Modular active line mixer

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geoff004

Well-known member
Joined
Apr 6, 2008
Messages
343
Location
Bucks County, PA
Sorry to do this once again - it seems that at least once a month someone comes up with some kind of idea about building a mixer. After all, almost all of us have built a stack of other gear - eqs, comps, pres..... so isn't the mixer the thing that brings it all together.

I think it begins with deciding what you need in a mixer. Then the design can begin.

this is what I'd like to see:

mono channels with pan
single stereo bus (left and right)
line input only
possibly HPF on a switch
3 aux sends -2 post , 1 pre fader
mutes
faders

IC op-amps for the auxes
Then what would be cool would to have the option of either an IC op-amp or a 2520 footprint for your main bus (kinda like how Purple sets up their mic-pres). this could give you a ton of sonic options as well as build-cost options.

Maybe squeeze 10 channels in to a 4ru box, with another 4ru box to house 100mm faders.
Totally modular - build 2 channels, build 32 - whatever you need.

As for the master section - what do you really need? I'd rather make up final gain with my top-shelf mic pre than some cheap IC amps. The fact is it really has to sound great for it to be worth building.

Am I beating a dead horse yet again?
 
[quote author="geoff004"]

Am I beating a dead horse yet again?[/quote]

No, not at all....beter start building the darn thing already, cause it takes time
...I tell ya :cool:
 
I think there was a discussion about building a modular mixer using 500 series modules here:

http://www.groupdiy.com/index.php?topic=27169&highlight=

and a discussion of the 500 series mixer frame by Tree Audio here:

http://www.groupdiy.com/index.php?topic=28820&highlight=

I think, rather than building a master section with a given (and all encompassing) set of specifications, that this section should be reduced to functional modules too. This is the approach that Tonelux went with and allows you to build up from some basic functional blocks and expand as your needs (or wallet) grow. The Tonelux modules have a 64 pin connector so they have some inherent flexibility over a 500 series module with only 15 functional pins defined, but as I suggested in one of the posts in the first thread, this could be overcome with some upward compatible additions. Here is a link to their product page:

http://www.tonelux.com/products.html

The basic rule would be that all modules need to communicate with all other modules by using the 600 Ohm balanced line standard. It wouldn't matter if the coupling was made with transformer, line drivers/receivers, or pseudo balanced op-amp circuits as long as the circuit can drive, bridge or terminate 600 Ohms at some reasonable level.
 
To allow the intermediate stages in a mixer to take advantage of the 2520 style discrete op amps without the necessity of large amounts of board real estate, I have repackaged the amplifier using smd output transistors instead of the TO225 transistors on the normal version. This places some limits on the output drive capability but when using this amp as a buffer or input preamp, I think this is an acceptable compromise. The unit is packaged as a 1.125 x .9 inch board with the power and IO pins arranged along the bottom edge. Here is a picture:

2510smd_proto_closeup.jpg


A photo showing the unit on a breadboard during initial testing:

2510smd_proto_testing.jpg


I have designated this version as the 2510SMD for want of a better name. Eventually, I would like to get rid of the through hole matched pair of input transistors. This would make the assembly very easy. So far, the performance is very similar to the 2520SMD showing gain at 60KHz down roughly 3db.
 
I've been thinking for while about all this and I recently completed a 9k preamp. This morning my projects collided.

I'm really impressed with the sound of the 9k pre and it's easy to build.
Here's my thoughts:
you could start with the existing 9k PCB
-on the input, a number of components can be omitted. Things like phantom-blocking caps and such. It could be configured to handle line-in sources.

-I can't imagine you'd need so much gain out of the unit, so possibly you could use easier to obtain dual ganged pots for trim.

-for the aux sends, take them right off of the output. Then send them to the balancer-boards to step them up to +4 outs.

-for the main channel outs use 1 balancer board and put a pan pot between the inputs. The caps on the balancer-board could be replaced by line-out impedance-matching resistors.

Anyone else out there have any experience with the 9k pre - enough to tell me if I'm on the right track? It seems that it wouldn't take much more than a revised BOM, and a couple more wires and pots. Maybe a bigger power supply too.
 
Ah yes, the mystery and allure of the modular mixer. I too have pursued this same vixen for three or four years now. And Burdij - Nice work on further SMD-ing the DOA's. This is the way to go. Attention all DIYer's - Never give up on the new mixer / modular mixer design project. This is our quest.
 
With regards to your through the hole transistors.

Just recently I got to see inside a rather expensive eq, which uses it's own DOAs. They were mostly surface mount apart from some through hole transistors. These were bent down so their flat faces were flush with the pcb, but the pairs were end to end. This is good because you can thermally couple them, but also if you make an area/pad on the circuit board that they lie against, kind of like a heatsink you can get better thermal coupling still !!
 
I have obtained some low noise supermatched transistor pairs is an 8pin SOIC package. I need to change the artwork to accommodate the SMD package and make a few more. The big problem with the thru-hole transistors is not a mechanical one but has to do with the matching that is very time consuming and has a fairly low yield on high gain transistors as the distribution of gains is very wide. This dual package is not cheap but it saves greatly in assembly time.

I have also made a set of boards that implements a fader/bus assign/bus output stage that fits in a 500-style module. I still need to stuff these and make the bracket and panel. Each one of these modules requires 3 2510SMD style opamps and if the bus output option is populated, a 2520SMD style opamp to drive the output transformer.
 
Yo - Keep up the good work. Monolithic matched pairs are an excellent choice. Anything to reduce assembly time without compromising the resulting audio quality is a good thing. I am extremely jealous of the work you are actually getting done on these DOA's and modular mixer boards. I need to get off my ass and do some work like this. DW.
 
I have a box of mono line input channel strips from a PR&E broadcast console. They are NE553x based with Penny and Giles faders, pan, with 3 buss destinations. and two sends. They are all packed in antistat and bubble wrap and I think there are 10-14 strips in that box. They have logic on board for switching and edge card connectors.

I also have the schematics for the strips as well.
This might be what you need.

ron <AT> mtrecording <DOT> com

:Ron
 
Here is a picture of the progress on the bus assignment module. Needs more work. This is viewed from the metal bracket (left) side of the module. The controls are (left to right) the dual concentric pan and input fader, the upper row of bus assignment buttons, the lower row of buttons for stereo bus assign, pfl, solo, bus output mute, and the bus output fader. The vertical DOAs are (left to right) the balanced input buffer, the fader buffer, and the bus summing amp. The large DOA on the output board is the transformer driver.

580_proto_boards_2.jpg


From the back, the two cardedge connectors are visible. The upper "500" cardedge and the lower 60pin cardedge that carries the summing busses and extra inputs and outputs.

580_proto_boards_1.jpg


The quarter inch stereo phone connector facing the rear receives an external input fader.
 
Yer killin' me here. That is friggin' schweet. I recommend you lock up your shop at night because I may come out there and commit audio larceny. It looks like possibly you have attended the Tonelux School of modular mixers. There is also the Purple Audio Institute of modular mixers. And the JLM Academy of modular mixers. That appears to be somewhat analogous to a Tonelux MX2 module or the MX2D to be exact. Except you have eight Aux busses. But if that is just one input module, why does it have it's own summing amp? What is the summing amp summing? I don't know what your entire design is so it's not that important. Really nice. Keep up the good work. DW.
 
[quote author="Tubemooley"]I'm an idiot. It's a bus assignment module. I should learn to read more closely. That makes it more like a Tonelux TR8+. DW.[/quote]

It sais bus summing amp...and I don't get it :?
Why is there need for a bus summing amp on every channel input?
Or is it another buffer amp?
 
The configuration is similar to the TR8 in that it is a channel assignment module and can also contain the bus summing amp, bus fader and bus output balanced driver if you don't want to implement full bus strips for the mixer. Alternatively, the bus summing and output could be used for aux bus sends, effects sends, etc. The function of the bus section is assigned with a jumper lead on the module. For normal channels, you could leave the bus section off and just use the assignment section. The disadvantage of this bus section is that it doesn't include inserts and other features that a full bus strip would have.
 
The console of the modular mixer using 500 series modules is starting to come together:

console_proto_full.jpg


closeup_console_bins.jpg


And with the racks mounted, one can see what it will look like:

console_2.jpg


Next project - backplanes
 
Very, very cool!!
I've recently finished my attempt of building a mixer so I know how much work this all is.
Looking at the pictures I see a very well thought of project.
May I ask how much time is in there all ready?

Respect!!
 

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