I’ve found that there is some popping being introduced on my square wave output when I get into higher frequencies. What I’ve found when testing for loose connections is that the popping stops whenever I touch a bit of metal to the connection between pin 6 and 7 of the TL072.
I have checked for bridging, and I have replaced the resistor and op amp. Does anyone have any advice? Thanks!
Maybe here some infos 
Hello, there def might be something I don’t understand about the situation, but I think you’re trying to sum voltage the wrong way on the non-inverting opamp input. You should try a basic inverting summer configuration. Here’s a quick schematic snippet that will output approx 0 - 5V. That should give you 0 - 100% sweep of the 3340 PWM control input.
You can use a single op amp in a non inverting configuration, but the voltage appearing at the op amp non inverting input will be the average, not the sum, of the CV and the PW setting — so a 5 V sweep of the CV, for instance, will produce only a 2.5 V sweep at the op amp. So you’d probably want a 100k resistor from the inverting input to ground, to get a gain of 2. You also have to be careful with voltages and impedances, since you no longer have a virtual ground, but I think what you have is all right from that standpoint. Not sure why you have a 4.7k pulldown on the op amp output, is that needed?
On the whole, if you have an op amp to spare, the inverting configuration above is probably preferable.
Either way, you can end up with a voltage exceeding 5 V at VPWM if the PW knob and the CV are both up. You could consider a 5.1 V Zener to ground, to limit the voltage on the 3340 pin, though it might not really be necessary.
