Problem with DIY summing mixer

[fatcatmusic]

I DIY a summing mixer, simpliest solution, unbalanced inputs into two ADM 1922 line amps.

The problem is, the output of my Roland RD300 piano would drop slightly after I insert my Indigo DJ laptop soundcard outputs ( both unblanced ). And when I patch in my ASR-Pro or Yamaha EX5, the RD300 and Indigo Soundcard are not affected but the ASR and EX5 would drop to a very low level. If I connect the ASR and EX5 to my SSL SL504 line amp the outputs are very loud and no problem.

Is this an impedence matching problem? How can I solve it?

Thanks.

[Matt Syson]

Hi
For 'simple' summing to work properly, all signals must be a similar level and come from a low source impedance. Having them all balanced / unbalanced probably helps too.
Matt S

[fatcatmusic]

Thanks. But doen't grounding the ring and sleeve together give me an unbalanced signal? How can I match their impedence more accurately?

Thanks again.

[stellar]

what value resistor are you using for the summing?

[Matt Syson]

Hi
The SOURCES should be low impedance, most line outputs are in the 22 to 100 ohm region which are fine. The levels should all be similar from all kit that is contributing to the mix otherwise it will be quiet. The levels WILL change as you add or take away channels.
This is all part of the fun of passive mixing, you MUST define the operating conditions or keep playing with mix levels. These hassles are one of the reasons why Virtual Earth mixing became popular as interaction from different sources is minimal and you can 'scale' inputs for +4 and -10dB operation if needed. Having a variety of balanced and unbalanced kit mixed together can work but it is yet more compromise.
I presume your mix resistors are in the 4K7 to 20K area? 10K is a common value to pick and most gear would be happy driving this.
Having a sufficiently low 'common' resistor (say 600 ohms) helps reduce interaction if you only have a few sources. This will drop level by about 30dB.

Matt S

[stellar]

The one I built, (my first DIY project) has the signal split to two different 10k pots, one of them going to a 20k resistor and one of them going through pan switching resistors then to another 20k resistor. all 3 busses (stereo plus 1 aux) are being shunted with 220 ohms. somehow, this came out to requiring about 40 db of makeup on the preamp, which is actually perfect, because I don't have to worry about overloading the input transformers on the preamps.

[fatcatmusic]

Thanks for the advice. I was actually using 500 ohm resistors. But just this afternoon I changed them all to 5K and everything is working fine. Overall volumn has dropped as you all suggested but all the inputs are more or less 'balanced' and I don't have one module cutting the others significanly.

Thanks again.

[Matt Syson]

Hi
Glad to hear you are making progress. I think I would tend to go for 10K, especially if you have a lot of semi pro gear. Yes it drops the level a bit more but the interaction is reduced. As in so many things it is 'swings and roundabouts'.
Matt S

[stellar]

Hey matt,

in my box, which I detailed in a post up there, the signal is being split between two 10k pots, then in series to 20k and then 220 ohm resistors. what impedance do you estimate the source is seeing? I was thinking that maybe I should have used different values but so far its working great, with both balanced and unbalanced gear.

[Matt Syson]

Hi
With the pots at max it will see 2 pieces of 10K and 2 pieces of 20K all in parallel making it about 3K3 as a guess. If the pots are holding say 10dB then it will be nearer 5K load, which is what it will see with the pot at minimum. I can't be bothered to get the calculator out!
Should be OK as you have it now although there is always room to 'tinker'.
Perhaps increase the 220 to say 330 ohms or maybe more, up to say 500 ohms as you don't need to drop it so far

Matt S

[fatcatmusic]

Originally Posted by Matt Syson

Hi
Glad to hear you are making progress. I think I would tend to go for 10K, especially if you have a lot of semi pro gear. Yes it drops the level a bit more but the interaction is reduced. As in so many things it is 'swings and roundabouts'.
Matt S

Hi Matt,

What kind of interaction would there be with the gears? Would the sound quality be compromised, or maybe stereo separation affected?

Thanks.

[Matt Syson]

Hi
Interaction is diferences in level when you plug another signal onto the mix. If you disconnect one or more 'sources' and leave the input 'open', then it will affect the level of the rest of the mix. This may not actually bother you as you simply readjust the amplifier gain.
Crosstalk /channel separation will depend on exactly what you are doing and the different output impedances of equipment. Having a mono source with a mix resistor to left and right can reduce separation if the mono source's output impedance is high. Most gear will be in the order of less than 150 ohms so no real worries.
Forget about 'signal quality' getting degraded. If the mix bus assembly is in a metal box it will keep out most hums and buzzes and you are only going through a couple of resistors of which there are hundreds in the various other bits of kit.
Matt S