API 527 User Manual

527 Compressor/Limiter

Operator's Manual

API, 8301 Patuxent Range Road, Jessup, MD 20794 Tel: [phone]

www.apiaudio.com

Revised 06-07-12

Features

• Feed-forward or feed-back compression

• Hard or soft knee compression

•PatentedTHRUST®switchforfrequency
dependent side chain control

• Continuously variable detented
THRESHOLD control

• Continuously variable detented ATTACK
and RELEASE controls

• Continuously variable detented OUTput
level control

• Continuously variable detented RATIO
control

• 10 segment gain reduction (GR) meter

• Side chain input

• LINK switch for multiple unit
interdependent compression linking (stereo
compression)

• COMP BYPASS switch with hard relay
bypass when in the "Out" position

• Audio circuit uses the 2510 and 2520
discrete op-amps

API 527 Compressor

Page 1

The patented THRUST® function can be switched in and out
via the front panel as well, applying a high pass filter before
the RMS detector circuit that preserves that punchy bottom
end.

The 527 Compressor sections of two or more can b e 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 527 Compressor makes use of the 2510 and 2520 discrete
op-amps and exhibits the reliability, long life, and signature
sound which are characteristic of API products.

The API 527 Compressor takes its place alongside the family of
API VCA based compressors, the 225L and the 2500 Stereo
Bus Compressor. Anyone familiar with those units will
immediately be at home with the 527.

Features common to the line like "feed forward" (NEW) and
"feed back" (OLD) gain reduction methods selectable on the
front panel, provide a choice of "that old way", or "the new
way" of compression, for the highest level of flexibility in signal
gain control. The "old way" or Feed-Back method is what most
of the classic compressors used for the gain control circuit. The
"new way" 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" 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.

API 527 Compressor

Page 2

Compressor Controls

The 527 Compressor provides a comprehensive suite of
controls:

· OUT: Output level adjust

· THRESHOLD: The level at which compression begins

· ATTACK: The time it takes for the compressor to respond

· RELEASE: The time it takes the compressor to return to

unity gain

· RATIO: The amount of compression applied above the
threshold

· TYPE: NEW or OLD detection path topology

· KNEE: The characteristic of the response curve at the

onset of compression.

· THRUST®: Patented circuit that inserts a high-pass filter
before the RMS detector

· GR Meter (GAIN REDUCTION): 10 LED gain reduction
meter

· LINK: Activates the DC LINK for stereo/multichannel
processing with other 527 units

· IN: Inserts or completely removes the 527 Compressor
from the audio path (when not pressed)

527

API 527 Compressor

OV

VU

GR

-1.5

3

-4

2

-6

1

-8

0

0

-10

-1

-12

-15

-17

-19

-23

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

0

-3

4

-5

7

-7

10

-10

-20

1

5

.5

.3

1

ATTACK

mS

2.5

1.5

R

1

0

RATIO

0

O
U

T

10

dB

-3

-6

T

H

R
E
S

H

-20

dBu

1.5

15

2

20

2.5

25

3

RELEASE

S

3

4

6

10

0

0

OUT - Continuously variable 31 position

detented output level pot

METER - 10 segment LED meter shows gain reduction
or audio level

THRESHOLD - Continuously variable 31 position

detented control

METER SWITCH - Selects gain reduction or audio level

ATTACK/RELEASE - Continuously variable 31 position

detented controls

LINK - Selects linking of multiple units for stereo

TYPE - Selects Feed-Back (OLD) or Feed-Forward (NEW)

Compression

RATIO - Continuously variable 31 position control

KNEE - Selects Hard or Soft knee

THRUST - Inserts the Thrust Filter in the RMS detector
IN - Inserts the 527 into the audio path

Page 3

API 527 Compressor

THRESHOLD: Sets the level at which compression begins

· Continuously variable between +10dBu and -20dBu

· Detented 31 position rotary pot for easy recall

· Turning clockwise results in more Compression

ATTACK: Sets the time it takes the compressor to react when
the level exceeds the set THRESHOLD

· Continuously variable between 1 and 25 ms (milliseconds)

· Detented 31 position 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 .3 and 3 seconds (S)

· Detented 31 position rotary pot for easy recall

Attack and Release times are fully variable on the 527
Compressor and share a dual-concentric potentiometer for
control. Attack time is adjusted using the outer ring of the pot
and Release time is adjusted using the inner knob.

OUT: Sets the output level of the 527

· Continuously variable 31 position control

OUT

OV

0

O
U
T

0

0

10

dB

THRESHOLD

-3

-6

0

4

7

10

dBu

T

H

R
E
S

H

-20

ATTACK and RELEASE

1.5

15

.5

.5

1

5

.3

1

ATTACK

mS

5

1

ATTACK

mS

1

.3

RELEASE

1.5

RELEASE

2

20

2.5

25

3

S

15

20

25

2

2.5

3

S

Page 4

RATIO: Sets the ratio of input vs. output levels for signals that
exceed the set THRESHOLD

· Continuously variable between 1:1 and infinity (hard
Limiting)

· A Ratio of 10:1 or greater is generally considered to be a
limiter

· Detented rotary pot for easy recall

RATIO

Againreduction(GR)meterisprovidedtoindicatethe
amount of compression being applied when the METER switch
is in the GR position.

When no gain reduction is being applied, all LED’s are lit on
the Gain Reduction meter (GR). When compression occurs, the
corresponding LED’s extinguish to indicate the amount of gain
reduction. The following gain reduction increments are
provided:

· -1.5 dB

· -4 dB

· -6 dB

· -8 dB

· -10 dB

· -12 dB

· -15 dB

· -17 dB

· -19 dB

· -23 dB

2.5

API 527 Compressor

3

4

6

1.5

1

RATIO

10

0

0

METERING - Gain Reduction

OV

VU

GR

-1.5

3

-4

2

-6

1

-8

0

-10

-1

-12

-3

-15

-5

-7

-17

-19

-10

-20

-23

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

R

IN

Page 5

API 527 Compressor

An OUTput level meter is provided to indicate the 527’s
output in VU when the METER switch is in the VU position.

The audio output increments provided are:

· +3 dB

· +2 dB

· +1 dB

· 0dB Note: 0Vu = +4dBu

· -1 dB

· -3 dB

· -5 dB

· -7 dB

· -10 dB

· -20 dB

In addition, a PEAK (OV) LED is provided that will illuminate at
+27 dBu.

METERING - Output Level

OV

VU

GR

-1.5

3

-4

2

-6

1

-8

0

-10

-1

-12

-3

-15

-5

-7

-17

-19

-10

-20

-23

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

R

Page 6

LINK

LINK: The LINK switch activates the DC control voltage
summing with other units.

Instead, when each of the 527s’ Link switches are pressed, they will contribute
their control signals to be SUMMED into the common Link bus and each of their
front panel controls will affect the compression of any other linked 527’s audio
signal. Units that have the link switch in the OUT position are unaffected. Setting
the Link switch to the “IN” position connects the 527’s compression DC side chain
circuitry to a DC summing bus on the 500 series module motherboard. This
allows for any number of 527 compressors to “LINK” their side chain signals into a
common control bus for tasks such as maintaining stereo image or surround
image during compression, or affecting one audio signal with another’s dynamic
characteristics. Front panel controls that affect the DC side chain signal (and
therefore the compression parameters of ALL linked units), include the Threshold,
Attack,Release,Hard/Soft,New/Old,ThrustandRatiocontrols. AdjustingANY of
these controls will affect the signal that will be sent out and summed with the
Link switch, and the dynamics that affect all linked 527’s.

The DC link bus exists in the Luchbox and is tied from channel to channel with
zero ohm jumpers that can be clipped out if desired, to limit the ability to link
channels. The 500V rack and 1608 console have solder pads that one can connect
with insulated wire to create the DC Link bus.

The Link function of the 527 does NOT operate as other
master/slave linked compressor pairs do, where the master unit’s
controls become the master control for both units while the slave
unit’s controls are disabled.

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

API 527 Compressor

IMPORTANT NOTE!

R

Page 7

API 527 Compressor

The 527 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.

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

.

COMPRESSION TYPE

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

R

OLD: Feed-Back Compression

"OLD" or FEED BACK type COMPRESSION

2 VCA's

IN

2510

THRUST

FILTER

RMS DET

Page 8

PER

CHANNEL

THRESHOLD, ATTACK,

RATIO, RELEASE,
LINK CONTROLS

OUT

2520

NEW: Feed-Forward Compression

In a feed-forward compressor, the RMS detector normally gets its signal
from a split of the input signal. 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.

Knee determines the 527 Compressor’s response curve at the
onset of compression.

The 527 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.

"NEW" or FEED FORWARD type COMPRESSION

IN

2510

2 VCA's

PER

CHANNEL

API 527 Compressor

OUT

2520

THRUST

FILTER

RMS DET

THRESHOLD, ATTACK,

RATIO, RELEASE,
LINK CONTROLS

KNEE

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

R

Page 9

API 527 Compressor

SOFT: Rounded response curve

· Gradual onset of compression (fade-in up to the set ratio)

· Similar to an “over-easy” type knee

· Generally considered more transparent

HARD: Sharp response curve

· Immediate onset of compression (sudden

transition to set ratio)

· More aggressive and noticeable

KNEE

Soft Knee Compression

Hard Knee Compression

Page 10

THRUST

The 527 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 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.

THRUST®filterresponse

®

SOFT

IN

THRUST

IN

R

API 527 Compressor

Page 11

API 527 Compressor

IN: The In/OUT switch inserts or removes the 527
from the audio path via a relay bypass. When the
switch is in the OUT position, a relay connects the
audio Input to the Output – completely removing the
527 from the audio chain.

IN/OUT Switch

GR

METER

VU

IN

LINK

NEW

TYPE

OLD

HARD

KNEE

SOFT

IN

THRUST

IN

R

Page 12

API 527 Compressor

SPECIFICATIONS

Input Impedance: High Level In 160K ohms
Low Level In: 80K ohms
Output Impedance: High Level Out 85 ohms
Low Level Out: 50 ohms
Nominal Operating Level: -2dBu or +4dBu
Maximum Input Level: +26dBu
Maximum Output Level: +26dBu
Output Fader Control Gain: -Infinity to +10dB
Attack Times: 1 millisecond to 25 milliseconds (typical)
Release Times: 0.3 seconds to 3 seconds (typical)
Compression Ratios: 1:1 to 1:Infinity
Threshold Control Range: +10dBu to -20dBu

Metering:
10 RED LED VU scale: -20VU to +3VU (positive illumination indicator)
RED GR scale: -1.5dB to -23dB (negative illumination indicator)
RED Overload LED: +27dBu (clipping)

THRUST® Filter Action: -10dB/Decade slope, unity @ 1kHz
Frequency Response: +/- 1dB 20Hz - 20kHz
Total Harmonic Distortion: < .2% THD, any level below clipping, any comp. setting
Signal to Noise: Unweighted -100dB below clipping

(+26dBu@0.1% distortion, no compression)

Power Requirements: +/- 15-18VDC @130mA

Conforms to Automated Processes, Inc. VPR Alliance size and connectorization
layout specifications. This unit is made to install into the 500 Series frame VPR rack,
lunchbox®, or any API console with 500 series slots.

Size: 1.5" X 5.25" X 7.5" Deep
Size (Boxed for Shipping): 4.5" X 6.5" X 10"
Actual Weight: 1.75 lbs.
Shipping Weight: 2.15 lbs.

Page 13

= F E

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A P I , 8 3 0 1 P a t u x e n t R a n g e R o a d , J e s s u p , M D 2 0 7 9 4

T e l : 3 0 1 - 7 7 6 - 7 8 7 9 w w w . a p i a u d i o . c o m

I n a c o n s t a n t e f f o r t t o i m p r o v e t h e s u b t l e d e t a i l s o f o u r p r o d u c t s , s o m e a m o u n t o f c h a n g e i s i n e v i t a b l e .
A P I r e s e r v e s t h e r i g h t t o m o d i f y o r c h a n g e a n y o f t h e f e a t u r e s , s p e c i f i c a t i o n s o r p e r f o r m a n c e o f i t s p r o d u c t s w i t h o u t n o t i c e o r o b l i g a t i o n .