The Locomotive Audio Power-MU 500 is a true stereo, Class-A Vari-Mu tube compressor and limiter for the 500 Series format. Using matched NOS 6N3P dual-triode valves, precision 21-step controls, separate AC and DC threshold settings, variable timing and a sidechain high-pass filter, it provides smooth tube compression, dependable recall and high-end stereo-bus control in two 500 Series slots.
 
The Locomotive Audio Power-MU 500 brings genuine stereo Vari-Mu compression to the compact 500 Series environment. It is designed for mix engineers and mastering engineers who want the density, movement and harmonic depth of a real tube gain-reduction stage, but also require repeatable settings and stable stereo behaviour for revisions, recalls and critical bus processing.
At its core, Power-MU 500 uses two NOS 6N3P dual-triode valves—one dedicated to each channel—to create true Vari-Mu gain reduction and tube harmonics. Unlike a mono compressor pair that depends on external linking, this is a purpose-built stereo processor with a shared workflow for stereo mixing, stereo tracking and mastering. It can also operate in dual-mono mode when independent left/right compression is required.
 
The Class-A tube circuit is intended to provide smooth gain reduction rather than abrupt peak clamping. Applied gently, it can add cohesion, width stability and low-level detail to a mix bus. Pushed further, it delivers thicker programme density, harmonically enriched sustain and more obvious valve character. This makes the Power-MU 500 suitable for a wide range of material: stereo mixes, drum buses, pianos, acoustic instruments, overheads, keyboards, backing vocals and mastering chains.
 
A major advantage is the use of 21-position stepped potentiometers for the main controls. These precision-detented controls make it much easier to recreate a successful setting after a mix revision or mastering recall. In a 500 Series compressor, where settings are often documented manually, this is a substantial practical benefit: settings can be noted accurately, recreated quickly and matched more reliably across repeat sessions.
 
The AC Threshold and DC Threshold controls provide two complementary ways to determine compressor behaviour. AC Threshold sets the level at which programme material begins to trigger gain reduction. DC Threshold adjusts the bias point of the Vari-Mu tube stage, influencing its operating position, compression onset and harmonic response. Together, they allow the engineer to balance clean, gentle levelling against increased tube density and more pronounced gain-reduction character. This dual-threshold design is particularly useful when working with very different programme material, because the amount and character of compression can be shaped independently of simple input/output gain staging.
 
Variable Attack from 10 ms to 75 ms and Release from 150 ms to 1.5 seconds provide timing control for stereo-bus and source processing. Faster timing can offer firmer control and increased density; slower attack preserves leading transients, while longer release settings can produce a more relaxed, glue-like response. The sidechain high-pass filter helps prevent sub-bass, kick drum and bass guitar from dominating the detector path. It is especially useful on full mixes and drum buses, where preserving low-end energy without excessive pumping is often the goal.
 
Input and Output controls provide gain staging before and after the tube compression circuit. Drive the input more conservatively for understated bus cohesion, or increase the input and adjust output compensation for more active tube colour and denser compression. A hard-wire bypass supports direct before/after comparison, while the dedicated power switch allows the tube circuit to be turned off when the module is not in use, helping preserve tube life.
 
Power-MU 500 occupies two adjacent 500 Series slots and draws substantial current: +16 V at 260 mA and −16 V at 300 mA. It therefore requires a high-current, properly specified 500 Series chassis with sufficient reserve capacity. With its true stereo topology, stepped control system, NOS tube complement and comprehensive bus-compression control, Power-MU 500 is a compact but serious analogue processor for studios that want a recall-friendly tube compressor without moving to a larger rackmount format.
 
Key features
True stereo Class-A Vari-Mu tube compressor and limiter
Dual-slot 500 Series module
Stereo and dual-mono operation
Two matched NOS 6N3P dual-triode tubes
One Vari-Mu tube gain-reduction channel per stereo side
Smooth tube compression and harmonic enhancement
21-position stepped potentiometers for repeatable recall
Separate AC Threshold and DC Threshold controls
Variable Attack: 10 ms–75 ms
Variable Release: 150 ms–1.5 s
Sidechain high-pass filter
Input and Output gain controls
Hard-wire bypass
Dedicated power switch to protect tube life
22 kΩ input impedance
Output impedance below 100 Ω
Frequency response: 20 Hz–20 kHz
Maximum tube-based stereo compression in two 500 Series slots
High-current 500 Series rack required
Lifetime manufacturer warranty
 
Applications
Stereo mix-bus compression and final mix cohesion
Tube mastering compression with repeatable settings
Drum-bus density while retaining low-end impact
Piano, acoustic guitar and stereo keyboard compression
Vocal-bus and backing-vocal glue
Overhead and room-mic stereo control
Harmonic enrichment of stereo stems
Dual-mono compression of two independent line-level sources
Hybrid mixing workflows needing analogue character and easy recall
High-end 500 Series mastering or mix-bus chains