Guitar preamp problems

Question

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?

Answer 1

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.

Answer 2

The input impedance is probably too high which could be causing the output to drift. Or if it's a bipolar op amp, you could be hearing the output offset bouncing. The reason it works when you use the oscilloscope is because the proe input impedance is only 1M probably.

Get rid of R1 and reduce R2 to 1M. Also, ideally you should use a JFET input op amp like TL072.

Answer 3

You have to be super careful in these circuits about capacitance from input to output. A capacitance of only 16pF has an impedance of 1M at 10kHz, so it is rather easy to have stray paths that create unintended oscillation.

This is one reason that input impedance of these circuits is a bit of a compromise - while 1M or even higher is often quoted as an ideal input impedance for a guitar pickup, values of 500k to 1M are common (except in circuits designed for piezo pickups which usually have the preamp near to the pickup).

As other has stated, lose the 10M resistor R1, or move it the other side of the input cap as it is messing up the biassing. (Be aware that measuring DC voltage at the input with a DVM is probably misleading because teh DVM will load the cct.) The other 10M resistor R2 which goes to your artifical ground should be reduced to 1M initially, then check the bias point using a 10M scope probe and scope, or something else with an input impedance of 10M or more. You might want to try other values from 470k to 1M and see if they give acceptable results. Having a slightly lower input resistor will improve stability and also make the circuit a bit less susceptible to hum.