Analog mixer "extention" box????

[Georgy]

Hello everyone.
I am new to this forum but read loads of the threads already looking for a solution to a "problem" i got. Nothing found, so i thought i'd post a quick question.

So....i have an old analog mixer and i need to make a box that would take individually the 6 channels that the mixer has and send them to my sound card...so far easy! But i also need to build 2 sub groups, 1 aux send and 1 master mix send on that box. I thought its easy to make without any ic's or so but maybe its not and i have to find some diagram so that the channels wont be messed! Can u please give me a hand with this one? Just some links would be useful as i couldnt find some myself. Thanx in advance!

 

[dintymoore]

Some of what you want might be covered in this box I made:
https://homerecording.com/bbs/showthread.php?t=295627

I'm not sure what you need though... is this right? :

6 mono channels in
1 mono channel out (that would be the master)
each channel has an aux send pot on it?

You don't want all the channels or the master out to be stereo? If so do you need panning knobs?

Making a box like this, you need to add resistors to stop one channel from affecting another. Because of those resistors, the whole thing brings the volume down, which is fine if you can crank it somewhere else, but if you can't you'll need to add something (mic pre, compressor etc) to make up the loss.

 

[Georgy]

Hello dintymoore! Thanx for the quick reply and sorry for the confusion.

Here we go. i need a box with:

6 inputs (no need for any faders-pan options or so apart from 1 aux sent)

6 outputs (working as direct outs of the 6 ins)

1 stereo out for the main mix of those channels (the mix will be done from the console before it gets to that box)

1 stereo aux sent

2 stereo sub-groups (group 1/2 and 2/3)

Thats all i need it to do and i could do it in a simple way but the problem would be that i wouldnt be able to use the direct outs at the same time with sub-groups for instance! So i think i need some kind of circuit to not mess all the signals around right?

Hope i made things more clear to you. Thanx again n hope to another quick reply!

Take care.
George

 

[Georgy]

Oh...also i have to mention that the signal that i will take from the old mixer to send to the box, will be "stereo" in all 6 mono channels as i'm taking it from the pan pot and feeding it into the box. in that way i dont have to make any panning circuit in the box or any other control coz i can use the ones on the mixer. I just needed to have some extra options coz the mixer doesnt have direct outs and sub-groups which are really useful especially in live performances were i will be using it.

Thanx again!

 

[mshilarious]

It can be done with a passive mixing network(s), but by the time you are done buying all of those switches and faders and wiring up all of the resistors and then figuring out where you are getting the necessary makeup gain (which will be large if you want to minimize the change in network loss as you switch channels in and out of the subgroups), you'll wish you had just found an old 16 channel Mackie.

 

[Georgy]

Probably you are right mshilarious but i would like to give it a try as am really short on money right now.

Plus, i said i dont need any faders on the box and i already have an old 6 channel mixer (dynamix If you've ever heard it) from which i need to get 6 direct outs and just the option to send them to 2 stereo sub groups (even without any faders on the sub groups) and an aux send (without return) plus a final main mix out. Thats all n i think (maybe am wrong though) it should be easy. I have figured out how to get the direct out from the mixer and also still use the mixers faders on each one but i dont know how to drive them into sub-groups, aux send, main mix!

If you could give me some info on that i would be gratefull!

Thanx for the reply!!
George

 

[mshilarious]

OK, the direct outs, easy, no problem, find the postfader pre-panpot out on each channel and tap it. This would be right before the panpot but after the buffering IC for the fader (if there is one).

Your tap point for your external main mix and subgroups is post-panpot. This would be right before the mix bus, there should be a resistor on each channel that feeds the mix bus. You need to tap before that resistor. Each mix needs its own mix resistor (10K is typically used).

Since you have six channels going into one stereo main mix and two stereo subgroups, that's 64 mix resistors you will have to wire up. If you want each channel to have switches into each mix, now you need 18 DPST switches.

An important note on those switches--if you want them to be silent when switched, you can't use simple electromechanical switches, you really need to do FET switching or use a switcher IC. So if you plan on needing to reassign during a performance, you have to consider that.

OK, back to each mix bus. The summing network loss is 20*log(1/n) where n is the number of channels on the bus. So a -16dB loss for six channels (more for the main mix, as the subgroups have to sum back into that). But that will change as you switch channels in and out (down to 6dB for two channels), so the usual approach in a passive network is to add a 100 ohm shunt resistor. That yields just about a -40dB loss for any network size.

Thus, you have to be able to compensate for either a small but variable network loss, or a fixed but large network loss. Unless your mixer can drive the signal hot enough to where that loss is acceptable, you will need makeup gain. That's another six channels of amplifiers. Possibly your power amps have enough range in input sensitivity to compensate.

For the aux sends, basically the same thing as above except you use a pot instead of a switch, and you'd tap the channel before the panpot and pre- or post-fader depending on your preference.

That's an awful lot of trouble for a six input channel mixer . . . I would save up for the Mackie.

 

[dintymoore]

... by the time you are done buying all of those switches and faders and wiring up all of the resistors and then figuring out where you are getting the necessary makeup gain (which will be large if you want to minimize the change in network loss as you switch channels in and out of the subgroups), you'll wish you had just found an old 16 channel Mackie.

I would say that you are saying that without having heard what it can sound like in the real world.

An old Mackie? I spent a lot of time on a $200K Neve 8080 and I honestly think my $100 mixer with my $3000 worth of Summit compressors sounds as good. It might even work with an Art VLA. I'm getting rid of $4,500 worth of Yamaha mixers because my new stuff sounds great, and those Yamahas sound a bit better than "an old 16 channel Mackie".

 

[dintymoore]

... An important note on those switches--if you want them to be silent when switched, you can't use simple electromechanical switches, you really need to do FET switching or use a switcher IC. So if you plan on needing to reassign during a performance, you have to consider that.
...

I used dumb ol' switches from Radio Shack and Ace Hardware and they are completely silent with no "FET switching or use a switcher IC".

The FX in/out switches on my new mixer are 100% silent. The switches on my speaker box and my 8 switch box are silent too. I have no idea why and frankly I don't care because all I really care about is songs!

I hear what you're saying mshilarious but I think it's a case of what theoretically will happen versus what does in the real world. I've been using switches like that for several years through a few CD releases and there's no noise, none, dead silent.

 

[Georgy]

Ok...here we go.

mshilarious once more thank you for the help. your explanation helped me understand how much trouble it could be and i dont doubt what u say but still i think that what i need this thing to do can be done in an easier way.

Am not lazy or just waiting for u people to tell me everything i have to know or do, so i constantly read articles n download pdf files about that subject.

So, through reading i understood that i probably need to build 6 ACN (virtual ground (or earth)" summing buses). 2 for group 1/2, 2 for group 3/4 and finally 2 for the main mix out. I saw the schematics in this pdf http://www.forsselltech.com/downloads/design_discussions/summing_buss.pdf and it seems that it isnt that hard to make as the needs i have are not so exclusive.

I came up with a design that goes like this (correct me please if am wrong) :

1. 6 channels go from pan-pot of mixer to the box i wanna make
2. then they go to mute switches (sending the signal to the ground should do the job, right?)
3. after that they go to 6 3pdt toggle switches from where i chose to drive them to the acn circuits group 1/2 or group 2/3 (dont mind to be silent as am not going to change it while performing)
4. from sub-groups they go to the main mix acn circuits and i'm done.
5. the direct outs could also be made from the toggle switch by taping the centre pins where the signal of each channel goes already (right?). and i can also put some led indicators to show me in which group the channel is driven (thats easy. already solved that)
6. as it comes for the aux i respect ur perspective n i would save some time not making it.

Also, i dont need the main mix out to get back to the mixer so that makes things easier i guess (right?). I only need it to drive it to an amp or another mixer as this one will be mainly used for percussion items.

Bottom line is that i just need some extra functions in that mixer (6 direct outs, 2 stereo subgroups) that i am missing right now and i dont look for much more trouble than it already is! lol!! Its just that i dont have enough space in the mixer to do so and i need to make an extra box doing those things. I will still be able to use the faders etc of each channel but in that way i will also be able to do some extra combinations for instance recording with my delta 1010 in my DAW!

I hope i dont get advantage of ur kindness n u can both (dintymoore that reply goes to u as well) help me out a bit more!

Thanx a lot again...really!

George

 

[mshilarious]

You would be incorrect; I have built and sold passive summing mixers.

A box of resistors doesn't sound like anything, but a bunch of switches and pots in a separate box is not a practical way to mix a show, especially when the switches cause audible pops. And OP didn't spec subgroup faders, so I'm not sure what the purpose is there.

There is also the time and cost in box fab and the cost in cabling. Think about this cabling--it needs to come from multiple points on each channel to operate the way OP wants it--prefader, postfader, post panpot. That's eighteen cables, thirty-six jacks, plus the desired seven outputs. This is not a trivial project.

I also know exactly what a Mackie sounds like as I use one frequently. They are cheap and they work. Sure, I'd rather mix on a Midas or even my old heavily modded A&H, but that's mainly because the Mackie EQ bands are useless.

Again, there will be the need for makeup gain. This is an inescapable fact of passive summing. You have some $3000 compressors, that's why you like the sound, not because of the resistors in the box. Whatever is selected for makeup gain will add to the cost of the project.

If I needed a summing network now, I would build an active summing network, but that's a much harder project than resistors in a box that feed an external box that provides the gain--mic preamp, compressor, whatever. I happen to have a little PCB that can be used as an inverting amplifier, but I'm guessing OP does not.

Bottom line, if you're mixing a show you need to be able to mix the show. The crowd cares less about the sound qualities of Mackie vs. Yamaha then if there is sound at all. I've done shows with jerry-rigged submixers, and it's not fun. I hope to never have to do that again.

 

[Georgy]

mshilarious please read my reply above coz i think u might have misunderstood what i need to build.

 

[mshilarious]

So, through reading i understood that i probably need to build 6 ACN (virtual ground (or earth)" summing buses). 2 for group 1/2, 2 for group 3/4 and finally 2 for the main mix out. I saw the schematics in this pdf http://www.forsselltech.com/downloads/design_discussions/summing_buss.pdf and it seems that it isnt that hard to make as the needs i have are not so exclusive.

Forssell is good info, but don't neglect the need for a good power supply too. I would probably not tap the mixer's power supply without verifying it can handle the added load.

1. 6 channels go from pan-pot of mixer to the box i wanna make
2. then they go to mute switches (sending the signal to the ground should do the job, right?)

You can ground the panpot if the output isn't buffered and the panpot is fed through some resistance. Otherwise, lift the input and ground the mix resistor.

3. after that they go to 6 3pdt toggle switches from where i chose to drive them to the acn circuits group 1/2 or group 2/3 (dont mind to be silent as am not going to change it while performing)

Why 3P? You only have left and right signals. I would get an on-off-on DPDT switch then, if you never want a channel to go to both subgroups.

Bottom line is that i just need some extra functions in that mixer (6 direct outs, 2 stereo subgroups) that i am missing right now and i dont look for much more trouble than it already is! lol!!

As I said direct outs are easy, tap them while the signal is still mono. You don't need an external box for that at all.

I don't follow what your subgroups are doing, but based on what you've said, keep the main mix in the box and your external box can be limited to the two subgroups. That would be manageable.

 

[mshilarious]

I hear what you're saying mshilarious but I think it's a case of what theoretically will happen versus what does in the real world. I've been using switches like that for several years through a few CD releases and there's no noise, none, dead silent.

Dude, I assure you I am very, very real world. When stuff is out there in the real world that you've sold and people send you emails when your stuff doesn't work and they are not happy about it, I think that is pretty real world.

There are ways to minimize pops without resorting to active switching, basically you need to make sure there is never any DC potential across the switch. So you add 10M resistors, make sure that all inputs and outputs are decoupled, and so on. Building a passive network to put between two professional bits of gear designed to interface with the outside world is not the same thing as tapping points inside a mixer that manufacturer never intended to see the outside world, and then strapping your own DIY active network on that bus.

 

[Georgy]

I would probably not tap the mixer's power supply without verifying it can handle the added load

I will check it out though i think it would be fine

Why 3P? You only have left and right signals. I would get an on-off-on DPDT switch then, if you never want a channel to go to both subgroups

I need the 3p to put a double led indicator on. I have made a "millenium bypass, sort of" led indicator that switches with a 2n3906. I would use a 3p3d to send the signal to Gp1 or Gp2 or none, but here in Greece is really hard to find so i will just keep the option of Gp1 or Gp2.

I don't follow what your subgroups are doing, but based on what you've said, keep the main mix in the box and your external box can be limited to the two subgroups. That would be manageable.

Subgroups should send the channels i want to another mixer or pa or amp for using it live. If it isnt a problem that i would get the direct channels from the mixer then i dont need to have a main mix at the box. but if there is some sort of overload or whatever (dont know what) then i need to make that other main mix.

 

[dintymoore]

Georgy I'd study the stuff on this site and on Gearslutz. I'd do searches for "passive mixer".

I'm working right now and I can't take out and hour+ to design your mixer, scan it and post it here. But much of what you're asking is much like the passive box I made.

If this is for live, and that wasn't brought up originally, I'd forget it and buy any POS like a Behringer (or Mackie! ) What I'm talking about is for home recording. After all, that's what this site is called and you started out talking about your soundcard, so I'm not talking about doing gigs with a mixer like this.

Here's the basic principle of all this passive mixer stuff:

When you "Y" two signals together, put a resistor on each signal right before they join together, otherwise the current from one signal, when it gets louder or softer, will affect the other. It's a lot like putting valves on water pipes.

The higher the resistor value you use, the more make up gain you'll need. The lower the resistor value, the more you'll start to see interaction between the channels. 10k is a common one to use, and you can find things like stereo ganged 10k pots on eBay, 5k not so easy.

 

[Georgy]

dintymoore am sorry for any kind of confusion i caused.

Mainly my project is for a home recording studio and thats what i wanna use it for.

The things i said about live performances were to explain to mshilarious what i need the sub-groups for and what they would do.

i wanna be flexible in my projects and because i already plan to buy an A/H R16 for the studio i thought i could use this old one to drive some percussion and weird sounds (bottles, wood, metals) through it and then to the A/H. Thats why i needed the subgroups!

And i wanna have the option to do even more by connecting both the firewire interface of the A/H and the Delta 1010 on my pc and record live (not track by track) in my DAW. thats why i needed the direct outs.

i hope i made it more clear now. Of course i know what this forum is about and thats why i asked for help here!

Thanx for the info n for gods ache i never meant to put u in such trouble of designing my mixer. After all, though the troubles and the time to spend, its a true plesure to make ur own stuff and actually with low cost and matching ur needs. U cant buy that sort of pleasure.

I will search more and trust me i already did for 20 constant hours!
Thanx again.

Any other comments are very welcome of course and i promise i will let u know what i do with pics as well!

Take care

 

[mshilarious]

dinty: crosstalk between busses is a function of the source impedance of the bus, which if sufficiently low prevents the problem. So it's a question of whether or not the fader/panpot output is buffered.

As for summing loss, that's actually the same just about irrespective of summing resistor value. It's when you take into account the load (shunt) resistance on the network that higher values mean more losses. For example, you could build a high impedance makeup gain stage and the resistor value would matter not at all (except for noise).

Of course if you are designing your own following amp, you would use an inverting topology and not worry about crosstalk at all. So if OP is talking about building his own active network, most of those troubles go away. This is why just about every mixer on the planet uses active summing.

But since people generally feed their passive summing networks into existing devices with somewhere between 1K and 10K input impedance, then the summing resistor value matters a bit.

I'll tell you though, if you pick up an older model A&H, say the MixWiz v2, that's a pretty easy console to mod and get really nice results. It would be a much easier project.

 

[Georgy]

mshilarious...i have decided to build an active and not passive summing network coz of the reasons u mentioned already and coz generally to my ears it sounds as a much better solution from what i have read since now.

Only thing is that i have a bit of a "problem" finding out which is the best op-amp to use and the pinout wiring (i mean i dont know where do i connect the offset nulls and the Decompensation and Distortion Neutralization pins). i tend to believe the best for me is AD797 coz it operates at 15V -15V that my mixer as well does.

Also i find it a bit difficult to spot the right place to get the stereo signal of a mono channel (meaning after it gets to pan-pot) to drive it to the box i wanna make. Its an old mixer (1980 i think) and though it seems to have a very simple circuit it doesnt appeal to what i read now. For instance to find the pre and post stereo outs of a channel.

If u got more patience feed me with some info or some links or what u think it might help me know more and finally make it.

Thanx a lot! i appreciate it!