API TCS-II User manual

TCS-II
The Channel Strip-II
512C – 527A - 550A – 325
Operator’s Manual
Written for Automated Processes, Incorporated
by Dan Pfeifer
Rev. 23-03-19

Table of Contents
About This Manual ...................................................................................... 0
Important Safety Instructions .................................................................... 1
1.0 Introduction ......................................................................................... 2
1.1 TCS-II Overview ......................................................................................... 2
2.0 Signal Flow and Block Diagrams ........................................................... 3
2.1 Default Signal Flow..................................................................................... 3
2.2 Alternate Signal Flows ................................................................................ 3
2.2.1 Compressor Post Equalizer Signal Flow ....................................................................... 3
2.2.2 Compressor Bypass.................................................................................................. 4
2.2.3 Equalizer Bypass...................................................................................................... 4
2.2.4 Insert ..................................................................................................................... 5
2.3 512C Preamp Signal Flow............................................................................ 5
2.4 325 Output Section Signal Flow................................................................... 6
2.5 Rear Panel Signal Flow ............................................................................... 6
3.0 Input Section: 512C Preamplifier.......................................................... 8
3.1 512C Preamp Overview and Features .......................................................... 8
3.2 512C Preamp Controls ................................................................................ 8
3.3 512C Input Selection .................................................................................. 9
3.3.1 Input Switch............................................................................................................ 9
3.3.2 Microphone Input..................................................................................................... 9
3.3.3 Instrument Input ................................................................................................... 10
3.3.4 Line Input ............................................................................................................. 10
3.4 512C Polarity Inverter .............................................................................. 10
3.5 512C Level Indication ............................................................................... 11
3.5.1 Level Meter ........................................................................................................... 11
3.5.2 Peak Indicator ....................................................................................................... 11
3.6 512C Preamp Output................................................................................. 11
3.7 512C Preamp Block Diagram ..................................................................... 11
4.0 Dynamic Processing: 527A Compressor .............................................. 12
4.1 527A Compressor Overview and Features ................................................. 12
4.2 527A Compressor Controls........................................................................ 13
4.2.1 Threshold.............................................................................................................. 13
4.2.2 Attack and Release................................................................................................. 13
4.2.3 Ratio .................................................................................................................... 14
4.2.4 Compression Type.................................................................................................. 14
4.2.5 Compression Knee ................................................................................................. 15
4.2.6 THRUST
®................................................................................................................. 16
4.2.7 Gain Reduction Meter ............................................................................................. 16
4.3 527A Compressor Routing......................................................................... 17
4.3.1 Compressor Pre Equalizer (Default Signal Flow)......................................................... 17
4.3.2 Compressor Post Equalizer ...................................................................................... 17
4.3.3 Compressor Bypass................................................................................................ 17

4.3.4 Compressor IN Switch ............................................................................................ 18
4.4 527A Compressor Side Chain..................................................................... 18
4.5 527A DC Link ............................................................................................ 18
5.0 Spectral Processing: 550A Equalizer................................................... 19
5.1 550A Equalizer Overview and Features...................................................... 19
5.2 550A Equalizer Controls............................................................................ 20
5.2.1 Low Frequency Band .............................................................................................. 20
5.2.2 Mid Frequency Band ............................................................................................... 21
5.2.3 High Frequency Band ............................................................................................. 21
5.2.4 Band-Pass Filter..................................................................................................... 21
5.3 550A Equalizer Routing............................................................................. 21
5.3.1 Equalizer Post Compressor Pre (Default Signal Flow) .................................................. 21
5.3.2 Equalizer Pre Compressor ....................................................................................... 22
5.3.3 Equalizer Bypass.................................................................................................... 22
5.3.4 Equalizer IN Switch ................................................................................................ 22
6.0 Output Section: 325 Line-Driver ......................................................... 23
6.1 325 Output Section Overview and Controls................................................ 23
6.1.1 Output.................................................................................................................. 24
6.1.2 Soft Mute.............................................................................................................. 24
6.1.3 Insert ................................................................................................................... 24
6.1.4 Level Meter ........................................................................................................... 24
6.2 325 Output Section Routing ...................................................................... 25
6.3 325 Output Section Block Diagram ............................................................ 25
7.0 Rear Panel Interface........................................................................... 26
7.1 Audio Path Inputs..................................................................................... 26
7.2 Audio Path Outputs................................................................................... 27
7.3 Audio Path Half-Normals........................................................................... 28
7.4 527A Compressor Side Chain & DC Link Connections ................................. 29
7.4.1 Side Chain Input.................................................................................................... 29
7.4.2 DC Link ................................................................................................................ 29
7.5 Rear Panel Signal Flow ............................................................................. 29
8.0 AC Power ............................................................................................ 30
APPENDIX................................................................................................. 31
A1 TCS-II Technical Specifications.................................................................. 32
A2 Block Diagrams ......................................................................................... 34
A2.1 Default Signal Flow ................................................................................................. 34
A2.2 Post EQ Signal Flow ................................................................................................ 34
A2.3 512C Signal Flow .................................................................................................... 35
A2.4 325 Output Section Signal Flow ................................................................................ 35
A3 TCS-II Setup Sheet.................................................................................... 36
A4 API Limited Warranty and Service.............................................................. 37

About This Manual
This manual explains the operation and applications of the API TCS-II (The Channel Strip II) signal
processor. Incorporating four legendary API signal processors into a stand-alone configuration, the
TCS-II provides a flexible, high-quality audio signal chain for most production applications.
Legend:
• UPPER-CASE BOLD = SWITCHES, BUTTONS, and POTS
• UPPER-CASE = REAR PANEL CONNECTIONS
Manual Revision History: YY-MM-DD format
Original: 11-02-22
Revised: 16-02-01
Revised: 23-03-19 TCS-II update with 527A and other features
Dan Pfeifer is a Professor of Recording Industry
at Middle Tennessee State University

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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.

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1.0 Introduction
The API TCS-II (The Channel Strip II) is a complete input module with a 512C Preamplifier, 527A
Compressor, 550A precision EQ, and 325 output level section. With extensive signal metering, multiple
balanced insert points and an output mute switch, the TCS-II contains the best features of a large
console input channel in a single rack space. It's the perfect choice for anyone looking for that high-
performance API console sound.
1.1 TCS-II Overview
Features:
• API 512C Preamplifier
• API 527A Compressor
• API 550A Equalizer
• API 325 Line-Driver (output section)
• Internal routing supports multiple configurations
• Comprehensive rear panel interface with multiple insert points
• 10-segment LED gain reduction meter
• 10-segment LED level meter displays preamp and output levels
• Output clips at +30dBu
• Transformer outputs
The result of thoughtful engineering, the TCS-II includes everything needed to record an audio signal.
It all starts with the famed 512C Preamplifier, which includes the benefit of additional Mic, Line or
Instrument input switching. Next is the 527A multi-mode compressor with fully adjustable Threshold,
Attack, Release, and Ratio controls on detented pots. The patented THRUST
®circuit in the 527A
Compressor gives your track the "in-your-face" punch that has made this circuit an award winner. Also
included in the TCS-II is probably the most recognized and revered API EQ of all time, the 550A three-
band equalizer with its switchable frequencies and 12dB of boost or cut. The 550A Equalizer's High
and Low frequency bands can be individually switched to be shelving EQs, and the "B.P. Filter" switch
inserts a 50Hz to 15kHz Band-Pass filter. At the output of the unit is the 325 Line-Driver, taken
directly from API console designs with an output level pot and 10-segment level meter. The meter can
also be switched to show the output level of the preamp. Each of the TCS-II's processing pieces can
be switched in or bypassed individually and the 527A can be routed to be post EQ. Additional features
include a side-chain input and the famous API 2520/transformer combination.

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2.0 Signal Flow and Block Diagrams
2.1 Default Signal Flow
With no routing switches engaged, the default signal flow through the TCS-II is illustrated by the left-
to-right order of the unit’s front panel:
512C Preamp ----------->527A Compressor ------------------>550A Equalizer-->Insert -->325 Output
The block diagram below illustrates the TCS-II signal flow with no switches engaged except the CUT
button. This is the “default” signal flow through the unit.
2.2 Alternate Signal Flows
The default signal flow can be modified in four (4) ways without patching:
•The 527A Compressor can be moved after the 550A Equalizer (POST EQ)
•The 527A Compressor can be hard bypassed (COMP BYPASS)
•The 550A Equalizer can be hard bypassed (EQ BYPASS)
•Insert Return can be engaged (INSERT)
Since each of these routing functions are independent of each other, multiple signal flow configurations
are possible. Combined with the open interfacing available on the rear panel, comprehensive and
flexible routing possibilities are available to meet most production needs.
2.2.1 Compressor Post Equalizer Signal Flow
Engaging the POST EQ switch will move the 527A Compressor to after the 550A Equalizer. When
the POST EQ switch is engaged, the signal flow will be as follows:
512C Preamp --->550A Equalizer --->527A Compressor --->Insert --->325 Output

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POST EQ:Moves the 527A Compressor to after the 550A Equalizer
•527A will use Second Effect rear panel interface jacks
•Illuminates when engaged
The block diagram below illustrates the TCS-II signal flow with the POST EQ switch engaged. No
other switches are engaged except the CUT button.
NOTE: The order of the First and Second Effect jacks on the rear panel does not change
with the compressor when the POST EQ switch is engaged.
2.2.2 Compressor Bypass
Engaging the COMP BYPASS switch will completely remove the 527A Compressor from the signal
path using a relay-based hard bypass. The 527A Compressor will be bypassed regardless of its
assigned position in the signal flow. When the COMP BYPASS switch is engaged, the signal flow
will be as follows:
512C Preamp --->550A Equalizer --->Insert --->325 Output
COMP BYPASS: Removes the 527A Compressor from the signal flow
•527A will not be accessible via rear panel interface jacks
•Illuminates when engaged
IMPORTANT NOTE: The 527A Compressor has an “IN” switch that must be engaged in order
for the compressor to effect the signal when the COMP BYPASS switch
is not engaged.
2.2.3 Equalizer Bypass
Engaging the EQ BYPASS switch will completely remove the 550A Equalizer from the signal path
using a relay-based hard bypass. When the EQ BYPASS switch is engaged, the signal flow will
be as follows:
512C Preamp --->527A Compressor --->Insert --->325 Output

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EQ BYPASS: Removes the 550A Equalizer from the signal flow
•550A Equalizer will not be accessible via rear panel interface jacks
•Illuminates when engaged
IMPORTANT NOTE: The 550A Equalizer has an “IN” switch that must be engaged in order
for the equalizer to effect the signal when the EQ BYPASS switch is not
engaged.
2.2.4 Insert
Engaging the INSERT switch will route the signal present at the INSERT RETURN jack to the input
of the 325 Line-Driver. When the INSERT switch is engaged, the signal flow will be as follows:
Insert --->Output
INSERT: Routes the signal present at the INSERT RETURN jack to the 325 Line-Driver
input
•Replaces the SECOND EFFECT OUT as the input to the 325 Line-Driver
•Illuminates when engaged
NOTE: Since the signal can be externalized at multiple points in the audio path, the TCS-II
does not have a dedicated “insert send” output. PRE OUT, FIRST EFFECT OUT, and
SECOND EFFECT OUT are intended to be used as “insert sends” as needed.
2.3 512C Preamp Signal Flow
The block diagram below illustrates signal flow through the 512C Preamp section of the TCS-II with
no switches engaged.

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2.4 325 Output Section Signal Flow
The block diagram below illustrates signal flow through the 325 Line-Driver output section of the TCS-
II with only the CUT switch engaged.
2.5 Rear Panel Signal Flow
The TCS-II signal flow is illustrated by the right-to-left order of the unit’s rear panel. The order
of these interfaces remains constant, regardless of the order of the 527A Compressor and 550A
Equalizer in the signal path.
Second First
Output <---------------- Insert <---- Effect <-------- Effect <--------- 512C Preamp
• The PRE OUT is “half-normalled” to feed the FIRST EFFECT IN
• The FIRST EFFECT IN feeds the first processor in the audio path (normally the 527A
Compressor*)
• The FIRST EFFECT OUT is “half-normalled” to feed the SECOND EFFECT IN
• The SECOND EFFECT IN feeds the second processor in the audio path (normally the
550A Equalizer*)
• The SECOND EFFECT OUT is “half-normalled” to feed the 325 Line-Driver input
* Engaging the POST EQ switch will move the 527A Compressor to after the 550A Equalizer,
reversing the order of these effects in the audio path.
The PRE OUT, FIRST EFFECT OUT, and SECOND EFFECT OUT jacks are all splitting outputs.
This means the connection to the input they’re normalled to will not be broken when a plug is
inserted.

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The FIRST EFFECT IN and SECOND EFFECT IN jacks are all switching jacks, so connection with
the output they’re normalled to is broken when a plug is inserted. The input is instead fed with
the signal present on the inserted plug.
The INSERT RETURN is only active when the INSERT switch on the front panel is engaged.

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3.0 Input Section: 512C Preamplifier
The input section is equipped with the legendary API 512C
Preamplifier with microphone, instrument, and line inputs.
The 512C Preamplifier is designed to provide an unusually
good sounding front-end for all types of audio recording
applications. Sonically, it offers the distinct API sound,
high quality performance, and low noise.
The 512C Preamp remains faithful to the circuit designs of
API's founder, Saul Walker. Fully featured, it carefully
preserves the original sound character that made it so
much a part of the early days of recording.
3.1 512C Preamp Overview and Features
Offering high headroom and a wide variety of inputs and input access points, it is equally at home in
the commercial recording studio as it is in the home project studio. The 512C Preamp makes use of
the 2520 op-amp and therefore exhibits the reliability, long life, and uniformity which are characteristic
of all API products.
Features:
•Microphone preamp with 14dB to 68dB total gain range (with -20dB pad)
•Instrument preamp with 14dB to 50dB total gain range
•Line preamp at unity gain
•-20dB microphone pad
•-6dB line pad
•Switchable 48V phantom power
•Polarity inverter
•+22dBu preamp output peak indicator
•Routable to the 10-segment LED level meter
•XLR rear panel balanced, low-impedance MIC input
•¼” -10dBu front panel unbalanced, high-impedance instrument (INST) input
•¼” TRS +4dBu rear panel balanced, low-impedance LINE IN
•¼” TRS +4dBu rear panel balanced, low-impedance line Preamp OUT
•Low noise (-129dBm EIN)
•Famous API 2520 op-amp and transformer
3.2 512C Preamp Controls

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The 512C Preamp provides a comprehensive suite of controls:
• GAIN: Microphone and instrument preamp gain
• POL: Preamp output polarity inverter
• 48V: Phantom Power
• PAD: -20dB microphone pad / -6dB line pad
• INPUT: Mic-Line-Instrument input selector
• INST: ¼” unbalanced, high-impedance instrument input
• PEAK: +22dBu output peak indicator
• Input Source LEDs:Illuminated LED indicates the selected source
3.3 512C Input Selection
The 512C Preamp can receive three (3) input sources:
• MIC (Microphone):Rear panel XLR microphone input with switchable 48V phantom power
• INST (Instrument):Front panel ¼” jack, unbalanced, Hi-Z instrument-level input
• LINE:Rear panel ¼” TRS balanced, low-impedance, +4dBu line-level input
3.3.1 Input Switch
The INPUT switch is used to select the active input source. Only one source can be active
at a time. The INPUT switch is a momentary selector which cycles through the three
input sources, LINE-INST-MIC. The input source LEDs indicate the active source:
• LINE:Red LED
• INST (Instrument):Yellow LED
• MIC (Microphone):Green LED
3.3.2 Microphone Input
Select the microphone input (MIC) by pressing the INPUT button until the
red MIC LED illuminates. The MIC IN connector on the rear panel will be the
active input:
• Female, 3-pin XLR microphone input
• Balanced, low-impedance
• 48v phantom power
• 34dB to 68dB of gain
• -20dB pad
• Green MIC LED illuminates when active
48V:Provides 48 volt phantom power to the MIC IN female XLR connector
• Illuminates when engaged
IMPORTANT: Caution should be exercised when engaging phantom power!
Damage can occur if phantom power is applied to some audio
devices, including most ribbon microphones. The TCS-II output
should also be muted (CUT) when engaging the 48V switch.
-20dB PAD: Inserts a -20dB attenuator after the MIC input
• Illuminates when engaged

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GAIN:Microphone preamp level control
• 34dB nominal gain
• 68dB maximum gain
• 14dB minimum gain with -20dB PAD engaged
• 14dB to 68dB total gain range with pad
3.3.3 Instrument Input
Select the instrument input (INST) by pressing the INPUT button until the yellow
INST LED illuminates. The ¼” INST jack on the front panel will be the active
input:
• -10dBu instrument-level input
• Unbalanced, high-impedance input (¼” tip-sleeve)
• 14dB to 50dB of gain
• Yellow INST LED illuminates when active
GAIN:Instrument preamp level control
• 14dB nominal gain
• 50dB maximum gain
• No pad
3.3.4 Line Input
Select the line input (LINE) by pressing the INPUT button until the red LINE
LED illuminates. The ¼” LINE IN jack on the rear panel will be the active input:
• +4dBu line-level input
• Balanced, low-impedance input (¼” tip-ring-sleeve)
• -6dB pad
• Unity gain (no preamp gain)
• Red LINE LED illuminates when active
• Red peak indication when output exceeds +22dBu
-20dB PAD: Inserts a -6dB attenuator after the LINE input
• Illuminates when engaged
3.4 512C Polarity Inverter
A polarity inverter (sometimes referred to as a “phase reverse”) is available at the output of the 512C
Preamp.
POL (Polarity):Inverts the polarity of the signal at the output of the preamp
• Illuminates when engaged

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3.5 512C Level Indication
The output level of the 512C Preamp can be monitored in two (2) ways:
•Level Meter: The level meter in the output section can display the preamp output level
•Peak Indicator: The LED peak indicator will illuminate when the 512C output exceeds +22dBu
3.5.1 Level Meter
The output of the 512C Preamp will be displayed on the level meter
in the output section when the VU PRE switch is engaged.
VU PRE (VU Preamp):Routes the output of the 512C Preamp to the level meter input
•Illuminates when engaged
3.5.2 Peak Indicator
The 512C Preamp is equipped with a red LED peak indicator. When the output level of the
512C exceeds +22dBu, the PEAK LED illuminates.
3.6 512C Preamp Output
The output of the 512C Preamp is available at the PRE OUT jack on the rear panel. This jack is “half-
normalled” to the FIRST EFFECT IN jack on the rear panel.
PRE OUT: Direct output of the 512C Preamp
•Balanced, low-impedance, +4dBu line-level output
•¼” tip-ring-sleeve jack
•Plugging into the PRE OUT jack does not break the connection to the FIRST
EFFECT IN jack
3.7 512C Preamp Block Diagram
The block diagram below illustrates signal flow through the 512C Preamp section of the TCS-II with
no switches engaged.

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4.0 Dynamic Processing: 527A Compressor
The API 527A Compressor takes its
place alongside the family of API
VCA based compressors, the 225,
529, and the 2500+ Stereo Bus
Compressor. Anyone familiar with
those units will immediately be at
home with the 527A.
4.1 527A Compressor Overview and Features
The 527A Compressor is equipped with functions common to other of compressors in the API line like
"feed-forward" (NEW) and "feed-back" (OLD) gain reduction methods selectable on the front panel,
provide a choice of "that old type", or "the new type" of compression, for the highest level of flexibility
in signal gain control. The "old type" or feed-back method is what most of the classic compressors
used for the gain control circuit. The "new type" gain reduction is more typical of the newer VCA type
compressors that rely on RMS detectors for the gain control voltage.
There is a "SOFT"/"HARD" KNEE switch for an "over-easy" style of 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 patented THRUST
®function can be switched in and out via the front panel, applying a filter before
the RMS detector circuit that preserves a punchy bottom end.
A COMPRESSOR SIDE CHAIN INPUT is provided for keying and external detector path processing.
The 527A Compressor sections of two or more TCS-IIs or other 527/527A compressors can be linked
together via a DC LINK allowing multiple units to be combined for stereo and multichannel applications.
The output level remains fairly constant regardless of the threshold or ratio control, much like the
"more/less" Ceiling control on the API 525 Compressor. This allows for live adjustments without any
noticeable gain changes in the program level.
The 527A Compressor makes use of API 2510 and 2520 op-amps and exhibits the reliability, long life,
and signature sound which are characteristic of API products.
Features:
• TYPE switch for “feed-forward” or “feed-back” compression
• KNEE switch for hard or soft knee compression
• Patented THRUST
® function for frequency dependent side chain control
• Continuously variable detented THRESHOLD control
• Continuously variable detented ATTACK and RELEASE controls
• Continuously variable detented RATIO control
• 10-segment gain reduction (GR) meter
• IN/out switch
• COMP BYPASS activates hard relay bypass when engaged
• POST EQ routing moves the 527A to after the 550A Equalizer
• LINK activates multiple unit compression linking (stereo/multichannel compression)
• DC LINK to interface with other 527As for stereo/multichannel operation
• COMPRESSOR SIDE CHAIN INPUT for external keying (NEW mode only)
• Audio circuit uses API 2510 and 2520 op-amps

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4.2 527A Compressor Controls
The 527A Compressor provides a comprehensive suite of controls:
• THRESHOLD:Sets the level at which compression begins
• ATTACK:Sets the time it takes for the compressor to respond
• RELEASE:Sets the time it takes the compressor to return to unity gain
• RATIO:Sets the amount of compression applied after threshold
• TYPE:Selects NEW (feed-forward) or OLD (feed-back) detection path topology
• KNEE:Selects HARD or SOFT response curve characteristic at the onset of compression
• THRUST
®: Inserts the patented API high-pass filter before the RMS detector
• LINK:Activates the DC LINK for stereo/multichannel processing with other TCS units
• COMP BYPASS (Compressor Bypass):Activates the 527A hard bypass in the audio path
• POST EQ:Moves the 527A to post the 550A Equalizer in the audio path
• GR (Gain Reduction):10-segment LED gain reduction meter displays compression amount
4.2.1 Threshold
THRESHOLD:Sets the level at which compression begins
• Continuously variable between -20dBu and +10dBu
• Detented rotary pot for easy recall
4.2.2 Attack and Release
Attack and Release times are fully variable on the 527A Compressor and share
a dual-concentric potentiometer for control. ATTACK time is adjusted using the
outer ring and RELEASE time is adjusted using the inner knob.

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ATTACK:Sets the time it takes the compressor to react when the level exceeds
the set THRESHOLD
•Continuously variable between 1ms and 25ms (milliseconds)
•Detented rotary pot for easy recall
RELEASE:Sets the time it takes the compressor to recover to unity gain after the
level falls below the set THRESHOLD
•Continuously variable between .1s and 3s (seconds)
•Detented rotary pot for easy recall
4.2.3 Ratio
RATIO:Sets the ratio of input vs. output levels for signals that fall above the
set THRESHOLD
•Continuously variable between 1:1 and ∞:1
•A Ratio of 10:1 or greater is generally considered to be a limiter
•Detented rotary pot for easy recall
4.2.4 Compression Type
The 527A Compressor can be set to operate in two circuit topologies:
•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 compressor circuit topology is selected using the TYPE switch.
TYPE:Selects the compressor circuit topology
•OLD:Feed-back detection path topology (default setting)
•NEW:Feed-forward detection path topology (switch engaged)
•Illuminates when engaged
OLD: Feed-Back Compression
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.

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NEW:Feed-Forward Compression
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 (not
shown).
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.
4.2.5 Compression Knee
KNEE determines the 527A Compressor’s response curve at the onset of compression.
The 527A Compressor has two (2) knee settings that control how the compressor transitions into
compression:
• SOFT:Rounded response curve
• HARD:Sharp response curve
The knee of the compressor is selected using the KNEE switch.
KNEE:Selects the response curve at the onset of compression.
• SOFT:Rounded, more gradual response curve (default setting)
• HARD:Sharper, more aggressive response curve (switch engaged)
• Illuminates when engaged
Soft Knee Compression
SOFT:Rounded, more graduate response curve
• Gradual onset of compression (fade-in up to the set ratio)
• Similar to an “over-easy” type knee
• More transparent

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Hard Knee Compression
HARD:Sharper, more aggressive response curve
• Immediate onset of compression (sudden transition to set ratio)
• More aggressive and noticeable
4.2.6 THRUST
®
The 527A Compressor 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
®: Inserts the
THRUST
®filter before the RMS detector
• Illuminates when engaged
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 Pro-6 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 the 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 compression effect
that still reduces the overall gain, but the sound is much more punchy and the signal actually
sounds less compressed.
4.2.7 Gain Reduction Meter
A gain reduction (GR) meter is provided to indicate the amount of
compression being applied.