Fully balanced summing box, 16 channels, with level and pan

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bradb

Well-known member
Joined
May 5, 2005
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Location
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I'm looking at the NYD and Forssell schematics and thinking about the pan pots. Pan pots are a SUPER convenient thing on a summing box as it eliminates the need for stereo or 2 channel compressors, etc when you want something at other than R, L or center.

Its seems like youre gonna need a quad pot for a balanced pan. Am I right?

Any ideas of a place that would make a custom one?

thanks
 
Yes, if you wanted a minimum-loss panpot. You CAN do it with a dual pot if you don't mind a minimum loss of 12dB or greater. It would be a balanced implementation of the "Orban" pan circuit.

I've posted this before, but here's my panpot circuit (unbalanced version):
Image

It has a better panning law and a much more constant input impedance than the conventional type using a dual linear pot. The penalty is a minimum insertion loss of 6dB.
 
The loss youre talking about is basically a balanced to unbalanced conversion?

Yep this is the schematic I'm looking at and thinking about the pan... I thought I saw this similar schem at the Forsell site... I can't remember, i have a printout, but no text...

i'm thinking about doubling this circuit and using a quad pot so it would be like so

+ ---(circuit)--> L+ and R+

- ---(circuit)--> L- and R-

does that make sense?
 
I'm thinking of making each channel with level and pan pot on a modular pcb that can be plugged into a buss pcb via 4 pin molex connectors... stuffing this into a 2U rack case. Just need to see how viable it is to get these quad pots.
 
I think it would be great if a balanced panpot could be made with only a dual pot. You would have to make up anyway, so 6dB more gain won´t hurt...
 
This is what i'm thinking... quad pots for pan

http://show.imagehosting.us/show/877782/0/nouser_877/T0_-1_877782.jpg

Will someone talk me out of the expense of keeping it fully balanced?

Is there any benefit to keeping it balanced?

By keeping it balanced won't I run into slight problems with the tolerances on the quad pots, making panning problematic?
 
There is a way to do balanced panning with a dual pot..... series Rs in all the lines, and then connect one end of one track of the pot to pos phase L, the other end of the same track to pos phase R. Ditto the other track to neg phase left and right; then connect the wipers to ground!
(the outputs are from the ends of the tracks.)

Pls don't ask me for the values.... I seem to remember that the series R's were 2K2 or 4K7 for 20K lin pots, and you need to fit the mixing resistors to the ends of the pot tracks. The whole thing is a bit 'lossy' but I did use this system sucessfullyon some projects back in the 70s.

It works well except that the pots need to be good.... cheap pots have poor conductivity to the wiper and can cause crosstalk in this system; I used centre-tapped pots and had to put a spot of metallic paint on the tracks at the centre position to overcome this.
 
That's the balanced implementation of "Orban panpot" I was talking about earlier. Here's an example:

http://groupdiy.twin-x.com/albums/userpics/10031/balpanpot.jpg

EDIT: The "cw" and "ccw" markings on the pots are transposed. The clockwise end (cw) should be at the top. I'll fix the drawing.

OK, it's fixed now.
 
Hello everyone,

I appreciate all the help here, I'm about to begin my passive summer design, basically stealing other designs (NYDave's, Forsell's, etc) and rolling it into my own.. I have some stupid "mental speedbump" questions that I wanted to run by everyone first.

The main reason I'm building this is to integrate all of my outboard into my mixes. I plan on making stems in protools (16 outputs with an 002 Rack and an apogee 8 ch a/d) and sending them thru the many compressors and effects i have. I plan on having a patch bay that has the 002 rack and apogee outputs normalled to the summer's inputs. I then plan on patching compressors and effects in between.

I'm worried that there will be too many input impedance variables for the summer to have to deal with... like, the federal compressor gets patched in or the effectron gets taken out and all of a sudden the summer's settings all need to be changed. Is this a concern?

I worry that turning the level and pan pots will alter impedance drastically and disturb the summer.

I mean, are people patching things inline before their passive summers?

Has anyone else built a 16 channel passive summer with level and pan?

Finally, is there any benefit to putting transformers on the summer's inputs other than adding a coloration?

thanks and sorry for my "mental speed bumps"

by the way, here's the face plate I designed:

 
Can't really answer those questions without seeing the circuit you're planning to use. "Stealing other designs and rolling it into my own" tells us nothing useful.

One thing I CAN tell you is that everything affects everything else :wink:. That's why most professional mixers in the old days, when the mixing was passive, used constant-impedance devices despite the added cost. Without resorting to that option, the best we can do is try to design the mixing network to be relatively insensitive to different patching arrangements and control settings. The approach that presents itself most readily is to make the input impedance of the mixing network rather large compared to the source impedance of anything that's likely to be plugged into it.

If you want to add features like level and pan controls, it becomes so much easier with an unbalanced network. You can use input transformers if you must have a balanced or isolated input. I can draw up an example if you think you might like to take that route.
 
Hi NY Dave,

YES! I'd greatly appreciate it if you could draw up something for 16 inputs, level and pan. I had a circuit at home drawn out, but don't have it handy now to redraw on the computer and post.

im coming out of an 002 Rack which is 50Ohms and Apogee Rosetta which I've HEARD is 500 Ohms, but also will be coming out of various compressors by DBX, symetrix, a G1176, etc etc.

I was considering input transformers, but if I could get away without using them, i'd save several hundred dollars.

Looking forward to seeing your design.

Thank you!
 
Dave, I'd also be interested to see something of the unbalanced idea you mentioned. Was wondering about implementing pan and doubling your passive design (16ch in to stereo).

Slightly OT... maybe I missed, but haven't seen anything in the forum regarding multiple busses in a sum box. (In my case, stereo busses). Any links I missed, hints, suggestions, warnings would be great.
Thanks, Paul
 
I have been thinking of doing this in a console for a while, pretty much replacing the busses with passive busses and single or dual discrete amps for the makeup.. do you think it would be worth it?
 
Thank you Dave, you rule!

So input transformers are optional for this design? Would a 1:1 line level transformer be appropriate?
 
Instead of 20k could 10k or 50k be easily sub'd or should values be changed around them?

Also you spec the pot as a B part which is linear correct? just want to make sure..
 
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