Vintage Digital & Analogue Studio Effects

Vintage Digital

SPL Iron Mastering Compressor

SPL IRON Mastering Compressor

As a mastering compressor, the SPL IRON is inspired by the legendary vintage tube compressors à la Fairchild, Collins or Gates and has been developed in a way to give modern productions more energy and power with the advantages of the 120V technology.


The SPL IRON mastering compressor is not a copy of a classic unit, but rather an original concept in itself. Our goal was to conceive a compressor that provided a pleasant, melodic-sounding, transparent compression, inspired on the vintage compressors of the radio era. And we wanted it to be versatile enough to adapt perfectly to the needs of modern mastering studios.

Thus, the SPL IRON combines not only the sonic virtues of legendary vintage tube compressors with the advantages of the High Dynamic 120V operating voltage in a single unit. It also sets a new benchmark in terms of tube compressor technology, with the innovative implementation of a parallel dual-tube circuit. Thanks to the especially conceived Mu-Metal iron transformers, the signal of each channel is split across two different twin-triode tubes.

The combination of the different response curves of both tubes results in a transparent and musically pleasant compression. Additionally, peak signals of the control voltage are limited by a feed-forward resistive opto-isolator. Thus, the output signal remains lively even with a high gain reduction. The compression is only noticeable with extreme settings.

But mastering is not the only domain where the SPL IRON sets new standards. It can also be used to process individual instruments, like vocals, bass, guitar, strings, etc. The SPL IRON is also an excellent option for subgroups.

The IRON mastering compressor is a variable-bias limiter/compressor. The implementation of new technologies results in many improvements. Its basic operating principle as a variable-bias tube compressor was loosely inspired by the sonic and technical operation of Fairchild, Collins and Gates compressors, which used remote cut-off of tube biasing to achieve a well-balanced, well-compensated and musical compression. However, the IRON compressor features a second sharp-cutoff tube, a medium-variable Mu Triode, in its circuit design. This tube is connected in parallel to the remote cutoff tube and it has a considerably steeper characteristic curve.

The tube used to process the signal depends on the amplitude of the latter. This results in a more well-balanced sound and more controllable settings of the parameters. The pair of parallel connected tubes has been specially matched for the IRON. In order to guarantee that tube selection and pairing is perfect, we use the Weigl Roe Test for PC. The optimal selection of the tubes guarantees that all IRONs have the same sonic characteristics. Moreover, we use Lundahl custom-made balanced high-level dual-coil Mu-Metal iron transformers in the signal flow of the variable-bias tubes, which add to the overall sound.

The second new technology implemented is the independent feed-forward resistive opto-isolator in the control path of the variable-bias tube circuit. Its function is to limit signal peaks and, thus, get a smaller THD (Total Harmonic Distortion) within the variable-bias tube section. The result is a silkier, more homogeneous sound in the higher frequencies of the music signal. The optical control element does not work in the sense of an audio limiter, like in a conventional opto-compressor. It is built-in in the control path of the parallel connected variable-bias tube, not in the audio path itself. The IRON compressor works as a feedback compressor in the variable-bias tube circuit and as a feed-forward compressor in the opto-control circuit.

Thirdly, the complex rectifier circuit is also worth mentioning, since it is the basis for tube control. You can use the six-position switch to choose either of the six different control characteristic curves of the diodes within the rectifier. Given the specific characteristic curve of its elements, the combination of germanium, silicon and LED diodes produces different behaviors and characteristics for the Attack and Release times. Hence, compared to most compressors, the application scope of the IRON is clearly enlarged, resulting in new possibilities regarding the processing of music material.

The fourth exceptional feature is the comprehensive logical relay circuit that perfectly links both channels together, making the right channel the Master regarding Release, Attack, Threshold, Rectifier, Tube-Bias and Side-Chain EQ settings

120 Volt Technology
SPL‘s goal was to push analog signal processing to the limits. That‘s why we combined the best possible components with a high-grade optimized circuit design. We have been using the in-house developed 120-volt technology – the highest-ever operating voltage used for audio applications – in all our products from the Mastering series for years. Some of the most highly respected Mastering studios today revolve around SPL consoles and signal processors from our Mastering series (Bob Ludwigs Gateway Mastering & DVD in the USA, Simon Heyworth‘s Super Audio Mastering in the UK, Galaxy Studios in Belgium, and the legendary Wisseloord in the Netherlands, for instance).

The 120-volt technology is based on op-amps developed internally by SPL‘s co-founder and Chief Developer Wolfgang Neumann. The IRON features the most advanced generation of these op-amps. They boast better tech specs thanks to the thermal behavior optimization they underwent under the hands of Bastian Neu. Ultimately, the supply voltage is key for the overall dynamic response of a processor. Voltage is to an electrical circuit what cylinder capacity is to an internal combustion engine.



Analog inputs & output; XLR (balanced)

  • Maximum input & output gain: 32.5 dBu
  • Input impedance: 20
  • Output impedance: 75 Ω
  • Frequency range (-3 dB): 10 Hz – 40 kHz
  • Common mode rejection (0dBu, 1kHz): > 80 dB
  • Crosstalk (1 kHz): -108 dBu
  • THD (+10 dBu): 0.002 %
  • Noise (A-weighted): -98 dBu


Internal Linear Power Supply with Shielded Toroidal Transformer

  • Operating voltage for analog audio: +/- 60 V
  • Operating voltage for relays and LEDs: + 12 V


Mains Power Supply

  • Mains voltage (selectable, see fuse chamber): 230 V AC / 50; 115 V AC / 60 Hz
  • Fuse for 230 V: T 1 A
  • Fuse for 115 V: T 2 A
  • Power consumption: max. 40 VA


Dimensions & Weight

  • W x H x D (width x height x depth): 482 x 132 x 300 mm
  • Unit weight: 11 kg

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SPL Iron Mastering Compressor

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