I built the following preamp to use with guitars
Basically the idea is to keep it on a single rail so that I can use it off the same battery with this power amp
The Power amp is working fine, and the preamp seems to work fine when I run it across an oscilloscope, but when I connect a guitar to the preamp and the preamp to the power amp there is a repetitive thumping sound. The thumping sound dissappears if I put a 10k resistor between the output of u2 and ground but only on an 8 ohm speaker. If I use a 4 ohm speaker the thumping is still there. Also with the 8 ohm speaker and the 10k resistor when I turn the volume down on the guitar the hum of the system increases. Can anyone see what I'm doing wrong here?
Also, unrelated, but I want to add a volume pot into this preamp circuit but am unsure where to put it without affecting the input impedence which I want to keep high.
***UPDATE: So it's taken me ages to get back to this project, but today I went through and tried all the suggestions below. Still when I connect the 4 ohm speaker it is not working. With an 8 ohm speaker now everything is okay. Can anyone suggest why I would have this problem with the lower impedance speaker?