After focusing on synth modules, I wanted to explore the studio side of analog electronics — specifically, how to sum multiple audio sources with precision and warmth. This led me to design a 16-channel summing board, using high-quality op-amps and proper gain staging for clean, high-fidelity analog summing.
Concept
Modern digital mixes often sound sterile when everything is combined inside a DAW.
A passive or active analog summing mixer helps restore natural depth and width by combining signals in the analog domain. My goal was to create a compact 16-input summing circuit that maintains low noise, wide bandwidth, and transparent tone — ideal for analog mixdowns or modular synth sub-mixing.
Circuit Design
Each of the 16 input channels passes through a precision resistor network into a summing node, where an audio-grade op-amp performs the active summing.
I used low-noise, low-distortion op-amps (such as OPA2134 or NE5532, depending on test results) for their musical sound and reliability.
After the summing stage, the signal passes through a gain recovery stage — essential because each channel’s input resistor slightly attenuates the total mix. The gain stage restores proper output level without adding noise or coloration.
Stage Gaining
The board includes:
- Pre-gain trimmer per channel, allowing small balance corrections
- Main output gain stage, adjustable via a precision pot
- Optional balanced line driver for feeding external gear or audio interfaces
This setup provides full control over headroom and ensures the summed output remains clear, open, and distortion-free.
PCB & Layout
The PCB was designed with star-grounding topology to minimize hum and crosstalk.
All audio traces are short and symmetrical, using 2-oz copper for improved conductivity. Power rails are fully decoupled with low-ESR capacitors close to each op-amp.
The board runs on a ±15 V power supply for maximum dynamic range and professional-grade output swing.
Build & Testing
Once assembled, I tested each input with precision audio signals and measured the noise floor, THD, and frequency response.
The results were excellent: flat from 10 Hz to 40 kHz, THD < 0.002%, and an extremely low noise floor even at full load. The summed stereo output retained all detail and spatial information — clean, punchy, and musical.
Applications
- Analog summing for DAW stems
- Modular synth sub-mixing
- Hi-fi pre-mix distribution
- Experimental analog processing chain
Final Thoughts
This 16-channel op-amp summing board turned out to be a powerful, transparent analog tool — simple, elegant, and sonically pure. It’s ideal for anyone who wants to blend multiple audio sources without sacrificing fidelity.