Hi! Is it possible to wire the speaker out of a crackle box (running on a 9V battery) to a 3.5mm jack and then plug that into the Bela Audio In without damaging the Bela?

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well ... the output may be too hot, but if you divide it down with a a couple of resistors if should be safe enough ...
you probably don't want to mess around with getting the line out from the UA709, as that may affect the feedback behaviour of the instrument ...

Try something like this:

 C5
-||----+---------------+
       |               |
       |               z  2k2
       |_/|            z
        _ |  LS1       +--------Bela audio in
       | \|            |
       |               z 1k
       |               z
       |               |
J5-----+---------------+-----Bela GND

It worked! There are some interesting artefacts going on with the audio signal, but the interaction and the noises are pretty similar to what I get with just the chip and the speaker. Thanks!