The Meg

I’ve done only a bit of circuit bending, so I’m not up on everything that’s otherwise commonplace among benders. Apparently people have been bending toy voice changing megaphones for a couple decades now, or something like that, but I wasn’t aware of it until Soundbender uploaded his video on the LMNC Discourse. From there I found Simon the Magpie’s take on the idea. And I found the megaphones at Amazon Warehouse for $5.15 each. So I bought two — figuring the odds of my frying one of them were pretty high — and set out to turn one into a synth module. The Mega Module.

The toy has a trigger, which switches power on, a microphone in one end, and a speaker and eight LEDs in the other. The LEDs light up when you talk into the mic. Five slide switches control various distortions of the sound.

It took me several months to get the project to completion, but in the end here’s what I did:

I removed the PCB from the megaphone and mounted it on a sheet metal bracket behind a Kosmo format front panel. On the other side of the bracket I mounted a protoboard carrying various support circuitry — more components than are on the megaphone PCB itself, tail wags dog, but that’s okay.

I cut off the battery and replaced it with wires to a 9V regulator connected to the synth +12V rail. The trigger switch is gone, of course.

I cut off the mic and replaced it with an input jack followed by a buffer and a voltage divider to cut the synth signal level to what the megaphone wants.

I cut off the speaker and replaced it with an amplifier to bring the signal back up to synth level. I also put a 10R resistor from the board output to ground, because the megaphone has an LM386 that expects to drive a speaker. Don’t know how necessary that was but it works. The amplified output gets mixed with the input and sent to an output jack.

I cut off seven of the LEDs. I kept one for old times’ sake.

I found a pitch resistor on the megaphone board. I cut it off and added leads to a potentiometer plus fixed resistor. The pitch resistor was about 16 kΩ, I used a 2k resistor and a 20k pot.

In parallel with the pot I wired a VTL5C3 vactrol. I used an op amp circuit to drive the vactrol’s LED, following a control voltage.

I connected the megaphone output, via a 100k pot wired as a voltage divider and a 22k fixed resistor, to the megaphone input. Feedback. At the upper end of the pot’s range, you can disconnect the input and the module will keep screaming.

I left the original slide switches in place, but put them in the off position and wired five toggle switches, mounted on the panel, in parallel with them.

I made a snazzy panel.

I did all this, tested it and found it working, and did so without frying the sound chip. So I still have an intact megaphone sitting around.

And I have a module that does nasty things to any signal you feed it. It’s lots of fun.

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