Discrete Mixer

[joaquins]

Hi... I've started a topic in the mixer part but I think it would be better here...

Here is the old topic. http://www.groupdiy.com/index.php?topic=48042.0

I'm building a mixer with 2 500/51X slots for each chanel, the initial project is 12 channels and 2 sub-master (anyone could work like master)

Later I will add 4 channels for working with stereo FX and so, (I've started with 12 because is the number of outputs I have from the computer) and 2 more sub masters.

Here is my question of the day: How should I interconnect all modules? I'm using pinheads for all the mixer part: buses, input, fader, summing amp and output amp. I mean... Should I use ribbon cable or a board to plug all them in? I'd made a board for buses but I won't die if I don't use it...

In any case, I was thinking about using a heavy solid wire for ground going down each channel and then taking all together, like a comb, how should I connect my pin connector to the heavy clean ground? Any suggestions?

here some pics, more on the old topic and even more comming later (I have nothing intresting now for new pictures)

Pan & Assign

In the board

Input/fader (left) and Summing/output amp (right) with and without opamps. (sorry crappy photos, just the cellphone here... Hey, my little feet wanted to show in the pic!)

They are not the totaly final boards, just add a little mods (servo to the fader and insert points to use the input amps and then send to the slots) But are very close. Wating for comments...

JS

 

[berkleystudios]

very cool stuff man. im looking at doing something very similar, possibly 12 but i would like at least 24 channels. plan on having it be a lne in only console and taking outs of the 51x racks to the in of the board. (most likely 24 so i have 22 channels of pres and ill use the extra 2 channels for either DI or stereo or just make them the master)

waiting to hear on how you will inter connect it for summing and power distribution. what style op amp are you using?

 

[joaquins]

The op amps in the picture are MOD1 for the input (I really like how it brakes the input when you hit them hard) and 25-20 for the rest (fader/summing/output)

I was thinking in use a dirty ground bar and supply parallel for all channels (all desk long) and a clean ground going down the strip and then all together next to the busses with a heavy solid wire.

Output transformers that I will make (allredy made one, and works nice). Maybe somday I could make some inputs and make the whole thing completly insulated... By now simple electrnically balanced input with a trim for CMRR.

In this moment I have to go for the frontplates that I send cutting yesterday (just black fronts, then I'll use them as i like and make all the holes)

Regards.

JS

 

[joaquins]

Hi... Still with this project, some ideas here.

I'm thinking in use a rotary switch to change level instead of pot to have total recall, maybe I'll add some automation system later like niche or something I'd build.

For now the rotary switch:
First thing first, a reference scheme, I'd made some changes like a servo for the fader amp.

My idea is to use two 12 pos rotary for the input faders and one 12 pos for the buses, other posibility for buses a 6x2 to use them stereo.

Here is the thing, at the input channel I want to use one switch where is the fader to do 0.5dB steps, so feedback resistor of the amp doesn't change too much and get better stability thanks to RC time constant (a cap will be parallel with the switch). The other one would be the resistor next to it, the one from the output of first opamp to inverting input of the second, here I'd like to have 6dB steps so I can use 60dB range here plus 6dB in the first switch and still have one position to mute the channel. If I'm making some mistake please tell me. Values between 5K1 and 10K for the feedback resistor, 1K6 to 1M6 for the input resistor and open for mute.

The other way would be using 0.5dB step for the input and 6dB for the feedback but may be problems with stability because FB resistor would change a lot and RC time constant will vary a lot too, less noise with this aproach. In both cases unity gain will be set in muted position, but in the first aproach could be avoided putting a resistor to ground instead of open and could get higher gain with the same technique in lower gains if required.

For the summing amp I was thinking in have 3dB steps to have 33dB range or 15dB if I use 6x2 switch for stereo, the switch in place of the fader of the scheme. Values between 22K and 510Ω for the feedback resistor, summing resistors are 22K.

I'm taking active level control so I don't have to use an extra opamp in the summing and because it only adds the gain you need so less noise gain around.


Another Idea I have is to use all 4 subs and no master, and summing all 4 buses with transformers, all four left buses goes each one for one output transformer primary and all secondary conected in series, from bottom of the first to top of the last is the output. The same thing for the right ones of course. Would it be ok?

Thanks

JS

PD: here is an xlsx file I'd just made for calculating the active atten for a inverting amp or the summing, they are basicaly the same but I made both for convinience. http://www.mediafire.com/view/?scb96azk6jgb968