Ok, a quick pass over the circuit:
- The input amplifiers use the standard non-inverting configuration, where gain is given by 1 + R1/R2 (so max 1+100/15=7.6 in your case). There may be some subtle issues here (more below), but I don’t think that’s what you’re seeing.
- Next you seem to connect all the outputs to each other? This isn’t great, because now the amplifiers will fight with each other; if say three go high and one goes low, the latter will have to sink almost all the current that the others can source, without any limit, which isn’t healthy. If the idea is to mix things up, you need resistors on the outputs to turn things into a mixing node (summing up currents).
- The lower TL072 has a floating input.
- You have a non-inverting input stage and an inverting output stage, so the output will be out of phase with the input. Consider using inverting amplifiers also on the way in.
- They also have the advantage that the opamp inputs are kept at 0 V (since one of them is grounded, and the opamp strives to keep both inputs the same) which keeps you far away from the subtle issue I mentioned earlier, which is that most opamps don’t like it when the inputs get too close to the supply rails. This is especially important for the TL07x series, which cannot handle inputs below 4 V above the negative supply (i.e −8 V if you use a −12 V supply).
- (also, not sure what tool you used, but the editor probably has a snap setting that you can set to a larger value to make it easier to keep the wires strictly horizontal or vertical.)
See e.g. this page for a bit more on summing amplifiers. @Caustic also did a bit of detailed circuit analysis here: