I'm trying to improve on my engineer skill so I'm building a guitar tuner circuit. I'm trying to build this circuit http://www.vuitservices.com/elec/Figure_01.gif [Broken] This circuit is from http://electronicdesign.com/Articles/ArticleID/6264/6264.html [Broken]. So far I have done this much. I used a breadboard and wired the following circuit. http://www.vuitservices.com/elec/schem.gif [Broken] The problem that I'm having is that the circuit doesn't respond well to the sound of the first string or "E" string on a guitar. I scoped the pin 6 (output) of the op amp while striking string 1 on my guitar and it looks really erratic. The peaks are small and erratic. I can't tell the frequency by looking at the scope. The frequency of the 1 string on a guitar is around 400hz (I don't remember exactly). On the other hand, the circuit responds well to the other 5 guitar strings. The amplitude is good and the signal is clear on the scope. I can see the peaks (fundamental frequency) and I could determine the frequency of the strings by measuring the period of the waveform.