mixer, the DAW fades will be compromised by the post location of the comp. Not a big deal for me (I do automation in Samplitude and use the compressors in my hardware mixer) but something you must be aware of.

ok, could you arrange DAW routing so that this analog mixer channel strips and master section sits in DAW inserts as hardware inserts?
- One wouldn't need analog summing, unless you want a bus or two for color, analog pan or to work with limited io. Maybe auxes if you prefer zerolatency while tracking.
- No need for inserts in analog channelstrips since you could patch stuff before or after the analog channel strip
- DAW automation would work properly since analog channel is before DAW fader
- In DAW you can patch plugins before / after channel strip
- One analog channel strip design can do all duties: channelstrip, bus, aux send / return even 2track master, depends how you route your DAW / interface to the analog channel strips.
- Almost no patching at all since you could do most of it in DAW and save with project (analog channelstrips normalled to AD/DA in patch field).
- Still youd have proper analog mixer channels with hands on feel.

Does anyone do this? Is my brain too twisted already or do you see why this kind of setup wouldn't work? We'll, again, if you add a master bus this will become a proper mixer and back to square one

 

[abbey road d enfer]

That may be fine for mixing, but for tracking, it leaves a number of stones unturned.

 

[JohnRoberts]

Here is where I repeat my over used phrase, consoles are one of the hardest simple circuits to design (power amps are the other). Both are deceptively simple in concept, but the devil is in the details.

First I want to thank everyone in this thread for spelling bus correctly, one of my personal peeves.

Since this is basically a DIY project there is no need for severely sharp pencil design, or bias against hand labor, while it is smart to take advantage of modern technology (like IC mic preamps).

I would budget a lot a of time for research and review, as I see it there are several steps. #1 is to define a rough feature set and architecture (split, in-line, other), #2 technology (IC, discrete, other), #3 Optimal or preferred circuit approaches for sub blocks chosen, #4 integration (how to make blocks work together), #5 prototype/test (the best designer must adjust for reality after bench tests), #5 execution.. the relatively easy part. The features don't need to be cast in stone early, but will impact the circuit blocks used. For example an 8 bus master sum amp is much easier lifting than 80 (I did one console with 100+ feeds to L/R) .

This should leave you with a list of circuit blocks needed. Mic input, line input, Insert 1/0, EQ (perhaps multiple types of EQ), fader gain stage, bus topology (perhaps two if aux bus is simpler (cheaper) than L/R bus, output drivers, power supply, metering, O/L indication, etc..

Within those circuit blocks or categories there are local and global concerns for things like power supplies, and a subtle globals concern for how do you maintain signal integrity between the sundry circuit blocks (hint use differentials).

FWIW console manufacturers didn't use multi gang or specialty pots just to make the EQ circuit hard to copy, but because they do something better. Covering the function with common off the shelf parts will generally result in some compromise or lesser performance.,

I don't wish to be a wet blanket (well maybe a little), but this is a significant undertaking to realize a one off with passible functionality. To cover the features of a decent 20 YO console, with good performance is IMO a massive undertaking and if you value you own time at more than pennies on the dollar, likely to cost you more than just buying a decent console.

Good luck... Don't let me talk you out of anything, but do try to appreciate the scope of your project.

JR

PS: I have seen a few novel features in custom one-off consoles that weren't crazy, but generally you are wise to study the feature sets and architecture decisions made by designers after working in the discipline for decades, while technology is evolving, so perhaps some features go away, some others get enhanced. 

 

[Nishmaster]

Here is where I repeat my over used phrase, consoles are one of the hardest simple circuits to design (power amps are the other). Both are deceptively simple in concept, but the devil is in the details.

Indeed. In fact I quoted you in my post. I am not taking this undertaking lightly, or even making the bold assumption that I will finish. However, sometimes the road is as rewarding as the destination.

I would budget a lot a of time for research and review, as I see it there are several steps. #1 is to define a rough feature set and architecture (split, in-line, other), #2 technology (IC, discrete, other), #3 Optimal or preferred circuit approaches for sub blocks chosen, #4 integration (how to make blocks work together), #5 prototype/test (the best designer must adjust for reality after bench tests), #5 execution.. the relatively easy part. The features don't need to be cast in stone early, but will impact the circuit blocks used. For example an 8 bus master sum amp is much easier lifting than 80 (I did one console with 100+ feeds to L/R) .

My general feeling is this. Channel inputs could switch between line and one of the buses. Channels themselves are unbalanced, with balanced line drivers coming off the channel to the bus assigns as well as to the aux sends. My goal is 32-48 channels, so anything traveling into any buses is going to be balanced. I don't see the need to keep the channel balanced until it hits a send or a bus, but you may have a better idea than I.

The main mix bus will be active virtual earth summed differentially in groups of 12 (one bucket), then each bucket summed into a final stage. I'm not sure about additional mix buses here, as 8 stereo buses means 16 mono buses x2 (differential) x3 or 4(number of buckets). For 4 buckets, that 128 amps. There is also the problem of selecting your input as bus 1 and sending out to bus 1 by accident (feedback loop). Not sure how to tackle those without complex logic circuitry. Perhaps the most simple solution is to defeat the bus assigns and go straight to L/R when the channel is in "bus" mode.

This should leave you with a list of circuit blocks needed. Mic input, line input, Insert 1/0, EQ (perhaps multiple types of EQ), fader gain stage, bus topology (perhaps two if aux bus is simpler (cheaper) than L/R bus, output drivers, power supply, metering, O/L indication, etc..

Here are the blocks I have jotted down in my pre-planning:

Channel

• Channel Input
• Channel Insert
• Bus Input
• Hi/Lo Cut Filters
• 4 Band EQ
• Auxiliary Send
• Bus Assign
• Pan/Fade/Solo/Mute
• Channel Output
• Channel Meters

Master

• Aux Bus
• Aux Send/Receive
• Mix Bus Inputs (from channel assigns)
• Mix Bus Outputs (to channel inputs)
• Main Bus
• Main Bus Fader
• Main Bus Output(s)
• Main Bus Meters

Power supply I may try and source a bulk +-18v unit. I can do simple power supplies, but I don't trust my abilities to do something that will need to be quite that beefy. Who knows, that's looking a bit far ahead.

FWIW console manufacturers didn't use multi gang or specialty pots just to make the EQ circuit hard to copy, but because they do something better. Covering the function with common off the shelf parts will generally result in some compromise or lesser performance.

I know. They're certainly not superfluous. However, if I could achieve convenience and musicality within the realm of off the shelf parts, then that what I'm going to try to do. If not, then we'll see what happens.

I don't wish to be a wet blanket (well maybe a little), but this is a significant undertaking to realize a one off with passible functionality. To cover the features of a decent 20 YO console, with good performance is IMO a massive undertaking and if you value you own time at more than pennies on the dollar, likely to cost you more than just buying a decent console.

I know it's silly to value my time at zero, but in this case I consider the knowledge gained as the payment. The scope of the project is not lost on me. However, at this time I have access to some exceptional metalworkers, helpful electronics folks, and decent funds. It's certainly not a sprint, it's a marathon.

Volker and Abbey,

Good call on the pot getting scratchy with DC coupling like that. I was hoping to keep caps in the signal path to a minimum, but I may have to not be quite so inflexible on that. The inverting gain stage was to get an inversion out of the way since the bus summing will produce an inversion also. Perhaps not entirely necessary at this stage. The reason for the 12dB boost is to set a 0dB point at a convenient place on the channel fader and to make the top of the fader +12dB.

Looking at that now, though, that will reduce my EQ headroom prematurely, but could also provide a nice boost over the EQ noise floor. Any suggestions?

-Matt

 

[JohnRoberts]

My general feeling is this. Channel inputs could switch between line and one of the buses.

? I guess you mean, direct to L. or R or both buses? vs. EQ etc.

Channels themselves are unbalanced, with balanced line drivers coming off the channel to the bus assigns as well as to the aux sends.

Most aux/efx buses are unbalanced, and sent from a different node than L/R buses.. (typically fader/pan/+10dB gain, while aux/efx are mono).

My goal is 32-48 channels, so anything traveling into any buses is going to be balanced. I don't see the need to keep the channel balanced until it hits a send or a bus, but you may have a better idea than I.

Inputs?

Indeed differential receiver to SE the signal at the input for local internal crunching.

The main mix bus will be active virtual earth summed differentially in groups of 12 (one bucket), then each bucket summed into a final stage. I'm not sure about additional mix buses here, as 8 stereo buses means 16 mono buses x2 (differential) x3 or 4(number of buckets). For 4 buckets, that 128 amps. There is also the problem of selecting your input as bus 1 and sending out to bus 1 by accident (feedback loop). Not sure how to tackle those without complex logic circuitry. Perhaps the most simple solution is to defeat the bus assigns and go straight to L/R when the channel is in "bus" mode.

I think i wrote about using a similar approach in an old article... This summing together outputs from groups of buses, was to address limitation of open loop gain in opamps (circa early '80s while this hasn't changed that much). There are other ways to address this, but using this approach the optimal number in groups may depend on specific opamps, or whatever IC are used and their open loop gain plot.  FWIW, I never used this approach because of cost, real estate, power consumption, etc.

This should leave you with a list of circuit blocks needed. Mic input, line input, Insert 1/0, EQ (perhaps multiple types of EQ), fader gain stage, bus topology (perhaps two if aux bus is simpler (cheaper) than L/R bus, output drivers, power supply, metering, O/L indication, etc..

Here are the blocks I have jotted down in my pre-planning:

Channel

• Channel Input
• Channel Insert
• Bus Input
• Hi/Lo Cut Filters
• 4 Band EQ
• Auxiliary Send
• Bus Assign
• Pan/Fade/Solo/Mute
• Channel Output
• Channel Meters

Master

• Aux Bus
• Aux Send/Receive
• Mix Bus Inputs (from channel assigns)
• Mix Bus Outputs (to channel inputs)
• Main Bus
• Main Bus Fader
• Main Bus Output(s)
• Main Bus Meters

Power supply I may try and source a bulk +-18v unit. I can do simple power supplies, but I don't trust my abilities to do something that will need to be quite that beefy. Who knows, that's looking a bit far ahead.

FWIW console manufacturers didn't use multi gang or specialty pots just to make the EQ circuit hard to copy, but because they do something better. Covering the function with common off the shelf parts will generally result in some compromise or lesser performance.

I know. They're certainly not superfluous. However, if I could achieve convenience and musicality within the realm of off the shelf parts, then that what I'm going to try to do. If not, then we'll see what happens.

I don't wish to be a wet blanket (well maybe a little), but this is a significant undertaking to realize a one off with passible functionality. To cover the features of a decent 20 YO console, with good performance is IMO a massive undertaking and if you value you own time at more than pennies on the dollar, likely to cost you more than just buying a decent console.

I know it's silly to value my time at zero, but in this case I consider the knowledge gained as the payment. The scope of the project is not lost on me. However, at this time I have access to some exceptional metalworkers, helpful electronics folks, and decent funds. It's certainly not a sprint, it's a marathon.

Semi seriously, this is dinosaur technology so mastering it will be useful if dinosaurs come back. There are still companies making analog boards but i doubt that is great career path these days.

Volker and Abbey,

Good call on the pot getting scratchy with DC coupling like that. I was hoping to keep caps in the signal path to a minimum, but I may have to not be quite so inflexible on that. The inverting gain stage was to get an inversion out of the way since the bus summing will produce an inversion also. Perhaps not entirely necessary at this stage. The reason for the 12dB boost is to set a 0dB point at a convenient place on the channel fader and to make the top of the fader +12dB.

Looking at that now, though, that will reduce my EQ headroom prematurely, but could also provide a nice boost over the EQ noise floor. Any suggestions?

-Matt

You may be disappointed to learn how many caps are used routinely in premium audio paths. The typical end user considers any clicks or scratchy pots as something wrong. Of course any controls you use during a mix need to be silent, but many DC blocking caps are added for IMO cosmetic concerns (just like turn on/off transients, etc), but the customer is always right.  8)

JR

 

[abbey road d enfer]

The inverting gain stage was to get an inversion out of the way since the bus summing will produce an inversion also.

Since you want to include an insert point, you'll have to deal with the phase inversion.

The reason for the 12dB boost is to set a 0dB point at a convenient place on the channel fader and to make the top of the fader +12dB.
Looking at that now, though, that will reduce my EQ headroom prematurely, but could also provide a nice boost over the EQ noise floor. Any suggestions?

There's no discussion here; you need the headroom. As I've said earlier, you NEED to produce a level dgm.

 

[pstamler]

Nishmaster, have you read Douglas Self's Small Signal Audio Design? The individual chapters have very useful information, and the one on console design has incredibly useful ideas on the small stuff you need to sweat -- for example, the problem of making sure that a switch that's turned off is really turned off.

Peace,
Paul

 

[Nishmaster]

Revision 2 of the Channel Input is attached.

? I guess you mean, direct to L. or R or both buses? vs. EQ etc.

I mean, the input of a channel can either be the line in, or the output of one of the mix buses. Thus, only one strip design is needed across the whole board. You can see what I mean in the Revision 2 schemo.

Semi seriously, this is dinosaur technology so mastering it will be useful if dinosaurs come back. There are still companies making analog boards but i doubt that is great career path these days.

Good thing I've already got a very non-dinosaur career path then.  People still take Latin, and I'll still have an end result (should I make it).

Since you want to include an insert point, you'll have to deal with the phase inversion.

Indeed. An inverting stage is best left for later.

There's no discussion here; you need the headroom. As I've said earlier, you NEED to produce a level dgm.

My back of the cocktail napkins say that after +4dBu converters (let's be generous and call that 1.5v for extra hot poorly calibrated devices), the knob full up gives us around 6v. Gain remaining is going to be then 3, or around 9.5dB headroom. Not quite enough breathing space for extra loud signals. Then again, who keeps the trim knob maxed on a hot signal? For now I've reduced the gain stage to 2 (6dB instead of 12dB), which gives some extra wiggle room. Might not be enough for quiet signals, but then again there is always the possibility of turning it up in the box first before sending it to the desk.

I also blew away the buffer in light of removing the inverting stage. A quick clip detector circuit was added, might be bogus, I'm somewhat tired.

Nishmaster, have you read Douglas Self's Small Signal Audio Design? The individual chapters have very useful information, and the one on console design has incredibly useful ideas on the small stuff you need to sweat -- for example, the problem of making sure that a switch that's turned off is really turned off.

Indeed I have that on my desk as well as the Handbook for Audio Engineers, especially the Steve Dove stuff.

-Matt

 

[haima]

As an aside, everyone should check out the Undertone Audio console (Google / Youtube and watch Eric Valentine's episode on Pensados Place) for some and interesting take on "diy/custom" console design...

The flexible EQ / Filters and the attempt to minimise the acoustic reflections seem particularly interesting to me.

I saw the 24ch version at AES last year - what a beauty! :-* :'(

Perhaps out of most of our reach for personal DIY... but worth a look!

Carry on...!

 

[JohnRoberts]

Revision 2 of the Channel Input is attached.

? I guess you mean, direct to L. or R or both buses? vs. EQ etc.

I mean, the input of a channel can either be the line in, or the output of one of the mix buses. Thus, only one strip design is needed across the whole board. You can see what I mean in the Revision 2 schemo.

OK, it doubles as a sub master.. got it.

Semi seriously, this is dinosaur technology so mastering it will be useful if dinosaurs come back. There are still companies making analog boards but i doubt that is great career path these days.

Good thing I've already got a very non-dinosaur career path then.  People still take Latin, and I'll still have an end result (should I make it).

Quod agitas hodie? (not sure of spelling, it's been a while).

[

Indeed I have that on my desk as well as the Handbook for Audio Engineers, especially the Steve Dove stuff.

-Matt

+1 on the Dove stuff, I'm not familiar with Self's console work. but he seems highly regarded. 

JR

 

[abbey road d enfer]

A quick clip detector circuit was added, might be bogus, I'm somewhat tired.

This detector generates current spikes that are very likely to propagate in the whole channel, even in the whole mixer.
Common practice is to have a detector operating at very low current, followed by a LED driver. Can be two transistors.

 

[Joechris]

Is it realy ok to short the output of the bal reciver and the 5532 to ground when not in use, like you do with the relay??

 

[abbey road d enfer]

No. Again, high-current loops...

 

[JohnRoberts]

One thing I did in a console with lots of audio inputs and no room for meters on every input was to use bicolor LEDs. A green for signal present and red for 3-6dB before clipping was very handy to see what was going on where without burning a lot of front panel space. .

I think those 393s are open collector outputs so you need and external pull up on the output. 

I think I probably uses a cheap opamp to make my bicolor LED display, but that was long ago and I don't recall the details. . 

JR

 

[0dbfs]

Some ideas:

Don't look at it like an integrated "console". Approach it as if it's a bunch of modules you are wiring together via patchbay.

Channelstrip, bus-module, monitoring. Power-supply.

Modular channelstrip design is most realistic in my opinion.
MTR buses can be either passive or active. PCB's or modules. Something similar to a folcrom passive resistor network can easily be patchbay'd. 16 or 32 inputs with signal patched as required.
Modules with two inputs, two main outputs, and LCR for discrete assign or pan.
Modules could be designed to have three select-able inputs. Mic, Line, Passive-bus (2 DB-25 connectors = 16 channels).
Module outputs could be pre/post. Perhaps with switchable outs... Like 1-8 Pre, 1-8 Post, 1/2 main.
Direct out with trim?
THAT1646 can drive outputs.
Transformers optional.
Channel modules based on a 19" RU form factor make mounting vertically or horizontally easy.
Additional channels of passive summing can be had fairly inexpensive with channel amps, EQ's, etc added as time/funds permit. This can easily add many more inputs without taking up too much space to your mixer albeit without much control. Many consoles have "monitor returns" which do not have as many features.
Simple input, channel-amp, fader, passive or active bus can save a lot of room and be high quality.
60mm or 100mm faders?
Main 1/2 bus. Main 3/4 bus. Solo bus. PFL?
DAW can also insert outboard if you have enough I/O for pre-console-fader compression.
If you use standard 19" RU dimensions you can make a very nice console frame out of rack rails and wood. You can also part it out or repurpose fairly easily if you change your mind and only get 6 modules built.


Best,
jonathan

 

[Nishmaster]

This detector generates current spikes that are very likely to propagate in the whole channel, even in the whole mixer. Common practice is to have a detector operating at very low current, followed by a LED driver. Can be two transistors.

I like the simplicity there, quite nice. However, I do like the full-wave capability of the dual comparator setup. Would adding a driver to LED1 be sufficient? I do definitely see how there would be some in-rush current when LED1 is driven in the current setup. I suppose the comparators slamming the rails might be noisy also, but I do have it decoupled. Is it current to ground that is your concern?

Is it realy ok to short the output of the bal reciver and the 5532 to ground when not in use, like you do with the relay??

:-[ It is most certainly not. There should be a 1Meg to GND on the ground leg of the relay which I forgot to add.

That brings up another question, though. What do I do with the inputs of U1 and U2 when they aren't connected to anything?

One thing I did in a console with lots of audio inputs and no room for meters on every input was to use bicolor LEDs.

I definitely hadn't ruled that out, it had occurred to me too.

I think those 393s are open collector outputs so you need and external pull up on the output.

Hmm, hadn't noticed that on the datasheet. I may scrap this whole dual-comparator business anyhow and simplify.

-Matt

 

[boji]

Hey Nish, great thread, I look forward to seeing your project come to life.

In my slow work towards my mixer, I am simply stealing a pair meter buffers from an old Wheatstone console to drive some old modutecs.
They include peak indicators at the VU. I know going about it this way is- in the spirit of your design choices- a form of cheating, but it saves me a little more time to concentrate on the music-sending parts of the console. 
I have yet to get to a point where I worry about what the VU in will do to the signal. On the wheatstone, I think it was simply y-split after the line driver, one to the vu buffer, one to the sends/eq.