Vintage Guitar Preamp re-design

I have seen this circuit many years ago and I did planned to build this circuit but I never got a chance. This time though let’s try to re-design it to work with lower voltage and use an smaller amplifier for practicing. I am planning to use it for 12V or 9V amp so we need to modify it to work below 9V instead of the 60V supply which is the original design.

About the Ciruit

This circuit is all over the internet now and you can find it in many website. It is quite simple although the biggest problem in building this circuit is the operating voltage which is 60V. I will try to modify it to work with 9V supply. This task should be easy by using simulation software such as simetrix. I’m too lazy to compute so I just modify it by trial and error on the simulation by intelligent guessing of course! The main focus here are the high side resistors such as R3, R6 and R17. The goal is to decrease its value until you will come up with same bias level with the original supply voltage.

There is a small error I spot on this design – C11 should be connected to ground and not on the emitter of Q3. I wonder how they made this error.

Simulation Circuit

Again, I am lazy to match the names of the device on my simulation model so let’s keep it this way. 🙂

I did not included the “Harmonic Modifier” circuit on the simulation circuit because I want to keep it as is.

Bias Levels

I only have noted those points of interest here:

The level at Q3-emitter is around 26V, since our goal is to operate it at 12V, it wont be able to reach 26V when VDD is only 12V. We just need the voltage at this point to be half the supply. Same is true with Q2-emitter voltage.

Frequency Response

Here’s the frequency response of the original circuit.

Here’s the 12V modified version:

There are minimal changes, the base bias of Q1 and Q3 have been modified to make it simple. Added R20 and C7 to make up the gain at low voltage operation.

Simulation result of the modified circuit

The circuit has slightly higher gain. You can try to experiment on the values of the resistors and capacitors. You can download the simulation file below. Simetrix is used for this simulation. It is a free software.

PCB Design

The PCB design is made using the kicad software, it is also free so anybody can download and use it. The design is very simple because I have no plans to build it for now. I just wanted to share the design for those who want to build this kind of project. You are free to edit it for your own requirements.