Doepfer A-138a Linear Mixer – A simple but essential signal summing tool for modular synthesists

The Doepfer A-138a is a four-channel linear mixer designed to combine either audio signals or control voltages in a Eurorack modular system. Despite its simplicity, it plays a crucial role in any patch, allowing you to sum multiple sources into one output with clean and predictable results.

Each of the four inputs features an individual level knob, enabling precise control over the mix. Because the mixer operates with linear response curves, it’s particularly well-suited for CV mixing—ensuring that modulations blend proportionally without distortion. Of course, it also works beautifully for audio, providing clean and transparent signal summing without coloration.

Key Features:

• 4 linear input channels with individual level controls

• Ideal for mixing control voltages (CV) or audio

• Clean, neutral summing with no added coloration

• 8 HP wide – compact and rack-friendly

• Passive design with simple, effective functionality

Whether you're blending modulation sources like LFOs and envelopes or mixing multiple audio oscillators, the A-138a is a utility workhorse that belongs in every serious modular setup.