[line outputs]

"Line" outputs, the electronic audio signals usually fed to a mixer or amplifier, can be derived from the wires going to the speaker of the unit you're working on. Simply solder two more wires to the speaker terminals and solder the other ends of these wires to an output socket of some type (1/4" "guitar" jack outlet, "RCA" phono jack, etc.) mounted on the instrument's case. A standard cable can then be used to make the connection between the new instrument and the other equipment. BUT...

Use a test amp first! This can be an inexpensive, low-watt amp, bought 2nd-hand and driving a small non-critical speaker. Such a system can be found for $20 at Goodwill & Salvation Army outlets, yard sales, pawn shops, the classifieds, etc. As long as the unit has a standard line input to plug into ("tape", "tuner" or "accessory" phono jacks, usually), it will serve the purpose.

The idea here is that unknown signal levels will be sent into the amp during various circuit-bending experiments. This might risk the well-being of the amp or speaker if certain precautions are not followed. So, an expendable amp/speaker is best.

Be sure to have the amp turned all the way down when first determining if the speaker-derived line output will work. Connect the extended speaker wires to the amp's line input. This can be done by clipping one end plug from an input cable (like a standard phono cord) and stripping the insulation off to expose the two wires within. Connect these two wires to the wires you soldered onto the speaker terminals. With the other end plugged into the amp's line input and the new instrument making its sounds, slowly turn up the amp.

If the sound from the amp is louder than the usual line-input signal from a standard source (tape deck, guitar, etc.), the new instrument's output level, coming from the speaker wires, may be too high or "hot". To tame this output, a resistor of the correct value can be soldered between one of the instrument's speaker terminals and then to the wire that leads to the amp. Better yet, a miniature potentiometer, called a "trimmer", can be soldered in place of the aforementioned resistor. The   trimmer can then be adjusted to set the instrument's output level precisely. Experiment with trimmer values around 1 MEG, but have higher & lower values at hand as well.

Creating line outputs is very important in circuit-bending. The small speakers that most of the circuit-bendable devices come supplied with cannot come near to reproducing the frequencies that the electronics are creating, even before circuit-bending. And after circuit-bending, frequency response can be mind-boggling since clocking speeds are commonly altered. This results in ranges of frequencies that can surpass human hearing at both the high and low ends. A hi-fi reproduction system can illustrate the power of the circuit-bent instrument's voice in  wonderful ways. Also, line outputs open the circuit-bent instrument's voices to signal processing: reverb and EQ, namely. These standards of the electronic music studio can expand and sharpen the circuit-bent instrument's voice, as with the voice of any electronic instrument.