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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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
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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”.

11
General InformationGeneral Information
Ground Li switch to avoid ground loops
On the rear panel of the IRON mastering compressor (see details on page 13) is also a
“GND LIFT” (Ground Li) switch to avoid any ground loops. Ground loops take place when
gear connected in the same network have dierent potentials.
The GND LIFT switch disconnects the equipment ground from the service ground to avoid
such problems. The Ground Li function is activated (= equipment ground disconnected)
when the switch is depressed.
Tube Warm-Up
IRON includes several tubes in the signal path. We recommend you to warm up the tubes
of the IRON about 30 minutes before processing any audio material with it. The sound
and compression characteristics of the IRON compressor might change especially within
the rst 15 minutes aer powering on.
Suggestions for setting the compressor
The operation of the IRON strongly depends on the input signal. Normally the following
start values are a good starting point:
Attack: Position 2 or 3 (clockwise from “fast”)
Release: Position 1 or 2 (clockwise from “fast”)
Rectier: LED
Side Chain EQs: O
Tube Bias: Low
Threshold: 20
If you switch through the dierent rectiers, you have to adjust the other parameters. The
remaining rectier circuits tend to provide faster/slower time values. When the program
material stays the same, slower/longer time values should be chosen and/or the thresh-
old should be raised.
Especially when it comes to group applications, the high bias setting can be interesting.

12
Cabling: Rear SideCabling: Rear Side
XLR inputs and outputs
We used exclusively Switchcra/Neutrik XLR input and output plugs and jacks to guaran-
tee perfect connectivity in the studio. These plugs and jacks provide an optimal connec-
tion thanks to their electromechanical design and large contact surface.
The image shows the XLR connectors pinout. They are balanced and have three conduc-
tors or wires. Conductor 2 (Pin 2) corresponds to the (+) or hot Signal.
In case an unbalanced connection is necessary, the correct polarity of the conductors
needs to be observed.
Input Output
balanced unbalanced balanced unbalanced
1=GND
2=hot (+)
3=cold (-)
1
2
3
1
2
3
12
3
12
3
Side-chain Insert
This insert serves to connect an external external stereo equalizer into the side-chain of
the IRON Mastering Compressor. This can be used to focus the response of the compres-
sion to certain frequency ranges. This is also referred to as frequency selective compres-
sion (for more information, see page 18).
Send features two XLR connectors, balanced. Return features two XLR connectors – one
for the le and one for the right channel. The pinout is identical to the input and output
connectors.

13
Cabling: Rear Side
Serial
Number
Made in Germany
Voltage Selection:
V AC: T  A L  V
V AC: T  A L  V
1. Remove Fuse Holder
2. Change Fuses
3. Flip Over
4. Reinstall
IRON
XLRs:
Balanced +4dBu
1
= GND
2 = HOT (+)
3 = COLD (–)
TO REDUCE RISK OF FIRE OR ELECTRIC SHOCK.
DO NOT EXPOSE THIS UNIT RAIN OR MOISTURE.
DISCONNECT MAINS BEFORE REMOVING COVER.
THIS EQUIPMENT MUST BE EARTHED.
WARNING
~ V AC / ~V AC,  Hz /  Hz, P max.  W
Fuses:
AC MAINS
AC MAINS
AVIS: RISQUE DE CHOC ÉLECTRIQUE • NE PAS OUVRIR
RISK OF ELECTRIC SHOCK
DO NOT OPEN
CAUTION
GND LIFT
GND LIFT
GND LIFT
GND
INPUT OUTPUT INPUT OUTPUT
INPUT OUTPUT
EXT SIDE CHAIN
EXT SIDE CHAIN
INPUT OUTPUT
SEND RETURN
SEND RETURN
EXT SIDE CHAIN
EXT SIDE CHAIN
SEND RETURN
SEND RETURN
230
Serial
Number
Made in Germany
Voltage Selection:
V AC: T  A L  V
V AC: T  A L  V
1. Remove Fuse Holder
2. Change Fuses
3. Flip Over
4. Reinstall
IRON
XLRs:
Balanced +4dBu
1 = GND
2 = HOT (+)
3 = COLD (–)
TO REDUCE RISK OF FIRE OR ELECTRIC SHOCK.
DO NOT EXPOSE THIS UNIT RAIN OR MOISTURE.
DISCONNECT MAINS BEFORE REMOVING COVER.
THIS EQUIPMENT MUST BE EARTHED.
WARNING
~ V AC / ~V AC,  Hz /  Hz, P max.  W
Fuses:
AC MAINS
AC MAINS
AVIS: RISQUE DE CHOC ÉLECTRIQUE • NE PAS OUVRIR
RISK OF ELECTRIC SHOCK
DO NOT OPEN
CAUTION
GND LIFT
GND LIFT
GND LIFT
GND
INPUT OUTPUT INPUT OUTPUT
INPUT OUTPUT
EXT SIDE CHAIN
EXT SIDE CHAIN
INPUT OUTPUT
SEND RETURN
SEND RETURN
EXT SIDE CHAIN
EXT SIDE CHAIN
SEND RETURN
SEND RETURN
230
1Input
2Output
3Side-chain Insert Send
4Side-chain Insert Return
5Voltage (see details on page 10)
6Ground Li (see details on page 11)
4
6
5
4 22 4
33 11

14
Control ElementsControl Elements
Auto
Byp. Link
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Sound Performance Lab
Model 1520
VoltAIR
120V DC Audio Rail
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mastering
compressor
AirBass
Tape Roll-Of f
1Input
2Output
3Threshold
4Tube Bias
5Attack
6Release
7Rectier
8Side Chain EQs / External side-chain
9Auto Bypass
10 AirBass/Bypass /Tape Roll-O
11 Link
12 VU switch
13 Channel switch
1 2
13
3
4
5
6
12 7
10
89
11

15
Control Elements
Input
The operating value of each channel can be increased or attenuated in 2 dB steps via
the six- step rotary knob. A three-way switch allows you to select whether the value
is increased or decreased. In the center position, the Input switch is inactive; in other
words, no level increase nor reduction takes place. This position is the default setting. If
the switch is in the “-” position (le), the input level is reduced according to the chosen
setting. If the switch is in the “+” position (right), the input level is increased according to
the chosen setting.
The increase or reduction of the input level aects the overall response of the compres-
sor and it has a direct impact on the level attenuation.
Tip:
For audio material with a narrow dynamic range and a lot of information in the lower fre-
quency spectrum, attenuating the input signal with the Input switch, for example -4dB,
allows the compressor to work in a much more relaxed manner.
Output
Since the compressor reduces the dynamics of the incoming signal, the output level is,
generally speaking, lower than the input level. This audible level loss can be compen-
sated with the Output control, in order to make the best use of the recording medium
used. Just like with the input section, the increase or reduction is achieved via a three-
way switch. In the center position, the Output switch is inactive; in other words, no level
increase nor reduction takes place. This position is the default setting. If the switch is in
the “-” position (le), the Output level is reduced according to the chosen setting. If the
switch is in the “+” position (right), the Output level is increased according to the chosen
setting.
Threshold
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. Do note, however, that the
intensity of the compression depends also on the parameters Input, Tube Bias, Rectier,
Attack, Release and Side Chain EQs.

16
Control Elements
Tube Bias
The Tube Bias switch allows you to determine the bias of the tubes according to three set-
tings: 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.
Cathode
Grid
Bias voltage
Audio signal
Audio signal
Anode
The modulation of the bias is the sum of the Threshold, Rectier, Side Chain EQ, Attack
and Release parameters.
Attack
Attack determines the response time of the compressor. Put simply, it is the time that the
compressor needs to respond once a signal is above the threshold. It indicates how long
it takes the compressor to perform 63% of its work. The Attack time can be adjusted in
six steps, from Fast to Slow.
The IRON does not oer the possibility to set an exact Attack time, since it is not a con-
stant value and it depends on the rest of the parameters.
Medium Attack times
To get an inconspicuous compression, set a middle-of-the-road Attack time and then
reduce it carefully until you can hear some distortion. Right at that moment you should go
back a bit and you will have reached an ideal compromise.
Longer Attack times
If you want to use the compressor to shape sound and to highlight the attack phase of an
instrument, you should use longer attack times.
Long Attack times and short Release times
The use of extremely long Attack times and shorter Release times is called Leveling,
because the compressor then hardly reacts to to any level changes and always keeps the
signal at the same level. That way, short dynamic changes in the music are not aected,
only long-term volume variations are processed.

17
Control Elements
Release
The counterpart of the Attack is Release. The Release parameter determines how fast the
compressor eases processing the signal. To be precise, it determines the time in which
63% of the reduced gain is restored. Similarly to the Attack time, the Release time can
also be set in six steps from Fast to Slow. Likewise, exact Release time settings are not
possible. Once again, there are no constant values, since the Release time depends on
the rest of the parameters, too.
Although the Attack and Release times can be considered xed intervals, the control-time
behavior and operating mode of the tubes is very dierent depending on the music. That
is why these values should not be considered absolute values.
On page 23 you will nd an overview for orientation, which shows the time values
depending on the chosen rectiers.
Tip: Compression during vocal recordings
The attack time should not be too fast, otherwise plosives could be distorted, resulting in
the vocals sounding unnatural. Many sound engineers compensate these level variations
by automating the fader. The actual peaks of vocals are not at the beginning of a syllable,
but rather later, when long vocals come along, which ought to be limited as well. That is
why the Attack time should be relatively slow and the Release time relatively fast.
Rectier
To produce the bias voltage to control the parallel connected tubes we use a rectier.
This circuit has six dierent operating characteristics (dierent rectiers), which can be
selected with the corresponding switch. They have a direct impact on the Attack and
Release times.
In comparison to the other rectiers, the rectier circuit LED delivers the longest/slowest
time values. You can nd more information at the time values overview on page 23.

18
Control Elements
Side-chain lter / External side-chain
Side-chain lters can be used to focus the response of the compression to certain fre-
quency ranges. This is also referred to as frequency selective compression. The side-
chain lters are only in the control signal path – not in the audio path. The IRON Mas-
tering Compressor oers a six-position switch to choose between the O position, four
side-chain lter presets or an external side-chain insert. For instance, if the low frequen-
cies are attenuated, the compressor will not react as promptly to kick drums and bass
lines. This can prove very useful when these elements are very present in the material
used. The same applies the other way around. If you increase certain frequencies, the
compressor will respond more resolutely to them. The side-chain lters are only in the
control signal path – not in the audio path.
TheIRONMasteringCompressor oersasix-positionswitchtochoose betweenthe Oposi-
tion, four side-chain lter presets or an external side-chain insert. In position 1 (O), only a
capacitor is integrated into the side-chain signal to lter out frequencies lower than 20Hz.
-20
+
20
-16
-12
-8
-4
-0
+4
+8
+
12
+
16
d
B
Hz
u
10 20k 20 50 100 200 500 1k 2k 5k 10k
EQ O
The next switch positions can be used to switch more complex lters into the
side-chain signal. These can be used to focus the response of the compression to certain
frequency ranges. This is also referred to as frequency selective compression.
Position 2 (EQ 1) integrates a lowpass lter.
-20
+
20
-16
-12
-8
-4
-0
+4
+8
+
12
+
16
d
B
Hz
u
10 20
k
20 50 100 200 500 1k 2k 5k 10k
EQ 1
Position 3 (EQ 2) integrates a highpass lter.
-20
+20
-16
-12
-8
-4
-0
+4
+8
+12
+16
d
B
Hz
u
10 2
0k
20 50 100 200 500 1k 2k 5k 10k
EQ 2

19
Control Elements
Position 4 (EQ 3) integrates a bandpass lter.
-20
+2
0
-16
-12
-8
-4
-0
+4
+8
+1
2
+1
6
d
B
Hz
u
10 20
k
20 50 100 200 500 1k 2k 5k 10k
EQ 3
Position 5 (EQ 4) integrates a lter curve empirically designed by developer Wolfgang
Neumann.
-20
+2
0
-16
-12
-8
-4
-0
+4
+8
+1
2
+1
6
d
B
Hz
u
10 20
k
20 50 100 200 500 1k 2k 5k 10k
EQ 4
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 lter curves can be created to focus the response of the compression to
certain frequency ranges.
In Link mode, only the side-chain insert of the right channel is active.
Tip:
You can also use a mono equalizer with two parallel outputs in the side-chain insert. The
input signal then has to be received from the send of the right channel – not from the le
channel!
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.

20
Control Elements
Ratio
Compressors of this design do not have a xed compression ratio.
The lower the Threshold and the higher the input signal, the stronger the compression.
This is actually one of the main factors that make the IRON‘s compression so musical.
Auto Bypass
To be able to make an objective judgment of the processed material, it is best not to have
to be toggling between the original and processed signals by yourself, but rather have it
done automatically. Plus, the fact that you do not have to move from the sweet spot and
can concentrate better on the music to optimally assess the processing is a huge advan-
tage. The Interval control determines the time that needs to elapse before the compres-
sor toggles between the processed and unprocessed signals. Hard le is the shortest
set- ting. To increase the interval, turn the knob clockwise.
AirBass / Bypass / Tape Roll-O
Many times, you might want to give that distinctive touch to a music production at the
very end of the production process, without the need to modify or redo the entire signal
chain. It was with this in mind that we developed two specially matched passive lters
and integrated them into a 120V technology amplication stage.
AirBass:
This lter makes music rounder and more well-balanced with powerful lows and bright,
silky highs.
+5
+4
+3
+2
20 50 100 200 500 1k
Hz
2k 5k 10k 20k 50k
+1
0
-1
-2
-3
-4
-5
-6
+6
d
B
u
AirBass