Adding a monitor out to a strange Neotek live board

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Hello hello,
I picked up this really interesting neotek from a friend, he bought it for studio use, realized it was a live board and didn't want to deal with some maintenance issues so sold it to me. Here's a link to the original Reverb listing my buddy purchased, the listing has better picks than I do...

https://reverb.com/item/14791340-neotek-24x8-mixing-console?utm_source=ios-app&utm_medium=share

24 ch 8 busses, It seems like it's the series 1 EQ, but the mic/line amp doesn't look the same to me. Input modules need to route to a buss to get to the master out.  It's all really straight forward, and almost suitable for my meager studio needs. The problem is there's no monitor out, and the master buss doesn't have solo logic . There is a mono headphone module that uses the solo buss and logic.  If you look at the first picture in the reverb listing theres a black and red knob towards the top, just to the right of some buss assign switches. This is just a pot on a panel wired to nothing.  I would like to make this into a monitor module that responds to the solo logic signal.

It would be very easy to grab the signal from the left right busses before the master modules, it would also be very easy to grab the signal at the out put jacks. I think I would like the monitor signal to be independent of the main left right faders.  My concerns are loading the master buss with wonky impedance stuff, and how to do the solo logic switch. I think that's about it.

I'm looking at these baxandall volume controls...
https://sound-au.com/project01.htm
http://www.ti.com/lit/ug/tidu034/tidu034.pdf

Can I just strap something like this to the master buss?
Any better ideas?

Thanks!

 

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Anyone?

 

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I swear Winston responded to this, seems like his reply straight up disappeared before I had a chance to say thanks.

 

[Winston OBoogie]

I swear Winston responded to this, seems like his reply straight up disappeared before I had a chance to say thanks.

Yes I did respond but don't see it now, strange?
I basically said that using the Baxendall active volume with buffers on the input should give no loading problem if you strapped it across your master bus.  Or any bus. 
For  monitor logic that flips between solo bus and master bus, it's probably easiest to use the same type of circuit topology as is currently used for the other stuff on your desk. 
I bet Neotek studio desks of the same era use very similar logic circuits as the live desks so maybe take a peek at the schematics.

 

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Thanks Winston, that’s helpful.  It’s funny, on the headphone module there is a manual toggle switch to select the solo or master buss but that’s it, there’s no logic switching anywhere (that I’ve seen)

However on the back of the buss assignment cards there is a solo logic buss. I haven’t taken a meter to it yet, I suppose I could just use a relay potentially.

 

[Winston OBoogie]

See what you find with a meter, it's possible they used standard modules which had provisions for logic on more elaborate desks.

If you can stretch to using another couple of chips, might be better to implement U1A and U1B from the T.I. scheme permanently across both the master and solo buses and do the switching after them.  That way, nothing is disturbed or changed on your master bus when you switch your monitor path to 'solo' mid that killer take! 

Baxendall active volume is def a good way to go for monitor though, good call

 

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Spent a little time poking around the solo logic buss, something is weird. The buss measures 13.5v regardless of wether a solo switch is activated or not. It seems like that voltage is just used for activating the LED on the solo assign switches. I'm going to need to spend a little time diagramming what's going on. Some pictures would probably help too.

 

[Winston OBoogie]

If there are no channels solo'ed does the headphone strip not get any signal from the desk?  In other words, is there no logic switching involved at all there?

 

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There is no logic switching, there is a manual toggle switch to select the solo buss or master buss (mono). In other words, to solo something you have to press solo on the channel, then toggle the switch on the headphone module.

I've attached a drawing of how the channel solo switches are wired, hopefully it makes sense. SL means solo logic, SS means solo signal. On the upper switch the pole and top throw are connected via PCB trace, the bottom throw is the SL Buss that carries 13.5v to light the indicator LED. On the bottom switch the prefader signal comes in on the pole and toggles between nothing and the SS buss.

The only thing I can think of is to hang a diode off the poles of the SL half of the switches and buss together all the cathodes, then send that signal to whatever logic/relay switching I can come up with. This would be doable, physically, but would be no fun at all.

 

[Winston OBoogie]

Blimey, so there's no solo/pfl detect bus at all.  Then it's basically a matter of how much work you want to put into it or think the desk warrants.  Simplest would be to put your monitor on the output of that headphone module switch and have to switch both the individual solo/pfl  source, and then the master/solo switch.  Just as it is now except with a monitor output.  Not great, but not a total disaster I suppose.
Or create a solo/pfl detect bus, which could be as simple or elaborate as you felt like dealing with.  I suppose if you make sure to keep dirty switching ground currents away from audio then you could do something a bit more rough and ready, rock 'n' roll slap-dash. 

 

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Then it's basically a matter of how much work you want to put into it or think the desk warrants.

That really is the crux of it. I have plenty more work to do cleaning it up before I put it into use so I’ve got time to think it over and experiment. I’ll make a post about my results. Thank you for you’re help thus far, I’ll definitely implement the switching after the buffer stages like you described.

 

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Just wanted to follow up. I ended up designing a little board using the texas instruments active volume circuit but with Winston's suggestion for buffering each input to the circuit, 2mix, solo, and 2 track return. Solo functionality is achieved with relays, it all works great and I even upgraded the headphone amp with an elliot sound headphone board. Thanks again for the help.