Power

Syntsamix

A modular, analog stereo mixer for live electronic music: synthesizers, drum machines and Eurorack modular gear on one desk, with a resonant ladder filter on every channel and a compressor built for bass pumping.

Status: in design. The specification is complete and the schematics for all four prototype boards are done. Nothing has been built yet: no PCB is laid out and the filter breadboard is still waiting for parts. Figures below are design targets, not measurements.

Goals

• Made for playing live. Few controls, each with an obvious musical effect; the mixer is part of the performance, not only a utility.
• Modular. Identical stereo channel cards, added one at a time from 1 to 16. No card depends on another, and there is no backplane: cards are joined by multi-drop flat cables in 8-card sections, with the master card at one end.
• Synth-friendly inputs. Balanced inputs take everything from consumer line level to hot Eurorack signals of ±12 V peak, with no external attenuator.
• Analog signal path with pro headroom: +4 dBu nominal, about +20 dBu internal maximum, +24 dBu at the balanced outputs.
• Cost-effective. A tight budget: cheaper parts wherever the audible difference is small.
• Open design for anyone to build for themselves (non-commercial license, see below).

Features

Channel strip (stereo, every element an L/R pair)

• Balanced 6.3 mm TRS inputs (L/MONO and R) on a separate, swappable passive input module; a mono source in L/MONO feeds both sides
• One wide input trim (about -20 to +20 dB) with soft clipping, so overloads saturate gently
• Switchable 100 Hz low-cut
• 24 dB/octave ladder low-pass filter (Sound Semiconductor SSI2144) with cutoff (20 Hz to 20 kHz), resonance and bypass. Driven "hot" for character, with resonance capped below self-oscillation and the bass loss half compensated
• Cutoff CV input at about 1 V/octave for modulation from modular gear
• 60 mm fader driving a VCA (SSI2162) with a console-style law: +10 dB at the top, 0 dB at 75 % of travel, fully off at the bottom; click-free mute
• Two stereo AUX sends (post-fader, jumper for pre-fader)
• Main or compressor bus assign
• PFL (pre-fader listen): headphones switch to the cue bus automatically while any PFL is on
• SC send to the sidechain bus and DUCK, which lets the channel duck with the sidechain trigger
• 8-segment LED level meter
• Lit latching buttons; stereo functions switch electronically, so buttons carry only logic

Master and compressor

• Performance compressor on a dedicated compressor bus: Amount (threshold and makeup together), Release, Mix (parallel compression), click-free on/off and gain-reduction LEDs. Feed-forward log peak detector, about 1 ms attack, 4:1
• Sidechain from the compressor bus itself, the channels' sidechain bus or an external jack (audio or Eurorack gates and envelopes), with a 40 to 500 Hz low-pass so the kick drives the pumping. An SC listen button puts it in the headphones
• Sidechain ducking: an envelope follower (threshold, depth, decay) ducks any channel with DUCK on, independently of the compressor
• Two stereo AUX returns (level, mute, main or compressor bus; jumper for pro or pedal levels) and two AUX send masters
• Master level through a VCA, soft clipping on the master bus
• Balanced main outputs (TI DRV135, +24 dBu) and a headphone amplifier (TPA6120A2, 32 to 600 Ω)
• Stereo 12-segment master meter
• Output protection relays: silent power-up and power-down on the main and headphone outputs

Construction

• Desktop unit: one PCB and a 35 mm FR4 top panel per channel strip, on an aluminium rail frame cut to length (about 650 to 700 mm wide at 16 strips)
• Powered by an off-the-shelf DC brick through a separate power board (24 V brick, non-isolated converters to ±20 V); every card regulates its own supply
• Separate audio and power ribbons, with audio and power grounds joined at a single star point
• RoHS compliant: every part needs a maker or supplier RoHS statement before it is chosen, and the boards are built lead-free

Status

Also done: ngspice simulations of the filter gain structure, the compressor detector, the sidechain/ducking circuits and the fader level control (simulation/); the mechanical contract for strips, panels and frame (docs/MECHANICAL.md); a RoHS audit of the parts register. Next: the SSI2144 breadboard, then PCB layout, starting with the channel card. The prototype has 4 channels.

The full roadmap and backlog (more input modules, direct outputs, 32 channels, and more) are in docs/ROADMAP.md.

Repository

Schematics are generated by Python scripts and checked pin by pin against an independent reference netlist; see hardware/channel-card/scripts/README.md. The design is developed with Claude Code; docs/WORKING-WITH-CLAUDE.md describes the workflow.

License

Source-available for non-commercial use; the author keeps all commercial rights. Hardware and documentation are CC BY-NC-SA 4.0, code is PolyForm Noncommercial 1.0.0. You may build, modify and play the mixer (paid gigs included), but not sell boards, kits or units without permission. Details in LICENSE.md.