Use Any Pocket Recorder As An EMG

Read on for instructions on making a simple buffer so you can use any WAV-capable recorder, instead of shelling out for a Zoom.

The whole reason for using the Zoom H4N was because it had a high input impedance (2MOhm) on the 1/4″ jacks. This means that it could pick up even an incredibly low-power signal… the Zoom’s 1/4″ inputs are light and nimble like a sheet of paper, whereas most recorders’ inputs are more like a plank of wood.

The problem is the Zoom recorder is expensive: several hundred Euros (or dollars) at least, and it’s not so easy to find in shops.

On the other hand, 100 Euros will get you a quite functional 44kHz/16bit PCM WAV recorder, just about anywhere.

So how can you make that 44/16 PCM WAV recorder light and nimble like the Zoom?

Make a buffer. A buffer is just an amplifer that doesn’t amplify: you use an amplifier chip (which has very high-impedance inputs), and leave out some parts that tell it to amplify the signal. Since the signal goes through the amplifier chip nevertheless, the low-impedance ‘plank of wood’ recorder isn’t getting pushed around by the fragile EMG signal anymore. It’s getting pushed around by the strong output stage of the amplifier chip.

(The chip doesn’t need to amplify because the recorder is already designed to pick up low-voltage signals. In fact, I had to add some resistors to the output to REDUCE the signal level coming into my recorder! I left those out here for simplicity, but it’s not hard. Look up a “voltage divider” if you want to try it. Just be sure to use fairly low-value resistors — between 100 and 1000 ohms probably — so the amp can still drive the recorder’s input stage whatever its impedance is.)

The amplifier chip I used for this is the LM358, a really cheap chip you can get anywhere. It’s not perfect, so if you zoom in really close on a high-frequency signal you’ll see “ringing” and “overshoot”, but that doesn’t matter too much for our purposes. And ringing and overshoot can even help you analyze a signal that’s too fast for the recorder to capture otherwise — the worse the ringing is, the faster the signal was!

Apart from the chip, you’ll need two 4.7K resistors, two 2M ohm resistors (you can make a 2M ohm resistor by putting two 1M ohm resistors back-to-back), some capacitors, a 9V battery clip, and some audio jacks. You’ll also need some prototyping circuit board — get the kind with the little copper doughnuts around each hole, not the kind with strips.

(Instead of audio jacks, you can get a headphone extension cable — the kind with a jack and a plug — and cut it in half. Solder the wires from the jack half to the board where the input jack would go, and solder the wires from the plug half where the output jack would go. This means you don’t have to mess about plugging and unplugging another cable from the buffer to the recorder.

Even better still is if you put SMA or MCX plugs on the end of the RG-174 cable from the electrodes, and then solder short lengths of RG-174 with SMA or MCX jacks on them to the board.)

You’ll also probably want some kind of metal box or enclosure for the buffer, and you’ll want to connect a wire from the enclosure to ‘ground’ on the circuit. (I’d suggest the ground at the input jack.) This can be anything — I wrapped the board in wax paper, and then covered that in adhesive foil, with a wire running from the foil to ground. The idea is to keep radio interference and things like that out of the circuit: the whole point is that it’s sensitive.

Here’s the schematic as well as one way of arranging the parts on the board and wiring them up.

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