a440 mixer schematic advise

[waldenpotato]

This is my first attempt at designing a module and I'm looking for eyes to see if things look correct.

I have a polyphonic modular - four voices - and I will use these modules to tune each voice. Each voice will feed two oscillators into this module. You will be able to turn on and off each oscillator separately. Each oscillator has its own output. In addition to being able to turn on and off each oscillator you will be able to route an a440 square wave to either oscillator - a/off/b. The a440 square wave is what I'll use to tune each oscillator by ear. This module will be very helpful because I will be able to turn off any oscillator I want and turn on and off the a440 tuning wave. Also, the a440 square wave will have a level pot.

Attached is my first attempt at building this. The a440 square wave was designed by Dave Brown here: https://modularsynthesis.com/a440/A440.htm. The schematic consists of the square wave and a mixer for each oscillator. Each oscillator has an on/off switch. I want to make sure that the connections between the three different parts make sense or if there is anything obvious I'm missing. Also, can you imagine any issues with the mechanical switches? I've decided that I'm okay with a bit of clicking.

[davebr]

Email me and I'll walk you through some stuff.

Dave

[waldenpotato]

Email me and I'll walk you through some stuff.

Just sent the email. Thanks.

[devinw1]

Nice! Glad you started to dig in!

Any reason you aren't using a non-inverting scheme on those op amps? Cascading 2 inverting unity gains seems like not the simplest way to cut this pie

[waldenpotato]

Any reason you aren't using a non-inverting scheme on those op amps? Cascading 2 inverting unity gains seems like not the simplest way to cut this pie

I just saw this. I was using the cascading inverting op amps because that's what I saw in most of the mixer schematics I was looking at. Dave from above helped me realize that with audio signals you don't always have to do the double-inversion. Buuuut, how would you suggest doing a single op amp unity gain without inverting? I'm still new to the different op amp configurations.

[devinw1]

If you don't want any gain, the simplest thing would be to just use a buffer (since that seems like what you are trying to do is buffer the high pass filter you have created with the 1u cap right? So that first part would be like this:

buff.png

[waldenpotato]

If you don't want any gain, the simplest thing would be to just use a buffer (since that seems like what you are trying to do is buffer the high pass filter you have created with the 1u cap right?

I'm just trying to mix two audio signals. I don't think I need the capacitor. Would this be a legitimate unity gain non-inverting mixer? I'm curious because most of the schematics I've seen around use the inverting opamp input.

Screen Shot 2021-01-11 at 7.48.16 PM.png

[devinw1]

Oh, I didn't know you wanted more than 1 signal. You CAN add signals with a non-inverting configuration, but it is more complicated, and you would add feedback resistors. But, things are not as clean cut. So, yeah, that's why it's easiest to add with an inverting summer, at the virtual ground point. Here is what a non-inverting summer would look like, plus the equation for Vout relative to Vin1 and Vin2:

noninv.gif

noninv2.png