MIDIBox Mixer - A PGAx311 Audio Mixer

[Sasha]

That is great! I`m glad you back on track Lyle.

If anyone has questions or suggestions, now is the time to ask!

well, if you can post some screenshot of the design we might have some suggestions. Ultra did it for his ultracore and I say it is quite improved from initial idea.  :)

[lylehaze]

That's a great idea.

I should be able to do that in about

12 hours from now.

LyleHaze

[nILS]

[me=nILS Podewski]sets the alarm clock[/me]

jk, glad to see you're back here and back on it! ;D

[dj3nk]

I'll try to upload them today

[lylehaze]

OK, the channel board is just about finished.

The master board is about 70%

4311 board description

The target application of this board is to

mix two incoming stereo pairs onto the stereo buss

of the mixer. (Volume and balance only).

By leaving off certain parts and adding an extra

connector, you can change that to other applications

if you want.

The board contains a PGA4311 chip. There are also

two dual amplifiers, used to buffer the incoming

signals. All three chips are SMT, along with all

resistors and capacitors.

There are six ten pin connectors around the board.

Five are normally used, CN5 is only needed if you

re-configure the board.

Starting from the top:

CN1 has the audio line inputs for normal operation.

There are a few ground pins, and the analog power

connections in case you want to attach special

input circuits here.

CN6 carries the +,-, and GND analog power supplies.

It also carries the right and left Mix buss if you are

mixing everything down to a stereo pair.

CN2 is a copy of CN6 for the next board in the chain.

CN5 is usually not used. It has direct connections

to the inputs and outputs of the PGA4311, in case you

want to bypass the input buffers, or the output mixing

stages.

CN4 carries the digital power and signals that control

the PGA chip. These include +5 volts for the digital

circuits, the digital ground (separated from the analog

stuff for lower noise), "Serial Data In", "Chip Select",

"Serial Clock" and lines to select "MUTE" and "Zero

Crossing Enable".

CN3 carries copies of all of these to the next board,

except that the "Serial Data Out" replaces the "Serial

Data In" so the boards can be "daisy chained".

The plan is modular. Most people will use one "Master"

board, which includes three voltage regulators, the

summing amp for the left and right mixer outputs, and

all the connections to link this to the MidiBox.

Then you connect as many 4311 boards as you like to the

right side, each adds two stereo pairs to the mix. Up to

8 boards can be added, for up to 16 pairs of audio.

The main Left and Right outputs will come right off the

master board. If you like building your own, you could

hand build the master board instead of buying one.

Comments are invited.

[lylehaze]

These boards link into a chain.

I think we have three options for linking them.

We can put male headers in all positions, then

use 2 IDC connectors on two inches of 10 conductor

ribbon cable. Simple, easy to do, Smash has

all the parts available. Boards arrange from Left

to Right.

Or we do the same thing with about five inches of

cable between the connectors. Now we can "stack"

the modules into a cube. All the mounting holes

match. Simple, modular, if a bit "Borg" in style.

Finally, there has to be an offbeat option.

On the right side of each board we mount a female

header, facing up. On the left side connectors we

solder a male header, coming from the BOTTOM of the

board. Now the boards really "piggy-back" together,

with no cables between them at all.

The connectors align with the mounting holes for

just that reason. We can add spacers and permanently

piggy back the boards together.

So, there's three options for joining boards together.

Time for bed. I'll check for comments in the morning,

LyleHaze

[Sasha]

Lyle, those PCBs looks really good. I personally need PCBs horizontally for my project because I`m planing to house it together with double MIDI controller so I don`t have enough space, otherwise I would love to piggy-back them. I always liked that simple and clean approach. Tell me, are those regulators need any kind of heat sink if all boards are hooked on? If do, would you need to move them from each other to make some a room for it especially the negative regulator? Thank you for your work. I love it, especially way of chaining. :)

[stryd_one]

Sweet as F**K!!!

[lylehaze]

re:heatsinks

I don't think it will need them, but I have not built up a full 16 pair set yet,

and it will also depend on how high the unregulated V+ is.

I usually hang the + regulators off the side, so they can be mounted with

their backs to the case. The - regulator tab is not at ground, so it will

be either no heatsink or a small, ungrounded one.

I like the idea of the mixer carrying it's own regulators, to keep it simple.

I thought of adding a bridge and caps, but there is NO ROOM on such

a small board for big capacitors. As it is now you have the transformer,

bridge, and caps off board, then attach the V-,GND, and V+ to the

master board. I may work more on that today.

It's off to work I go!

LyleHaze

Follow Up:

Based on the datasheets, the + and - 5 volt analog regulators will be supplying about 270ma for a full 16 channel mixer.

The +5 digital will be supplying almost nothing.

I have moved all 3 to the left edge of the master board. 2 can be case mounted, 1 MAY need a heatsink.

[offe]

Really nice work! Any plans of selling/bulk ordering the pcbs?

/offe

[dstamand]

Outstanding boards! I'll want one.

[lylehaze]

re Bulk Order:

Not likely. Just ordering one set of boards will cost

more than I can easily afford. (Prototype costs are

a bit much). I will probably make the gerber files,

eagle files and documentation available. I want these

to be available to everyone, but I can't afford to

manage the project. If giving them to one person makes

that possible, then I'll do it. If giving the files to

everybody is a better way, then I'll go that way.

I don't want any profit from this, but I can't afford

to invest much into it either. I'll spend $85+ for a

set (one master, four channel boards, eight pairs stereo

mix) That will "prove" the board files, then I'll figure

out what comes next.

At least, that's the plan now. I hope to finish the

master board design tomorrow, then spend the weekend checking my work. The hand wired ones have been working great, but I never went past four pairs. I'm looking forward to the upgrade!

LyleHaze

[DrBunsen]

Superb work lyle.  Just a couple of quick questions:

Can the channel board be configured as 4 mono inputs?

What would be involved in providing FX sends and returns?

Thanks for your hard work so far - the design looks very clean.

[lylehaze]

4 Mono inputs? Yes. Minor changes in hardware and software.

Hardware change for four SEPARATE outputs: Leave 4 of the SMT resistors out when assembling the board. "pick" the 4 separate outputs off of the right side of CN5.

For four inputs mixed into a single channel, just short the left and right mix terminals together. (it's OK, really!)

Software: The current mixer software keeps separate tables for sixteen volumes and sixteen pan/balance settings. It re-calculates all of these into 32 volume settings which are sent out to the PGAs. So you just take that "re-calculate" call out, and load your values directly into the 32 output registers. There are LOG conversions available to keep the levels right.

Effect Loops. Hmm, I've never done those.

If you want two stereo busses (main and FX) then just separate the outputs as above and use two summing amps. (one on the master, you make the other). Or make two master boards and call the second one "FX". (on a different MIDI OUT if you go above 8 pairs). You CAN tie the inputs together (VERY high impedance, esp if you leave 4 resistors off each of the second set of channel boards)

If you want an individual FX loop for each channel (or pair) then you might have to cut four traces. But this is guesswork off the top of my head. It would help if I could see a function diagram of where the controls are in the picture.

If you need a better answer, I'll need to see exactly how you want it to be arranged, like whether you have separate volume controls for the main and FX send (and FX return) and if we are stereo or mono.

LyleHaze

[edit] Duh, I missed the obvious one.

Mono input panned into the stereo mix.

Yes, easily done, very minor HW changes, no software changes needed. BUT.. Only two inputs per channel board

instead of four, and no MIC preamps on board, though there is a provision for adding them.

[DrBunsen]

Here's how I imagine it functionally:

       |
input [ ]
buffer |
       |----,
       |    | pre/post switch
       |    |
 vol  (/)    \----(/)--
       |    |
       |    |     FX send level
       |----'
       |
pan   (/)
      / \
     |   |
     L   R 

Repeat the FX send circuit for however many FX sends you want (FX1, FX2 etc).

       |
      [ ]
       |
       |----+---+---+
       |    |   |   |
       |     \--)---)----(/)--- FX1
       |    |   |   |
       |    |   |   |
      (/)   |    \- )----(/)--- FX2
       |    |   |   |
       |    |   |   |
       |    |   |    \---(/)--- FX3 
       |----+---+---'               etc
       |
      (/)
      / \
     |   |
     L   R 

And sum each FX channel to a master send volume.  Return is pretty much the same as another input channel I guess.

Leaving out the pre/post switch would make it a lot simpler.

I suppose what I am talking about here is pretty much another master bus.

[lylehaze]

Interesting.. to me at least.

It may be old hat to others here.

Now, to change from the "functional" diagram to an actual signal path.. Consider this:

For the moment, ignore the FX loop. Let's focus on the main outputs. You have taken a mono signal, through a input buffer for good impedance, then through a single volume control and then a panpot.

In reality (for some cases of reality, anyway) we would eliminate the level control, and only have two "volume pads" one feeding main to Left, and one feeding main to right, The current code does exactly that. It converts a volume setting(both channels), then applies a balance/pan setting, resulting in volume left and volume right.

Your controls are separate, but the end result requires only two PGA channels.

Now we add an FX loop. Whether it is pre or post fader is complely under software control. It makes no difference to the hardware. You are now sending the incoming (mono) signal to three possible destinations. Left, Right, and FX. Each level may be software controlled.

Since these things always come in even numbers, it would be easy to mix a mono signal into a strereo buss plus two effects sends. that would require one channel board for each channel.

Or you could re-wire the thing for one fx send and the other for an FX return level, but no control of FX return pan. Maybe better to bring that back into a separate channel board.

You've made this much clearer for me, is there anything I can help to explain for you? Everything I described above can be done, mostly with software changes.

Minor issues: The current software uses CC7 and CC10, I think, for volume and pan/balance. Are there "standard" controls for FX send, or do we hack something nonstandard up?

I am lucky to have LOTS of extra MIDI ports available, but I realize that most people don't. (LOVE my UM-880)

LyleHaze

[edit] it looks like CC91 and CC92 would be fair choices for FX1 and FX2. I wonder how many people want to mix stereo pairs, and how many want to mix mono sources into a stereo mix?

[lylehaze]

After a full day of considering options, I'm going to delay sending the proto order out for a few more weeks.

I'm changing the circuit to make it

a> More Affordable (slightly)

b> Higher Fidelity

c> Easier to reconfigure to different applications,

based on the comments I have received so far.

And also hopefully more compact. (GAAA!)

Besides, I just had to replace the power supply

in my AmigaOne, so I'll need a few more weeks to

get my hobby cash together.

But I'm still open for more suggestions..

LyleHaze

[Sasha]

Improvement is always good excuse. ;) Good luck!

[lylehaze]

After some discussion in another thread, I want to release some info on the redesign. Only the channel board has been done yet, but the changes have been pretty big.

I'm about 90% sure that I'm done with the channel board. Now I need to make a matching master board, and I'll be ready to get started again.

The attached files will describe the options of the board. What it doesn't say much about is the size. This board is now <4 inches by <1 inch. If you get your boards from Sparkfun, it should bill out as 4 square inches. If you get them from Olimex, you should be able to fit 6 of them on a single board.

In the interest of making a document that will read on any computer, I zipped it up as an HTML file. I hope it carries the images properly to everyone.

As always, I'm open for suggestions,

LyleHaze

chanbrd.zip

chanbrd.zip

[DrBunsen]

Whatever combination up to 4X4 that you want, it’s all user configurable./ less than four inches long, and less than one inch tall.

A big round of applause for Mr Haze!  Superb work.

Am I reading right that it can also be configured as 4 mono ins mixed to 4 mono outs (or one stereo master and two mono sends)? 

[Sasha]

Maaan... that is beautiful work!!!  :D ...so many configuration possibilities!  8)

Thank you very much for this great work.

[stryd_one]

Woh!

And the doco is the icing on the cake. Fantastic!

[lylehaze]

@DrBunsen

Yes, 4 separate inputs can be buffered, volume controlled, and each brought out on a separate wire, OR each mixed to any of bus A,B,C, or D. If you leave out the SIP resistors in the last step, then the 4 signals will just sit there and wait.

Probably a more likely use is to use the entire channel board for one mono input. Just build up a pair of input caps, a single op-amp, then drop the signal into all four channels of the PGA. Then we re-name the four mix busses as Left, Right, FX1, and FX2. If you want a "private" FX loop for each input, then wire directly to that board instead of bussing them together.

As was discussed previously, each FX send may be pre or post fader, that's just a software change.

Finally, the size. I think the max number of channel boards we can safely gang together is probably 8, due to signal fan-out. But those would assemble into a 4" by 4" square that is only an inch tall. It would still fit in a 1U rack case.

re:Docs. It's the first time I've tried html format. Glad it worked. I like formats that can be read on any OS.

LyleHaze

[DrBunsen]

Yes, 4 separate inputs can be / mixed to any of bus A,B,C, or D.

And/or ALL or any combination of bus A, B, C and D in separate amounts?  I've had a look at the PGA4311 data sheet and your doc, and I still can't figure out if a true 4x4 mix is possible on one board.  I can see 1 into 4, or 4 into 1, but I can't see how 4 into 4 works with only 4 volume controls.  Wouldn't that require 16 volume levels in a matrix configuration?

Please excuse the extreme dimness of the question. 

[lylehaze]

You are correct. Sorry if I described it poorly.

There are only 4 volume controls in each PGA chip.

Lot's of options, but none of them involve stacking

PGA chips onto a single board :-)

With the "4 in to 4 out" option there is no mixing involved. Just simple audio level control of four separate channels. That might be useful for some projects, but it's not a mixer at that point.

I hope I didn't mislead.. I am a bit excited about this project.

LyleHaze