API 2500 User manual

2500 Stereo Bus Compressor
50th Anniversary Edition
OPERATOR’S MANUAL
Written for Automated Processes Incorporated
by Dan Pfeifer
Rev. 19-07-18

©2019
8301 Patuxent Range Road
Jessup, MD 20794 USA
301-776-7879
http://www.apiaudio.com

Table of Contents
About This Manual ...................................................................................... 4
Important Safety Instructions .................................................................... 5
1.0 Introduction ......................................................................................... 6
1.1 Features ..................................................................................................... 6
2.0 Quick Start............................................................................................ 7
3.0 Overview .............................................................................................. 8
4.0 Compressor Section ............................................................................ 10
4.1 Compressor Section Controls .................................................................... 11
4.1.1 THRESHOLD.......................................................................................................... 11
4.1.2 ATTACK ................................................................................................................ 11
4.1.3 RATIO .................................................................................................................. 11
4.1.4 RELEASE............................................................................................................... 12
5.0 Tone Section ....................................................................................... 13
5.1 Tone Section Controls ............................................................................... 13
5.1.1 KNEE.................................................................................................................... 13
5.1.2 THRUST
®................................................................................................................. 15
5.1.3 TYPE .................................................................................................................... 16
6.0 Link Section ........................................................................................ 18
6.1 Link Section Controls ................................................................................ 19
6.1.1 Left/Right Link....................................................................................................... 19
6.1.2 Link Shape............................................................................................................ 19
7.0 Output Section.................................................................................... 20
7.1 Output Section Controls ............................................................................ 20
7.1.1 Make-up Gain........................................................................................................ 20
7.1.2 Blend and Mix........................................................................................................ 21
7.1.3 IN/out and Bypass ................................................................................................. 21
8.0 Meters ................................................................................................ 23
8.1 VU Meter Controls..................................................................................... 24
8.1.1 Stereo Gain Reduction Meters.................................................................................. 24
8.1.2 Stereo VU Meters................................................................................................... 24
8.1.3 VU Meter Source.................................................................................................... 24
9.0 Side Chain Input ................................................................................. 25
10.0 Power Switch & Rear Panel Connections........................................... 26
10.1 Power Switch.......................................................................................... 26
10.2 Rear Panel .............................................................................................. 26
10.2.1 Audio Connections................................................................................................ 26
10.2.2 AC Power ............................................................................................................ 27
10.2.3 Commemorative Serial Number Plate...................................................................... 27
APPENDIX................................................................................................. 28
A1 Specifications and Block Diagram............................................................... 29
A2 Recall Chart............................................................................................... 30

A3 API Limited Warranty and Service.............................................................. 31
About This Manual
This manual explains the operation and applications of the 50th Anniversary Edition of the API 2500
Stereo Bus Compressor. This manual is organized in sections that correspond to the front panel
controls and rear panel interface.
Legend:
• UPPER-CASE BOLD = POTS, SWITCHES, BUTTONS
• UPPER-CASE = REAR PANEL CONNECTIONS
Manual Revision History: YY-MM-DD format
Original: 19-07-18
Revised:

Important Safety Instructions
1. Please read these instructions
2. Keep this information in a safe place
3. Do not use this device near water
4. Clean only with a dry cloth
5. Do not block any ventilation openings
6. Do not install near any heat sources such as radiators, heat registers, stoves, or other
devices that produce heat
7. Do not defeat the safety purpose of the polarized or grounding type AC plug
8. Protect the AC power cord from being walked on or pinched
9. Use only attachments/accessories specified by the manufacturer
10. Unplug this device during lightning storms or when unused for long periods of time
11. Refer all service to qualified personnel
ATTENTION: Exposure to extremely high noise levels may cause
permanent hearing loss or damage. Individuals vary considerably
in susceptibility to noise-induced hearing loss, but nearly everyone
will lose some hearing if exposed to sufficiently intense noise (this
may include music) for a period of time. Be safe.
WARNING – To reduce the risk
of fire or electric shock, do not
expose this apparatus to rain
or moisture.

50th Anniversary 2500 Bus Compressor API
6
1.0 Introduction
To commemorate our 50th year of providing high-quality audio equipment to the professional
community, API has produced the 50th Anniversary Edition of the API 2500 Stereo Bus Compressor.
The 2500 Bus Compressor is API’s flagship compressor and the 50th Anniversary Edition adds a few
unique features to this highly-valued unit. These unique features include flexible output mix blending,
LED gain reduction meters, premium VU meters, and a 2ru, gold-faced chassis. A limited run of only
fifty 50th Anniversary Edition 2500 compressors will be produced, so each unit is uniquely identified as
one of this exclusive class.
The 50th Anniversary Edition of the API 2500 Stereo Bus Compressor provides the unmistakable sound
of API’s compression technology combined with unmatched parametric control. The result is one of
the world’s best sounding and versatile stereo bus compressors that’s capable of handling an infinite
number of production applications. Carefully designed to deliver a wide range of compression options
and features not found on the standard 2500, the 50th Anniversary Edition of the 2500 delivers
warmth, clarity and punch, whether used for subtle adjustments or heavy compression effects.
1.1 Features
• Limited 50th Anniversary Edition: Only 50 units built
• “Over Threshold” LED illuminates when input audio crosses the set threshold
• Patented THRUST
®filter with 3 settings for frequency dependent side chain control
• 3 selectable compressor curve KNEE settings
• Selectable NEW or OLD type switch for feed-forward or feed-back operation
• Preset or variable release time control
• Variable Left/Right link with peak and RMS control voltage filters
• Selectable automatic or manually variable make-up gain
• Variable input-output blending for onboard crossfaded or parallel compression
• Audio circuit uses 2510 and 2520 discrete op amps with transformer output
• Premium stereo VU meters show selectable input or output
• 10-segment stereo gain reduction meter
• Full hard-wired relay bypass
• Commemorative serial number plate
The 50th Anniversary 2500 has several unique features that will allow you to tailor the sound of the
compression in many different directions.
The patented THRUST
® feature has been used for many years in the famed ATI Paragon and Paragon II
consoles, as well as the Pro-6 Input Strip. This circuit places a filter in front of the RMS detector, with
a slope of 10dB per decade, which is the inverse of the pink noise energy curve. In acoustics, the
pink noise curve is used to equalize energy vs. frequency over the audio spectrum, as sound requires
more low frequency energy than high frequency energy to sound correct to your ear. In Hi-Fi
equipment, a "LOUDNESS" contour is used to equalize the music at lower levels so it sounds correct.
Even with this curve, there is still a substantial amount of low frequency information compared to high
frequency information in the audio signal path. When that signal is fed into an RMS detector, the
detector will process the signal into a DC control voltage based on those louder low frequencies,
resulting in a control voltage that favors the low frequencies of the signal, causing pumping and a loss

50th Anniversary 2500 Bus Compressor API
7
of punch. Sometimes, this is not desirable. By switching the THRUST
® button in either the MED or
LOUD positions, this inverse filter is placed in front of the RMS detector, evening out the energy by
lowering the energy in the low frequencies and increasing the energy in the high frequencies, so each
octave has the same energy instead of each octave having half the energy as the one lower. This
creates a unique compression effect that still reduces the overall gain, but the sound is much more
punchy and the signal actually sounds much less compressed.
The L/R LINK feature is also unique to the API 2500. First, there is a variable link control, ranging
from IND (independent) to 50% through 100% in six steps. Variable linking allows combining of the
Left/Right control voltages over a range, minimizing the interaction between channels, while still
linking them to retain the stereo image. While engaged, the link control has selectable detection
speeds, FAST (peak) and SLOW (RMS). The 50th Anniversary 2500 has two complete, identical
compressor circuits, from the input through the RMS detector, the VCA's and to the output section.
When many compressors are linked to the "STEREO LINK" mode, only one channel is controlling the
compression. When the 50th Anniversary 2500 is linked, the control voltages are summed together,
but both channels are still detecting their own control voltages. This also eliminates changing of the
tone of the compressor when linking is used. Many popular units actually change when linked!
Additionally, each channel uses multiple VCA's to minimize noise and distortion found in single VCA
compressors. The input and output stages are ALL DISCRETE, using the API 2510 and the API 2520
op amps.
A TYPE switch that lets you choose between classic and modern compression characteristics: OLD for
vintage-style feedback compression and NEW for today's more common feed-forward compression.
The 3-position KNEE switch adjusts the shape of the curve at the onset of compression for an "over-
easy" type compression resulting in a very natural, uncompressed sound or a typical sharp knee type
that lends itself to a much more severe limiting effect.
The auto make-up GAIN function lets you adjust ratio and threshold controls without affecting the
output level. Alternately, make-up GAIN can be set manually.
BLEND is a new feature that allows mixed or parallel compression to be achieved on-board by allowing
the input and compressed signals to be “blended” in two different ways: input vs. compressed dry/wet
percentage (Crossfade) or input plus compressed percentage (Parallel).
2.0 Quick Start
One tip to get started: We try to design all of our processing modules with the understanding that
there is never enough time to read a manual and learn all of the features in the 10 seconds that is
allowed during a setup. If you simply set all of the parameters as follows the 50th Anniversary 2500
will have a useful, but conservative effect on the signal.
• Set all of the knobs in the COMPRESSOR section to the 12:00 position
• Set KNEE to MED
• Set THRUST
®to NORM
• Set TYPE to NEW
• Set L/R LINK to 50%
• Set SHAPE to SLOW
• Turn off MANUAL make-up gain (auto make-up gain on)
• Set BLEND to off: X(Crossfade) or ll(Parallel) not engaged
These settings will provide a good starting place for additional tweaking to achieve the desired sound.

50th Anniversary 2500 Bus Compressor API
8
3.0 Overview
The 50th Anniversary Edition of the API 2500 Stereo Bus Compressor provides a comprehensive suite
of controls:
• THRESH (Threshold): The level at which compression begins (+20dBu to -20dBu)
• Threshold LED: Indicates when the input audio crosses the selected threshold level
• RATIO: The amount of compression applied after threshold
o 1.5:1, 2:1, 3:1, 4:1, 6:1, 10:1, and ∞:1
• ATTACK: The time it takes for the compressor to respond
o .03, .1, .3, 1, 3, 10, and 30ms
• RELEASE: The time it takes the compressor to return to unity gain
o Fixed Times: .05, .1, .2, .5 , 1, and 2 seconds
o Variable: 50ms to 3 seconds
• KNEE: Sets the characteristic of the response curve at the onset of compression
o HARD: Aggressive onset of compression
o MED (Medium): Moderate onset of compression
o SOFT: Smooth onset of compression
• THRUST
®: Activates the patented circuit that inserts a filter before the RMS detector
o LOUD: Gradual, 10dB/decade filter
o MED: Slight low-frequency attenuation and a slight high-frequency boost
o NORM (Normal): THRUST
®filter not applied to side chain (flat)
• TYPE:Selects detection path routing
o NEW: feed-forward detection path
o OLD: feedback detection path
• L/R LINK: Sets the percentage of Left/Right control voltage link
o IND (Independent): Channels operate independently of each other or
o Percent: 50, 60, 70, 80, 90 or 100% linkage between channels
• SHAPE: Sets the speed of the Left/Right control voltage link
o FAST: Peak acting: RMS control voltage influence attenuated
o SLOW: RMS acting: peak control voltage influence attenuated
o Both: Both peak and RMS link control voltage influence attenuated
• MANUAL make-up GAIN: Selects MANUAL or automatic make-up gain
• GAIN: MANUAL make-up gain control (0dB to +24dB of output gain when MANUAL is engaged)

50th Anniversary 2500 Bus Compressor API
9
• BLEND: Selects output mixing mode: 100% compression, X=Crossfade, or ll=Parallel
o MIX: Sets the percentage of compressed signal and input signal sent to the outputs
• IN: Toggles to switch gain reduction IN or out
• BYPASS: Toggles to engage the hard-wire bypass
• VU (stereo VU meters): Stereo Left and Right analog VU meters
o METER: Selects output or input as the source for the stereo VU meters
• GR (gain reduction meters): Stereo Left and Right 10-segment gain reduction meters

50th Anniversary 2500 Bus Compressor API
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4.0 Compressor Section
The COMPRESSOR section of the 50th Anniversary 2500 sets the basic compression parameters for
both the Left and Right channels.

50th Anniversary 2500 Bus Compressor API
11
4.1 Compressor Section Controls
4.1.1 THRESHOLD
THRESHOLD: Sets the level at which compression begins
• Continuously variable range between +20dBu and -20dBu
Threshold LED: Illuminates when the input audio crosses the level set by the
THRESHOLD control
• Red LED illuminates when input level crosses the set THRESHOLD
4.1.2 ATTACK
ATTACK:Sets the time it takes the compressor to react when the level
exceeds the set THRESHOLD
• Fixed attack times: .03, .1, .3, 1, 3, 10, and 30 milliseconds (m/sec)
4.1.3 RATIO
RATIO: Sets the ratio of input vs. output levels for signals that fall
above the set THRESHOLD
• Fixed ratios: 1.5:1, 2:1, 3:1, 4:1, 6:1, 10:1, and ∞:1 (x:1)
• Compression with ratios of 10:1 or greater is generally
considered to be limiting

50th Anniversary 2500 Bus Compressor API
12
4.1.4 RELEASE
RELEASE: Sets the time it takes the compressor to recover
to unity gain after the level falls below the set THRESHOLD
• Fixed release times: .05, .1, .2, .5, 1, and 2 seconds
(sec)
• Variable release times continuously variable between
50m/sec and 3 seconds (sec)

50th Anniversary 2500 Bus Compressor API
13
5.0 Tone Section
The TONE section of the 50th Anniversary 2500 controls the shape of the response curve at the onset
of compression (KNEE), inserts the patented THRUST
®filter before the detector, and set the detector
path routing (TYPE). These controls all influence the tonal characteristics the compression applies to
the audio.
5.1 Tone Section Controls
5.1.1 KNEE
The KNEE function determines the shape of the 50th Anniversary 2500’s response curve when the
input audio crosses the THRESHOLD.
The 50th Anniversary 2500 has three (3) KNEE settings that control how
the compressor transitions into compression:
• HARD: Sharp response curve
o Red LED
• MED (Medium): Slightly rounded response curve
o Yellow LED
• SOFT: Rounded response curve
o Green LED

50th Anniversary 2500 Bus Compressor API
14
HARD: Sharp response curve
• Immediate onset of compression (sudden transition to set ratio)
• More aggressive and noticeable
• The red LED illuminates when engaged
MED (Medium): Slightly rounded response curve
• Moderate onset of compression (firm, but less sudden transition to set ratio)
• Less aggressive and but still noticeable
• The yellow LED illuminates when engaged

50th Anniversary 2500 Bus Compressor API
15
SOFT: Rounded response curve
• Gradual onset of compression (fade-in up to the set ratio)
• Similar to an “over-easy” type knee
• More transparent
• The green LED illuminates when engaged
5.1.2 THRUST
®
The 50th Anniversary 2500 includes API’s patented THRUST
®circuit that can be switched in or out as
needed. This places the THRUST
®filter before the RMS detector that decreases the compressor’s reaction
to low frequency content. The result is a noticeable increase of punch and low frequencies, but a
uniformly compressed signal. It’s the “little more punch” switch!
The THRUST
®circuit can be engaged using the
THRUST
®switch.
THRUST
®:Cycle to select the
THRUST
®function
• LOUD: Inserts the LOUD THRUST
® filter before the RMS detector
o Red LED
• MED (Medium): Inserts the MED THRUST
® filter before the RMS
detector
o Yellow LED
• NORM (Normal): No THRUST
® filter before the RMS detector (flat)
o Green LED
The patented THRUST
®circuit has been used for many years in the famed API 2500 Stereo Compressor,
ATI Paragon and Paragon II consoles, as well as the Pro6 Input Strip. This circuit places a filter in front
of the RMS detector with a slope of 10dB per decade (-3dB/8va), which is the inverse of the pink noise
energy curve. In acoustics, the pink noise curve is used to equalize energy vs. frequency over the
audio spectrum, as sound requires more low frequency energy than high frequency energy to sound
correct to your ear. In Hi-fi equipment, a “LOUDNESS” contour is used to equalize the music at lower
levels so it sounds correct. Even with this curve, there is still a substantial amount of low frequency
information compared to high frequency information in the audio signal path. When that signal is fed
into the RMS detector, the detector will process the signal into a DC control voltage based upon those
louder low frequencies, resulting in a control voltage that favors the low frequencies of the signal,
causing pumping and a loss of punch. Sometimes, this is not desirable. By engaging the THRUST
®switch,
this inverse filter is placed in front of the RMS detector, evening out the energy by lowering the energy
in the low frequencies and increasing the energy in the high frequencies, so each octave has the same
energy instead of each octave having half the energy as the one lower. This creates a unique

50th Anniversary 2500 Bus Compressor API
16
compression effect that still reduces the overall gain, but the sound is much more punchy and the
signal actually sounds less compressed.
LOUD: A gradual, 10dB/decade filter is applied to the signal feeding the RMS detector, equalizing the
energy going into the RMS detector. This decreases the way lower frequencies act on compression.
The overall difference is a noticeable increase of punch and low frequencies, but a uniformly
compressed signal. It is the “little more punch” function.
• The red LED illuminates when engaged
MED (Medium): A slight attenuation of the low frequencies and a slight boost in the high frequencies,
with a flat mid-range are applied to the signal feeding the RMS detector. This reduces the low
frequencies from pumping the compressor as much and increases the sensitivity of the RMS detector
to the higher frequencies, affecting the higher frequency peaks of the signal.
• The yellow LED illuminates when engaged
NORM (Normal): The THRUST
®filter is not inserted in the path to the RMS detector and the 2500
compresses like most units on the market today (the signal in the detection path remains flat).
• The green LED illuminates when engaged
5.1.3 TYPE
The 50th Anniversary 2500 can be set to operate in two circuit topologies that determine where the
signal that feeds the RMS detector comes from:
• OLD: Feed-Back topology: The RMS detector receives the signal from after the VCA
• NEW: Feed-Forward topology: The RMS detector receives the signal from before the VCA
The detection path routing is selected using the TYPE switch.
TYPE: Toggle to select detection path routing
• NEW (F FWD): Engages the feed-forward topology
o Yellow LED
• OLD (F BACK): Engages feed-back topology
o Green LED

50th Anniversary 2500 Bus Compressor API
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NEW (Feed-Forward)
In a feed-forward compressor, the RMS detector normally gets its signal from a split of the input
signal. (The detector path can alternately get its signal from a SIDE CHAIN input jack.) With this
method, the RMS detector sends a signal to the VCA that is an exact ratio of the desired compression
set by the RATIO control. This is how many new VCA based compressors work. This can yield more
aggressive compression and a harder, more affected sound.
• The yellow LED illuminates when engaged
OLD (Feed-back)
In a feed-back compressor, the RMS detector gets its signal from the output of the gain reduction
device (VCA). This is how older API 525, 1176 type, and 660 type compressors work. This yields a
smoother, softer, more transparent sound.
• The green LED illuminates when engaged

50th Anniversary 2500 Bus Compressor API
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6.0 Link Section
The LINK section of the 50th Anniversary 2500 controls the amount each channel will influence the
gain reduction of the opposite channel. This allows “center-image shifting” caused by differing
amounts of gain reduction on each channel to be reduced or eliminated.
Most stereo compressors only allow 100% linking between channels. The 50th Anniversary 2500 allows
for linking to start at IND (independent), where each channel operates as a separate compressor with
0% influence between channels. 50% to 100% in six additional steps can be applied the amount of
linkage between channels. 100% will yield equal gain on both channels. At the same time, each
channel is still controlled by its own detector, preventing loading and slaving from one side, which
many times creates errors.
In addition, the 50th Anniversary 2500 allows the response speed of the Left/Right control voltage link
to be filtered so the link acts on peak information (FAST) or RMS information (SLOW). If both are
selected, both peak and RMS information is attenuated in the control voltage link, resulting in very
subtle linkage.
In combination, the 50th Anniversary 2500 offers very accurate compression while providing the
flexible and precise control needed to achieve exacting results in any application.

50th Anniversary 2500 Bus Compressor API
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6.1 Link Section Controls
6.1.1 Left/Right Link
L/R LINK: Sets the percentage of the control voltage link between the Left
and Right channels
• IND (independent): No linkage between channels; channels operate
independently of each other with no influence from the opposite
channel
• Percent: 50, 60, 70, 80, 90, and 100% fixed steps
6.1.2 Link Shape
The speed of the control voltage link between the Left and Right channels is selected using the SHAPE
switch.
SHAPE: Cycle to select the speed of the Left/Right control voltage link
• FAST: Peak acting: RMS control voltage influence attenuated
o Green LED
• SLOW: RMS acting: peak control voltage influence attenuated
o Green LED
• BOTH: Both peak and RMS link control voltage influence attenuated
o Both green LEDs

50th Anniversary 2500 Bus Compressor API
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7.0 Output Section
The output section of the 50th Anniversary 2500 controls make-up gain, input/output blending, and
compressor IN/out and bypass functions.
7.1 Output Section Controls
7.1.1 Make-up Gain
MANUAL make-up GAIN: Toggle to select manual or auto make-up gain
• Disengages “auto” make-up gain
• Engages the manual make-up GAIN pot
• Illuminates in red when engaged
The “auto” make-up GAIN function applies make-up gain based upon the settings
of the RATIO and THRESHOLD controls.
GAIN: Manual make-up gain (output level)
• Active only when the make-up function is set to MANUAL
• Continuously variable between 0dB and +24dB