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
https://www.eleccircuit.com/bridge-amplifier-bcl-18w-by-tda2009/
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?
Best Answer
If this op-amp uses a single-ended supply then it should work ok unless your resistor values are too high as as mentioned in the comments. Change 10M to 500K. Delete R1 as it would reduce the bias voltage, which should stay at 1/2 Vcc, as set by R3 and R4.
If your using a quality JFET op-amp it needs a negative supply rail (ideally) so it can bias itself, but there is a way to cheat and you have already stumbled into it. Install a 10K resistor from the outputs of U1 and U2 to the 12 volt supply rail.
This forces an internal bias current to correct for the slight offset the 10K resistor creates and stabilizes the op-amp. I replaced a lot of LM324 op-amps in car stereos with TL074 JFET op-amps which had MUCH better qualty sound, and this was the trick I used to get it to work with just a single supply rail.
NOTE: If you add an input volume control it needs to be ten times less than R2, and before the input capacitor, or you will upset the bias voltage at the + input of U1-A. Note that this pre-amp is not designed to drive a speaker of any standard impedance. It is designed to drive a medium to high input impedance power amplifier.