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Passive mixers, I don't undertand why the resistor/capacitor Answered

Hi, when I was young, our teacher show us a passive mixer, each input had only one green capacitor in series for each input, it was not electrolytic but ceramic or polyester, it sounded so good for just being there 1 simple component per channel. Now taking a look at google, can see that the most passive mixers, uses resistors to isolate sources, with values since 10k to even 100K. Other schematics shows a resistor and capacitor in series (see image below) what does that capacitor? and why some schematics only has a simple resistor? is it easy to calculate its ohm value?.

Is it better using this configuration supposing that the source is weak (coming from cellphone 3.5 mini jack or RCA) and this mixer targets the signals to a pre amp? So I need to from weak sources like iphone, dvd, any RCA output, etc... anybody know whats the best to use in this case? Thanks so much. Should I put a resistor as minimum as possible? I undertstand that a resistor there, in the input, will reduce the volume input, and I dont want to do that, so can I simple connect everything direct? or the resistor isolates the inputs, otherwise it will sound ugly? I am very confused, why the resistor? why I have seen resistors from some ohms to 2km and even a lot more, 50k, 100k. I want to mix two weak sources, as a signal coming from a walkman and cd player, or cell phone, etc, so whats the law for not reduce signal intensity and mix everything well? all channels will come in into a pre amp IC.


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4 years ago

The input resistors prevent interaction between the outputs (you are mixing together). "Crosstalk" is a good word for it (kudos to Downunder35m).

If you really want to understand it, you'll need to learn something about both output impedance and input impedance of audio stages. It tends to be somewhat difficult to get a handle on...

Output impedance can be thought of as "signal strength" or the amount of (apparent) resistance present, i.e., between the output and next channel. A low output impedance is generally best. I.E., low output impedance means there is plenty of current available to drive the next stage.

Input impedance can be though of as "the amount of load placed on the incoming signals" by the next stage. A high input impedance is generally best. I.E., a high input impedance means the input doesn't load down the incoming signal.

These are general guidelines only -- speakers, a legitimate "audio stage" have very low input impedance and obviously they interact greatly with the previous stage (hint: all the power needed to move the speaker cone has to come from the previous stage -- they are passive devices!)

How those resistor are chosen for the resistive mixer stage depends on what comes next. Is it a high impedance Op Amp? That's usually a good choice. A simple common-collector binary junction transistor amp? Not so great. Low input impedance usually results in both signal loss and loss of high frequencies... There isn't a "law" here, other than a fairly well understood interaction between the two types of impedances.

If your audio sources are significantly different levels, you might want to create custom channels for each. Or replace them with POTs.

Using caps and decoupling might be needed, but it might not. Depends on the outputs you are using.

Impedance, BTW, is just a fancy word for "AC resistance" -- because audio is AC, and the "apparent resistance" resulting is greatly effected by inductance and capacitance, which is a big part of signal processing.


4 years ago

First you don't have a line out on those device, which means you should do something to get the correct voltage levels for the amp.
Easiest way would be resistors or good audio potentiometers if need an adjustable input for other sources.

The resistors on the input are there to avoid "cross talking".
With such high values there is not much that can go from one source to another - Ohm's law comes into mind.
The capacitors help if you have an AC input as they block off the DC part of it.

I am sure some of audio and electronics guru's can explain it better, I just tried to keep it simple ;)