Capacitor dyslexia

Posted by Frank_NH on Mar 10, 2015; 1:36pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Capacitor-dyslexia-tp18467.html

So last night, I thought I'd do a quick build of a circuit I've been meaning to try out for a while, the Jack Deville Deuce Coupe:



I just got some charge pump chips last week, and this was a simple project.  Everything went together very smoothly.  So I fired it up and...no overdrive!  Just a unity gain signal.  

I then spent the next two hours probing, testing, measuring voltages, changing chips...nothing.  WTF???

I was sitting down later looking over the schematic when it suddenly dawned on me.  That "1u" capacitor on the left??  Yep - I had used a "1 nF" capacitor instead!!  Oooops....

It so happens that this capacitor is in the ground leg of the op amp feedback loop.  The op amp gain for the non-inverting configuration in the Deuce Coupe is:  Gain = 1+Zf/Zg, where Zf is the impedance of the feedback loop and Zg is the impedance of the ground leg attached to the inverting (-) input.  Using a 1 nF cap made Zg increase by several orders of magnitude (over the normal effect frequency range of 50 Hz - 10000 Hz)!    Hence Zf/Zg ~ 0 --> Gain = 1 --> no amplification.  Duhhh.  

So if you ever have a problem build and can't figure out what's going on, keep an eye out for capacitor dyslexia...  

(There is also a related malady known as "resistor dyslexia"  where 470K ohm resistors are used instead of 47K, 4.7K, or 470 ohm resistors...)