This is a continuation from a previous post. Read here if you have further questions.
I have decided to makee a simple guitar amplifier with the circuit seen in the attached figure.
What maximum value of variable resistors should I use for the variable volume (R1) and variable gain (R2)? The amplifier I will use is the LM386.
I want the maximum gain to correspond to an output voltage of 10V. How can I figure out what the voltage coming from my guitar might be?
For R2, just follow the datasheet or application examples. There must be some useful info available. Any value would work I reckon as it'll not appear on the signal chain.
For R1, the story is a bit different. The volume pot of the guitar interacts with the tone control circuitry and the pickup itself. And the pot is almost always the final divider therefore it connects to the output jack (directly, or through the selection switch, if there's any).
So if you place a pot directly to the signal input, it'll interact with the guitar's volume pot (for active-pickup guitars it's usually a few tens of kΩ, but for passive-pickup ones it can be as high as 500 kΩ) so you may experience some tone changes as you play with the R1 or guitar's volume pot. Now even if you place the highest possible value (e.g. 10 MΩ) to make the interaction minimum, it'll be limited by the input impedance of the LM386 which is about 50k, so an unwanted tone and maybe volume change appears to be inevitable.
Instead, buffer the input first (e.g. TL07x or TL08x) and feed the output to a potentiometer. This makes the interaction minimum, and allows you to select any (almost) pot value. 1k to 10k would then be sufficient because the buffer will isolate the guitar from the signal chain.
Electric guitars that do not have a built-in preamplifier are typically high impedance devices, and for the best quality/volume of sound delivered into the amplifier, they need the amplifier's input to present a high impedance to the guitar's pickup(s).
With the simple input volume control in your schematic, typical controls are audio taper potentiometers with values of 500k to 1 meg ohms. I've seen 250k ohms in a few amplifiers, but that can start to alter the coloration of the signal from the pickups.
Your R2 gain control is only connected to the amplifier circuitry, it has no effect on the input impedance seen by the guitar pickups, so the appropriate value will be found in the application notes for the amplifier chip.
