Piezo Preamp Part 1: The Making

This project is almost ready so lets start with what I have already done long time ago (few months or so). Long story short when I bought items for the bass envelope follower I bought op-amps and I bought them from ebay 10 per order. One of the op-amps were TL071 which I now have ten only needing one for the envelope follower. I also sometime ago watched Make Magazine videos by Collin Cunningham and one of the projects he did was piezo preamp. He demonstrated it by placing the piezo on different things like a comb and what not. I thought this was cool so I planned to make one someday. It used the MPF102 MOSFET which I was unable to find so I thought about finding a replacement MOSFET. At some point it came to me that it was just the amplifier of the circuit and what are op-amps good for? Amplifying! So I thought why not google for piezo preamp schematics with TL071 and what do you know I was able to find some drawing using that component, the one I originally used at the beginning can be found here.

I started putting the circuit together on the breadboard. It uses two 2.2M resistors as voltage divider and I did not have those, I didn’t even have 2M resistors, but I did have 1M and 220k so I thought that I solder these in series into a perfboard to make 2.22M resistor or close enough. I soldered two of these resistors. I could not measure these because my multimeters only go up to 2M, but it doesn’t really matter. I used 5% tolerant resistors so they would be close enough to 2.2M in any case. These pictures shows the result. You can see from that one photo that the breadboard has IR receiver in it and it is part of the upcoming trip wire project that I promised for my son, I am not yet sure will it be IR or laser (as shown here by Afrotechmod):

piezoresistor1 piezoresistor2

It wasn’t schematics that I started with. It was a  picture showing how to put the circuit together on a stripboard. It turned out to be kind of hard to follow it and being lazy I did not trace it into schematics, but I turned to google so I found this schematic looking thing, which had drawn components instead of schematic symbols, which was basically the same circuit, but it used 2M resistors in the voltage divider. I used that to put together the rest of the circuit. This process took months because I only did it a short period at a time at night. But I eventually got it together and I tested it. I did not have amplifier at home so I plug it into my audio interface, hit record and tapped the piezo. I was able to record something so I thought that the circuit works and I can put it together. I already had those two resistors ready in a perfboard so I started to solder parts into it. I was able to solder the parts and some wires into it in one sessions trying to keep track that everything was in its correct place. I finally got it together with only the piezo, power switch, volume pot and audio jack missing and here is the result:

piezopreamp1 piezopreamp2

As you can probably see from the pictures I am still in need of several hours of soldering practice, but the quality has improved over time although I solder very rarely. The next step is to verify that everything is there. I originally soldered this long time ago taking one part from the breadboard and soldering it to the prefboard. Now the parts are not on the breadboard anymore, but I am quite sure that I got it all in place. Before I start to drill holes into the enclosure, like I would have done some time ago, I should test the circuit by connecting the cables using alligator clips. If the circuit does not work then I have to troubleshoot it. Sometimes I can remember the first rule of electronics as stated by Dave Jones from EEVBlog: “Thou shall measure voltages” which is something I should have done when the Andy’s Simple Fuzz did not work. When I get the circuit to work it would be time to design the layout of the components when fitting it all into the enclosure. I have these ABS project boxes into which I will put this device.

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