DIY Audio Build: SURGEON 25 (Part 3) - Mid-Side Processing, and Learning by Necessity, After a First Major Mistake

Mid-Side toggle switch on the front panel

Mid-Side Integration and Relay Control

Part 3 continues from the core circuit discussion in Part 2 and focuses on how mid-side functionality was integrated into the system. Along the way, I made my first major mistake on this project, one that changed the direction of the build and made it clear I would need to step beyond assembly and learn new skills to move forward.

The sections that follow cover how mid-side was incorporated into the signal flow, and how that first major mistake led me to design my first schematic and PCB (printed circuit board).

Mid-Side Processing: Context and Intent

Mid-side processing treats a stereo signal as two components: what is common to both channels (mid), and what differs between them (side). It is also sometimes referred to as sum-difference processing.

Basic illustration of stereo vs. mid-side

In mastering, this provides another way to work with the stereo image. It allows adjustments to the center without affecting the sides, or subtle changes to width without disturbing the overall balance. If you’re not familiar with mid-side, it can help to think of it like separating the foreground and background of an image and adjusting each independently. It’s not a perfect analogy, but it’s the closest one I can think of to describe what mid-side makes possible.

How would you adjust the foreground and background differently to make this image pop?

For this project, mid-side was intended as an option I could engage when needed, without changing how the EQ behaves the rest of the time. In stereo mode, the two channels operate as a conventional left and right pair. When mid-side is engaged, those same channels process mid and side instead, and are converted back to left and right at the output. The entire signal path remains analog throughout.

Signal flow of stereo and mid-side modes

The KA Electronics MS Board

For the mid-side encoder and decoder, I chose the MS Mini board from KA Electronics. KA offers several mid-side designs, ranging from simple encoder-decoder matrices to more elaborate boards with independent control over encoding and decoding stages. Those larger boards are useful when working with incoming mid-side signals, or when a unit is expected to output mid-side directly.

For this EQ, I was only looking for a straightforward way to convert left and right into mid and side, process those signals, and convert them back again. The MS Mini did exactly that, with a simple circuit and a lower cost. It seemed like the right fit.

The MS Mini board by KA Electronics, fully assembled. The top half is the MS encoder, the bottom half is the MS decoder.

What I hadn’t fully thought through at the time was how that board would be engaged and disengaged in practice. My initial assumption was that the MS matrix could sit between the I/O and the EQ boards, with a bypass switch determining whether it was in the signal path or not.

Uroš pointed out the error in my thinking and helped me understand what was actually required. Switching between stereo and mid-side means rerouting multiple balanced signal paths in a coordinated way, both before and after the EQ stages.

This block diagram that Uroš drew shows the switching logic.

In practical terms, four routing decisions need to change together.

• The audio input must feed either the EQ directly or the MS encoder

• The EQ input must select between the audio input and the encoder output

• The EQ output must feed either the audio output or the MS decoder

• The audio output must select between the EQ output and the decoder output

Each of these choices has to happen on both channels at the same time, all controlled by a single toggle switch on the front panel.

This block diagram shows how the relays divert the signal flow to engage or bypass the mid-side encoder and decoder.

Why a Relay Switcher Became Necessary

By the time the routing requirements became clear, I had already assembled the MS Mini kit. Rather than replacing it with a larger all-in-one solution, I designed the missing piece myself: a relay-based switching system capable of handling the required routing reliably and predictably.

Stereo/mid-side selector

Relays were the practical solution. They keep the audio switching local and quiet, while the front-panel switch simply selects the mode. The relays handle the routing close to the boards, which made it possible to solve a fairly involved routing problem without turning the internal wiring into a liability.

Designing the relay switcher meant turning the signal paths and control logic into a schematic and PCB that could actually be built. It became the link between the MS encoder, the EQ circuits, and the outputs, and the first custom circuit board in the project.

Designing and Building the Relay Switcher

Up to this point, I had spent a lot of time reading and studying schematics, and assembling PCBs designed by others. Designing a schematic of my own was new territory. I downloaded KiCad, a free and open-source schematic and PCB design tool, and started learning my way through it one mistake at a time.

The circuit schematic for the relay switcher

The process was iterative and sometimes slow. I translated the signal flow I had already worked out into a schematic, revised it as my understanding improved, and gradually learned how to turn that schematic into a PCB layout that made sense both electrically and physically.

The PCB design for the relay switcher

Once the board was finally ready, I sent it out for fabrication. I chose a red board because… why not? That board became my first original PCB design, and a clear marker of how far this project had pushed me beyond assembly alone.

My first circuit design and PCB: MS Relay Board - bare

My first circuit design and PCB: MS Relay Board - assembled

Looking back, the larger KA Electronics board with integrated relays would have simplified the build, and I would likely choose that route next time. Designing and building the relay matrix myself, however, forced me to learn schematic design and PCB layout, and deepened my understanding of how the system fits together. The experience taught me things I wouldn’t have learned otherwise. This project was, quite simply, a significant learning experience for me.

The relay switcher board wired up to the MS board

With the mid-side system in place, the focus shifts next to areas of the build where mistakes carry real consequences.

Surgeon 25 internal overview