diy 12+4 in 2 out summing / mixdown console

[audiomixer]

Hi,

I have started to document my build efforts for my 20 channel stereo mixdown console. I hope its going to be a highend quality build, that's what I am aiming for. no fancy stuff in there, just faders and stereo summing and a dedicated monitor controller - here's my build log:

https://audiomixer.wordpress.com

feel free to have a look and comment, but don't tell me I it will be cheaper to buy a commercial unit, I know that already ;-)...

cheers,

- michael

 

[audiomixer]

Hi,

I have been working a lot on the electronics, but here is a (preliminary) layout of the console. I need to test the switches and the pcb design. I indent to do separate pcbs each two channels, so that you can do different channel counts. two mono or two stereo channel would be on a channel pcb. inputs will be eight channel d-sub 25 with the associated passive components. the mix / aux pcb will host the mixamps and masterfader, and the monitor section on a next pcb. power supply, controller microprocessor, still a lot to do.

PDF here:
http://audiomixer.files.wordpress.com/2012/03/mixer_v0-20.pdf

- michael

 

[geoff004]

I did something similar a couple years ago.
You can read up on it in this old thread:
http://www.groupdiy.com/index.php?topic=36438.msg449335#msg449335

I use it all the time - here's a picture from yesterdays mix session:

the only problem I've had is a 1510 IC went bad.

Looking at your block diagram you might reconsider how you've got your panning set up.  there are a bunch of threads here discussing how to do panning in a mixer.  And there are countless schematics to borrow from to get it all together.

It is cheaper to buy something, but you won't get what you really want.

 

[audiomixer]

Thanks for the feedback! nice build indeed. still need lots of work to get there myself. Schematic is conceptual only - not the actual circuit. the input buffer is much more complex, and the schematics have no RF protection and such....

I have been closley following some of douglas selfs designs, very encouraging book....

right now I have build the input stage with 5532, waiting for more chips to arrive this week - I have become a digikey slave...

how do you do summing? opamp? I build a transistor - opamp mixbus amp but I am still not sure about the performance. will continue to check. how much gain do you have on the faders, I presume the pencil line is 0dB?


- Michael

 

[geoff004]

how do you do summing? opamp? I build a transistor - opamp mixbus amp but I am still not sure about the performance. will continue to check. how much gain do you have on the faders, I presume the pencil line is 0dB?

My summing is basically passive.  The board has no master section.  I use another preamp for that.  The auxes hit a THAT1646, so they're balanced sends.
Not sure of the gain on the faders either, but they're simply a THAT1510 into a OPA134.  The lines drawn on are really just for visual reference.  My budget didn't allow for fancy graphics.

 

[audiomixer]

heres some new about my build - got two channels on a breadboard, almost full functional, still some bulding blocks to check, but I have sound running thru it!

http://audiomixer.wordpress.com/2012/04/04/progress/dsc_6389_sm/

I have started my summing stage, but right now it's only input / mute / fader / pan / balanced output.....

- michael

 

[audiomixer]

been working on the power supply...



http://audiomixer.files.wordpress.com/2012/04/dsc_6413.jpg?w=450


schematic will follow, I did this one on breadboard, filtered lm317 / lm337 combo.

- michael

 

[wolfgang]

Good progress!

I read on your thread that you are using the J112 Fet for audio-switching. Is it working well? how have you implementet the j112?

regards,
Wolfgang

 

[audiomixer]

Hi Wolfgang,

I've looked up some studer and ssl schematics, but in the end I have been implementing two different modes: full dual Fets (one to ground, one in series) for the mute to get best attenation and a simpler approach with single fet in series for the PFL / AUX switching. Good information about this in douglas self's book on small signal design (very worth buying indeed). Mute is almost perfect, PFL Switching is a little less, but very still good. the control logic is based around optocouplers to switch / toggle between the control voltages (groung / - 15V for the mute and +15V / -20V for the PFL / AUX switches). they need lower switch voltage, otherwise you loose tons of headroom....
I have finalized the schematics for the audio portion of the mono boards still got to put this on the blog, but here's a preview, uncommented. I bought both J112 ans J111 but the difference is not obvious to me, got to check further.

- michael

 

[wolfgang]

Thanks for reply!

I have used the Classic API Mute system in my build and i am thinking of the FET-Solution for a future upgrade.
I hav the Book from Douglas and will have a closer look to that chapter!

Regards,
Wolfgang

 

[jensenmann]

Wolfgang, have a look at the Studer 900 series desks. They did use very well implemented FET switching for everything.

 

[audiomixer]

Hi all

I've added some schematics and details on the control system on my blog, I've  got my Atmega48 micro to talk to the fets - works quite nicely. the firmware is nothing near final right now but I have the basic functionality up and running. have a look here:

http://audiomixer.wordpress.com/2012/04/19/control/

next thing is the layout of the 'real' boards, lets see how long this is going to take me. by the way - I changed the physical layout of the board and made it smaller - now with 30mm fader pitch instead of 35mm. 35mm was simply to large and looked wrong when I printed the design 1:1 on our new office plotter. the new design is +/- 750mm wide which is much better. I am going for a non 19" design on purpose - why restrict myself to 19" when I am never going to put this baby in a rack anyway.

here's a preview of the basic control functionality:

- michael

 

[audiomixer]

little update today - I got the switches! I think they look nice and I am going to try to make it happen with this model:

http://www.mentor-bauelemente.de/db/tk/2271.4009.html

there is a more detailed description of whats going on on my blog.

Today I took the time to browse this thread here http://www.groupdiy.com/index.php?topic=45781.120

seems lots of work to make that happen when I look at my project and compare both.... good luck. I found many valid points in this thread - just wondering if it will be a real project one day. I can recommend the Jfet switching and the microcontroller, I found Arduino to be a nice and helpful community so that I am confident to be able to make that part of the console work.....

- michael

 

[audiomixer]

I've asked for a quote on the metal work - will update when I know what this thing will cost me to laser the bits out of it.... I had to perfect the autocad drawing and then I just went all the way and got a proper 3D model of it all:

http://audiomixer.wordpress.com/?p=93&preview=true#

and here's one of the pics:

http://audiomixer.files.wordpress.com/2012/04/mixer_v0-65-3d-model.png?w=700&h=


I can provide the model if someone wants to look under the hood.....

- michael

 

[Joechris]

Hi, great project! What 3d program do you use for your model? You should render it in perspective mode, not parallel..it looks kind of strange
You migth not have landed on a color scheem, but I realy liked the way it looked with the colored buttons on a cream white paper..
Looks aside, could you share some more info on the integration with the arduino, and the mute solo arangement?

j

 

[audiomixer]

Hi Joechris,

3d is done in standard Autocad - it's quite powerful and does 3d 'on request'.... I use the extrude command quite a lot to build 3D volumes. 'on request' because most of the time people don't know it has extensive 3D features. volumes can be subtracted or summed to build more complex forms, but here I had to use this only with the complicated bottom sheet that has non right angle bends to meet the top panel. I wanted to make this precise enough to get a decent result. with Autocad you can put objects on layers - and give them a dedicated color (per layer or block or object) the color scheme is not meant to be final at all, besides the switches that I kind of like.... I will try to put a 'parallel' version of the desk online, but it's really much more helpful in 3D when you can rotate and deselect layers to show the inner working of the design. I had to reassign the connectors a few times to make it all fit on the back side, it's quite crammed with the large power supply heat sink.

here's a update on color scheme with the right colour of the knobs... the only thing I know how it schould look like in the end...

the switching is done with fets, as mentioned earlier. I use two modes - mute and switching. both designes originate from the studer / ssl and douglas self designs floating around. douglas selfs book on small circuit design has been particularly helpfull and I can only recommend it.
the mute switch uses a parallel and a serial fet, driven with opposite controls - the parallel fet will be high impedance when the signal is passed through and close to 50 ohm when the signal is muted. the serial fet is driven the inverted way. in my drive circuit I use two optocouplers to generate the opposite control voltages (-15V / 0V). both optocouplers are driven at the same time with their leds in sereis. one channel has 0V potential, the othe -15V. when off these are reversed. because the fets are voltage controlled the hole drive circuit can be high impedance. I have choosen this approach because it is a nice way to provide the -15V / 0V required with regular logic / microcontrollers and provides a completely decoupled control l/ audio circuit. the microcontroller just has to drive some leds....

the switching is a little easier, I can do it with one fet per channel, but I had to get the control voltage lower to maintain the control voltage below the music signal, therefore the additional -23V. the fets are driven with the same 0V / -23V arrangement through the optocoupler. because the logic is inverted, I have a different arrangement of the switch leds - these switches are active low and the leds are driven on when the microcontroller output is low.

Mute switch:
                      control voltage  control voltage  switch indicator  audio state
uC Output      parallel FET      serial FET          LEDs 
----------------------------------------------------------------------------------------------
LOW                  0V                -15V                      ON                      Muted

HIGH                -15V                0V                      OFF                    Audio On


select switch:

uC Output    control voltage serial FET            Switch indicator LEDs    audio State
------------------------------------------------------------------------------------------------------
LOW                  0V                                        ON                            Audio ON

HIGH                -15V                                      OFF                          Audio OFF


I have chosen the arduino framework because I thought it might be a good thing to use a framework with a large DIY community.... many people posting code..... right now I have not continued work on that part, I am just sensing the input ports and switching the corresponding outputs... but it lights up my swiches and drives the optocouplers, so it does 90% of what I need to do.  one might want to do this simple logic with regular ttl logic driving the optocouplers, that would be a option, or with cmos +-7.5V, but that's nothing for me.....

I have a dedicated microcontroller for each two channels with eight switch inputs and eight outputs for the audio switching and some bus inputs to react to global commands. this bus system senses the state of the mutes, pfl, aux sends - the main microcontroller will check these and switch the monitor section accordingly. this is still not final, but the bus thing works already, if a channel has a mute condition the bus will be high and I can work with that. the pfl is used to switch the monitor section to the pfl audio bus, this is also working already. the aux needs this signalling to unmute the aux when a signal is present. otherwise it will stay muted to reduce unwanted noise. the channels microcontrollers will have input for some global functions, like mute all / unmute all, clear pfl / solo, change to DAW mode (listen back the DAW stereo output without any mix on channels 1 and 2) so that I can check the mix that whent into to DAW. all these functions are triggered by the master microcontroller.

I hope this is of some help to clarify what I am doing with these switches and microcontrollers.....  - michael

 

[audiomixer]

Stuck!

I have reached the point where I am stuck! I have begun thinking about the connections inside the mixer and find it difficult to make decisions, specially regarding bus, power and ground distribution. let me explain:
I have decided to use a small ribbon cable for the control logic, 20 pins or the like. no problems there, I can make this work nicely. but now I come to the point where I must route the audio signals and power to all my boards. now I have read the nice 'Ground' thread going on in the drawing board section and I am getting confused about how to get it right. I wanted to put the power and ground (grounds or one ground....) on a larger AWG 24 or so cable in a fully star distribution manner from the power supply, but is this really usefull or just a waste of copper. I have a nice focusrite 19" channelstrip that has something like 15 cables distributing ground to the various subsections of the PCB, all inside a 19" 1HE enclosure.... but then again, power is through smallish traces on the PCB. So this would suggest to use a ribbon connection to the independent PCBs and and have Power and Audio there and a star ground distribution besides it to all PCBs - any thoughts on this? I also looked for a larger IDC ribbon cable but this seems a rare species, most of these are targeting PC connections and are AWG26-28...

so right now I am having three bus / star connections per PCB (power+ ground, control, audio outputs bus), one audio input from the d-sub board and a connector for Pan / Fader.... this is just to much to fit on the allocated space... after that there is no space left for components.

An other question came up while trying to layout the boards: ground fill or not to ground fill..... the old school PCBs were always traces with large void areas, most probably because of the PCB technique using masking tape of the old days. but today, should I fit a ground plane on top and bottom between the traces or will this do bad regarding capacitance of the traces?

kind regards,

michael

 

[Joechris]

Thanks Michael for your answer. I really has to pick up some books on digital systems to get my head around this..I fell green like an apple
Regarding earthing and layout I cant contribute much, but I have some questions regarding the analog side of things. It puzzels me why you dont want pots for the aux sends.
Do you intend to use the auxes as mono subgroups? if so I would add them to the monitor section. For reverb and echo stuff you realy need a pot on the Aux, more than a switch.
It looks like you have two solo busses. one prefade and one after. Is that realy needed? Since you have the mute swith digitaly controled, it would be easy to make a solo in place function insted.
Save som fets and busses, or use the two busses for a stereo sub group instead...well thats just me talking away

 

[audiomixer]

Hi Joechris,

I use the aux to drive some old school reverb effect, for me it's more a routing then switching application, like a effect reverb on a ***single*** channel - but of course I don't want to reconfigure stuff so I will have switches on all channels. I can also use it to drive a parallel compressor or a parallel distortion unit - thats fed by one channel only of course, return on one of the returns. might think about adding the option of replacing the mute function on the board by a volume pot, that might be doable and would make the board more universally usable, true. the send level of the aux can be adjusted of course, there's a master level for each aux send. If I want to mix on the aux bus I can still output a feed from the DAW.

Solo bus as PFL / AFL comes from my live sound time, I guess. don't know If I really need it, but I thought it would be cool to do it, not to much of a hassle for me here. You can choose the function in the monitor section, it's either PFL or AFL, to monitors (default) or tho headphone only (option). I'am not sure if this is something I will use much, but hey, got to try. the difference between PFL and Solo in place is of course the position of the send in regards of the fader..... I can leave the fader untouched even if it is down quite a bit an listen to each signal as it leaves the DAW. we will see if this is any good. of course I can do the same in the DAW... but hey, if you have real switches who wants to work with software ;-)

Regarding the microcontroller, the Arduino framework makes it quite easy to start, everything you need is on a smart little board and you can start with something like this:

http://www.adafruit.com/blog/2012/05/02/new-product-adafruit-menta-mint-tin-arduino-compatible-kit-includes-mint-tin/

be sure to get a bunch of leds, resistors and switches as well and happy soldering


cheers, -Michael

 

[audiomixer]

Had a nice weekend with my electronics... but alas the only thing worth mentioning int the end is my foot operated PCB drill switch.... Aaarghh!

So the overall Mixer project has kind of stalled due to pending decisions regarding the cable harness and proper grounding technique - everything hangs on this to continue design work of the PCBs. and from this weekend on I definitively know I need to get all PCBs, even preliminary design ones from a professional source.... I am no good at etching PCBs.... no real link except my superduper footswitch:

anybody on grounding and power dsitribution, any clues - what are your two do / dont's ?

- Michael