Evaluating Coil Values in an Guitar Sustainer Circuit and Not Frying ICs - Page 1
Forum section: electronics / beginners

author: spirituallyinsane
author_rank: Newbie
timestamp: October 20, 2014, 09:29:34 am
content: Good day, all!This is my first post, and I'm very excited to be here!I've recently gotten over the "hump" in my electrical engineering degree where things start to make sense, and all the little bits on the schematics mostly make sense, but I'm still having trouble with how reactive devices interact with circuits. I am working on a project that popped into my head the other day, and won't leave me alone.The basic idea is a string sustainer for steel-stringed instruments, that works similar to the way an Ebow works. An Ebow basically picks up the motion of the string using a pickup coil, and then amplifies that signal, feeding it back into the string. There are a number of circuits, but they all center around LM386 audio amplifiers and tiny coils. The most successful DIY versions use little speaker coils from buzzers. I attached a graphic of the schematic I found. It's in this video (careful, loud audio in the vid):http://youtu.be/1iccGqmEC-MThe biggest parts that I don't understand are a few of the capacitors, and how the coil impedances affect things. Does a higher-impedance (and I'm assuming, generally, more turns of wire?) coil increase the sensitivity on the pickup end? Does it increase the strength of the electromagnetic field when it excites a string (I assume so, because the magnetic field is proportional to the current and number of turns on the coil)? Do coils with more turns act as antennas, and risk more interference? Finally, on an amplifier chip, how do I know how much impedance will damage the chip, or not have a usable output? I don't want to buy a bunch of chips and toast them one after another because I have no clue what I'm doing. How would you select a coil in this situation?Also, I'm not sure about two of the capacitors. From the datasheet for the LM386,http://www.ti.com/lit/ds/symlink/lm386.pdf, I think that the one between pin 1 and 8 is just setting the gain to maximum. Pin 7 is called "bypass", but I don't know what it does, or why it would need a capacitor, unless it's there to filter out high frequencies by shunting them to ground? Finally, the 1nF capacitor in parallel with the input coil; I'm guessing it is a noise filter for high-frequencies and low amplitudes?I haven't worked with ICs much yet, and I'd like to understand the process that you all go through when using a new chip, to understand and learn how to use it without frying half a dozen in the process. Or--perhaps that is the process? I read somewhere that the LM386 is obsolete. Should I look for a newer chip to use, and adapt the circuit for that? I'm just trying to get some idea about the thought process.Thank you all for your time.

author: Rerouter
author_rank: Super Contributor
timestamp: October 20, 2014, 09:48:19 am
content: To me it looks like an almost direct copy of the Amplifier with gain = 200 on the datasheet, with the capacitor between pins 1 and 8 setting the gain, and the R/C on the output of the op amp to stabilize the op amp driving wiring with capacitance (this makes a large number of op amps unstable), the 1nF cap on the non inverting input forms an low pass filter with the coil,Having more turns on your coil should increase the voltage generated for a given movement, and should reduce the noise, you even gain from the fact that the op amp is completly buffering the signal so it does not greatly matter how thin a wire you use,If you want to prevent a risk of blowing up the op amp, add 1K resistor between the 1nF cap and the non inverting input plus a diode from your non-inverting input to your positive supply to clip the signal if it is too great in amplitude, this way the diode will not distort until you at-least equal the supply voltage,To me this circuit should be very easy to make up with any old op amp, providing it can match or exceed the current output,