A-151

Doepfer 4HP
SWITCH SEQUENCER UTILITY

Verified sheet

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Quad bi-directional sequential switch. Each trigger steps a common O/I jack through 4 I/O jacks; reset returns to step 1. Version 2 adds Steps switch (2/3/4) and handles full ±12V signals. LEDs indicate active step.

Patch Ideas · 6

★ 4-note pitch sequencer
Four offset voltages → I/O 1–4 · clock → Trig. In · O/I → VCO 1V/oct
▸ Show diagram
Patch diagramPatch diagram with 7 modules and 6 connections. Signals: 4 cv, 1 pitch, 1 clock.A-151Offset 1Offset 2Offset 3Offset 4ClockVCOSteps: 4I/O 1cvI/O 2cvI/O 3cvI/O 4cvTrig. InclkO/I1v/octCVcvCVcvCVcvCVcvClkclk1V/oct1v/octcvpitchclock
★ VCO waveform stepper
VCO saw/tri/sqr/sine → I/O 1–4 · LFO → Trig. In · O/I → VCF
▸ Show diagram
Patch diagramPatch diagram with 4 modules and 6 connections. Modules: A-151, VCO, LFO, VCF. Signals: 5 audio, 1 clock.A-151VCOLFOVCFI/O 1audioI/O 2audioI/O 3audioI/O 4audioTrig. InclkO/IaudioSawaudioTriaudioSqraudioSineaudioSqrclkInaudio11. timbral cyclingaudioclock
★ Rotating envelope across 4 voices
Gate source → O/I · I/O 1–4 → four ADSR gates · each note triggers a different envelope
▸ Show diagram
Patch diagramPatch diagram with 6 modules and 6 connections. Modules: A-151, Keyboard, ADSR 1, ADSR 2, ADSR 3, ADSR 4. Signals: 5 gate, 1 trigger.A-151KeyboardADSR 1ADSR 2ADSR 3ADSR 4O/IgateTrig. IntrigI/O 1gateI/O 2gateI/O 3gateI/O 4gateGategateGategateGategateGategateGategate11. advance on each notegatetrigger
Three-step ostinato with reset
Steps=4 · feed /3 clock → Res. In for explicit 3-step loop · O/I → pitch
▸ Show diagram
Patch diagramPatch diagram with 7 modules and 6 connections. Signals: 3 cv, 1 pitch, 1 trigger, 1 clock.A-151ClockClock÷3Pitch APitch BPitch CVCOSteps: 4Trig. InclkRes. IntrigI/O 1cvI/O 2cvI/O 3cvO/I1v/octClkclkClktrigCVcvCVcvCVcv1V/oct1v/octcvpitchtriggerclock
Krell-style voice rotator
Self-cycling EG end-of-cycle → Trig. In · four oscillator outs → I/O 1–4 · avoids clicks by switching between cycles
▸ Show diagram
Patch diagramPatch diagram with 7 modules and 6 connections. Signals: 5 audio, 1 trigger.A-151VCO 1VCO 2VCO 3VCO 4Function EGVCAI/O 1audioI/O 2audioI/O 3audioI/O 4audioTrig. IntrigO/IaudioOutaudioOutaudioOutaudioOutaudioEOCtrigInaudio11. new voice each Krell cycleaudiotrigger
Audio-rate wavefolder
VCO square at audio rate → Trig. In · four offsets → I/O 1–4 · O/I is new composite wave
▸ Show diagram
Patch diagramPatch diagram with 7 modules and 6 connections. Signals: 1 audio, 4 cv, 1 clock.A-151Offset AOffset BOffset COffset DVCOVCFI/O 1cvI/O 2cvI/O 3cvI/O 4cvTrig. InclkO/IaudioCVcvCVcvCVcvCVcvSqrclkInaudio11. audio-rate steppingaudiocvclock

Behaviors

4-to-1 multiplexer sources → I/O 1–4 · O/I → destination

Trigger steps through four source signals, presenting one at a time at O/I. Useful for cycling CVs, VCO waveforms, or audio sources at clock rate.

1-to-4 demultiplexer source → O/I · I/O 1–4 → destinations

Switch is bi-directional: a single signal at O/I is routed in turn to one of the four I/O jacks. Useful for rotating a trigger across four voices or envelope generators.

Reset-shortened loop periodic pulse → Res. In

Send a pulse to Res. In every N trigs to loop just the first N steps. Combined with the Steps switch, produces arbitrary short patterns like 1→2→3 or 1→2.

Audio-rate wave mangler VCO square → Trig. In · waveforms → I/O 1–4

When Trig. In is driven at audio rate, the A-151 switches so fast that the ear hears a composite wave — effectively waveform synthesis by time-slicing between four VCO outputs.

Voltage range clipping input exceeds ±8V on V1

V1 modules malfunction above ±8V; V2 handles the full A-100 ±12V range. Attenuate or offset large signals before V1 units, or identify the version by the presence of the Steps switch.

Controls

Sequence length Steps Limits the step count to 2, 3, or 4 — shortens the switched cycle before wrap-around. V2 only.
positions: 2 · 3 · 4
Indicators LED 1–4 Four status LEDs light the currently active I/O jack.
one-hot display

I/O

IN · 2

  • Trig. In trigger/gate GATE
    Advance input. Each rising edge steps the switch to the next I/O; after the last step it wraps to I/O 1.
  • Res. In trigger/gate GATE
    Reset input. Rising edge immediately returns the active connection to I/O 1.

OUT · 5

  • O/I v1: ±8V · v2: ±12V
    Common jack — acts as output when I/O 1–4 are driven as inputs, or as input when I/O 1–4 are used as outputs. Direction is determined by patching.
  • I/O 1 v1: ±8V · v2: ±12V
    Sequential tap 1 — routed to/from O/I on step 1.
  • I/O 2 v1: ±8V · v2: ±12V
    Sequential tap 2.
  • I/O 3 v1: ±8V · v2: ±12V
    Sequential tap 3.
  • I/O 4 v1: ±8V · v2: ±12V
    Sequential tap 4 (skipped when Steps set to 2 or 3).