DIY Contact Mic - Collin's Lab 

yes. yes it has

Frankie Romero lol

You were probably missing a connection between one of those three bottom resistors. I came across the same problem while making the circuit. It was interesting, to say the least.

Two years late, but... This is because of the type of cable you used between the disk and the rest of the components. The cable he used (an old audio cable) is "shielded", meaning the audio signal won't be influenced by outside sources and won't pick up other signals. If you use standard wires or speaker wire, you have this problem because it doesn't have that shielding. (The shielding happens because the negative "signal" is on the outside.) This is important when buying regular cables for an instrument as well, specifically when you need an instrument cable (which is essentially was made here) versus an amp cable (which is usually not shielded). Source: I've been getting into this recently and needed to learn the difference when purchasing equipment.

Good to know. I'm fairly certain I bought a nearly identical cable for the project, and the interference did go away after I realized I had missed a connection and corrected the circuit. It may have been a combination of the two somehow, though I suppose going back through the project again with new parts and components would show it was the cable all along as you said. Anyways, thanks for the info.

Can i shield the audio cable with aluminum foil? Will it work?

They are now on Adafruit.

qual a solução?

Então como fica no esquematico!!???

Very fun!