audio mixer circuit that works when one supply is off

[nathfy]

I have a very basic audio mixer circuit with two audio pairs in and one audio pair out. using the following diagram (for each pair):

<pre>
        INPUT 1 -------\/\/\/\---+
                ---+   R1  10K   +------------ OUTPUT
                   |             |       +----
        INPUT 2 -------\/\/\/\---+       |
                ---+   R2  10K           |
                   |           (shields) |
                   +---------------------+


</pre>

BUT i have found that if one supply is physically off then no audio is outputted at all - and that isn't good. So the question is how can i get it to work if one supply is off (no power) and when both sources are on (mixed)?

any pointers appreciated.

 

[Nigel Goodwin]

There's no reason that shouldn't work, the whole point of the resistors is so they don't affect each other.

 

[audioguru]

There are no coupling capacitors to block DC so the DC is messed up when one input is turned off.

 

[nathfy]

so if i add a capacitor after the resistor as shown here:

<pre>
        INPUT 1 -------\/\/\/\--||-+
                ---+   R1  10K     +------------ OUTPUT
                   |               |     +----
        INPUT 2 -------\/\/\/\--||-+     |
                ---+   R2  10K           |
                   |           (shields) |
                   +---------------------+


</pre>

that should sort it? if so do you know what value capacitor i should add, or how to measure it to find the value i need?

or (in reply to nigel's comment) would upping the resistor values help? (say 30K)

 

[Nigel Goodwin]

It really depends on exactly what is connected to all three connections, if they are properly designed it should work fine, there should be no need (or use) for capacitors, which will already be in all three units.

 

[Torben]

Hi Nathfy,

Perhaps it would help if you explained exactly what devices you have hooked up and how exactly they are connected (a diagram or schematic would be nice), and what is not working the way you expect it to work.


Torben

 

[nathfy]

Currently setup like the attached
Two sources, one (old) xbox and a Wii - connected via the mixer circuit to the speakers

With the xbox power off, then no audio from speakers - xbox on then audio mixed from both sources.

audioguru suggested coupling capacitors to block DC - which i want to try, but need some advice on possible values to try with it.

 

[audioguru]

If it works only when the xbox is turned on then the input of the mixer uses the DC voltage from the xbox for its DC bias. Use a single coupling capacitor in series with the Wii for both channels.

 

[nathfy]

no joy with adding capacitors - tried a 100pf and 270pf - connected before the resistors in the circuit diagram i posted previously, when connected no sound at all. Anyone any idea of a possible work around to this? are the capacitor values too high?

 

[Roff]

no joy with adding capacitors - tried a 100pf and 270pf - connected before the resistors in the circuit diagram i posted previously, when connected no sound at all. Anyone any idea of a possible work around to this? are the capacitor values too high?

Waaaay too small. You need about 1uF.