SPL IRON User manual

Manual
Version 2.0
IRON
Mastering Compressor

Index
IRON 4
Version 2.0 – 11/2022 4
Package Contents 4
Introduction 5
Operating Principles of a Compressor 5
IRON Mastering Compressor 5
Technical Aspects 6
The 120V technology 6
Innovating Compression 8
Signal Flow 9
Installation 10
First Steps 10
General Information 11
Ground Li switch to avoid ground loops 11
Tube Warm-Up 11
Suggestions for setting the compressor 11
Cabling: Rear Side 12
XLR inputs and outputs 12
Side-chain Insert 12
Control Elements 14
Input 15
Output 15
Threshold 15
Tube Bias 16
Attack 16
Release 17
Rectier 17
Side chain lter / External side-chain 18
Ratio 20
Auto Bypass 20
AirBass / Bypass / Tape Roll-O 20
Link 22
VU switch 22
Channel Switch 22
Time values 23
Time values depending on the rectiers 23
Specications 24
Measurements 24
Security Advices 25
Contact 30

4
IRONIRON
Version 2.0 – 11/2022
Developer: Wolfgang Neumann
This manual includes a description of the product but no guarantee as for specic charac-
teristics or successful results.
It describes the IRON Mastering Compressor in the hardware revision v2. IRON v2 diers
from the rst hardware version in the following details:
1. IRON v2 features balanced side-chain inserts with send and return XLR sockets.
2. IRON v2 features dierent side-chain EQ presets.
3. IRON v2 is equipped with WathenCryoTone™12AX7-WCM Long Plate tubes.
Unless stated otherwise, everything herein corresponds to the technical status at the
time of delivery of the product and user manual by SPL electronics GmbH.
The design and circuitry are under continuous development and improvement. Technical
specications are subject to change.
Package Contents
IRON Mastering Compressor
Power cord
Manual
The IRON Mastering Compressor is available in dierent colors.
Black: Model 1520
All Black: Model 1523
Red: Model 1524
Do consider keeping the original packaging. It can come in very useful whenever you
need to transport your gear. If there is ever the need to send it in for repair, the original
packaging guarantees a safe shipment.
The SPL IRON Mastering Compressor was developed and manufactured in Germany.

5
IntroductionIntroduction
Operating Principles of a Compressor
The basic operating principles of a compressor/limiter can be easily explained.
The level of an audio signal is reduced according to the specied Attack time and Ratio
whenever it exceeds a given threshold. This reduction ceases when the Release time
elapses, while the compressed signal is amplied with the Make-Up Gain.
Compressors basically dier from each other in the technology used. This technology –
tubes, opto, FET, or VCA – is what gives a compressor its particular character. Some units
sound so and silky, some sound pounding, while some others make sound fatter, and
there are those that make sound clearer, harder or more percussive. The trick resides in
how the unit is technically designed, in the signature of the maker. Dierent compressors
with the exact same settings might work and sound completely dierent. They provide
dierent sounds for dierent applications and music styles.
Nowadays, the compressor has become a key element when it comes to provide dynam-
ics and punch to any production. The number of compressors available is huge and it‘s
easy to succumb to the promises made by soware emulations and analog recreations of
vintage gear as the perfect solution. Unfortunately, many of these emulations and recre-
ations dier quite a bit form their original counterparts. You must simply accept that the
components used today, like the transformers, tubes and all other passive elements, are
dierent to the ones originally used and that they can‘t be digitally emulated. No so-
ware (DSP-emulated compressors) or hardware replica will ever be able to sound like the
original.
An authentic sound can only be achieved with the original unit.
IRON Mastering Compressor
The IRON mastering compressor is not a copy of a classic unit, but rather an original con-
cept in itself. Our goal was to conceive a compressor that provided a pleasant, melod-
ic-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 Iron combines not only the sonic virtues of legendary vin-
tage 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 espe-
cially conceived Mu-Metal iron transformers, the signal of each channel is split across
two dierent twin-triode tubes. The combination of the dierent 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 IRON sets new standards. It can also be
used to process individual instruments, like vocals, bass, guitar, strings, etc. The IRON is
also an excellent option for subgroups.

6
Technical AspectsTechnical Aspects
The 120V 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.
The 120V technology is our reference technology. The 120V technology is unique in the
world. It operates at a DC voltage of 120 volts. This is four times that of IC-based semicon-
ductor op-amps.
The highest possible audio quality requires the highest possible audio
operating voltage.
The 120V technology works with +/-60 V. To be able to handle such a high voltage, we
have developed special proprietary operational ampliers that can operate with a DC
voltage of +/-60 V: the SPL 120V SUPRA operational ampliers. This high voltage would
destroy conventional components and operational ampliers.
The 120V technology achieves exceptional technical specications and sonic benets.
Technically, in terms of dynamic range, signal-to-noise ratio and headroom. Sonically, in
terms of richness of detail and an absolutely relaxed listening experience.
By the way, the “120V” in the name of the technology has nothing to do with the local
mains voltage from the mains power socket. This is about the operating voltage inside
the device with which the audio signals are processed.
The mains voltage from the mains power socket is transformed to the required secondary
voltage in the device’s internal linear power supply with toroidal transformer. Rectiers
convert this AC voltage into DC voltage required in the audio device.
Ultimately, this supply voltage is key for the overall dynamic response of a processor. Vol-
tage is to an electrical circuit what cylinder capacity is to an internal combustion engine.
You can‘t replace cylinder capacity with anything else, except more cylinder
capacity.
The 120V 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 optimiza-
tion they underwent under the hands of Bastian Neu.
Some of the most highly respected Mastering studios today revolve around SPL devices
with 120V technology (Bob Ludwigs Gateway Mastering & DVD in the USA, Simon Hey-
worth‘s Super Audio Mastering in the UK, Galaxy Studios in Belgium, and the legendary
Wisseloord in the Netherlands, for instance).

7
Technical Aspects
120V technology – Comparison
Most audio devices work with an internal operating voltage of +/-15 volts and can thus
process a maximum input level of +21.5 dBu. If a DAC, for example, has an output level of
+22 dBu at 0 dBFS, level peaks of the music material would already cause overloads in
the input stage of the device. All components in the audio device oen operate at their
limits.
SPL devices with 120V technology can handle input levels of +32.5 dBu thanks to the
higher internal operating voltage of +/- 60 volts – thus oering 12 dB more headroom.
All components consequently operate continuously in the optimum operating range. The
result is a very pleasant, natural and relaxed sound experience.
These diagrams clearly show the superiority of the 120V technology in comparison to
other circuits with lower, common operating voltages.
The direct relation between operating level and maximum level is fundamental for the
classication: the higher the operating level, the higher the maximum level a circuit can
handle. And since virtually all essential acoustic and musical parameters depend on this
relation, a higher operating voltage also has a positive impact on the dynamic range, dis-
tortion limit and signal-to-noise ratio.
Do bear in mind that dB scales do not represent linear but rather exponential increases. A
3 dB increase corresponds to doubling the acoustic power, +6 dB correspond to twice the
sound pressure level, and +10 dB correspond to twice the perceived loudness.
120
125
130
135
140
145
dB
124,2
141,4
120 V30 V
Dynamic Range
0
5
10
15
20
25
30
35
dBu
21,5
33,2
Maximum Levels
120 V30 V
0
20
40
60
80
100
120
Volt
Voltage
120 V30 V
-115
-113
-111
-109
-107
-105
dBu
-106
-114,2
THD&N
120 V30 V
When it comes to volume, the 120V technology exhibits a performance that is twice that
of common components and circuits, in regard to maximum level and dynamic range,
with values that are approximately 10 dB higher. THD measurements of the the 120V tech-
nology more than 8 dB better than a device with a operating voltage of 30V.

8
Technical Aspects
Innovating Compression
The IRON Mastering Compressor is a variable-bias limiter/compressor from the basic
concept. Through the integration of new technologies, this concept gets signicantly
improved.
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-o of tube biasing to achieve a well-balanced, well-compensated and
musical compression. However, the IRON compressor features a second sharp-cuto
tube, a medium-variable Mu Triode, in its circuit design. This tube is connected in paral-
lel to the remote cuto 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. 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 modies the molecular structure in electronic
components by reducing or eliminating voids and imperfections in the material. Wathen
utilizes a cryogenic treatment process that is specically 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. 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 cir-
cuitry achieves its full sonic potential.
A complex rectier circuit is the basis for the tube control. You can use the six-position
switch to choose either of the six dierent control characteristic curves of the diodes
within the rectier. Given the specic characteristic curve of its elements, the combina-
tion of germanium, silicon and LED diodes produces dierent behaviors and characteris-
tics for the Attack and Release times. Hence, compared to most compressors, the appli-
cation scope of the IRON is clearly enlarged, resulting in new possibilities regarding the
processing of music material.
A comprehensive logical relay circuit perfectly links both channels of IRON together, mak-
ing the right channel the Master regarding the parameters Release, Attack, Threshold,
Rectier, Tube-Bias and Side Chain EQ.

9
Technical Aspects
Signal Flow
The following diagram depicts the signal flow within the IRON compressor. It is meant to
clarify how it works and to show the relation between its dierent parameters. The audio
signal flow is in blue and the control voltage signal flow is in green.
R
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AU
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ECC
Input Output
Output Gain
Sidechain Selection
Equalizer Input
Sidechain Send Sidechain Return
Equalizer Output
Attack Release
Input Gain Rectier
Threshold Tube Bias
Air Bass /
Tape Roll-O
Sharp Cut-O Tube
Remote Cut-O Tube
Opto Peak
Limiter Transformer Transformer
Cathode
Anode
Grid
Cathode
Anode
Grid
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TUBE BIAS
Low
Mid
High
AirBass
Tape Roll-O
O
U
T
P
U
T
External Equalizer
Audio
Control voltage
higher bias =
higher Gain
reduction

10
InstallationInstallation
First Steps
Before turning on the IRON you must rst connect the included 3-pin power cord to the
3-pin IEC socket. The transformer, power cord and IEC socket all comply to the VDE, UL
and CSA regulations.
The IRON should not be installed in close proximity to equipment that emits magnetic
elds or emanates heat. Avoid exposure to heat, moisture, dust, and vibrations. Do not
install the IRON close to any power amps or digital processors. Instead, install it in a fully
“analog rack” where any interferences can be avoided (Word Clock, SMPTE, MIDI etc.).
The unit should be powered o before connecting or disconnecting any cables or equip-
ment to it.
Use the On/O switch on the rear panel to turn the unit on or o. The illuminated VU-me-
ter on the front panel indicates the unit‘s operating status. The On/O switch was placed
on the rear panel to avoid any emissions due to voltage-carrying conductors running
across the unit and aecting sound. When powering on or o, there‘s no need to observe
a specic sequence regarding the connected devices. However, like with any audio sig-
nal chain, power ampliers should always be powered on last and powered o rst. The
IRON can be powered on and o with the use of a circuit breaker, as long as the total load
does not exceed the rating of the latter.
Voltage Selection
Before connecting the IRON to the mains, make sure that the voltage selection corre-
sponds to the values of your local power grid (230 or 115 volts). Inside the power connec-
tor, to the right, next to the on/o switch, there is an opening that displays the voltage
selected. If the voltage indicated does not correspond to the one required, change it by
following this procedure:
Open the power connector lid with a small screwdriver (use the tiny slots on the right
hand side). Use the screwdriver to lever the red fuse holder from above until you can
grab it. Take the fuse holder out and replace the fuse with one corresponding to the local
power grid specications. You can nd the adequate values on the rear of the unit or on
page 13 of this user‘s manual. Turn the fuse holder around 180 degrees and place it back
again. When you close the lid again, you should see the correct voltage displayed in the
opening.
On the product site on our website (https://iron.spl.audio) you will nd the video “Chang-
ing the mains voltage” concerning this topic. If you ever have to exchange a fuse, we rec-
ommend the video “Exchange defective fuses”.