Cooper 106/108 mix bus summing not working?

[jawharp]

Hey guys,

 

I have 2 coopers one a 106+1 and one a 108+1.  I'm trying to connect them via the db9 connector on the side of the mixer that in the manual is referred to as the mix bus connector.  The manual says that multiple mixers can be daisy chained via this connector so you can use more channels.  The wiring appears to be the same on the 106 and 108, there's only one diagram given in the manual for both.  

 

I wired 2 DB9 connectors with pins 1-5 identical on both sides, and the cable's shield wired to the connector shell as the diagram suggests.

 

When i connect the cable to each mixer, I get a loud hum and very low/distored signal on the mix bus.  I tried floating pins 1-3 cause I don't care about PFL functionality, just joining the mix busses and was greeted by what i can only describe as the scream you hear when you open a portal to hell.

 

I'm trying to figure out if I'm doing something wrong, or if my mixer(s) have some kind of problem.  

 

Has anyone had any experience with joining two Cooper's together via their mix buses?  I'm at a loss for options, I've never dealt with this connector on the mixer before.  

 

Any experiences or wiring advise would be greatly appreciated. 

 

Thanks

[The Documentary Sound Guy]

I don't own a Cooper of any variety, but if you wired pin-to-pin, you tied the inputs together, and the scream you heard is feedback.

Note that this these pins are *inputs* to the busses, which means you need to connect the other end to the corresponding outputs on the slave mixer.

[jawharp]

Hey thanks for your reply. 

 

I was operating under the assumption that this connector links the mix busses before the mix module so they are neither an input or an output.  By connecting the busses they would be sharing the same mix bus/aux bus/pfl signal/ and pfl voltage as if they were one mixer.

 

Do you think that's not the case?  I've never dealt with this connector before and was hoping i wouldnt have to lose a channel by running the line output of one mixer into a channel of the other mixer.

 

Thanks for your help

[The Documentary Sound Guy]

You couldn't daisy-chain them if they were meant to connect them all pin-to-pin, and the diagram clearly refers to the bus pins as "this input".

 

This is also the way it works on Sound Devices models, so I guess I just expect it to work that way.  Whenever I've connected mixers via a bus, I've always connected the main outputs to the input bus on the master mixer.

To answer your question more directly, you shouldn't lose any channels:

The slave mixer's outputs get routed to the appropriate inputs on the DB9 connector.  You wouldn't want to use the slave mixer's outputs for anything else, because it's only half a mix, and you don't lose an input, because that's what the DB9 input is for.

You do need a custom cable that breaks out the DB9 into appropriate input connectors though (presumably XLR-F, if you are plugging it into the main output).

[jawharp]

Aaaaah, okay thanks I'll give it a try.  Thanks for your help!

[jawharp]

That worked!

 

I'll worry about pfl later, but for now this works for my needs.

 

Thanks!

[The Documentary Sound Guy]

Glad my educated guess was worth it.

Here's some more guesses:

For PFL, I'd try plugging the slave mixer's headphone output into PFL signal, and then ... I don't know, maybe try tying PFL VC together on all of them?  If I were to guess, VC is probably the signal that switches the headphone output away from the mix so it can receive the PFL bus, which would need to be propagated to every mixer in the chain simultaneously.  So, if there's any pin that I think would need to be tied together across all of them, it would be that one.

Come to think of it, the headphone output from the slave mixer will be stereo running to a mono input, so that probably isn't right.  The headphone output would be the same signal on both sides whenever PFL is active, so I suppose you could just sum to mono in the cable ... but I don't know what the official advice would be here.

[The Documentary Sound Guy]

After a bit more Googling:  Vc probably stands for Control Voltage, which would be used to trigger a transistor.  So I think tying the PFL VC together on all mixers is the right approach.  When no PFLs are active on any mixer, VC will be low (zero), and the transistors that control what goes to the headphone bus will be in their ordinary state of sending the mix to the headphones.  When a PFL is triggered on any mixer, that will make the voltage high (non-zero), which will tell the transistor in every mixer to switch to the PFL bus for the headphone source.

As a circuit, that makes sense to me.  Let me know if you try it and tell me if my guess is right!

[jawharp]

Quick update, 

 

So while that did work, I was getting a lot of hiss and way too loud signal at first.  I'm more of a hobbyist with this stuff and was at a loss so I took some cooper schematics I have and fed them into chatgpt with a description of the problem. 

 

It pooped out an answer saying that the mix busses are much lower than line level and the mix output was being loaded incorrectly, hence the hiss and way too hot signal.  Also worth noting the hiss was independent of output level from the source mixer.  It recommended I put a 47k series resistor on the hot pin and it worked perfectly.  Tone coming in perfect on the second mixer, and all is well.

 

The final wiring was:

 

XLR Pin 1 to DB9 Shell

XLR Pin 2 to 47k resistor to DB9 Mix Bus L pin

XLR Pin  to XLR Pin 1

[The Documentary Sound Guy]

Makes sense.  Is it possible there's a mix bus output somewhere on the mixer that we've missed?  Perhaps pins 6-9 on the DB9 are outputs?

[Patrick Farrell]

The manual is quite thorough. It specifies the mix bus inputs are current driven which is why you need the resistors. Here's the relevant sections.

[Philip Perkins]

Be glad you. can figure a way to do this sort of modular add-on.  I find it very useful and digital mixers have left this sort of thing behind.  (Miss Sound Devices "Link i/o")

 

[jawharp]

Yeah, this solved my problem of adding more tracks really nicely.  Though my real dream is to get 2 106's, saw one end off, and join them into a single mega cooper.