RRP A$11,763.53
13.00 KGS
0.25 (cm)
0.46 (cm)
0.55 (cm)



Mastering Compressor

The IRON Mastering Compressor combines the sonic benefits of legendary vintage tube compressors with the advantages of the SPL 120V technology in one device. The result is a pleasant, melodic-sounding, musical compression inspired by vintage compressors of the radio era, yet very flexible and perfectly adapted to the needs of modern mastering studios and productions.

IRON adds power, size and depth to any signal – without being obtrusive.


Basic Concept

The IRON Mastering Compressor is a variable-bias limiter/compressor from the basic concept. Through the integration of new technologies, this concept gets significantly improved.

IRON works on the principle of the bias controlled remote cutoff tube. Sharp-cutoff tubes with a steeper characteristic are parallelly connected to these.

Depending on the level of the signal amplitude, one or another tube is in charge for signal processing/limiting. The positive result is a well-balanced sound and more controllable settings of the parameters.

All four tubes in the IRON are a specially matched set to the overall system.


The Wathen CryoTone™12AX7-WCM Long Plate tubes are not only a perfect match as a pair – but also the perfect complement to the selected 12AU7 tubes.


The tubes are integrated into the IRON’s signal flow via special Lundahl manufactured balanced high-level double core mu-metal-iron transformers.

An independent feed-forward resistive vactrol-opto-isolator in the control path of the variable-bias tube circuit intercepts extreme signal peaks, ensuring that the tube circuitry achieves its full sonic potential.

Thanks to SPL 120V technology, tube compressor technology sets new standards in IRON.



The input level is switchable in a control range of +/- 12 dB in 2 dB steps. An increase of the input level leads to more intensive compression.

Tip: For audio material with a narrow dynamic range and a lot of information in the lower frequency spectrum, attenuating the input signal with the Input switch allows the compressor to work in a much more relaxed manner.



„SPL’s IRON allows sculpting compression at the circuit design level. I could customize this compressor very specifically to produce the perfect style and amount of processing for any job.“



The output level is also adjustable in a control range of +/- 12 dB in 2 dB steps. This allows the audio level to be increased again after compression.



Attack determines the response time of the compressor.

The Attack time can be adjusted in six steps, from Fast to Slow.



The counterpart of the Attack is Release. The Release parameter determines how fast the compressor eases processing the signal.

Similarly to the Attack time, the Release time can also be set in six steps from Fast to Slow.



“Once you’ve used it you don’t want to let go.”



Threshold determines the level beyond which the compressor starts to compress. The compressor begins to process the signal once the threshold value has been exceeded. Only signals that exceed the threshold level are compressed. Signals whose level is beneath the threshold value are not processed.

The Threshold parameter of the IRON can be adjusted in 41 steps with the detent potentiometer.


Video with Audio Examples

Play Video
SPL IRON Compressor Review by Audio Animals


The tube stage is controlled by a rectifier circuit. There are six diode control characteristics generated by germanium, silicon and LED diodes or combinations thereof.

The attack and release times vary depending on the rectifier circuit selected.


Side Chain EQ Presets

Side-chain filters can be used to focus the response of the compression to certain frequency ranges. This is also referred to as frequency selective compression. The side-chain filters are only in the control signal path – not in the audio path. The IRON Mastering Compressor offers a six-position switch to choose between the Off position, four side-chain filter presets or an external side-chain insert.

In position 1 (Off), only a capacitor is integrated into the side-chain signal to filter out frequencies lower than 20 Hz.

Position 2 (EQ 1) integrates a lowpass filter.

Position 3 (EQ 2) integrates a highpass filter.

Position 4 (EQ 3) integrates a bandpass filter.

Position 5 (EQ 4) integrates a filter curve empirically designed by developer Wolfgang Neumann.


Side Chain Insert

In position 6 (Ext) the external side-chain insert is activated. Via insert send and insert return, an external stereo equalizer can be integrated into the side-chain signal. Individual filter curves can be created to focus the response of the compression to certain frequency ranges.

The side-chain equalizers in the IRON Mastering Compressor only process the control signal – not the audio signal! If no external equalizer is connected to the side-chain insert or if the send and return of the side-chain insert are not bridged with a cable, the audio signal in the IRON will be interrupted when the switch position (Ext) is selected.


Video with Audio Examples

Play Video
Smooth Tube Compression on Drums​

Tube Bias

The Tube Bias switch allows you to determine the bias of the tubes according to three settings: Low, Mid, and High. The bias of a tube is the voltage present on the tube‘s grid. The higher the voltage, the less signal makes it from the cathode to the anode of the tube, which means the compression is stronger.

The modulation of the bias is the sum of the Threshold, Rectifier, Side Chain EQ, Attack, and Release parameters.



IRON uses four tubes in its circuit. A parallel connected pair for each channel. To receive mastering grade sound and performance it is necessary to use the best tubes available. It is not only crucial to use perfectly matched tubes – they also need to stay true to their specs as long as possible. Therefore IRON is equipped with matched 12AU7 tubes, which get paired with preselected matched pairs of Wathen CryoTone™12AX7-WCM Long Plate tubes at SPL.

Wathen Audiophile’s CryoTone™ tubes are cryogenically treated tubes. Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Cryogenic treatment modifies the molecular structure in electronic components by reducing or eliminating voids and imperfections in the material. Wathen utilizes a cryogenic treatment process that is specifically designed to maximize all aspects of the sonic performance in audio tubes. Wathen CryoTone™ Long Plate versions of the 12AX7-WCM have unequaled soundstage, depth and frequency balance. Silky, clear and large from top to bottom with the quietest noise floor and an extended life-span.

IRON combines the best of modern technology and vintage magic.