I assume it's OK to reduce down to only 2 audio inputs, simply by "not including" the corresponding bits of circuit (audio jack/ganged pot/capacitor/resistor)? That is, should I assume a reduction in inputs shouldn't change the quality of the various components (I still need exactly 1uF capacitors in their corresponding spots, etc...)?
Same as question 1, except with only one output instead of 3 (Keep "Headphone Monitor", leaving both "Line Out"s out).
Is it safe/reasonable to swap out the TL072 for a LM358? (and are there any tips you might have for such a switch?)
What is the box in the top right? ("Futurlec Dual Supply Mini Board") What is -5v and how/where should I supply it?
Do I duplicate the whole schematic for two channels (so, for example: right off the ganged pot, I'd have two separate 1uF capacitors, two separate 10k resistors, etc... leading up to 2 separate TL072s (or in my case, hopefully 2 LM358s))?
Any tips in general? This is my first attempt at implementing someone else's design from a schematic!
Absolutely not, the LM358 is completely unsuitable for audio, its extremely slow. Are you wanting rail to rail operation?
A +/-5V supply for the opamp.
Yes, apart from the power supply.
Where did this circuit come from? Its plain wrong.
Right the important part, this circuit will not work and has many issues.
The TL072 cannot drive any headphones its not powerful enough, it doesn't like driving loads of less than 2k ohms and will distort heavily into a low impedance load.
The output is taken from the wrong pin, it should be from pin7, not pin6. This is a show-stopper but easily fixed.
The output potentiometer is 10k yet is supposed to feed headphones (32 ohm loads). Should be more like 1k log pot.
The input resistors are pointless as the input pots are resistors anyway.
10uF output capacitors are too small for a headphone load, 470uF or more would be reasonable.
I think for each headphone monitor output you'll need a separate headphone driver opamp, one of the best being the TPA6120, for which cheap modules are available.
BTW if you only want line-level outputs you just need to fix the pin7/pin6 issue. A better opamp to use would be the NE5532, but for +/-5V operation there are some true rail-to-rail audio opamps available which would give a lot more signal headroom.
WOW. Well, good thing I asked I guess! Thanks a ton for all the corrections. I had pulled this from an "electronics tutorial" website a bit ago. (It's a bit shitty they're supplying beginners with a broken schematic...)
I'll look into those changes, but it might be easier/better for me to also jut find a new schematic?
All I'm looking for is a 2-to-1 mixer for headphones.
Well its half wiring diagram and half schematic, which is why they made the mistake.
You probably want the TPA6120 module(s) to drive the headphones, but the basic circuit they give is a
standard current-summing mixer circuit. You look for another circuit and it could be even worse!
You haven't answered about rail-to-rail - what voltage supplies are you happy to use? If +/-9V to +/-15V then
I'd say NE5532 for the mixer using that circuit, add decoupling (you always add decoupling everytime, BTW), and have its output routed to the TPA6120 modules, then all the high power handling for headphones is done there.
+/-5V is pushing it (not enough headroom) with most opamps, you'll have to find a rail-to-rail audio opamp that can run from 10V supply.
regarding the "rail to rail" question- yes? I think? honestly I had to google it to see what that even means, and I think I might be in over my head with this problem. I had assumed a 2-to-1 audio mixer was as "beginner" as an electronics project could get, and I just don't think that's the case hah.
So I'm probably going to go back to square 1 and figure out how to start on more solid footing. (I've done plenty of arduino projects in the past, but as I'm a programmer and not an electrical engineer, they've leaned much more heavily on the logic end of things...)
Any tips/resources for where to start in building a 2-to-1 audio mixer would be appreciated!
+/-15V supplies are traditional, but these days +/-5V is more likely to be used with all the low voltage devices.
Traditional opamps can only work upto a few volts short of both voltage rails, rail-to-rail ones can go very close to the voltage rails, providing a lot more signal headroom from low-voltage rails such as +/-5V (or even single rail 5V).
