Stereo Tube Mastering Compressor with Variable Bias and 5 Modes - All Black
The SPL Iron All Black is a variable-bias tube stereo mastering compressor, designed from the ground up as an original concept rather than a replica of a classic unit. SPL’s goal was to achieve pleasant, musical, and transparent compression, inspired by tube compressors from the radio era, but with the versatility required by today’s mastering studios. Iron combines the character of vacuum tubes with the advantages of SPL 120V technology and introduces a dual-tube parallel circuit that makes the compression more balanced and controllable.
The operating principle is inspired by the variable-bias compressors from Fairchild, Collins, and Gates, which utilized remote-cut-off tubes for smooth, well-balanced compression. Iron pairs each channel with a second dual-triode tube connected in parallel, featuring a sharper cutoff and a steeper characteristic curve. Depending on the signal amplitude, one or the other valve is activated: the combination of the two characteristic curves produces transparent and musical compression, with parameters that are easier to adjust.
The valve pairs are selected and matched by SPL using a dedicated testing system, ensuring that every Iron unit has the same sonic characteristics. This is an important detail when working with two channels that must behave identically when processing stereo material.
The Tube Bias control offers three settings and, together with the input gain (up to ±12 dB) and threshold, allows you to adapt the tubes’ behavior to any material, from a full mix to individual instruments. Attack and Release each feature six positions ranging from Slow to Fast, with times that vary depending on the selected rectifier.
In the variable-bias tube path, the Iron uses custom-built, balanced, dual-winding transformers with mu-metal cores from Lundahl, which contribute to the unit’s overall character and split the signal between the two tubes in each channel.
A feed-forward resistive optoisolator (vactrol), inserted into the control path rather than the audio path, limits control voltage peaks and reduces harmonic distortion in the tube section. The result is a smoother, more uniform high-frequency response and a signal that remains lively even with significant gain reduction. The circuit has its own selector switch: with the Iron variable-bias tubes, it operates as a feedback compressor; with the optical control, it operates as a feed-forward compressor.
The rectifier forms the foundation of the tube control. With the six-position selector, you can choose from different characteristic curves achieved by combining germanium, silicon, and LED diodes: each combination alters the attack and release behavior, offering you multiple compression characteristics within the same unit.
In Stereo Link mode, a relay logic circuit links the two channels, and the right channel becomes the master for Threshold, Tube Bias, Attack, Release, Rectifier, and Sidechain EQ; the compressor operates on the sum signal. In Dual-Mono mode, the two channels remain independent, providing a wider stereo image for stereo material. The Sidechain EQ affects only the control signal: you can choose from four internal presets with dedicated frequency responses, use an external EQ, or disable it.
At the output, you’ll find a passive equalizer featuring 120V technology and two presets. AirBass gently boosts high and low frequencies by approximately 2 dB, while Tape Roll-Off replicates the response of a tape machine with low-pass and high-pass filters. The ±12 dB adjustable output gain makes it easy to integrate Iron into any mastering chain.
All functions are controlled via toggle switches, and the threshold via a stepped potentiometer: precisely reproducing a setting from one master to the next is immediate. Each channel has its own illuminated activation switch, and Auto Bypass mode automatically alternates between the compressed and original signals at an adjustable interval, enabling reliable A/B comparisons within the context of the track.
Iron is designed for mastering but also works very well on vocals, bass, guitars, strings, and subgroups, where its tube-like character and smooth compression help create cohesion without flattening the dynamics.
The All Black version offers the same features as Iron in an all-black finish, coordinated with the other SPL units in the black-finish mastering series.
SPL Iron All Black Technical Specifications
Compressor Type
Tube
Channels
2 (Stereo)
Hardware format
Rack
€ 4.913,18 Ex VAT
For European customers, select your country to view the correct price including VAT.
Normally shipped in 10-15 working days
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