I got these schematics from an arcade game released in 1983.

These two circuits are responsible for generating two different sounds even though they look very similar.

First one (sounds like a whistle)


Second one (sounds like a fart, sorry for the lack of better word)

enter image description here

These were manually designed so I am trying to "reverse engineer" the logic.

The second one seems like a comparator that will trigger after a delay and then generate a series of distinct peaks in the output. The input triggers the charging of the capacitor, the output voltage split more or less in two, compared, and boom.

The first one has 2 JFETs in Darlington(?) which should just amplify the other sound. I tried simulating them in CircuitLab but the chart doesn't seem to be meaningful at all.

What is going on? How do I learn to "read" this? My plan is to add these to the game emulation available in MAME.


  • \$\begingroup\$ As both circuits produce sound, what have you got wrong in your simulations? \$\endgroup\$
    – Solar Mike
    Jun 8 at 6:38
  • \$\begingroup\$ I would try to think it through based on principles of op amps and basic components. It looks as if the input to each block on the left is an enable signal. So with that signal high or low, what is going to happen at the output? remember that an op amp basically does everything it can to keep keep the two input terminals at the same potential. (hint : any oscillator circuit must have some positive feedback, bot of these have that, so you are on the right track). \$\endgroup\$
    – danmcb
    Jun 8 at 7:56
  • \$\begingroup\$ @SolarMike what I got from circuitlab was nothing like a wave at all. \$\endgroup\$
    – Paperino
    Jun 8 at 17:21
  • \$\begingroup\$ @danmcb, thanks, I will look at op-amps oscillators. My questions was more on why the JFETs alter the wave-form but I seem to read between the line that the JFETs might have nothing to do with the generated waveform. \$\endgroup\$
    – Paperino
    Jun 8 at 17:23
  • \$\begingroup\$ "what I got from circuitlab was nothing like a wave at all" I'd have expected you to post simulate-able CircuitLab schematics here, then. Share your work! Then we can easily fix it if possible. \$\endgroup\$ Jun 8 at 20:05

1 Answer 1


made with FREE microcap v12.

Don't know if "sounds like a whistle" ...
Low-frequency period : ~ 5.8 ms , high-frequency period : ~ 1.07 ms.

enter image description here

And a little "zoom" inside ...

enter image description here

And here is the behavior of the second circuit ... which can also be "cut" "logically" at 0.4 s for a "shorter" wave.

enter image description here

Added link wave1 wave2

  • \$\begingroup\$ Chirping sound? \$\endgroup\$
    – winny
    Jun 8 at 9:30
  • \$\begingroup\$ The frequency inside the "red" is varying from "low" to "high" ... Will add in answer a "zoom" inside ... \$\endgroup\$
    – Antonio51
    Jun 8 at 10:20
  • \$\begingroup\$ @Antonio51 what did you use to generate the output wave? The MAME emulator doesn't support the specific opamp and bjts, so I am looking to see if I replace with what's already available and emulated leaves a big difference. Would it be possible to save it as WAV? \$\endgroup\$
    – Paperino
    Jun 8 at 17:20
  • 1
    \$\begingroup\$ The simulator used is microcap v12. I add in answer. LM339 is a "old" comparator. BJT does not matter, I think, used as a "switch". Don't know MAME :-) Saving as WAV never tried. I will search if possible. \$\endgroup\$
    – Antonio51
    Jun 8 at 18:20
  • 1
    \$\begingroup\$ Shared dropbox dropbox.com/s/l00bxxh8hzfm68y/… dropbox.com/s/tvwmcy89wx05z4k/… \$\endgroup\$
    – Antonio51
    Jun 9 at 18:42

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