I am going to attempt to build a small practice guitar amp with a couple of op amp circuits I already have. I know that the LM386 is probably the standard for guitar amps, but I've already got the LM358 circuits and I have read a couple of tutorials so I know it can be done just fine. I am basing my design off of this schematic but as I am a bit new to this sort of thing I had a couple of questions.

1. I know that the two 100K pots are used to control gain and master settings respectively. Would it be a problem if I substituted 10K pots in their place, and should they be log or linear?

2. I know that I will try to drive an 8 ohm speaker, but I am unsure of what power rating I will need. I have a couple of 0.5W speakers laying around that I'd like to use, would one of those work okay?

Any help would be appreciated Cheers!

 

10 k pot increases the load on the op amp, but only 1ma at 10 volts, I would guess ok.
log tape is standard for volume control, but 10 k resistor into the op amp modified its characteristics ( and will differ for the 10 k resistor ) You might try it and see if working of volume control seems ok to you.

Using the 358 for output to a speaker seem dubious to me. Short circuit current is only 50 ma. There are lots of little audio amps out there, similar to op amps with more muscle ( lm384 is one, may not be the best ) . Or you can use push pull emitter follower booster.

 

You definitely need more output drive for a guitar amp. I used to build these years ago and the push-pull Class B design is the way to go. Look on this forum for a Class B or a push-pull amp.

Cheers, DPW [ Everyone's knowledge is in-complete...Albert Einstein]

 

The LM358 is no good for audio. It is noisy (hiss), it has up to 3% of crossover distortion and at full output its low slew rate reduces its output at frequencies higher than only 2kHz. We can hear up to 20kHz.

A guitar pickup needs a load of at least 3M ohms. But since you used inverting opamps the input resistance is only 10k ohms which is like a short circuit to a guitar pickup. A TL07x opamp has Jfet inputs and when it is non-inverting its input resistance can easily be 3M ohms. The TL07x opamps are low noise and have a full output frequency of 100kHz.

You are feeding half of the noise on the supply to the non-inverting inputs of the opamps. A filter capacitor should be used.

 

Non unity gain circuits can have much high impedance than inverting circuits, you may want 2 unity gain non-inverting buffers feeding an inverting adder. I knew someone would dump on the LM358, I am sure the TL07x is a better amp.

That said you may find the lm358 ok, my understanding is that guitar guys like distortion and my calcs suggest the lm358 can swing 10 v peak to peak within its slew rate at 10k hz. I may have blown the calc, but so far it suggests for a pratice amp this may be ok. In any case I am old, and male and can not here to 20 khz any more, and most of my speaker probably cannot do it either.

I have done a lot of stuff with the much maligned 741, some it I think impossible according to this forum. I would not use it now but some of this old junk is better than it gets credit for. I would at least consider prototyping the preamp with what you have in hand.

 

An 8 ohm speaker was mentioned but a power amplifier IC was not.
An opamp will have a max output of only 0.0025W at clipping into an 8 ohm speaker. Barely audible because an opamp cannot drive a speaker.