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The Fender Mustang LT-25 is a versatile practice guitar amplifier. However it lacks a effects loop (a break between the pre-amp and the power-amp stages where the user can plug in additional effects). I reverse engineered these two stages and created buffer circuits to go between the two stages and create a differential audio signal which the power amplifier stage requires. The buffers utilize the existing power supply from the guitar amp.
Currently the PCB for the first prototype has been tested and improved upon to create a second prototype.
First prototype
First prototype
Update: 1/23/2025
The first prototype proved that the overall design works. However, since the audio signals are biased with a voltage divider, it couples power supply noise into the audio input.
The power supply noise was measured to be a 1.25kHz sawtooth wave with 0.5V amplitude.
Two possible solutions were considered:
Use an virtual ground IC to bias the audio signal.
Add a LC lowpass filter on the power rail.
Solution 2 was chosen for lower cost of implementation.
In addition, the unused op-amp was also properly disabled in prototype #2, and additional capacitor were added to prevent instability at unity gain.
Prototype #2 was ordered and will arrive soon for testing.
Second prototype
Second prototype
The second prototype was installed and tested for functionality with and without effect pedals. The signals of the differential audio output were measured and are low in noise and very close to amplitude to the signal generated by the pre-amp.
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