Summing Mixer with Custom Resistor Values for every channel

Hi there, I have been experimenting quite a bit with passive summing in the past, but I am no technician and I get easily lost/confused when dealing with formulas.

Having just got an 8 extra outs from my adat, I decided to expand my DIY passive mixer to 16 ins. What happened next was that the DAC (Aphex A141) had a much louder signal than the Motu 828, when both drive the passive mixer's 10K input resistors.

Now I came up with the decision to put 3K resistors on the 828 dedicated channels instead the 10K, is that "healthy" to have a summing network with different load resistors?

The volumes almost match now, but I wonder hot to calculat the right shunt value for the outputs that will load my mic preamp. Obviously the same value I used before (202 ohm I think it was) it's too small.

I don't know about passive mixers... but my interface (Echo Layla) has an option for the outputs to be +4 dB or -10 dB. Software selectable. Might be worth checking if the Aphex & Motu have this option set differently.

No problem there.

Calculation is the same, just take the parallel result of the first resistor value and the parallel result of the second resistor value, then calculate those two results in parallel, and feed that into the shunt resistor calc.

See http://johnhardyco.com/pdf/990-2007.pdf

Look to Fig. 6 at the bottom of page 5. The first paragraph addressing that figure discusses using different summing resistor values (referred to as Rin by the article)

For 8 channels (4 per channel), I used 2K4's as my summing resistors

skipwave said:

No problem there.

Calculation is the same, just take the parallel result of the first resistor value and the parallel result of the second resistor value, then calculate those two results in parallel, and feed that into the shunt resistor calc.

See http://johnhardyco.com/pdf/990-2007.pdf

Look to Fig. 6 at the bottom of page 5. The first paragraph addressing that figure discusses using different summing resistor values (referred to as Rin by the article)

Click to expand...

Great! So if I have 3K and 10K, I use a value of 13K for the calc (13K/n.channels)*200/(13K/n.channels)-200 to get a 200 output impedance I guess?

Also I noticed in that figure you mention a capacitor for stray resistance. Someone explain me the use of that? I see happening a noticeable high freq rolloff when routing things in the summing mixer, so I wonder if that has anything to do with wrong impedances or is just my bad internal wiring and too many solder connections, or any other idea?

@DMP: unfortunately my MOTU has no way of selecting between +4 and -10, while the Aphex has a -12 db attenuation trim on each output, but that won't be enought to set the two level equal using equal values resistors.

Thanks for your help by now!

Just wanted to bump this and ask an on topic question...So I'm interested in doing a similar thing, that is, a summing box.  I'm just a bit confused about the impedances though because I'm very new.  Every schematic I see has a different resistor in line.  Forssell had 5k, NYD's had a formula, someone else posted a schematic I just looked at JLM audio i believe that said 2k resistors for each channel...  Now what do I want to choose?  I get the formula and all, but what is the ideal input impedance and then what should I strive to make the output, or is it called load, impedance so that it matches the mic pres that I plan to add makeup gain with?  Maybe this is more an issue with my lack of fully understanding impedance matching...any help would be great!

well, I am inexperienced as well but for what I figured out building 3 of those is that  high res values are not realy needed if you drive the channels from your DAC (audio card). I recall reading that the input resistors should be at least x10 the output impedance of your DAC, so I am sure that 5k1 for unbalanced channels is going to be more than enough. 

Suggestion: Build a smaller summing network, like 4 unbalanced channels, and try out how this works with the gear you have. If you notice you need too much gain from your preamp - assuming you ARE using a mic preamp to drive the suming network outs, (and therefore increase the noise volume too much) you might want to lower the resistor value. On the opposite side, if you notice you get too much gain even with the minimum gain settings of your pre, you might want to make those resistors larger.

I wish there would be a perfect universal receipt but I am noticing trying things out is the only way to start to understand this (to me) whole new world and how to adapt it to your setup. Moreover, trying and experimenting is fun, unless you are rushing to finish your last track and you feel you can't wait...in this case, ehm, just be patient, in one year you'll find yourself knowing so much more already!

good point.  I'll try that.  I suppose it's not THAT difficult to just change resistors through trial and error!  Thanks