A-151 - Synthesizer Doepfer - Free user manual and instructions

Product Type	Quad Sequential Switch (Eurorack synthesizer module)
Brand	Doepfer
Model	A-151
Width	8 HP (approx. 40.5 mm)
Height	3U (approx. 128.5 mm)
Depth	Approx. 40 mm
Weight	Approx. 120 g
Power Supply	+12V: 30 mA, -12V: 10 mA (typical)
Voltage Range (Version 1)	-8V to +8V
Voltage Range (Version 2)	-12V to +12V (full A-100 range)
Main Functions	Quad sequential switch, trigger and reset inputs, four bi-directional I/O sockets, step number selection (2/3/4 on version 2)
Indicators	4 LEDs showing active I/O
Additional Controls	Steps switch (version 2) to limit addressed steps to 2, 3 or 4
Connections	Trig In, Res In, Common O/I, I/O 1–4
Switching Function	Each trigger pulse advances common to next I/O; reset returns to first
Typical Applications	Switching VCO waveforms, four-step sequencing, switching filter types
Maintenance	Clean with dry cloth; no user-serviceable parts
Safety	Use only within specified voltage range; avoid exceeding power supply limits
Spare Parts / Repairability	Contact Doepfer or authorized dealer; module should only be repaired by qualified personnel
General Information	Part of Doepfer A-100 system; designed for modular synthesis

Frequently Asked Questions - A-151 Doepfer

How does the A-151 trigger input work?

Each rising edge of a pulse at the Trig In socket advances the common O/I to the next I/O socket. After the fourth I/O, the next trigger returns to the first, creating a cyclic sequence.

What is the voltage range for signals on the A-151?

Version 1 (without steps switch) handles -8V to +8V. Version 2 (with steps switch) handles the full A-100 range: -12V to +12V. Exceeding the range may cause malfunction.

How do I reset the A-151 to the first I/O?

Apply a positive pulse to the Res In socket. The rising edge immediately switches the common O/I back to I/O 1, overriding the normal stepping sequence.

What is the Steps switch used for?

On version 2, the Steps switch limits the number of addressed I/O sockets to 2, 3, or 4. For example, set to 2 to cycle between I/O 1 and I/O 2 only.

Can the A-151 switch audio signals?

Yes, the A-151 is bi-directional and can switch audio signals as well as control voltages, provided they stay within the module's voltage range. Very fast triggers can produce audio-frequency modulation.

What do the four LEDs on the A-151 indicate?

Each LED corresponds to an I/O socket (1–4). The lit LED shows which I/O is currently connected to the common O/I socket.

How can I create a repeating three-step sequence?

Set the Steps switch to 3 and connect a reset pulse to Res In at the desired point. For example, cycle through I/O 1, 2, 3, then reset to 1: sequence becomes 1→2→3→1→2...

Is the A-151 compatible with the Doepfer A-100 system?

Yes, it is designed for the A-100 system. It fits standard Eurorack cases and uses the standard power connector (16-pin ribbon cable with +12V and -12V rails).

Can I use the A-151 as a multiplexer?

Yes, it can act as a 4-to-1 or 1-to-4 multiplexer depending on signal direction. Connect multiple inputs to I/O 1–4 and common as output, or vice versa.

What should I do if the A-151 behaves unexpectedly?

Check that all connections are secure, trigger signals are proper, and voltage levels are within range. If issues persist, consult the manual or contact Doepfer support. Do not attempt repairs yourself.

USER MANUAL A-151 Doepfer

1. Introduction

Module A-151 (Quad Sequential Switch) is like an electronic four-position rotary switch.

It includes trigger and reset inputs, four in / outputs, and a common out / input.

Each time a pulse is received at the trigger input socket, the common out / input is connected to the next in / output. After the fourth in / output, the next trigger makes it step back to the first again, and so on (see Fig. 1).

Voltages in the range -8V...+8V at the O/I resp. I/O sockets can be processed by the module.

Four LEDs indicate the active in / output (ie. the one that is connected to the out / input at any particular time).

A positive pulse at the reset input switches the out / input immediately back to the first in / output (see Fig. 1).

Version 2 of the module (from about 2005) is equipped with an additional switch that is used to set the number of steps to 2, 3 or 4. On top of that the new versions allows to switch audio or control signals within the full A-100 voltage range (i.e. -12V...+12V).

2. Overview

Indicators:

① LED: Indicator for in / output ④
② LED: Indicator for in / output ⑤
③ LED: Indicator for in / output ⑥
④ LED: Indicator for in / output ⑦
⑤ Steps: Switch for the limitation of the number of addressed steps to 2, 3 or 4 (available only in version 2)

In / Outputs:

① Trig. In : Input for trigger pulse
② Res. In : Input for reset pulse
③ O/I : Common out / input
④ I/O 1 : In / output 1
⑤ I/O 2: In / output 2
⑥ I/O 3 : In / output 3
⑦ I/O 4 : In / output 4

3. Indicators

① LED • ② LED • ③ LED • ④ LED

LEDs ① to ④ are the status indicators, showing which of sockets ④ to ⑦ is connected to the common out /input ③ at any moment.

⑤ Steps

This switch is used to limit the number of addressed in/outputs to 2, 3 or 4. The switch is available on in version 2 of the module (about since 2005).

4. In / Outputs

① Trig. In

Socket ① is the A-151's trigger input. With each new pulse (at the rising edge), the common out / input is switched to the next in / output.

After stepping to the fourth in / output the next trigger pulse sends the common out / input back to the first in / output again (see Fig. 1).

Don't forget that with a very fast repeating trigger, the switching process on the A-151 can produce audio-frequency modulation (see chapter 5, user examples).

② Res. In

If you want to over-ride the stepping sequence, and send the common out /input back to the first in / output, send a reset pulse to the reset input ② (see Fig. 1). The rising edge of this pulse immediately resets the common out / input to the first in / output.

Using the reset input, you can create repeated sequences which switch between three inputs or outputs (4 → 5 → 6 → 4 → 5 → 6 → 4...).

③ O/I

Socket ④ is the common out /input. Each time the A-100 is switched on, or a reset pulse is received, this socket is connected to the first in / output socket ④ (see Fig. 1).

④ I/O 1 • ⑤ I/O 2 • ⑥ I/O 3 • ⑦ I/O 4

Sockets ④ to ⑦ are the in / outputs.

Fig. 1: How the A-151 switching works (■ : in / output connected)

The switches are bi-directional, so switching can take place from four inputs to one output, or four outputs to one input. Which process is happening at any one time will always be clear from looking at the modules connected.
Any signal from -8 V to +8 V at the O/I resp. I/O sockets can be processed by the A-151. Voltages less than -8V or more than +8V will lead to malfunction of the module ! It is possible to modify the module so that voltages in the range 0...+12V can be processed. You find a modification of the A-151 for signals in the range 0...+12V on our web site www.doepfer.com in the FAQ section (click to the FAQ button on the left side of the page).
Another solution for signals beyond -8V resp. +8V is to attenuate and/or to change the offset voltage of the signal. The A-129/3 attenuator/offset generator can be used for both purposes.
From version 2 of the module (from about 2005) the full voltage range of the A-100 (i.e. -12V...+12V) can be switched without any modification. Version 2 can be identified with the additional switch ⑤ Steps.

5. User examples

Switching VCO waveforms

In the example in Fig. 2, every time a trigger pulse hits the A-151's trigger input socket, the VCO switches to a new waveform. Since very fast switching of the A-151 (by an LFO set to its fastest range, or even by a VCO) can produce audio frequency modulation - that is, changes so fast that the ear can't resolve them - the result is in effect a new waveform.

graph TD
    A["A-110 VCO"] --> B["LFO"]
    B --> C["SEC SWITCH"]
    C --> D["I/O 1"]
    C --> E["I/O 2"]
    C --> F["I/O 3"]
    C --> G["I/O 4"]
    D --> H["Ground"]
    E --> H
    F --> H
    G --> H
    H --> I["Output"]

Fig. 2: Switching VCO waveforms

As an alternative to using an LFO or VCO to trigger the waveform sequencing, it's possible to get the VCO to trigger the switching itself. Simply connect the square wave output into the trigger input ① of the A-151.

Four-step tone sequencing

The patch in Fig. 3 on page 6 produces a four-step tonal sequence, with a different filter envelope for each of the four steps.

An LFO simultaneously triggers ADSRs 1 to 4 and the switch on the A-151, so that for each step the VCF is affected by a different one of the four envelopes.

ADSR 5 is also triggered by the LFO, and controls the VCA.

It's crucial to make the envelopes of all the ADSRs fit the LFO's speed.

graph TD
    A["VCO V"] --> B["FFVCA"]
    C["LFO"] --> D["Gate"]
    E["ADSR 1"] --> F["Gate"]
    G["ADSR 2"] --> H["Gate"]
    I["ADSR 3"] --> J["Gate"]
    K["ADSR 4"] --> L["Gate"]
    M["CV"] --> N["A-151 SEQ SWITCH"]
    N --> O["Trg. In"]
    N --> P["Res. In"]
    N --> Q["C1"]
    N --> R["I0 1"]
    N --> S["I0 2"]
    N --> T["I0 3"]
    N --> U["I0 4"]
    N --> V["Output"]
    W["Gate"] --> X["ADSR 5"]
    X --> Y["Output"]

Fig. 3: Four-step tone sequencing

If you use a keyboard gate as the trigger instead of the LFO, each note played steps through to the next envelope.

Switching filter characteristics

With the patch in Fig. 4, you can use an A-151 to step through each of the types of output in the A-121 multimode filter, one step for each note played.

graph TD
    A["Gate"] --> B["CV"]
    B --> C["VCO A-21"]
    C --> D["FCV In"]
    D --> E["ADSR"]
    E --> F["VCA"]
    F --> G["A-151 SEC SWITCH"]
    G --> H["Pin 1"]
    G --> I["Pin 2"]
    G --> J["Pin 3"]
    G --> K["Pin 4"]
    F --> L["VCA"]
    L --> M["Output"]

Fig. 4: Switching filter characteristics