Airfield Audio LIMINATOR User manual

LIMINATOR
CLASS A DUAL TRANSFORMER COMPRESSOR
OWNER’S MANUAL
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TABLE OF CONTENTS
SECTION PAGE
INTRODUCTION 2
INSTALLATION 3
FRONT PANEL CONTROLS 4
OPERATION 5
SPECIFICATIONS 7
WARRANTY AND REGISTRATION 8
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INTRODUCTION
Thank you for choosing Airfield Audio.
Our many years of specializing in tech service, custom build and modifications to high
end studio and professional audio installations are now being expanded with an exciting
new product line. For the Airfield Audio Liminator we have analyzed more than a dozen
classic and vintage audio compressors, and have distilled the best of the best into this
device. Your ears will love the sound of this unique analog processor.
Class A discrete transistor circuitry is used exclusively, with classic input and output
transformers deployed as an important element of the design. The best sound possible is
realized with a complete reduction of cable-induced noise. And, in fact, a main feature of
the Liminator is the inclusion of 2 switchable input transformers for a great range of
flexibility and tone. All front panel controls as well as internal components have been
selected for ruggedness and long life. We take pride in quality construction; all units are
handcrafted with care, thoroughly bench tested, calibrated and burned in for 48 hours.
The Airfield Audio Liminator exemplifies state of the art analog audio design, and
delivers natural, musical characteristics to the recording process. One of the most
versatile compressors ever invented, it can be used with confidence on voice, all
instruments, any sound source, and with an additional unit, on stereo mix and mastering.
For reasons not even science can fully explain, the best analog devices create a sonic
magic that flows from the sound source through real parts and circuitry into the human
ear with the highest clarity and resolution. Airfield Audio is dedicated to this natural
process. It sounds very very very very good.

INSTALLATION
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51 2 6 4 3
REAR PANEL VIEW
1. IMPORTANT :SET THE MAINS SWITCH ON THE REAR PANEL TO THE
CORRECT VOLTAGE.
2. Be certain that the unit has a ½ AMP fuse (115 VAC mains)
or ¼ AMP fuse (230 VAC mains).
3. Connect a line level audio input cable from your source (microphone preamp, EQ,
guitar or bass preamp / effects box, analog console or analog output from your
DAW) to the 3 pin XLR / ¼” female connector. The input connector is Balanced
Transformer Coupled, pin 2 hot.
4. Connect an audio output cable to the 3 pin XLR male connector and plug it into
the input of your recording or mixing device. The output XLR is Balanced
Transformer Coupled, pin 2 hot.
5. Connect the supplied IEC power cable to the IEC mains socket on the rear panel.
USE ONLY A 3 PRONG GROUNDED POWER CABLE. For safety reasons do
not lift the ground on the power plug by using a 3 to 2 ground lift adapter.
6. Link Jack: connect a standard ¼” unbalanced guitar type cable between 2
Liminator units for stereo operation. Y-cables should be used in a daisy-chain
configuration for multi-channel operation of 3 or more Liminator channels.
7. Turn on the unit and allow 30 minutes warm up.

FRONT PANEL
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A B C D E F G H I J
A. THRESHOLD Continuously variable adjustment.
This control determines the amount of gain reduction.
Fully counter-clockwise (ccw), gain reduction is off.
Fully clockwise (cw), gain reduction is at maximum.
B. ATTACK 5 position rotary switch, ccw: fastest, cw: slowest.
C. RELEASE 5 position rotary switch, ccw: fastest, cw: slowest.
D. RATIO 5 position rotary switch, select for 1.5 (gentle), 2 (moderate),
4 (medium), 8 (heavy), or 20 (extreme) compression.
E. OUTPUT Continuously variable adjustment.
This control determines the amount of gain make up.
Fully ccw, audio is off. Fully cw, gain is at maximum.
This control knob goes to 11.
F. METER Measures in decibels amount of gain reduction, input level, or
output level, according to meter switch.
G. METER 3 position rotary switch, select meter function: In (Input Level),
SWITCH GR (Gain Reduction), Out (Output Level).
H. T1 / T2 Input transformer switch. T1 (Jensen) is very clean and transparent.
T2 (Sowter) sounds richer with more harmonics.
I. MODE 3 position rotary switch, select Link, Compress, or Bypass
J. POWER Power is On when switched up. Amber bezel and meter
will illuminate.

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OPERATION
All front panel controls are designed to be functional and uncomplicated.
Initial control settings:
1. Set METER switch to “In”, and MODE switch to “Bypass”.
2. Set THRESHOLD control off (fully counter clockwise).
3. Set OUTPUT control to unity gain position (12 o’clock noon).
4. Set ATTACK, RELEASE, and RATIO switches straight up (12 o’clock noon).
The Liminator has been designed so that this position is a useful starting point.
These switches can be adjusted to taste from this position.
5. Apply an analog line level signal from your source (microphone pre-amp, EQ,
guitar or bass preamp, audio console or DAW). Adjust the output of the source to
get a nice hot level coming into the Liminator. VU meter reading should average
around 0 and up into the red zone.
6. Now, set METER switch to “GR” and set MODE switch to “Comp”.
7. Slowly turn up the THRESHOLD control, and observe gain reduction of signal on
the VU meter. Normal gain reduction is 2 to 6dB. Higher RATIO settings will
result in deeper compression.
8. While listening, adjust the OUTPUT control to maintain an appropriate level back
into your signal path. To view the output level on the VU meter, set METER
switch to “Out”. After “In” and “Out” levels have been checked it is
recommended that the METER switch be normally left in “GR” position.
9. Adjust ATTACK. This control represents the amount of time a compressor needs
to react to an input signal change. In general, fast settings are often used while
tracking percussive sounds (drums, piano, choppy rhythms). Mid settings are
good for voice, brass, acoustic guitar. Slow settings work well with strings, synth
pads, creamy guitar solos, vocal chorus and on the stereo mix and mastering.
10. Adjust RELEASE. This control determines the amount of time for the gain of a
compressed signal to return to normal (or 0dB reduction). Fast release returns the
gain to normal quickly, which is useful for short-duration signal peaks.
Slower release settings are less audible, useful in mixing and mastering.

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11. RATIO. This control indicates the amount the output will change in relation to
changes in the input level. The overall dynamic range of the program signal is
reduced or “squeezed”. (In theory, a ratio setting of 2 means that a 2dB input
level change above threshold will produce a 1dB output level change).
A ratio setting of 1.5 or 2 is gentle or moderate compression, very useful when
mixing or mastering. At a setting of 4, compression becomes a bit more audible,
which is normal and useful for vocal recordings and general purpose medium
compression tasks. A setting of 8 would be considered heavy compression, for
example live drums, loud guitars, controlling background vocals, etc.
At setting 20, you are using extreme compression. At this setting the output level
will essentially remain constant despite increases of input level above threshold.
Ultimately, your best choices of various settings will come from using your
ears…..they will tell you more than the meter.
12. T1 / T2 switch. Select one of two input transformers. T1 is a medium impedance
Jensen transformer. Select it for clean, uncolored, accurate, flat response; perfect
for mixing, mastering and pristine tracking. T2 is a lower impedance Sowter
transformer; select it when thicker musical harmonics and richer tone are desired
for tracking or mixing. When using a pair of Liminator compressors linked for
stereo operation, both channels should be set to same input transformer.
13. MODE. Select Comp (compress) for normal unit operation, or Bypass (true relay
bypass) for comparison between selected compression settings and the
unprocessed signal.
Link setting can be selected when 2 or more Liminator units are used together
for stereo or multi-channel recording, mixing or mastering. The gain reduction
control voltages are thus mixed, which stabilizes the sound-stage image and
prevents drift between channels.
For stereo or multi-channel operation, connect a ¼” Link cable or Y-cables
between units (see page 3). Each channel’s controls will remain individually
operational for compression settings, and are usually set identically.
14. Unbalanced operation. The Airfield Audio Liminator works fine in unbalanced
audio systems. Typically, input and output XLR cables (with pin 1 and 3 joined
together) wired to ¼” or RCA connectors will do the job. For proper operation of
THRESHOLD control, ensure that the output of your source device delivers a
good hot +4dB (0 VU) signal level.

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SPECIFICATIONS
INPUT IMPEDANCE: 15KΩwith Transformer 1, Balanced, Pin 2 hot
1.2KΩwith Transformer 2, Balanced, Pin 2 hot
OUTPUT IMPEDANCE: 75ΩBalanced, Pin 2 hot
FREQUENCY RESPONSE: ± 0.25dB, 20 Hz to 20 kHz
- 2dB at 10 Hz, -2dB at 68 kHz with Transformer 1
- 2dB at 10 Hz, -2dB at 120 kHz with Transformer 2
NOISE: less than -80 dB, 20 Hz to 80 kHz, -92 dB typical below operating level
DISTORTION: less than .02% THD at unity gain, 4 dBu input level
.1% THD at 24 dBu input level
MAXIMUM OUTPUT: + 32 dBu into 600Ω
GAIN REDUCTION RANGE: 0 ~ 20 dB
ATTACK: 5, 30, 60, 100, 150 milliseconds
RELEASE: .1, .3, .6, 2, 6 seconds
GAIN REDUCTION ELEMENT: LD1 forward biased optical resistive module
METER: dB Gain Reduction
dB Input, dB Output (0VU = + 4dBu)
POWER REQUIREMENTS: 15 watts, 115 / 230 VAC, 50 / 60Hz
FUSE TYPE: 3AG slo-blo, ½ Amp for 115 VAC mains, ¼ Amp for 230 VAC mains
DIMENSIONS: 19” Rackmount 2U Chassis
DEPTH: 7” (180mm)
WEIGHT: 15 pounds (7 kg)

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WARRANTY AND REGISTRATION
Airfield Audio equipment is covered by a limited warranty against manufacturing defects
in materials and craftsmanship for a period of two years from date of original purchase.
Under the terms of this warranty, Airfield Audio Inc. will repair or replace the part or
parts of the product which prove to be defective (except incandescent lamps, which are
guaranteed for 6 months from date of purchase), provided the purchaser gives Airfield
Audio prompt and satisfactory proof thereof.
If the equipment requires warranty repair, please contact your dealer or Airfield Audio by
telephone, fax or email. The purchaser’s sole remedy shall be to return the product to
Airfield Audio, where the defect will be repaired at no charge for parts and labor. All
returns to Airfield Audio require a return authorization number and must be in original
packaging, shipped to Airfield Audio via insured freight at the owner’s expense. Charges
for transportation costs and damage in transit are not covered by this warranty. The
equipment will then be returned via prepaid, insured freight, method and carrier to be
determined solely by Airfield Audio.
Airfield Audio Inc. is not responsible for damage from abuse or for any claim including
but not limited to property damage, loss of revenue, incidental or consequential damage
of any kind whatsoever which may result from failure of this product.
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