The design is slowly starting to take shape for my guitar amplifier circuit.  Here is the power amplifier stage with the simple effects loop and the tone stack portion of the pre amp that I will be testing next:

Tone Stack


This is essentially the same three knob tone stack found in most big name amplifiers. I’m still working on tweaking some values and have not yet wired this one up.
Using the very useful Duncan Amps Tone Stack Calculator this is the frequency graph for the above diagramed circuit:

Tone Range

One of the challenges to designing a guitar amplifier from scratch is balancing what can be done with what is realistic to do for the particular project. In a full sized amplifier there is the space and perhaps even the need to put in plenty of control over the sound. Many two channel amplifier designs have a tone stack for each pre amp channel and although I could implement that, I keep having to remind myself that this is a tiny battery-powered 1/2W amplifier meant for testing DIY effects pedals and low-volume practicing. The whole assembly is intended to be small and although I am including two channels and an effects loop – the part count should be kept as minimal as possible.
I’ve not been super enthused with the performance of the one and two knob tone controls I’ve played with so far which prompted me to pick up some more potentiometers at the store yesterday and work on the above revision/compromise to the design. Part of the issue was I did not have any pots that were of large enough value to dramatically affect the tone.
In the keep-it-simple mindset my aim now is to implement a single pre amp tone stack before the effects loop that will be used by both the clean and dirt channels. I’ll be probably going with a three control design for greater shaping of the sound – my thinking being that if there is going to be only one tone stack, it should be a good one.

Even simplified down to this level there will still be six potentiometers in the circuit: Clean Volume, Dirt Gain, Dirt Volume, Bass, Mid, Treble. Coupled with a power switch, input jack, headphone jack and channel selector switch this is going to be a crowded control panel as it is without having to add more pots for a second tone stack.

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