DIY Summing Mixer Part 2

[Saudade]

quote from other thread:
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"Joe Malone (of JLM audio) suggested you can mix 'outside the box' simply by jigging up a resistor bank from the 8 or so analog outputs and mixing them to the analog input. Here is a quote:
quote:
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Neve bussing is very easy to do. But even easier below to get you started.
How to make the simplest 16channel balanced passive buss mixer with 32 x 1k or 2.2k resistors.
1. Mix all DAW outputs as left & right mixes (Drums, Bass & keys ,etc)
2. Connect every DAW output XLR pin 2(Jack tip)& XLR pin 3 (Jack ring)to one end of a 1k or 2k resistor.
3. Join other end of all Left output pin 2’s resistors together and the same for all left pin 3. Do the same for the right. This now gives you a balanced left and right mix which is down 18dB. Using Digi001 for an example with it’s +18dBM max output. You will have a balanced mix at about 0dBM. Run this into your Tape or DAT or back into your DAW. (Digi001 push up the soft faders in Pro tools about 18dB so those two inputs clip at about 0dBM). This gives you a place at the DAW outputs to insert analog gear as well as a balanced passive mix which is only used in the most expensive mixers. All fading & panning is done still inside the DAW so it is remembered next time you return to that song. Doing the Neve bussing will give you passive Fading and Panning which will improve things again but you will have to manually remember the setting like the good old days. "

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Don't quite understand the part about doing the panning inside the DAW.......

I can pan inside my DAW, but this passive summing mixer will ALWAYS assign each input into the centre position instead of hard left or right. I am under the impression that no matter how I pan, every input will still end up in the “Center” position when summed into the L/R outputs! This is because each input's tip and ring (pins 2 & 3) are always connected to both the left and right busses. Why else would a Folcrom have switches that lets you choose L, R, C or none?

Anyone knows how I can work around the problem? I was thinking of buying a Folcrom but it is still too expensive for me since I am just a hobbyist in the quest of “good sound” for my home setup. I figured considering the low cost, I should try building one first and if it works well enough I can make do with it until I have money to splash on a Folcrom......

The other thing is, in the above DIY summing mixer, would I need to always connect 16 inputs? Would disconnecting any inputs or sending no signal to some inputs affect the impedance/loading or stuff? Sorry I am an electronics idiot......

One last thing, which online shop would you guys recommend to get the parts (resistors etc) and what brands are good?

Thanks a million......

[frist44]

I built my patchbay version for pretty cheap. go to mouser.com or digikey.com or something like that to get supplies. Use metal film resistors. they last longer and aren't that much more expensive. when i built mine, I just had every odd input go to the left master channel and every even input to go the right. It's like setting up 8 stereo channels sends. I chose to sends kick and snare to the first stereo out, OH, toms, and rooms mics go to 2, guitars to 3, clean guitars to 4, solos and leads go to 5, vox to 6, effects to 7, and 8 was whatever else wasn't included. If you think of them as stereo groups, the group will control the panning. my OH's for instance are panned hard left and right, because they're sent to a group channel, the group channel will control the amount of pan in the left and right and will send the appropriate amount of signal to each of the left and right inputs on the summing. don't think about it as kick on channel 1, and snare on channel 2, and OH on 3 and 4.

It would be easier to create a template in your software and just have 8 stereo group channels setup up supplying the 16 inputs on your box. then in each channel settings, send them to whatever group you would like.

technically, i think not using some inputs would change the inpedance the summing sees, but there are many more electronics experts on this board than me. i think that's part of the reason why some other boxes are more expensive, they may compensate for the input variation.

hope this helps,
brandon

[jeronimo]

But using stereo pairs... you cannot use analog compression, etc... on stuff like kick, snare... or anything that goes on center...

[Helsing]

Hi,

This is a response for FRIST44 who wrote:

"The other thing is, in the above DIY summing mixer, would I need to always connect 16 inputs? Would disconnecting any inputs or sending no signal to some inputs affect the impedance/loading or stuff? Sorry I am an electronics idiot......"

You're definitely not an idot. This is critical to the design. The sort of summing design you are discussing likes to see a low impedance source. If you have nothing hooked up to an input the impedance is infinite=not good. You want something low. I build the Folcrom with Ulysses and in our design it is important for unused channels to be disengaged for this very reason. Thats one of the main reasons we have all those switches.

If you are doing a 1/4" patchbay design you can make dummy plugs out of TRS connectors to solve this issue. No wire, just the connector. Just solder the tip to the sleeve inside the connector. This yields a 0 ohm source which the overall summing network agrees with just fine. You can also connect the tip and sleeve to the shell which ties the signal to ground. Experiment with both and see if you hear a difference.

Jon Erickson
www.helsingaudio.com

[Saudade]

Thanks dudes for the concise replies, now I understand how to go about it much better. And that "dummy plug" trick, damn cool workaround, especially coming from someone who helped built the Folcrom and sells it too!

Now I am getting greedy, and thinking of building a 24 channel version instead.....still workable? Any potential pitfalls?

[Saudade]

forgot to ask, referring to the building instructions above, only pin 2 and 3 are mentioned. What about pin 1? I mean they were talking about balanced TRS connectors here right? What do I do with the remaining pin? Connect to the chassis for grounding? Just a wild guess....I know nuts about electronics....

I was thinking of building this whole thing into a cardboard box or something temporary. Don't think I can find any metal chassis that can fit TRS jacks here.....so do I need grounding of some sort? I have to ensure the three pins are never connected in any way am I right?

[Fletcher]

If you're even marginally handy with a power drill with a 3/8" bit... Middle Atlantic products builds a 19" Rack mountable project box in pretty much all heights... personally... it's more of a pain in the ass than I would want to go through, especially with the Folcrum on the market... but if you feel like going for it, the local "electronics supply" [or possibly the local music store] will either have these boxes in stock, or can order them for you.

Anyway you slice it... I wouldn't suggest you use a cardboard box for a myriad of reasons.

Best of luck.

[frist44]

Helsing,
very useful information, thanks for the response. the idea of the impedance changing without some inputs connected made sense, but I guess I just didn't take it to the next level and figure out what to do about it. the little box I made yielded some pretty good results compared with the computer mix. If i didn't have a console and use outboard, i would definitely be wiring up a box for myself.

the folcrum has sparked a lot of good discussion and ideas in terms of both design and products. thanks for the introduction to a middle of the road alternative to in the box mixing.

regards,
brandon

[Saudade]

Thanks everyone for sharing so much info unselfishly, this is one of the best forums I have seen! Problem is, I know too little to be of help to anyone........

sorry if I come across as a kid bugging you for candy....but I was hoping someone could answer my earlier question:

"referring to the building instructions above, only pin 2 and 3 are mentioned. What about pin 1? I mean they were talking about balanced TRS connectors here right? What do I do with the remaining pin? Connect to the chassis for grounding?"

[Helsing]

If you are referring to the 1/4" "dummy plug" I wrote about you dont need to worry about the ground. If you want to connect pin 2 and 3 to ground, like the second method I suggested, just tie pins 2 and 3 to the sleeve off the jack which mates the shell and finds its way to ground through the patchbay. Use as good a patchbay as possible and try and get some steel shielding around it. Fletcher's suggestion to use a rackmount box and drill holes for individual trs jacks is a far better route than the patchbay in my opinion.

Jon Erickson
www.helsingaudio.com

[JLM Audio]

Hi All
Don't get confused by the Mackie example. It just showed that externally mixing the separate D/A outputs improved the sound and gave a convenient way of A/Bing this. And then prompted the question of how good would this sound if the Mackie wasn't inline.
All pin 1 XLR or Screen connections on TRS are used for screening and can be connected together but you will find they are probably connected internally in the D/A so you will only need one screen connection to link the ground of the A/D to the what ever the passive mixer left and right are plugging into. Resistors should be 1% tolerance types to keep the balancing accurate. Also the passive mixer can be used with as little as 2 (2L & 2R) summing points to about 32 (32L & 32R). If you only sum 2 outputs the signal will drop 6dB and will drop 6dB for every doubling of outputs so. So up to about 8L & 8R running at pro output level can be summed and run back into to A/D inputs switched to -10 non pro or DAT input etc without any makeup amp. Or if you want to mix 32L & 32R use two 1272 modules and you basically have a Neve mixing buss but running fully balanced. Only inputs to the summing mixer that are plugged into outputs of equipment or shorted to ground count in the summing loss.

2 outputs summed = 6dB loss
4 outputs summed = 12dB loss
8 outputs summed = 18dB loss
16 outputs summed = 24dB loss
32 outputs summed = 30dB loss

Hope this clears some things up. I have only just become aware of this forum so I have not read the whole thread. So if I missed any questions feel free to ask:-)

[Wiggy Neve Slut]

Welcome aboard Joe...!!!

Your inputs allways welcome around here
Wiggy

[hammer]

Welcome Joe!

[doug_hti]

Any suggestions of brand/model of resitors to buy?

what is the real world sonic result of running a 1k or 2k resistor for 16channel summer(8 pairs).

[JLM Audio]

Hi All

[JLM Audio]

Hi All
The sonic impact of different type of resistors here is minimal, But what is important is tolerance or match of the resistors to keep the balancing as accurate as possible for noise cancellation. The resistor summer is as pure path as you get compared to going through a full mixing console. The thing you do not want to do is overload equipment that can only run 2k loads with 1k load resistors as the mixer will still be running pure but your equipment will put out much higher THD which will be a step backwards rather than forwards in performance. For those who are not sure if all there gear can run 1k loads use 2k resistors as almost all opamp output equipment can run this load with low THD.

[spectrasound]

Joe,

What are your thoughts on an output stage for this passive resistor summing circuit?

Do you like the Neve 1073 output stage with transformer ? Would this add too much "color" ? Love that big class-A sound!

Thanks!

Jim McGee

[bassman]

Just saw this thread and had to chime in as that is exactly what I had a friend of mine build: 24ch passive summer with four channels that are switchable like the folcrum. We used DB 25 connectors and put it inside the volume/monitor box he made for me some years back. I use the SSL AlphaLink so the combo works real nice. The mono channels are up a few DB which helps on the LV/kick/bass channels.

I've just been doing some mixing through it over the past few weeks, from English Folk to Neo-goth metal, and have loved the results. I am making up gain with a Sytek using the Burr Brown opamps. Nothing like bringing a little amp clipping back into the mix!!! METAL.

Something about the punch I'm now getting that does not happen ITB. Also, imaging is....... different. I like it. I could go on but it would be rambling... Suffice to say, I can accurately A/B the ITB and Summer mixes, level matched, and notice subtle musical things that are different, hard to describe well, but..... I like it! Have yet to try the other pre's lying around API, Avalon.... hmmmmmm

-bassman

[msquared]

This sounds like a nifty project. Does anyone have a schematic anywhere to illustrate the overall idea?