Passive mixer - Help me choose resistor values for Firepod

[AdamZuf]

Hi!
I'm going to use the two Presonus Firepods outs to passive summing, unbalanced, as simple as it gets.
That forms 16 analog outs, but I would like to take the output of the mixer back into the computer so I can record and monitor the return. So actually it's a total of 14 outputs dedicated to summing.

I read various views about resistor value and I'm kind of lost. I've seen recommendations between 1k to 10k for 16 channels. Still trying to decide if I should buy "hi-fi" resistors or just the basic stuff. 1% or 5%?

I'll be using TS inputs (that can break a circuit if needed)

Goals:
1.Keep noise floor to minimum.
2.To be able to use other mic pres later other then the presonus. (this is a function the output resistors, isn't it? Maybe a stereo pot?)
4 inputs acting as mono, splitting inside the box. Should affect resistor values for these splited channels? The Firepod pre's impedance is 1.3k ohm.
3.crosstalk no more then a usual good spec'd active mixer.

Should I keep the cables always connected to maintain performance?

The Firepod's outs are 51 ohms. However it might be that I'll add 4 more D/A from the S/PDIF outputs later, so that device will probably have different output impedance. If making the box specifically for 14 channels, 51 ohms each will make it worthwhile, then I can drop this idea. Would be preferable to be a bit flexible, if performance won't suffer.



Many thanks for your help
Adam

[Matt Syson]

Hi
10K is probably a good value, low enough not to be noisy and high enough not to overstreach any likely output stage. Aiming for a total combined resistance of say 200 ohms may possibly reduce the noise, if your preamp is best at this level, in which case your resistors could be about 3K6.
You could agonise over this for days, or just build it and get on with using it.
1% and 5% are tolerance rating not 'quality' but normal 'metal oxide' types 1% should be fine. iIf you were feeling flash then you should make one with Hi Fi resistors the other normal, you probably won't be able to tell the difference except in the wallet.
Crosstalk is an issue when the sources were traditionally from a fader which would be 2K5 ohms at worst setting (for a 10K fader), and when it was feeding more than 1 bus. With single sources as you say then crosstalk is not an issue.
Terminating them with a 'short' is handy if you want to keep the following preamp gain constant but you could benefit from having less noise if they were left open for the times where you only have a handful of sources on the mix, it is up to you.
In this situation you would need less 'make up' gain thus less noise.
Matt S

[AdamZuf]

About shorting, so it's either retuning the gain when I plug/unplug a cable or a worse noise floor? It's better to retune.. So I should just make it as simple as it gets I understand, like this:



If the output stage of the D/A are not streched, what is the benefit of lower valued resistors? (won't there be more gain, resulting in better SNR?).

The Presonus's pre get unlinearly more noisy with higher gain. Therefor, I wouldn't like to push it hard.
Can there be any benefit with using a stereo pot for the "master" resistors? (maybe for other pres?)

Also, what about the mono channels? Or perhaps I should Y split, to keep all options open?
EDIT: maybe I'll drop the idea of the mono channel, to avoid errors if I want an easy total recall just by loading the project..

Thanks!
Adam

[Matt Syson]

Hi
The gain required will not change that much as it is a similar fraction 1 /16 or 1/18 or whatever. Perhaps go to 4K7 resistors.
Yes the picture looks fine as a construction method but perhaps keep the bus wires closer if balanced. Obviously if unbalanced you only have the 2 bus wires.
Yes you can use a pot for a variable level but its effect (level change) may be different if you are not 'back terminating'
Matt S

[AdamZuf]

Originally Posted by Matt Syson

Hi
The gain required will not change that much as it is a similar fraction 1 /16 or 1/18 or whatever. Perhaps go to 4K7 resistors.
Yes the picture looks fine as a construction method but perhaps keep the bus wires closer if balanced. Obviously if unbalanced you only have the 2 bus wires.

Oh this is a balanced box, then? So how should I wire exactly if it's unbalanced? 1 resistor for the hot lead of each channel and what about the value and implemetation of the output resistors...?
I'm sorry I looked for a schematic but everything I've found had a dead link or something..

Yes you can use a pot for a variable level but its effect (level change) may be different if you are not 'back terminating'

I have no idea what you just said..

Thanks for your time and help!
Adam

[Matt Syson]

Hi
The picture shown earlier is a balanced version and so has 4 bus wires (and all the grounds are linked).
An unbalanced version would only have a resistor from the tip of each jack and being stereo would have 2 busbars plus the grounds.
The variable resistor thing is simply that you should use a pot as a variable resistor (use top and wiper contacts) but it's gain control effect will depend on how many other inputs you are using (a bit). You should use something like a 2K2 resistor and a 10K pot in series to give you about 6dB boost and about 6dB cut in level (roughly speaking).
Matt S

[AdamZuf]

Is there a disadvantage with the balanced version? Because when I think of it, the signal leaves the presonus balanced anyway, it's not like I'm adding anything to the chain to keep it balanced...

[Matt Syson]

Ho
A mic input would prefer to be balanced anyway. Build it like the picture with 4K7 resistors. Play with a pot at a later date if you want.
Matt S