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[Moved] Advise with Stereo Mixer, specifically a TL072

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tommyjay

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Hey All!

I was checking out an Instructable at https://www.instructables.com/id/Altoids-Tin-18-Stereo-Mixer/ because I have some problems with my Y adaptor, so I wanted to find a better solution, and found this instructable. Right now I have one output, and two inputs, but I’m planning on adding a third device to the mix, so just in case I want to make it with 4 inputs.

Somewhere in the comments, someone mentioned to check out this link: https://sound.westhost.com/articles/audio-mixing.htm to read more about mixers, how they work, and how they should be wired. I don’t understand circuits and all the components that well, and I definitely have problems understanding all the calculations that go on between volts, watts, amps, ohms and so on. Though I have recently been introduced to the world of Arduino, and I’m loving it.

I definitely want to have a passive circuit, I don’t want to add batteries or a wall wart to the mix. I also don’t need to add volume control or faders or anything, Just want to make a little box to which I would connect my laptop, AirPort Express, and in the future my Homey ( athom.com ).. and that box would be hidden and connected to my HiFi. It’s not even about mixing sounds together, it’s just about having them all plugged in and not keep unplugging and changing the cable.

The diagram on the audio mixing website first takes the level down with resistors, and then back up with an amp… Specifically a TL072. I live in the Czech Republic, and pretty much the only place to buy parts is the store I found a Texas Instruments chip like that: https://www.gme.cz/tl072-dip8-texas-instruments-p311-013 It has two of these amplification triangles (like in the circuit diagram) which isn’t an issue, but it does have a + and -… And I guess it wouldn’t work passively. So….. basically, what I am asking is what component could I use to add to that circuit that is attached, that can amplify the signal up again, without having to add voltage to it… Basically, I want to make the same thing, without having to add external power.
audmix-f2.gif


Any chance you would know what I can use instead of that?

P.S. I understand that the ground of the audio signals is joined without any resistors or anything in between. But since the diagram has an extra ground output shown, does that mean I should put it to an external ground, like the one in my 230V plugs?


Thanks in advance,
Tommy
 

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KlausST

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Hi,

Most probably you have stereo signals, that means two channels.
You need two mixers.

You want a passive solution without volume control.

The easiest way is just to use simple resistors.
For each channel use four equal resistors. Try 10k Ohms at first.
Connect it as a star. On each end there is one input, on the star there is the output. Connect all grounds without resistors. Don't connect it to earth.

The signal level of each input is lowered, maybe you need to adjust overall level.
Maybe you hear some noise caused by ground loops and power supplies.
Maybe you hear plops when you switch of a signal source.

Hope this helps

Klaus
 

tommyjay

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Hi Klaus,

Thanks for the reply. You got everything right, that's where I want to go. about the ground loops/power pops; This is actually one of the main reasons why I want to make a better solution then just the Y plug. My fridge, window shades, fan etc. make loud pops whenever they turn on, and I would prefer using some sort of adapter/analogue circuit that is earthed, to try and get rid of these pops and clicks. My HiFi sadly doesn't have earth, and I don't want to have to take it apart and try to put one in there.

If there could be a solution with the above pictured circuit diagram, then that would be perfect. Just not sure about that amplifier part.

Tommy
 

KlausST

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Hi,

the obove circuit is the same as i recommended to start with.
There are two benefits of the obove solution.
1) output impedance is low. So voltage is more stable and thus less influenced by noise.
2) the TL071 has gain of 2 .. to adjust volume level.

***
Test the "resistor only" solution. This needs not much time and is not expensive. See what happens.

Tell us if it is OK, or if you need some improvements.
Take a photo of your circuit.

***
For a first test put the mixer near the amplifier (destination). Especially the GND lines should be short and solid.

Klaus
 

tommyjay

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Hi Klaus,

I made the simple circuit. I have lost some volume (as opposed to the Y adaptor) nevertheless everything seems to work, the audio is very clean, no hum or noise when my computer plays music. Thank you for your advice!


Here are the photos: 2015-06-19 20.46.04.jpg2015-06-19 20.47.15.jpg

But what seems even worse now with the hum when I disconnect my computer, there is very very loud hum. I tried to connect the audio ground to the earth of a plug, but it was even worse so I scrapped that idea. This problem only occurs when a cable is connected to the mix box, but not to a device on the other end. This is quite an inconvenience, is there any way around this?

Thanks,
Tommy
 

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