SPL IRON User manual

Manual
IRON
Mastering Compressor

1
Content
IRON 2
Version 1.1 – 11/2015 2
Package Contents 2
Introduction 3
Operating Principles of a Compressor 3
IRON Mastering Compressor 3
Technical Aspects 4
120 Volt Technology 4
120 Volt Technology - Diagram 5
Innovating Compression 6
Signal Flow 7
Installation 8
First Steps 8
Voltage Selection 8
General Information 9
Ground Lift switch to avoid ground loops 9
Tube Warm-Up 9
Cabling: Rear Side 10
XLR inputs and outputs 10
Sidechain Input 10
Control Elements 12
Overview 12
Input 13
Output 13
Threshold 13
Tube Bias 14
Attack 14
Release 15
Rectier 15
Side Chain EQs 16
Ratio 16
Auto Bypass 17
AirBass / Bypass / Tape Roll-Off 17
Link 18
VU Switch 18
Channel Switch 18
Time Values 19
Timevaluesdependingontherectiers 19
Specications 20
Measurements 20
Security Advices 21
Notes on Environmental Protection 22
Contact 23

2
IRON
Version 1.1 – 11 /2015
Developer: Wolfgang Neumann
This manual includes a description of the product but no guarantee as for specific charac-
teristics or successful results.
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 specifications are subject to change.
Package Contents
IRON Mastering Kompressor
Power cord
Manual
The IRON mastering compressor is available in different colors.
Black: Model 1520
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 packa-
ging guarantees a safe shipment.
The SPL IRON mastering compressor was designed, developed and manufactured in
Germany.

3
Introduction
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Ra-
tio 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 differ 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 soft 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. Different compressors
with the exact same settings might work and sound completely different. They provide
different sounds for different applications and music styles.
Nowadays, the compressor has become a key element when it comes to provide dyna-
mics and punch to any production. The number of compressors available is huge and it‘s
easy to succumb to the promises made by software emulations and analog recreations of
vintage gear as the perfect solution. Unfortunately, many of these emulations and recre-
ations differ 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
different to the ones originally used and that they can‘t be digitally emulated. No software
(DSP-emulated compressors) or hardware replica will ever be able to sound like the origi-
nal. 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, melo-
dic-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 vinta-
ge tube compressors with the advantages of the High Dynamic 120 V 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 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 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.

4
Technical Aspects
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 ope-
rating 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 gene-
ration 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. 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.

5
Technical Aspects
120 Volt Technology - Diagram
These diagrams clearly show the advantages of our 120-volt technology in comparison
to other circuits with a lower operating voltage. 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, distortion limit and signal-to-noise ratio. The
result is a clearly more laid-back and natural sound with less unpleasant coloring.
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 +10dB correspond to twice the perceived loudness.
When it comes to volume, the 120-volt 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 SPL op-amps
show a difference of more than 3dB compared to the OPA134 at 36V — in terms of sound
pressure level, that corresponds to an improvement of more than 50%.
The operating level most commonly used for audio equipment is 30 volts.
120
125
130
135
140
145
dBu Dynamic Range
OPA 134@30 VOPA 134@36 VSPL-OP@120 V
124,2
129,1
141,4
0
5
10
15
20
25
30
35
dBu
OPA
134@30
VOPA
134@36
VSPL-OP
@120
V
21,5 22,5
33,2
Maximum Levels
0
20
40
60
80
100
120
Volt
+/- 15 Volt +/- 18 Volt +/- 60 Volt
30 V
36 V
120 V
Operational Voltages
-115
-113
-111
-109
-107
-105
dBu TL 071@30 VOPA 134@36 VSPL-OP@120 V
106
111,7
114,2
THD&N

6
Technical Aspects
Innovating Compression
The IRON mastering compressor is a variable-bias limiter/compressor. The implementati-
on 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 paral-
lel 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. Mo-
reover, we use Lundahl custom-made balanced high-level dual-coil Mu-Metal iron trans-
formersinthesignalowofthevariable-biastubes,whichaddtotheoverallsound.
The second new technology implemented is the independent feed-forward resistive op-
to-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 li-
miter, like in a conventional opto-compresor. 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rectier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rectier.Giventhespecic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, compa-
red 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,Rectier,Tube-BiasandSide-ChainEQsettings.

7
Technical Aspects
ThefollowingdiagramdepictsthesignalowwithintheIRONcompressor.Itismeantto
clarify how it works and to show the relation between its different parameters. The audio
signalowisinblueandthecontrolvoltagesignalowisingreen.
O
U
T
P
U
T
2
4
6
8
1
0
1
2
I
N
P
U
T
T
H
R
E
S
H
O
L
D
Threshold
m
i
n
2
4
6
8
1
0
1
2
1
4
1
8
2
0
2
2
2
4
2
6
2
8
3
0
3
2
3
4
3
6
3
8
4
0
m
a
x
Input Gain
Input Output
Rectifier
1
m
F
2
m
F
3
.
3
m
F
3
3
0
n
F
2
2
0
n
F
1
0
0
n
F
R
E
C
T
I
F
I
E
R
G
e
G
e
L
E
D
S
i
G
e
G
e
/
S
i
Side Chain EQs
O
F
F
E
Q
1
E
Q
2
E
Q
3
E
Q
4
E
X
T
.
S
I
D
E
C
H
A
I
N
E
Q
s
Anode
Variable-µ /
Sharp Cut-off Tube
Variable-µ /
Remote Cut-off Tube
Grid
Cathode
Tube Bias Higher Bias =
More Gain
Reduction
Audio
Control Voltages
Attack
f
a
s
t
•
•
•
•
s
l
o
w
A
T
T
A
C
K
Release
f
a
s
t
•
•
•
•
s
l
o
w
R
E
L
E
A
S
E
Transformer
TUBE BIAS
Low
Mid
High
Anode
Grid
Cathode
– +
0
1
2
3
5
8
1
2
Output Gain
– +
0
Transformer
Opto
Peak Limiter
AirBass/
Tape Roll-Off
AirBass
12AX7
ECC83
Perkin Elmer
VTL 5C1
12AU7
ECC82
Signal Flow

8
Installation
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no
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 off before connecting or disconnecting any cables or equip-
ment to it.
Use the On/Off switch on the rear panel to turn the unit on or off. The illuminated VU-me-
ter on the front panel indicates the unit‘s operating status. The On/Off switch was pla-
ced on the rear panel to avoid any emissions due to voltage-carrying conductors running
across the unit and affecting sound. When powering on or off, there‘s no need to observe
aspecicsequenceregardingtheconnecteddevices.However,likewithanyaudiosignal
chain,powerampliersshouldalwaysbepoweredonlastandpoweredoffrst.TheIRON
can be powered on and off 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 corres-
ponds to the values of your local power grid (230 or 115 volts) Inside the power connec-
tor, to the right, next to the on/off 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18
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.

9
General Information
Ground Lift switch to avoid ground loops
On the rear panel of the IRON mastering compressor is also a „GND LIFT“ (Ground Lift)
switch to avoid any ground loops. Ground loops take place when gear connected in the
same network have different potentials.
The GND LIFT switch disconnects the equipment ground from the service ground to avoid
such problems. The Ground Lift function is activated (= equipment ground disconnected)
when the switch is depressed.
Tube Warm-Up
The IRON compressor 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afterpoweringon.
Suggestions for setting the compressor
The operation of the IRON compressor strongly depends on the input signal.
Normally the following start values are a good starting point:
Attack/Release: Position 2 or 3 (clockwise from „fast“)
Rectier: LED
Side Chain EQs: Off
Tube Bias: Low
Ifyou switchthroughthe differentrectiers, youhavetoadjust theother parameters.
Theremainingrectiercircuitstendtoprovidefaster/slowertimevalues.Whenthepro-
gram material stays the same, slower/longer time values should be chosen and/or the
threshold should be raised.
Especially when it comes to group applications, the high bias setting can be interesting.

10
Cabling: Rear Side
XLR inputs and outputs
We used exclusively Switchcraft/Neutrik XLR input and output plugs to guarantee perfect
connectivity in the studio. They provide an optimal connection thanks to their electrome-
chanical 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.
Sidechain Input
The Sidechain input serves to connect an external control signal to the compressor. To
that end, each channel features a TS jack. Please use only TS (mono) not TRS (stereo)
plugs.
The image shows the pinout of the TS connectors.
Tip : Signal
Sleeve: Ground
Input Output
balanced unbalanced balanced unbalanced
1=GND
2=hot (+)
3=cold (-)
1
2
3
1
2
3
12
3
12
3
Tip Sleeve
Tip Sleeve
Ring
TRS
TS

11
Cabling: Rear Side
GND LIFT
GND LIFT
GND LIFT
GND
INPUT 1
INPUT 1
OUTPUT 1
OUTPUT 1
SIDE CHAIN 1
SIDE CHAIN 1
Use Mono Jack
(Unbalanced)
Use Mono Jack
(Unbalanced)
INPUT 2
INPUT 2
OUTPUT 2
OUTPUT 2
SIDE CHAIN 2
SIDE CHAIN 2
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.
AC MAINS
AC MAINS
XLRs:
Balanced +4dBu
1 = GND
2 = HOT (+)
3 = COLD (–)
XLRs:
Balanced +4dBu
1 = GND
2 = HOT (+)
3 = COLD (–)
WARNING
AVIS: RISQUE DE CHOC ÉLECTRIQUE • NE PAS OUVRIR
RISK OF ELECTRIC SHOCK
DO NOT OPEN
CAUTION
Serial
Number
Made in Germany
~ 115V AC / ~230VAC, 50 Hz / 60 Hz, P max. 100 W
Fuses:
Voltage Selection:
115V AC: T 2 A L 250 V
230V AC: T 1 A L 250 V
1. Remove Fuse Holder
2. Change Fuses
3. Flip Over
4. Reinstall
IRON
230
GND LIFT
GND LIFT
GND LIFT
GND
INPUT 1
INPUT 1
OUTPUT 1
OUTPUT 1
SIDE CHAIN 1
SIDE CHAIN 1
Use Mono Jack
(Unbalanced)
Use Mono Jack
(Unbalanced)
INPUT 2
INPUT 2
OUTPUT 2
OUTPUT 2
SIDE CHAIN 2
SIDE CHAIN 2
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.
AC MAINS
AC MAINS
XLRs:
Balanced +4dBu
1 = GND
2 = HOT (+)
3 = COLD (–)
XLRs:
Balanced +4dBu
1 = GND
2 = HOT (+)
3 = COLD (–)
WARNING
AVIS: RISQUE DE CHOC ÉLECTRIQUE • NE PAS OUVRIR
RISK OF ELECTRIC SHOCK
DO NOT OPEN
CAUTION
Serial
Number
Made in Germany
~ 115V AC / ~230VAC, 50 Hz / 60 Hz, P max. 100 W
Fuses:
Voltage Selection:
115V AC: T 2 A L 250 V
230V AC: T 1 A L 250 V
1. Remove Fuse Holder
2. Change Fuses
3. Flip Over
4. Reinstall
IRON
230
1Input
2Output
3External Sidechain
4 Voltage (see details on page 8)
5 Ground Lift (see details on page 9)
11223
3
4
5

12
Control Elements
Auto
Byp. Link
Byp
I
n
t
e
r
v
a
l
IRON
O
F
F
E
Q
1
E
Q
2
E
Q
3
E
Q
4
E
X
T
.
f
a
s
t
•
•
•
•
s
l
o
w
A
T
T
A
C
K
S
I
D
E
C
H
A
I
N
E
Q
s
S
I
D
E
C
H
A
I
N
E
Q
s
VU
VU 0
GR
VU +10
Cal
TUBE BIAS
T
H
R
E
S
H
O
L
D
+–+–
m
i
n
2
4
6
8
1
0
1
2
1
4
1
8
2
0
2
2
2
4
2
6
2
8
3
0
3
2
3
4
3
6
3
8
4
0
m
a
x
2
4
6
8
1
0
1
2
2
4
6
8
1
0
1
2
2
4
6
8
1
0
1
2
I
N
P
U
T
0
Low
Mid
High
f
a
s
t
•
•
•
•
s
l
o
w
R
E
L
E
A
S
E
O
U
T
P
U
T
Sound Performance Lab
Model 1520
VoltAIR
120V DC Audio Rail
dB
0
dB
TUBE BIAS
Low
Mid
High
R
E
C
T
I
F
I
E
R
O
F
F
E
Q
1
E
Q
2
E
Q
3
E
Q
4
E
X
T
.
f
a
s
t
•
•
•
•
s
l
o
w
A
T
T
A
C
K
VU
VU 0
GR
VU +10
Cal
T
H
R
E
S
H
O
L
D
+–+–
m
i
n
2
4
6
8
1
0
1
2
1
4
1
8
2
0
2
2
2
4
2
6
2
8
3
0
3
2
3
4
3
6
3
8
4
0
m
a
x
2
4
6
8
1
0
1
2
I
N
P
U
T
0
f
a
s
t
•
•
•
•
s
l
o
w
R
E
L
E
A
S
E
O
U
T
P
U
T
dB
0
dB
R
E
C
T
I
F
I
E
R
A Wolf
Neumann
Design
1
m
F
2
m
F
3
.
3
m
F
3
3
0
n
F
2
2
0
n
F
1
0
0
n
F
G
E
R
M
A
N
I
U
M
L
E
D
S
I
L
I
C
O
N
G
e
G
e
/
S
i
1
m
F
2
m
F
3
.
3
m
F
3
3
0
n
F
2
2
0
n
F
1
0
0
n
F
G
E
R
M
A
N
I
U
M
L
E
D
S
I
L
I
C
O
N
G
e
G
e
/
S
i
mastering
compressor
AirBass
Tape Roll-Off
1Input
2Output
3Threshold
4 Tube Bias
5 Attack
6 Release
7 Rectier
8 Side Chain EQs
9 Auto Bypass
10 AirBass / Bypass / Tape Roll-Off
11 Link
12 VU Switch
13 Channel Switch
1 2
3
4
5
6
7
89
10
11
12
13
Overview

13
Control Elements
Input
The operating value of each channel can be increased or reduced 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 (left), 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 affects the overall response of the compressor
and it has a direct impact on the level reduction.
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 (left), 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 be-
neath 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Input,TubeBias,Rectier,Attack,Release,and
Side Chain EQs parameters.

14
Control Elements
Tube Bias
The Tube Bias switch allows you to determine the bias of the tubes according to three
settings (Low, Mid, High). The bias of a tube is the voltage present on the tube‘s grid. The
higher the voltage, 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,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 has exceeded the threshold. The Attack time
can be adjusted in six steps, from Fast to Slow.
The IRON does not offer the possibility to set an exact Attack time, since it is not a cons-
tant value and it depends on the rest of the parameters.
Moderate Attack times
To get an inconspicuous compression, set a middle-of-the-road Attack time and then re-
duce 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 transient 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 can hardly react to to any level changes and always keeps the
signal at the same level. That way, short dynamic changes in the music are not affected,
only long-term volume variations are processed.
Cathode
Grid
Bias Voltage
Audio
Audio
Anode

15
Control Elements
Release
The Release parameter determines how fast the compressor eases processing the signal
and returns to a 1:1 ratio once its level has fallen bellow the threshold. 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 different depending on the music. That
is why these values should not be considered absolute values.
On page 19 you will find an overview for orientation, which shows the time values depen-
ding on the chosen rectifiers.
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 pe-
aks 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differentoperatingcharacteristics(differentrectiers),whichcanbeselec-
ted 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19.

16
Control Elements
Side Chain EQs
TheSidechainlterscanmaketheresponseofthecompressionbeinuencedbyagiven
frequency range. Some call this frequency-selective compression. For instance, if the low
frequencies are reduced, 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 ma-
terial used. The same applies the other way around. If you increase certain frequencies,
thecompressorwillrespondmoreresolutelytothem.TheSidechainltersareonlyinthe
control signal path.
The IRON mastering compressor‘s Side Chain EQ features a six-step switch that allows
youtochoosebetweenOff,foursidechain-lterpresetsoranexternalsidechainsignal.
IntheOffpositiontheonlylteringthatappliesisduetoacondenserthatltersoutfre-
quenciesbelow20Hz.Position3-5provideempiricallydetermined,presetltercurves.
In the last position, an external signal, fed through the corresponding jack on the rear of
the IRON, is used to trigger the compressor. Please use only TS (mono) plugs to connect
outboard gear to the rear jacks of the IRON.
Inthefollowingdiagramyoucanseethefrequencyresponsecurvesofthedifferentlter
presets.Thefrequencyresponsecurveofthelterpresetsareshownindifferentcolors
to make them easily distinguishable.
Ratio
Thistypeofcompressordoesnothaveaxed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.
02-
2+
81-
61-
41-
21-
01-
8-
6-
4-
2-
0-
d
B
u
02 k0305 001 002 005 k1 k2 k5 k01 k02
zH
Off
Preset 1
Preset 2
Preset 3
Preset 4

17
Control Elements
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 compressor
toggles between the processed and unprocessed signals. Hard left is the shortest set-
ting. To increase the interval, turn the knob clockwise.
AirBass / Bypass / Tape Roll-Off
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 120-volt SUPRA op-amp.
AirBass:
Thisltermakesmusicrounderandmorewell-balancedwithpowerfullowsandbright,
silky highs.
Tape Roll-Off:
Thislterisbasedonthefrequencyresponseoftapemachines.Itcanproveveryuseful
to provide a nice rounding-off in the high end when the material being processed is too
shrill.
In the following diagram you can see the frequency response curves for the AirBass and
Tape Roll-Off presets, as well as that of the Bypass switch.
6-
6+
5-
4-
3-
2-
1-
0-
1+
2+
3+
4+
5+
d
B
u
02 k05 05 001 002 005 k1 2k k5 k01 k02
zH
Bypass
AirBass
Tape Roll-Off

18
Control Elements
Link
The IRON mastering compressor has been designed as a completely independent
two-channel, dual-mono compressor and can be used to process two different mono sig-
nals at the same time. Nevertheless, you can also easily process a stereo signal, since all
settings are made with switches or a detent potentiometer (Threshold). This allows you
to effortlessly make the same settings on the left and right channels. The components of
both channels have been especially selected so that the difference between them is as
little as possible, considering a very small tolerance range.
However, if you activate the Link function. all settings of the link channel are applied
to the right channel thanks to an intelligent logical relay circuit. This is true for the Th-
reshold,butalsofortheAttack,Release,Bias,SideChainEQ,andRectiersettings.
The combination of both control voltages makes it easier to process a stereo signal more
precisely. It allows you to concentrate on the music without having to worry about correc-
ting a parameter on the other channel.
And it can also be put to use creatively. For instance, with the Link function not activated,
a sound that only exceeds the Threshold on the left channel would trigger the compressi-
on on the left channel only. However, if the Link function is activated, the right channel is
compressed, too. When processing stereo signals in dual-mono mode, the stereo image
is still acoustically perceived.
VU Switch
Use this three-way switch to toggle the display between Gain Reduction and Output level
(0dB and +10dB). This can be done for each of the two VU-meters separately. The meters
work independently for each channel, even when the Link function is activated.
The CAL trimmer allows you to calibrate the display of the Gain Reduction on the VU-me-
ter. The IRON‘s Gain Reduction VU-meter ought to show 0 dB after the warm-up phase.
0 dB on the VU-meter correspond to an output level of 0 dBu.
Channel Switch
The two, centrally located, orange-lit switches activate or deactivate the corresponding
left and right channels.