The Locomotive Audio Copperline is a dual-channel microphone and line signal conditioner for placement between a microphone and preamp—or between two line-level devices. It combines continuously variable, high-current phantom power up to 52 V with slow-start ramping, selectable attenuation, passive high-pass filtering, Vintage Load, polarity inversion and mute in a robust 2U rackmount unit.
 
The Locomotive Audio Copperline solves recording problems that a conventional microphone preamp does not always address. It is not a preamp, compressor or EQ. Instead, it is a two-channel utility processor designed to sit in front of a preamp or between line-level devices, providing direct control of phantom voltage, microphone loading, filtering, attenuation and polarity before the signal enters the main recording chain.
 
Its defining feature is variable phantom power. Each channel has an independent high-current, low-noise phantom supply adjustable from near 0 V to 52 V. Rather than relying on the fixed 48 V phantom circuit built into a preamp, Copperline lets the engineer choose a more appropriate operating voltage for a connected active microphone. This can be valuable with microphones that respond differently to reduced phantom voltage, equipment requiring careful power-up handling or situations where the connected preamp’s phantom supply should be avoided. Copperline also includes output capacitors that block phantom voltage from reaching the downstream preamp input.
 
The phantom supply uses a slow-start ramp of approximately five seconds. Instead of applying voltage abruptly, Copperline gradually raises and lowers phantom power, reducing stress on sensitive active microphone electronics and helping protect vintage or transformer-coupled preamp inputs. This is a practical advantage when using high-value condensers, older microphones, boutique ribbon-compatible preamps or complex patchbay workflows. It also makes the unit a safer central phantom-power source in studios where microphones are routinely moved between different analogue chains.
 
Each channel includes a five-position passive high-pass filter. The selectable positions are Off, 45 Hz, 75 Hz, 150 Hz and 300 Hz. Use it to remove rumble, stand-borne vibration, ventilation noise, excessive proximity effect or unnecessary low-frequency energy before a microphone preamp, compressor or converter. Because the filtering happens before downstream gain stages, it can preserve headroom and prevent low-frequency buildup from causing avoidable distortion or compressor pumping later in the chain.
 
The five-position attenuator provides Unity, −15 dB, −20 dB, −30 dB and −40 dB settings. It can be used to manage unusually hot microphone outputs, reduce high-level line signals before a sensitive preamp or create gain-staging options between two line-level devices. At attenuation settings from −15 dB to −40 dB, the input impedance is approximately 1.2–1.5 kΩ; at Unity, the effective input impedance is largely set by the device connected downstream. This makes Copperline useful not only as a pad but also as a practical tool for controlling the electrical interaction between microphone and preamp.
 
The Vintage Load switch places an 825 Ω resistor across XLR pins 2 and 3, intentionally loading the input signal. This can change how certain microphones interact with the following preamp, often producing a more controlled, denser or vintage-style response. It is especially relevant for engineers who use dynamic and ribbon microphones, transformer-coupled sources or vintage-inspired recording chains and want to explore loading as a tonal variable rather than relying only on EQ after recording.
 
Copperline also includes individual polarity inversion and mute switches for both channels. Polarity reversal is essential when combining microphones on one source—such as top and bottom snare, inside/outside kick, multi-miked guitar cabinets or DI-plus-amp recording. Mute supports quiet microphone changes, patching, troubleshooting and safe operation when phantom power is being adjusted.
 
The unit can also work between two line-level devices. Place it before a compressor, EQ, converter, tape machine, mix-bus insert or analogue processor to add high-pass filtering, level attenuation, polarity reversal and muting where the rest of the chain does not provide those facilities. This makes Copperline a valuable utility piece for hybrid studios, recording rooms, live tracking setups and engineers who routinely work with diverse microphones and vintage outboard gear.
 
With dual independent channels, XLR I/O, an internal power supply and a 2U 19-inch chassis, Locomotive Audio Copperline is a serious studio infrastructure tool. It does not try to add “colour” through an active audio circuit; its purpose is to give the engineer control before the signal reaches the tonal processors. Used thoughtfully, it can protect gear, improve gain staging and make difficult microphone/preamp combinations far easier to manage.
 
Key features
Dual-channel microphone and line signal conditioner
Designed for use between microphone and preamp or between two line-level devices
Independent variable phantom power per channel
Adjustable phantom voltage from near 0 V to 52 V
High-current, low-noise phantom-power supply
Approx. five-second slow-start and slow-stop phantom ramp
Output capacitors isolate downstream preamp inputs from phantom voltage
Five-position passive high-pass filter per channel
High-pass settings: Off, 45 Hz, 75 Hz, 150 Hz and 300 Hz
Five-position attenuator per channel
Attenuator settings: Unity, −15 dB, −20 dB, −30 dB and −40 dB
Vintage Load switch places 825 Ω across XLR pins 2 and 3
Approx. 1.2–1.5 kΩ input impedance with attenuation engaged
Polarity inversion per channel
Mute per channel
Balanced XLR inputs and outputs
True dual-mono channel architecture
Suitable for microphones, preamp front ends, line inserts and hybrid systems
Internal mains power supply
19-inch, 2U rackmount chassis
Handmade in the USA
Lifetime manufacturer warranty
 
Applications
Supplying controlled phantom power to condenser microphones
Protecting sensitive microphone and vintage-preamp workflows through slow phantom ramping
Operating active microphones below nominal 48 V when appropriate
Removing rumble and proximity buildup before preamp gain stages
Padding hot condenser microphones or line sources
Matching microphone level to high-gain or vintage microphone preamps
Loading microphones intentionally with the Vintage Load function
Polarity correction in multi-microphone setups
Safe muting for patching, microphone changes and troubleshooting
Conditioning line-level signals before compressors, EQs, converters and analogue inserts
Building a flexible front end for hybrid, vintage and high-end recording chains