With the Wur-monica, I have found a novel way to amplify a harmonica. The Wur-monica does not have a microphone or pickup of any kind inside. Instead, the Wur-monica relies on the electro-acoustic properties of the instrument itself as a signal source. In fact, the Wur-monica is based on the same elegant principle as my beloved Wurlitzer 200A electric piano (hence the name).
How it works:
The original Wurlitzer Electric Pianos of the 1960s were gorgeous machines with a really interesting way of generating sound: instead of strings, the piano had large reeds struck by hammers (see picture). The reeds produced a signal using an ingenious electrostatic principle: the reeds were charged up to hundreds of volts, so that they formed a capacitor with the adjacent ground plate. When the reeds vibrated, this capacitance changed, pushing electrons back and forth to magically produce ethereal honks, buzzes, crunches, and chimes. If you think about it, a condenser microphone also works in much the same way (except with parallel plates instead of vibrating reeds). In a sense, each Wurlitzer piano reed is its own condenser microphone.
The Wur-monica works using the same principle as the Wurlitzer piano. Each brass reed inside the Wur-monica is mounted on a brass plate, but does not touch it. This creates a very small capacitance between the reeds and the plate. When a reed vibrates, this capacitance changes, creating a signal that can be amplified. Again, no mics, pickups, or piezos are needed -- wires are connected directly to the reeds, so that the reeds themselves produce their own electrical signal.
The main advantage of this technique is that it is immune to feedback from the speakers, because the device does not pick up airborn sound, but rather the raw vibrations of the metal parts. Of course, I'm also hoping to maybe capture a little of that crunchy, magical, soulful Wurlitzer sound in a pocket-sized instrument.
This project is only a proof of concept, so I didn't spend much time refining the circuitry, the method for mounting the reeds, or the intonation, for that matter. In fact, I only bothered to mount seven of the reeds, and one of them is still kind of wonky and shorts out the circuit if I blow too hard (you can hear it shorting in the video). Still, after only a few hours of hacking, I was pretty pleased with how the prototype looks and sounds.
Please check it out and leave me some feedback! And if you like classic devices updated with new electronics, check out my USB Typewriter instructable, too. And please vote for this project in the Pocket Sized Contest! Thanks!
Final Test (just before getting dizzy and passing out):
Step 1: Parts Needed
Resistors (1MΩ, 2MΩ, 22kΩ, 10kΩ, and 4.7kΩ)
Capacitors (22nF and 1uF)
1/8 inch instrument jack
Power supply that produces both +48V and +12V (I used a Behringer Phantom Power Supply)
Harmonica (I used a Hohner "Bluesband")
An amp for testing (I used a computer speaker from RadioShack)
Tools and stuff
You will not need any magnets, microphones, piezos, guitar pickups, or anything like that to do this hack! That is what is so cool about this project: the harmonica will become its own microphone!
Step 2: Open Up the Harmonica
Step 3: Remove the Reeds
However, notice how the reed plates are designed -- each reed is riveted into the plate with a tiny steel nail. This means that each reed is already electrically shorted to the plate! That is bad news for our design. Therefore, we have to find a way to mount the reeds in such a way that they don't make electrical contact with the plate.
First off, we need to gently remove each reed. I found the best way is to grind down the head of each little nail, then gently rotate each reed until it loosens and falls off. Make sure you don't damage the reeds while doing this. Once the reed is off, grind down what is left of the nail so the surface is flush.
Step 4: Shave Down Each Reed
I did this by holding them on their side against a Dremel wheel. I narrowed the long part of each one by about 10%. This will probably mess up the tuning of the instrument, but I'm not so interested in that right now.
Step 5: Attach Reeds and Wires to Plate
While perfoming this step, make sure you use a thick enough layer of glue that the reed is not directly touching the plate. While the glue is drying, use a multimeter to check that they are not touching. It helps if you grind the two services flat before gluing them together.
The hardest part is making sure the reed stays lined up perfectly inside the rectangular hole while the glue dries. Remember -- it shouldn't ever touch the sides of the hole or else the sound will cut out.
Step 6: Build Preamplifier Circuit
The opamps used are LM385 but you can probably use any single-rail opamp. They are supplied by the 12V rail. The 48V rail is only used to charge up the capacitor formed by the reeds and the plate.
How the circuit works:
The 48V supply loads electrons into the capacitor formed by the reeds and plate. When this capacitance changes, the amount of charge the capacitor can store at that voltage changes (C=Q/V), and so the electrons have to move. They do so through the 22nF cap and into the input of the first opamp stage, which is a buffer amplifier. The buffer is designed to have a very high input impedance, so that even a small trickle of current from the reeds can produce a good voltage. The signal then goes into a second opamp stage, which is an amplifier with a variable gain set by the 100kohm trim pot. Its a very simple circuit, and probably not the best, but hey it worked first time so I don't complain.
Step 7: Reassemble and Rock Out
The setup is like this: Right now, a Behringer Phantom power supply provides the 48V and 12V rails to the circuit, while a dinky Radio Shack computer speaker outputs the sound. But there is no reason the circuit could not be battery powered with a tiny built-in speaker. I just didn't have the time or the parts to do all of that during the 24-hour Hackathon.
In this video you can hear that the sound cuts out occasionally -- that is because one of the reeds is not seated right and sometimes shorts out. Otherwise the thing works surprisingly well. It can produce some hum on some settings but there is a certain sweet spot on the trim pot dial where the amp is nice and quiet. The sound is not so different from a regular harmonica, although somehow more "robotic" and metallic, even when played on a clean setting.
One disappointing thing I found out is that you can't do wah effects with your hands -- this is an acoustic effect and does not really change how the reeds vibrate, so the signal is unchanged. Well, maybe that is a good thing for some purposes but it would be nice to hear my invention with cool wah effects -- hey maybe I should plug it into a wah pedal!
Alas, another challenge I faced while trying this out is that, when exhaling repeatedly into the "blow reeds," your breath creates condensation on the reads which can short them out or change the capacitance. Therefore this idea is only practical for amplifying the "draw" reeds (the ones you inhale to play). I have yet to think of a workaround for that problem, so maybe a different reed instrument like a harmonium or a melodica would be more suitable to this hack. Maybe I will try that at the next Music Hack Day, which I hope will be soon!
If you like this instructable, please vote for it in the Pocket Sized Contest. Thanks!
Final Test (just before getting dizzy and passing out):