SID audio output mixer?

[subatomic]

What module (if any) does audio mixing? 

And if none, anyone have a recommended mixer design?

I probably just haven’t seen the appropriate page that describes this, but if I wanted to have my 1/4" outputs on the back of the synth box be like this (5 plugs total):

[tt]- outA

• outB

• outC

• outD

• stereoMix (2 sids on left, and 2 sids on right)[/tt]

alternatively, I was thinking of just having the 4 outs (outA-D), and having some switches to route each sid to one of the 4 outs.

Also, and maybe this answers my question, but where’s the design for the LED/Button Matrix thing I see on everyone’s midiboxSIDs?  Maybe that handles the audio mixing?  Sorry, I haven’t seen info about the Matrix or how to build/wire it, so I don’t really understand it yet (though I’m assuming it does typical LFO type routing?).  There’s a lot of pages here, and I probably just missed it.

Thanks for any links that might enlighten me.

[machinate]

from this page:

http://ucapps.de/midibox_sid.html

you’ve chosen Step B:

http://ucapps.de/midibox_sid_csB.html

The matrix is in step C:

http://ucapps.de/midibox_sid_csC.html

Andreas / Machinate

[subatomic]

The audio outputs of all SIDs can either be cascaded (Audio Out SID1->Audio In SID2->Audio Out SID2->…) or connected in parallel via 10k resistors in order to get a 2-channel signal from 4 SID channels. Note that both methods are lossy. An active mixer, realized with operating amplifiers (OPs) would be better, but requires a biased voltage (+/- 9V or more). The best and most flexible solution is the use 4 seperate channel strips on a mixer or soundcard for every SID, because this allows you to change the position of the sound source across the stereo panarama and to lead every signal to a dedicated Fx unit.

ah, yes, I had missed that tiny section.

Ok, so here’s what I’m thinking now. 

I’d like simply 4 outs, just like the nord3:

[tt]

• outA

• outB

• outC

• outD[/tt]

but here’s the detail:  For flexibility, I want to use switches to route the SIDs to the outs…  Maybe a 4 position switch for each SID, not sure. 

this way I can do any combination of stereo or mono audio out.

I have ideas for doing this with standard audio opamps.  it shouldn’t be too hard… 

I’m thinking this:  http://www.dibsplace.com/design/CIRCATS/opamp04.gif  except duplicate the dual opamp section (right hand side) once per output, and then where the 4 inputs joins into one, have that controlled by 4 position switches (one switch per channel)… 

So doing a little photoshopping, it would be something like this:  http://www.subatomicglue.com/sidl0g/audiomixer.jpg

• If anyone’s already done a design for this, please post here. 

• If you have comments about the linked circuit, good or bad, also please let me know.  I like that the design is for balanced outputs, and it seems simple enough…

[subatomic]

so I’m going to prototype this design soon, and will post results here if I find somethig useful. :-*

If good, maybe it could make a good module?  4inx4out audio router…

[thefRont]

here is a link to another interesting audio mixer :

http://www.musicfromouterspace.com/analogsynth/stereomixer.html

the op-amp needs a regulated psu supplying +12V and -12V

a sample schematic for the psu can be found here:

http://www.musicfromouterspace.com/analogsynth/powersupply.html

[subatomic]

That’s interesting, I hadn’t thought of panning, that could certainly be useful.

Here, I’ve modified my frankenstein design to incorporate panning into the 4in 4out router.

Note that this design is more simple, it uses DPDT (2 pole, 2 position) switches instead of the 4 position switches of the previous design.

With this design, I can assign any SID input to any two channels (AB or CD), and then pan between those channels.

This should allow the greatest flexibility, since I can have discreet assignment SID1,2,3,4 -> A,B,C,D  or  mixed/panned assignments as needed.

->> Could someone with circuit design experience look at this and tell me if the input section will work here? 

I do expect I might need to adjust some of the resistor values…

I wonder if maybe my pan pots should be shorting to ground (like the previous post’s stereo mixer diagram) instead of fading between the inputs.

I’m going to prototype this one soon, probably with lm741’s since that’s what I got available. 

In the end, I plan to use a much nicer amp like the AD823 or OPA2227.

UPDATE: the frankenstein grows, I updated the image to use the input section from the stereomixer.gif (previous post), whose panpots tie to ground. 

[banneduser]

I wonder if maybe my pan pots should be shorting to ground (like the previous post’s stereo mixer diagram) instead of fading between the inputs.

Yes, that’s one way to do it.

Moebius

[subatomic]

Should I add anything to the output of the mixer to decouple the device from ground loops?

I’ve seen other designs add capacitors. 

If so… 

• i’ve seen polarized and non polarized caps used, does it matter? 

• what value of cap to use (is there some way to calculate it?).  I’ve seen other designs use 1uf, 10uf, 0.22uf, maybe it’s based on power xfer?

• I can see putting caps on the + and - balanced outs, what about the ground, do I even conect it to the jack?  Or just go with + and -  to the TRS balanced jack and leave the ground hanging?

sorry, I’m not experienced in audio interfacing…  does this isolation even matter?  (seems it would to me but…)

[subatomic]

well, lucky today’s a holiday (MLK day), so I’m home working on this… 

so through experimentation I think I’m going to answer my own questions, and post here so you guys can watch, or comment if you care to…

So I breadboarded one input and two outputs (using some old LM741’s I had laying around). 

The amp works, though like I predicted I had to update the resistor values on the outputs stage: they ended up changing from 10k and 59k.  I also used a 10uf polarized cap on the input instead of the .1uf…    Overall gain is 1.4 so you can make it a little hot if you like…  keep in mind I have no idea how hot the SID outputs will be so this may need adjusting later…

I’ll update the schematic when I’m happy with the design.

For decoupling ground, here’s my results:

• I placed a 1uf cap on each balanced output:  output+ and output-. 

• Signal seems to be transmitted regardless of whether the ground is connected or not.  It doesn’t seem to need a ground.

• Interesting observation:  only connecting one of the outputs transmits signal, though not as strong as using both.

• I notice no loss in signal quality using caps, vs connecting direct.

Cool, so looks like the mixer’s working, next I’m going to test out more than one input and see how well they mix… 

[subatomic]

Ok, here’s the updated schematic, let me know if you see any bugs/typos/problems:

Now, two last questions,

1.) can someone tell me how to attach the S leeve output connection? 

Do I tie S leeve to ground, or let it hang? 

NOTE: In testing, the output signal sounds correct, with no noticable differences, with or without ground connected to S leeve.

2.) for the + and - outputs, what orientation should the polarized caps be?  Right now I have the + end of the caps towards the opamp.  (see picture).  I worry about damaging them…  does it matter?

[subatomic]

So I created an eagle schematic of the 4in 4out mixer.

schematic file

also I did some thinking about the PCB layout (freeware eagle was too small to do this)

so far this is intended to be perfboard with wire jumpers.  I’d like to do a real PCB, but not sure if I’ll get around to it since I don’t have the equipment…

I also answered one of my previous questions.  I should use _non-polarized_ electrolytic caps for input/output decoupling…  kind of obvious since we’re dealing with AC +/- signals. 

[subatomic]

created some files I think that will work for the PCB layout, using a paint program (anyone know a free editor that doesn’t have restrictions?  Eagle was too small)…

Here it is, I thnk I might do some more work on the lables, but it should be useable like this…  Keep in mind to chop off the bottom so the 1/4 jacks fit flush with the panel.

Break out:

Front:

Back:

If this looks non-standard, it’s because I haven’t done this before, but I think this will work for etching from what I know of it. 

In the coming weeks, I’ll have a friend help me out with the etching (I don’t have a kit)…  I’ll report anything I find once that happens.

Keep in mind this is a double sided board, though you shouldn’t need through-hole plating.  I provided pin spots where you can stick a jumper in to jump the top to the bottom.  Just look for the O’s that don’t have a component associated with it.  There’s 13 spots…  Use some old cut ends from your components or plain wire.

[subatomic]

I created a Bill Of Materials (BOM) for the mixer here…

[DrBunsen]

My thoughts:  a headphone amp and a “cue” button for each SID would be handy.

Also, there are a few other mixer ideas floating around this forum - it might be worth doing a search for “mixer” in title.

[subatomic]

My thoughts:  a headphone amp and a “cue” button for each SID would be handy.

I agree about headphone amp, not sure about cue for my needs, but maybe you have a killer idea in mind?  I think a headphone amp could possibly be done in a separate module, and tap off the above pictured 4x4 mixer outputs.  If you want, feel free to extend my design to include the headphone amp.

Also, there are a few other mixer ideas floating around this forum - it might be worth doing a search for “mixer” in title.

Got some good links in mind?  I searched, found a lot of threads, but most seem to advocate using commercial.  I just want a simple module I can embed in the SID, customized to the SID (i.e. only 4 channel).

[DrBunsen]

This thread contains most of the links I dug up last time I searched. I started a search for the words “mixer chip” in the full message, in all forums, to see what else would show up, but I guess you can search as good as me

[DrBunsen]

Here’s an older one that turned up.