Simple 8x2 mixer

[herrmann]

Hi all
I want to build a very simple mixer : just a level pot, mute switch. For use in live to submix my synths, and at home for the summing possibilities.
Balanced inputs.
Can I use a DOA to drive each input (like the 2520 diagram attached) ? This is the 2520, but it can be any DOA, a faster may be better for my use.
Can I substitute the 10k resistor (just before the DOA input) with a 10k pot for the level ?

OR

I unbalance the input with a opamp, level, mute and use just 1 DOA per mixbus ?

Thanks

 

[conleycd]

Hi there,

If you look at the rest of the 2520 page there is a schematic diagram for a loseless combining amplifier.  That schematic is essentially a mixer diagram.  It is unbalanced.  You will need to polarity reverse at the end of this circuit either running a unity inverting buffer or using a transformer.  You can kill 2 birds with one stone with an opamp unity inverting buffer in terms of proper polarity reversal and balancing the output stage.  Just take your "-" output from the output of the summing amplifier opamp after a resistor and cap run the same output into the input of another opamp configured in an inverting buffer.  Mirror the resistor and cap that was in the summing opamp and take your "+" from the end of this chain.  Viola - balanced output stage. 

You could add 10k pots to the front of each channel for resistive volume.

If you want unbalancing (which I think you do) plus gain consider these circuits:

http://www.groupdiy.com/index.php?topic=29333.0

If you use a Discrete Opamps on each channel it could get expensive.  Or you could use some DIY discrete opamps - check out www.classicapi.com

I'm not too up on pan circuits. 

You could mute each channel on the mixer (i.e. the Loseless combining amplifier) by grounding each channel.  In fact, unused channels should be run to ground.

CC

 

[abbey road d enfer]

The circuit shown is balanced. You can't substitute one of the input res with a pot to control gain because it will not balance properly.
Do you really need balanced inputs?
Most synths have unbalanced outputs.
Anyway, the structure should be: input amp (balanced or unbalanced, your choice) - level pot - make-up amp - pan-pot - bus injection resistors.
And somewhere a Mute switch; it can be put almost anywhere in the signal chain. If you use a constant-gain mix amp (one whose gain doesn't change with the number of channels routed to it, such as the virtual-ground mixer), installing the mute switch between pan-pot and bus injection resistors is optimal in terms of noise (requires a DPDT switch).
I don't see the point in using DOA's in such a mixer, except maybe on the mix amp. 5532's and even TL072's have a proven record of excellent performance is such applications.
Proper level diagram and choice of pan-pot can influence the overall performance much more than the selection of opamps. Opamp speed is almost irrelevant since most synths sample at 44 or 48 k, resulting in ca. 20us rise-time.

 

[herrmann]

Thanks !
I want to use this baby at home too, that's why I prefer the balanced inputs.
As I want to keep this thing very simple, and because the case I have is small (10"x10"), I'll not use pan. Channel 1/3/5/7 to Left and 2/4/6/8 to Right. In the future I'll add some mono channels if needed.
I was thinking about using the 2 Channel ACA board for the mix amp : http://classicapi.com/catalog/product_info.php?cPath=22_46&products_id=81
And leave for now the booster part, to keep the price low.

 

[detonator]

try one of these for a simple mixer and adapt it to your own personal needs
http://sound.westhost.com/project94a.htm
http://sound.westhost.com/project94.htm

 

[conleycd]

Thanks !
I want to use this baby at home too, that's why I prefer the balanced inputs.
As I want to keep this thing very simple, and because the case I have is small (10"x10"), I'll not use pan. Channel 1/3/5/7 to Left and 2/4/6/8 to Right. In the future I'll add some mono channels if needed.
I was thinking about using the 2 Channel ACA board for the mix amp : http://classicapi.com/catalog/product_info.php?cPath=22_46&products_id=81
And leave for now the booster part, to keep the price low.

You can certainly use Jeff's ACA.  It is still unbalanced in - it is essentially that other circuit on the API datasheet for the Loseless combiner.  I have also posted over at JLMaudio's forums how to convert Joe's dINgO into a Virtual Ground mixer (much like Jeff's).  One advantage with the dINgO is you can go discrete or IC or a combination!

You still have the problem have a balanced input that you need to get to unbalanced.  You can do this with AudioX circuit and board and then run into the dINgO or ACA.  There are many ways to do this.  But... if you want to add gain to your channel before hitting your mixer (ACA or whatever) then you want to look at AudioX's diagram which allows debalancing plus gain with a dual opamp like the NE5532 or if you want a ton of headroom the OPA2604.

CC

 

[herrmann]

I'm reading your post at the JLM forum. This Dingo is very interesting, tons of possibilities.
I think I will use SSM2141 or INA137 for debalancing, there's some schem on the net.
I don't need gain after, my very first idea was to build it in the more passive way (except for the mix amp).
I'm gonna change some things : will go for 4 or 5 stereo inputs, and the others mono. I have to find dual gang pots, but this is a pain to find quality at a good price. Where to find carbon stereo Bourns ?

 

[sodderboy]

For synths I would think transformers, especially with live use.  It really eliminates the problems encountered with a setup that is different at every gig.  The isolation provided leaves your head empty for creativity rather than full of tech.  Much more than electronically balanced.  I integrate synths only with iron because it achieves the lowest noise floor.  A 10x10 case is quite a constraint, but there are some smaller sizes that don't break the budget.
And add one extra of each input over what you think you need.
Mike

 

[conleycd]

I'm reading your post at the JLM forum. This Dingo is very interesting, tons of possibilities.
I think I will use SSM2141 or INA137 for debalancing, there's some schem on the net.
I don't need gain after, my very first idea was to build it in the more passive way (except for the mix amp).
I'm gonna change some things : will go for 4 or 5 stereo inputs, and the others mono. I have to find dual gang pots, but this is a pain to find quality at a good price. Where to find carbon stereo Bourns ?

I had looked at SSM2141 or INA137 but to really do it right - it felt like I had to add a ton of stuff beyond the chip.  What about a good old Opamp and 4 resistors?  If you want lots of headroom you could use the OPA2604 (24-/+).  So for 6 channels that would be 3 OPA2604 plus the resistors.  Put a cap a the end of it and run into 10k audio taper single gang pots (cheap as dirt).  If you're in North America check www.smallbearelec.com for their pots.  I think they also have dual 10k audio taper.  For log reverse audio I'd go with JLMaudio.   They're about $10 there.  You could buy the dINgo just for debalancing or at least have a look at the schematic to see what is at play if you wanted to breadboard something.

The problem with Stereo stuff outside of switched pots is that the L and R channels on a dual gang pot might not increase/decrease at the same time because the tolerance is typically 10-20%.  If you can tolerate it you could consider separate pots.  I'm actually looking for some Concentric Pots which for taking up less space but still having separate L and R controls may be a solution.  Switched pots can get pretty expensive and annoying but would guarantee good simultaneous changes.

CC  

 

[herrmann]

Yeah, I have that idea too that the dual concentric was maybe the solution. Like the API stuff. But I didn't find any of them on the net !
I will look at the OPA2604.
Take a look at this schem (the first one) : http://www.sonelec-musique.com/electronique_realisations_desymetriseur_audio_005.html. There's just some caps.

Sodderboy, wich transformers are you thinking off ?

 

[conleycd]

Hey there,

I noticed I original posted "now" instead of the word "not" regarding stereo pots.  There were some concentric pots at Digikey but I haven't really looked myself.

I don't speak french so I can't super figure what that site is about.  The diagram looks appropriate.  If they have a cheap board then by all means go for it.

My point about these unbalancing ICs is that they require a fair bit of "stuff" compared to just an opamp (which you get 2 opamps on a NE5532 or OPA2604 or TL072 blah, blah) plus 4 resistors, maybe 2 caps (in the feedback) and I guess a single resistor and capacitor at the end. 

If you were making your own PCBs boards I think the standard OPAMP receiver is easier to design and the actual circuit is cheaper.

The THAT series of receivers have a circuit thought up by Jensen transformers owner/guru that has an incredible amount of Common mode rejection ratio (CMRR) but I think this is only necessary in really problematic setups.  Of course, an input transformer would do the same thing.

Transformer inputs and the circuits based on the THAT series of receivers will always have higher CMMR than Opamps but... thousands and thousands of great stuff was made with opamps for receivers.

If you were thinking of transformers I would consider something like a 10k:10k transformer something like a Jensen JT-11-1P.  Of course there are tons of transformers you could consider.

CC

 

[sodderboy]

Most things are built with op amps, but, when you are talking mixer design for MI, especially with live usage, transformers rule.  CMRR and isolation.  There are 3 good choices for a small box.  I think that you would be very pleased with these but I do not know where those vendors ship.  There are two good 600:600 transformers inside.  Another transformer available from the UK is OEP.  It is a good medium expense transformer, and their 1:4 would give some input boost to the level.  The top shelf would be Carnhill or Jensen at about $80 per input.  Big difference, right?  Anything built from the application schematics on the Jensen site will be primo at a price.
Another consideration is to omit the level controls in the mixer and adjust at the synth outputs- your setup is tight.  It really depends on your setup and how you play- on the synths or looking at a screen.
Mike

 

[herrmann]

Some news.
I've decided to put 5 stereo and 3 mono tracks. Stereo are debalancing with ina2134 (0dB), and mono with ina137 (-6dB).
Bus resistors 47k => classic API ACA-Booster card, wich have 3 simultaneous outputs. DOA are 2xGAR2520 and 2xJLM Hybrid.
I've found nice (but pricey) Sfernice dual and single pots.
Have to buy some caps, and it will be ready for test...