For the first time in years, I’ve had an uninterrupted week at the lab bench, with all the time and focus to get some serious progress made on the Magnetic Cello. I’ve spend most of each day tinkering in the lab, alone, except for the occasional ant. In this solitude (broken a few times by my EE friend), I was able to solve some nagging problems with the circuit.
Two huge problem I’ve always had with the instrument is how to amplify the coil and how to combine its output with the oscillator. In every prototype, I’ve depended on a $10 Radioshack pocket amplifier to pick up the output of the instrument. Plugging the Magnetic Cello directly into a regular amplifier would only result in noise or a very quiet signal.
After talking to a few EE professors, it looked like I was having problems with impedance. The output of the coil was mostly current, while most amps work with voltage signals. I looked up Wikipedia for a current-to-voltage converted, and in an hour I was able to turn the high-current low-voltage output of the coil into a usable voltage signal.
The next step was to combine the output of the coil with the output of the voltage controlled oscillator. After not being able to get a voltage controlled amplifier to work, I started to get creative. I stumbled upon using the output of the oscillator to change how the coil’s current-to-voltage converter worked:
As far as I know, this is not a common way to combine frequency and volume. But it is a very simple solution, and, with the right resistor values, I haven’t had problems with noise. Most importantly, the output can be feed into a regular audio amp. (I was also able to fix up a few more thing in the circuit, but I can tell that this post has already become too technical.) It still needs some optimization and a bit of testing, but the circuitry is now reliable and functional enough to be stuck into an instrument and sold.
I feel like this week has given me the time to research and solve problems that I’ve been putting off. There is still quite a bit of work to be put into the instrument, but this summer I have time to solve problems as they arise, while still having enough energy to do push-ups and play my autoharp and cello.
Oh, talking about the autoharp, I have a few ideas on how to rework the instrument. It’s a few years off, but keep your ears tuned for something called an “acoustic keytar”.
