Will this synthesizer work?

[BobK]

As others have pointed out, you cannot use the same resistor for 3 oscillators. What was your intent with the 3 oscillators anyway?

Bob

 

[BobK]

Here is a simulation of a single tone generator using a 555. I give three outputs, square, sine and triangle, scaled to be about the same amplitude.

R1 controls the frequency, with 1K giving you about 650 Hz. Note that in order to maintain the same wave shapes, R1 and R2 must be modified in concert, with R2 10 times R1.

The waveforms are not meant to be pure and undistorted, they are what you can get easily with a 555.

Also note that R5, R6 and R7 represent the load on your summer. The outputs are fairly high impedance, so use summing resistors of 10K to get the level of output I show.