Passive summing box -Mono Channels?

I'm going to build a passive summing box, with 8 stereo channels, and 8 mono (so 24 channels total.)

I have two questions:

1.) What value resistor is optimal? I'm looking to do something similar to the Forssel Tech thing. I was thinking 10k, originally, but I see 5k on their schematic, and some people have suggest 1k?

2.) Do I need to use a different value on the mono channels? Obviously, I want to avoid crosstalk as much as possible (from the stereo channels,) but I also want the levels to be in the same range as the stereo inputs. The mono channels would be for things like lead vocal, kick drum, bass guitar, etc, that the user might want to run through an outboard eq or compressor, but not have to use two channels, etc...

Thanks for the help, I've learned a lot on this board, but couldn't find (or understand) the answer for this little thing...

How are you going to get mono channels to sit centered in the mix? Assuming you are building a stereo out, you'll have to connect the mono channel to both output channels. Normally people are building these with every input going to either L or R. Not both. So for a mono (center panned) signal, you'd send it to two inputs, both L and R, and it would appear centered in the mix. Anyway, maybe you're asking how to connect one channel to both outputs to accomplish this with one input instead of two? I think that's what you're asking, if I understand... I don't know the answer off-hand.

Have you tried a search? There's lots of discussion on this topic.

yes, one input going to both outputs. I've searched a good bit, but didn't see this particular point, except to use something like 10k to prevent crosstalk. My thinking is that if I'm using 5k on the stereo channels, I should use 10k on the mono channels. Would that be correct?

Try searching for "pan law". Here's the quick Wiki def:

http://en.wikipedia.org/wiki/Pan_law

The short of it is, you want your mono channels attenuated by 3 dB, in order to balance their volume with stereo pairs. There's more detail and backstory and options than that, but that's the basic idea.

So, correct me if I'm wrong someone, but you'd want to build 3dB pads into your mono channels, before splitting them to the L and R outputs.

Leigh

Thanks for the response!

I'm thinking, though, that if you're mixing through your DAW, is that already compensated for by the software? It might not be, I'm just wondering.

If I was using 4.7k resistors on the stereo channels, I was thinking I'd use 2 in series on each leg of the mono channels. If I wanted to knock 3db off that, what would you recommend?

3 db are 1.4
formula 4700 x 1.4 ( 3db ) = 6580
so add 1K88 to your 4K7, you'll drop 3db
M.

[quote author="JohnSuitcase"]Thanks for the response!

I'm thinking, though, that if you're mixing through your DAW, is that already compensated for by the software? It might not be, I'm just wondering.

If I was using 4.7k resistors on the stereo channels, I was thinking I'd use 2 in series on each leg of the mono channels. If I wanted to knock 3db off that, what would you recommend?[/quote]

I think it does "know". Think about it like this, my outboard gear doesn't really know or care if I'm using 1 or 2 channels to push a signal into my DAW, but the DAW knows whether or not I have the input set up to accept a mono or stereo signal.
Now I am curious about the difference between the two as well?

I think there is some attenuation going on in the DAW. Because if I push a mono signal though a stereo channel it will be much quieter through that 1 side than if I were to setup a mono channel for the mono signal.

[quote author="JohnSuitcase"]I'm thinking, though, that if you're mixing through your DAW, is that already compensated for by the software? It might not be, I'm just wondering.[/quote]

Along the lines of what Josh said: a mono soundfile played through a stereo output of a DAW has been compensated, by the (virtual) panner of the DAW. In other words, it's already been attenuated by 3dB (or by 2.5 dB in Pro Tools, or by some user-selected amount in Nuendo, etc etc)

On the other hand, a mono soundfile played through a mono output of a DAW doesn't hit a "pan pot", so there's no compensation in that case. It's playing back at "full" volume, when it "should" (for perceptual purposes) be down 3 dB.

It can get confusing keeping track of all this. I tried navigating it a few months back, in this Tape Op thread.

Let me inject a caveat here, in case this is all seeming like too much to deal with: you don't *have* to deal with this by having your summing box attenuate your mono signals by 3dB. You could just do that attenuation in your DAW. The thing is, it becomes a pain if you have to switch back and forth between ITB and OTB summing. Mixing in the box, your mono channels are being attenuated by 3dB; now, fan that mix out to an "uncompensated" summing box, and all your mono channels are 3dB louder than they were ITB. If you know you'll always be working OTB, however, in the mixing process you would already have adjusted your mono faders lower, because that's what would sound right in the mix.

Make sense?

Leigh