DIY Audio Build: SURGEON 25 (Part 1)
- Eitan Brown
- Jan 12
- 6 min read
Building a Precision Mastering Equalizer, One Decision at a Time
Surgeon 25 started as a conversation.
I met Uroš Đorđević, an audio engineer from Serbia, who showed me several pieces of equipment he had built himself. Among them was a dual 5-band parametric mastering equalizer, which is an adaptation of the Sontec 250A topology, with modifications, based on schematics and main PCBs by Hieg Khatcherian.
Uroš had already built and used the unit, and he offered to share the schematics and boards if I wanted to take on the project. More importantly, he offered guidance throughout the build, and followed through on that promise with patience and generosity.
At the time, I had already been building audio gear for several years: microphones, mic preamps, DI boxes, 500-series modules, racks. Mostly kits, mostly well-documented, with a clear path from start to finish. I was also considering building a mastering equalizer of some kind.
This opportunity arrived at a point where I had the tools, the curiosity, and the willingness to commit to a project that would take longer and demand more care than anything I had built before.
This post is the first in a five-part series documenting the process of building Surgeon 25. It’s intended as an overview and is written to be readable even if you’re not deeply technical. The posts that follow will go into much more detail — covering circuit architecture, system-level decisions, mechanical design, and calibration — but this one is about setting the context and explaining how the project took shape.
Why DIY Audio Build, Why This EQ, and Why Then
I wanted to build a precise, transparent mastering equalizer, and this design was available, proven, and well suited to that goal. The Sontec 250A lineage is known for clean behavior, carefully chosen control ranges, and the ability to make small, controlled changes without imposing a strong sonic fingerprint — qualities that translate well to mastering work.
Just as importantly, this design already existed as a working circuit, with real-world precedent and support from someone who had already navigated the process.
I enjoy various DIY audio build projects, and this one seemed like the logical next step.
From Study to Commitment
Before ordering anything, I spent a month or two studying.
I went through the schematics repeatedly, traced signal paths, and familiarized myself with how the bands interacted, how gain was handled, how bypass worked, and what assumptions the design made about power, grounding, and control behavior. The goal was to understand the circuit well enough to build it responsibly.
Once the first major parts order went in, there was no turning back. At that point, the project became do-or-die. Half-finishing something like this isn’t really an option.
So the pace was deliberate. There was no deadline. The guiding rule was simple: if I’m going to build this, I’m going to build it as well as I possibly can.
Dual Mono as a Structural Choice
One of the earliest decisions was to build the unit as dual mono, rather than a permanently linked stereo EQ.
Functionally, this allows the unit to be used in several ways:
As a stereo mastering EQ
In mid-side mode
Or as two independent mono EQs
From a construction standpoint, this meant duplicated controls, duplicated signal paths, and a significant increase in physical work: more switches, more wiring, and more opportunities for errors that only reveal themselves later.
It also placed a strong emphasis on component consistency. Regardless of whether channels are controlled by separate switches or by multi-gang switches, predictable stereo behavior depends on closely matched components. That requirement became one of the defining aspects of the build.
Switch Assembly and Resistor Matching
The project uses 30 rotary switches across the two channels. Altogether, those switches require well over 750 resistors of various values.
To assemble them, I measured more than 10,000 resistors by hand, one by one, grouping them into matched pairs and quads so that each switch position behaved as closely as possible between channels. This process alone took over a month, not including wiring.
In mastering, small inconsistencies add up quickly. Even slight channel-to-channel differences can become audible long before they show up on a meter. Starting with tightly matched components reduces the need for later correction and makes behavior more predictable across the entire control range.
This part of the build was slow and repetitive, but it underpins everything that follows.
Mid-Side as a Selectable Mode
Mid-side processing works by converting a standard left/right stereo signal into two components: a mid signal (information common to both channels) and a side signal (information that differs between them). This allows spatial and tonal adjustments to be made in ways that aren’t possible using conventional stereo processing.
The MS encoder/decoder board came from KA Electronics. While it performs the conversion cleanly, it is designed for permanent MS operation. I wanted MS to be something I could engage or disengage from the front panel, switching cleanly between stereo operation and mid-side operation as needed.
Solving that meant designing a relay-based switching system capable of routing multiple balanced audio paths in and out of the MS matrix while maintaining signal integrity, all controlled by a single toggle switch. This marked the point where the project expanded beyond assembly and into circuit and PCB design.
Power Supply: Adapting to Location and Goals
Power was another area that required rethinking.
Japan runs on 100 VAC, which affects transformer selection and overall PSU design. Noise performance was also a priority, given the goal of a quiet, transparent mastering equalizer.
My friend, Shun Yoshino, found and recommended a linear power supply kit designed by a trusted audio engineer for upgrading high-end audio equipment. After evaluating the options, I decided to house the power supply in a separate external chassis.
That decision reduced magnetic and mechanical interference in the main unit, but it also introduced additional considerations: a second enclosure, grounding strategy across two chassis, and a reliable DC umbilical connection between them.
None of these problems are particularly exotic. They’re just unforgiving. Small mistakes in power and grounding tend to show up as noise, instability, or hard-to-trace behavior later.
Learning Beyond Assembly
Somewhere along the way, the project stopped being about only building an EQ and started being about understanding systems.
Designing a relay board meant learning schematic capture and PCB layout. Integrating that board meant thinking about signal routing, grounding, and mechanical constraints at the same time. Designing front and rear panels meant planning control spacing, mounting depth, and service access before anything was physically assembled.
None of this was planned at the outset. It simply became necessary.
That’s one of the quieter lessons of a project like this: as responsibility increases, problems stop being isolated. Everything interacts.
The Middle of the Build
Not everything worked the first time.
Some issues that appeared electrical turned out to be mechanical. Some solder joints passed visual inspection but failed under probing. Some switches behaved unpredictably due to subtle physical issues rather than wiring errors.
There was also a lot of repetition: measuring, adjusting, testing, listening, and measuring again. Comparing channels. Verifying bypass behavior. Checking power-up and power-down characteristics.
This part of the process doesn’t compress well into a clean narrative, but it’s where confidence in the finished unit is built.
What Surgeon 25 Represents
Surgeon 25 is the result of a long sequence of decisions, most of them small, many of them time-consuming.
It’s based on an existing design, extended where needed, adapted where required, and built with care appropriate to how it’s meant to be used. The common thread throughout the project was patience — taking the time to learn what was necessary and to address problems as they arose.
I learned a lot in the process of building the Surgeon 25 EQ. This project was school for me.
What Comes Next
This overview sets the stage. The next four posts will focus on specific parts of the build in more detail:
Understanding the Core Design: what was inherited and how it behaves
Architecture and Modifications: dual mono structure, mid-side integration, and relay control
Power, Grounding, and Mechanical Design: external PSU, chassis choices, wiring strategy
Assembly, Calibration, and Verification: issues encountered, solutions, and behavior in use
Each post builds on this one, adding detail without losing sight of the larger picture.
Closing
This was a careful project.
It required slowing down, repeating work that didn’t look productive from the outside, and spending time on details that only become visible much later, if at all. It involved learning new skills as they became necessary and revisiting earlier decisions as the system around them took shape.
Surgeon 25 exists because those choices were made deliberately, one after another, over the course of 8 months, and because of the generous guidance and expertise from Uroš Đorđević throughout the process.
The posts that follow go deeper into how that happened.
Congratulations on this incredible project Eitan!! Thanks for sharing this detailed narrative of the process. You have so much to offer the sound engineering community (and music community more broadly). Looking forward to listening to the music mastered with the Surgeon 25!