S-Trig vs V-Trig: MIDI to CV Adapters Explained

When bridging modern MIDI gear with vintage analog synthesizers, understanding trigger formats is critical for proper note gating. This article explains the difference between Voltage Triggers (V-Trig) and Switch Triggers (S-Trig), why standard MIDI-to-Control-Voltage (MIDI-to-CV) converters fail to trigger certain vintage instruments out of the box, and how an S-Trig vs V-Trig adapter functions to ensure seamless communication between your equipment.

Understanding V-Trig and S-Trig

Control Voltage (CV) setups require two primary signals to play a note: a pitch CV (determining which note sounds) and a gate or trigger signal (determining when the note starts and stops). Manufacturers historically used two competing methods to handle this gate signal:

• V-Trig (Voltage Trigger): Adopted by manufacturers like Roland, Sequential Circuits, ARP, and modern Eurorack systems. In a V-Trig system, the signal rests at 0 volts (ground) when no key is pressed and sends a positive voltage (typically +5V to +12V) when a key is held down to open the amplifier and filter envelopes.
• S-Trig (Switch Trigger): Famously utilized by vintage Moog, Korg, and Yamaha synthesizers. In an S-Trig circuit, the resting state is held at a positive voltage via an internal pull-up resistor inside the synth. When a key is pressed, the circuit is physically shorted to ground (0V) to trigger the envelopes.

The Incompatibility Problem in MIDI-to-CV

Modern MIDI-to-CV converters almost universally output standard V-Trig signals. When you play a note over MIDI, the converter outputs a positive voltage gate.

If you connect a standard V-Trig output directly into a vintage synthesizer requiring an S-Trig input, the synthesizer will misbehave. Because S-Trig relies on a ground connection rather than positive voltage, the instrument will typically either fail to produce sound, invert the gate behavior (sustaining indefinitely when released and cutting off when pressed), or risk damaging sensitive vintage trigger circuitry due to improper voltages.

How an S-Trig to V-Trig Adapter Works

An S-Trig adapter is an electronic circuit—often housed inside a small cable, an external box, or integrated into specialized CV interfaces—designed to translate positive voltage gates into ground switches.

1. Circuit Operation: The adapter typically uses an NPN transistor, an optocoupler, or an operational amplifier. When the MIDI-to-CV converter sends a positive V-Trig voltage pulse into the base of the transistor, the transistor conducts, effectively pulling the S-Trig line down to ground (0V).
2. Key Release: When the V-Trig signal drops back to 0V (note-off), the transistor stops conducting, allowing the vintage synthesizer's internal pull-up voltage to return to high, closing the envelopes.
3. Physical Connectors: In addition to electrical conversion, many adapters convert physical jack types, such as translating a standard 1/4-inch or 3.5mm TS plug into the two-pin Cinch-Jones connector historically used on vintage Moog synthesizers.

Using an S-Trig vs V-Trig adapter provides a safe, reliable bridge, allowing modern DAWs and MIDI sequencers to accurately articulate notes on vintage gear without modifying the original analog circuitry.