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tenorjazz 3rd January 2020 09:58 PM

Questions on Building a TL074 OPAmp Line Mixer
I want to build to build a very small line mixer for a bunch of synths.

Synth DIY Guy has a YouTube for building a dual Mixer with a TL074 OPAmp. Looks pretty simple and seems to sound OK. Search YouTube for Building a dual Mixer with OTTO's DIY (I tried to copy and paste the link but can't make the copy/paste work).

A couple of questions I am hoping you can help me with:

1. Does anyone know if there is a limit to then number of input you can have? I am thinking between 8 and 12.

2. Could I put a switch right after the 10k Pot and either before or after the 100k resistor that would temporarily divert the audio from the synth to a second output. This output would be connected to an RC505 looper.

3. If someone has a better suggestion for a line mixer, please let me know.

lame pseudonym 5th January 2020 04:49 AM

The only experience that I can share with you, and it's happened to me a few times, slow learner that I am, is that TL0-anything amps don't like to drive long or even medium cables.

The classic 5532 op amp is better for this but it's not a FET input, if the circuit requires that.

If there were a FET-input 5532-level op amp I'd be all over it.

Cabirio 7th January 2020 04:31 PM

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To reduce noise, I would use 12k input and feedback resistors instead of 100k. Then you can use 50k linear pots, which loaded with 12k will give a pretty reasonable approximation of a log taper and will have much better tracking between channels if you use stereo pots, while keeping the minimum input impedance at ~10k. There's no limit to the number of inputs you can add, but noise will increase with the number of inputs. Since we're talking line level and no gain, this may not be a problem. Try and see.

A 100R resistor at the output will help with driving capacitive loads (long cables). Arrange it as in the attached schematic and don't forget those 22p caps for stability (use C0G / NP0 ceramics or polystyrene here). Use DC blocking caps at the inputs and outputs as shown, I like Nichicon Muse ES bipolar caps for this, they are tiny and very linear.

Edit: yes, you can put a switch after each pot. If you feed the aux output to the inst. inputs of the RC505, which are high impedance, add a 12k resistor to ground at the aux output position of the switch to keep the same taper.