OCL-2 Vacuum Tube Compressor/Limiter- Operating
·Fast electro-optical input attenuator with an all tube
·Soft-knee design with fully adjustable compression ratio
·Transformerless high voltage class A signal path
·Low noise, wide bandwidth and superior transient response
·Custom optical cells matched for precise stereo balance
·Three sidechain operating modes: Fast, Presets and Manual
·Two completely independent channels with stereo linking
·Hard bypass switches for each channel
·Sidechain inserts for frequency-dependent compression
·Transformerless output stage capable of delivering +35dBu
·Soft-start warm-up with output muting
·Gold-plated switch contacts, I/O connectors and tube
·Fully regulated high voltage (300V), filament and bipolar
·Custom toroidal power transformer with shield for minimum
·Polypropylene capacitors and metal film resistors
·Single-sided audio pc boards to minimize capacitive interaction
Thank you for purchasing the Pendulum OCL-2, a modern two
channel electro-optical compressor/limiter designed for the utmost
in transparency, detail and versatility. Our short signal path
design uses a custom optical input attenuation network in front
of an all tube class A gain stage. Since the signal path is entirely
transformerless, we've eliminated the coloration inherent in
traditional transformer-coupled tube designs. The result is an
open, detailed sound with an expanded sound stage and clarity
that can only be achieved with modern tube circuitry.
The OCL-2's soft-knee design offers smooth, effortless dynamic
control with the ratio adjustable from light compression to limiting.
Our unique sidechain circuit linearizes the optical gain element
to reduce distortion, and achieves much faster attack and release
times than encountered with conventional optical compressors.
Stereo linking is accomplished with the flip of a switch .
For maximum versatility, the OCL-2 has three modes of dynamics
processing: In the FAST mode, the compressor works as fast as
the optical element will allow, and is remarkably free of the
'pumping' artifacts normally encountered with slower optical
compressors. It's the ideal choice for invisible dynamics processing,
either for tracking or tight program compression.
The PRESET mode consists of six classic attack/release time
settings that are optimized for program compression, including
two settings with program-dependent release times.
The MANUAL mode offers total control over the attack and release
times, which is particularly useful for creative compression
effects or for processing bass guitar.
Most important, the hard bypass switches on the front panel
conveniently allow a quick determination of exactly what the
OCL-2 is, or in most cases, isn't doing to your signal.
The unit was carefully packed at the factory to protect against
damage in transit. Nevertheless, be sure to inspect the unit
and shipping carton for any signs of damage that may have occurred
during shipment. If there is any damage, notify us immediately
for further instructions. It's also a good idea to save the carton
and packing materials should you ever need to return the unit
for repair. The shipping carton should contain the following
items: the OCL-2 Vacuum Tube Compressor/Limiter, a 3 prong IEC
power cord, and this operating manual.
The OCL-2 uses two EIA-standard rack spaces, and can be
mounted in any standard 19 inch (483mm) equipment rack. If the
OCL-2 is mounted in a mobile rack or road case, it is important
that the rear of the chassis is supported to prevent possible
damage from mechanical shock and vibration. Excessive shock and
vibration can cause damage or premature failure of the vacuum
tubes, or cause them to shaken loose from their sockets. Please
avoid rough handling.
For proper operation, it is very important that adequate
ventilation is provided. Vacuum tubes produce a significant amount
of heat that must be removed from inside the chassis. The side
panel vent holes and top panel slit vents should never be blocked
in any way. Never mount the OCL-2 below a rack unit with
a depth greater than 7 inches (178mm). Do not mount the OCL-2
near other heat-producing equipment such as power amplifiers
or other vacuum tube products. If possible, leave open at least
one rack space above the unit, and use a rack spacer with a ventilation
grille. Never operate the OCL-2 inside a road case where
the side panels are cushioned in foam.
Preventing Ground Loop Hum
One of the reasons the OCL-2 sounds so good is that unlike
many other vacuum tube products, it operates single-ended, Class
A, and has a signal path that is completely transformerless.
However, it does not benefit from the galvanic isolation provided
by input or output transformers. For this reason, a few precautions
are necessary to insure hum-free operation:
· Isolate the front panel from the rack rails. Use plastic
shoulder washers to prevent electrical contact between the rack
ears of the OCL-2 and the metal rails of the equipment
· Isolate the OCL-2 from units mounted above
or below it in the rack. Make sure the front panels are not in
electrical contact and that the top or bottom cover screws of
the OCL-2 are not touching those of any other units.
· Connect the 3 prong IEC power cord to the single-point
star grounded electrical source for your
The idea here is to make sure the OCL-2 seeks ground
at only one point. For safety reasons, do not lift the ground
at the IEC power cord. Keep in mind that in a properly grounded
hookup, the OCL-2 does not hum. Please, take the time
to do this right, and you will be rewarded with hum-free operation.
Call us if you have any questions. Please note that pin 2 is
The OCL-2 is equipped with a 3-prong IEC power connector
and detachable cord. Never operate the OCL-2 with the
ground on the power cord defeated. Unless otherwise stated, this
unit operates from 120V/60 Hz at 30W. The ac fuse is accessible
from the rear panel and is rated at 1A/250V (3AG) SLO-BLO. To
check or replace the fuse, make sure the unit is unplugged. Operation
at 240V/50Hz is available as an option. Please contact us for
Other than changing the tubes, the user should not attempt to
service the OCL-2 beyond that described in this manual.
Never remove the covers or attempt to replace the tubes until
the unit has been disconnected from the ac power source, and
all circuits inside have been allowed to discharge for a period
of at least 30 minutes. The vacuum tubes become very hot once
the unit has been turned on, and they should not be touched until
they have cooled to room temperature. To reduce the risk of fire
or electrical shock, do not expose to rain or moisture, or operate
it where it is exposed to water. Since potentially lethal voltages
are present inside the unit, it should only be opened by qualified
service personnel. Refer all servicing, or any questions about
servicing, to Pendulum Audio, Inc.
While the operation of the OCL-2 may appear to be rather
straightforward, there are a few features which may differ from
what you're accustomed to seeing on other vacuum tube compressors.
You may find it useful to refer to the Condensed
Operating Instructions to quickly identify the operation
of the front panel controls. However, we suggest you read through
this section to take advantage of all its features, and to make
sure you are operating the OCL-2 in the way most appropriate
for the type of dynamics processing you're doing.
Please refer to the rear panel layout (see the Condensed Operating
Instructions) for the location of the inputs, outputs, and ac
power. Make all connections to the OCL-2 before applying power.
On the right hand side of the rear panel are the input jacks
for channels 1 and 2. The female XLR connectors on top are 3
pin unbalanced inputs with Pin 1 + Pin 3 = ground, and Pin 2
= positive phase input (+).Use them for connecting the OCL-2
to +4dBu transformer-balanced or active-balanced sources. If
you encounter ground loop hum when connecting to transformer-balanced
outputs, lift the shield from Pin 1 on the input end of the cable.
The input impedance is 10k.
The two 1/4" jacks below the XLR connectors are unbalanced
input connectors, wired in parallel to the XLR inputs, with Tip
= Pin 2, Sleeve = Pin 1. Use them for connecting the OCL-2 to
unbalanced (-10dBV) sources. Do not attempt to use the XLR and
1/4" inputs simultaneously.
To the left of the input jacks are the output jacks for channels
1 and 2. The male XLR connectors on top are 3 pin unbalanced
outputs, with Pin 1 + Pin 3 = ground, and Pin 2 = output. Connect
these outputs to XLR- or TRS-balanced line-level inputs. If you
encounter ground loop hum when connecting to balanced inputs,
lift the shield from Pin 1 on the input end of the cable. When
connecting to a balanced patch bay, be sure that Pin 2 = Tip.
The two 1/4" jacks below the XLR connectors are unbalanced
output connectors, wired in parallel to the XLR outputs, with
Tip = Pin 2, Sleeve = Pin 1. Use them for connecting the OCL-2
to unbalanced (-10dBV) inputs or mixer inserts.
On the left side of the rear panel is the IEC input socket. Connect
to a 120V/60Hz receptacle with the 3 prong IEC power cable supplied
with the OCL-2. For safety reasons, do not lift the ground on
the power plug by using a 3-to-2 ground lift adapter.
Turn on the power to the unit using the ac power switch located
above the power inlet socket. Although it might seem a bit awkward
to put the ac power switch on the rear panel of the OCL-2, there
is a very good reason for it. By keeping the ac away from the
front panel, hum induced by the ac line into the front panel
circuitry can be virtually eliminated. This is especially important
with the OCL-2, since the optical gain control circuit is located
on one of the front panel circuit boards.
If necessary, replace the 1A/250v 3AG SLO-BLO fuse only with
the same type and rating.
To prolong tube life, the OCL-2 goes through a soft-start
sequence for gently applying power to the tubes and stabilizing
the circuit before engaging the outputs. When the power switch
is turned on, the outputs are relay-muted to ground and the dc
voltage on the tube filaments is ramped up to 12.6Vdc. Next,
the high voltage supply is slowly increased to 300V and the circuit
is allowed to stabilize for about 2 minutes. Finally, the relays
lift the outputs from ground and the blue 'on' led on the front
panel is illuminated. For best results, please allow the OCL-2
to warm up for 10 minutes or longer before use.
OCL-2 Front Panel Controls
Please refer to the front panel layout (see the Condensed
Operating Instructions) for the location of all switches and
controls discussed below. The operation of both channels is identical,
unless the two channels are linked. All toggle and rotary switch
contacts are gold-plated for high reliability. All panel rotary
controls are long-life conductive-plastic potentiometers.
Threshold: Off to -20dB
The THRESHOLD control determines how much gain reduction is applied
to the source material. Since the OCL-2 is a 'soft-knee' compressor
that uses both feedforward and feedback sensing, it does not
have a strictly-defined 'threshold'. Rather, the dial markings
indicate the amount of gain reduction that is applied to a steady-state
input signal at +4dBu. Use higher settings for -10dBV inputs,
and lower settings for input levels that exceed +4dBu. A maximum
of 27dB of gain reduction is available.
Ratio: 1.5:1 to 15:1
The Ratio control sets the 'slope' of the gain reduction, defined
as the signal level above threshold (in dB) that produces a 1dB
increase in the compressor output. Since the OCL-2 is a soft-knee
compressor, its ratio determines the degree of gain reduction
that occurs for signal levels above the knee. In the case of
the OCL-2, the ratio starts at 1:1 and makes a smooth transition
to the value indicated by the ratio control after approximately
5 dB of gain reduction. So, at moderate compression levels, an
adjustment to higher ratio will be heard as a greater amount
of 'restraint' applied to peaks. At higher compression levels,
the sound will become markedly more 'dense' as the ratio is increased.
Output: Off to +20dB
Use the OUTPUT control to boost the signal level after compression.
Up to 20dB above the level of the input signal is possible. The
unity-gain setting (0dB) is 12:00. The OUTPUT control on the
OCL-2 is a passive level control that is positioned immediately
after the electro-optical input attenuator. For this reason,
it is impossible to overload the input of the OCL-2 tube line
Meter: Input/Gain Reduction/Output
The illuminated ANSI VU meters are electronically isolated from
the signal path, and can be switched to measure input, output
or gain reduction. Keep in mind that a VU meter is a mechanical
device, designed in accordance with a with a well-accepted ballistic
standard, to indicate an average loudness level. On the other
hand, the led meters on your mixer or digital recorder are reading
a peak program level, and faithfully register all those short
transient spikes that add little to the perceived loudness of
the program material. The ratio of the peak to average levels
can be 20dB or greater depending on the source (e.g. drums).
So, if you're wondering why the led meters on your recorder are
flashing near zero, but the input and output levels on the VU
meters of the OCL-2 are hovering at or below -10, you're simply
seeing the difference between the peak and average program level.
When the METER switch is in the INPUT position, the VU meter
indicates the signal level at the XLR and 1/4" input connectors.
The meter is calibrated to 0dB = +4dBu (1.23vrms). Use this setting
to monitor the average program level into the optical attenuator
at the input of the OCL-2. For the best signal to noise ratio,
the input level should be in the range of -10 to 0 VU.
When the METER switch is in the GAIN REDUCTION position, the
VU meter indicates the amount of gain reduction applied to the
input source. Keep in mind that the meter is indicating an average
gain reduction, and does not reflect how the OCL-2 is responding
to peaks faster than the meter's response time. When the peak
to average ratio is high (e.g. drums), trust your ears to be
the ultimate judge.
A screwdriver - adjustable trim control, located below the
METER switch, is used for zeroing the meter when it is set to
read GAIN REDUCTION. Use a small, flat-bladed screwdriver, and
adjust the meter to indicate 0VU with the THRESHOLD control set
to OFF. Gain Reduction is measured directly off the electro-optical
cell, so it is normal for some drift of the meter to occur until
the temperature of the cell stabilizes. Let the unit warm up
at least 20 minutes before making any adjustment.
When the METER switch is in the OUTPUT position, the VU meter
indicates the signal level at the XLR and 1/4" output connectors.
The meter is calibrated to 0dB = +4dBu (1.23vrms). Use this setting
to monitor the average program level sent to a tape machine,
mixer input or channel insert.
The OCL-2 uses a unique sidechain circuit, which linearizes the
optical gain element (reducing distortion) and achieves much
faster attack and release times than normally encountered with
optical compressors. For maximum versatility, there are three
distinctly different modes of operation.
In the FAST mode, the optical cell operates with very fast attack
and release times, which are peak-averaged by the rapid operation
of the cell. This mode is remarkably free of pumping artifacts
even at high compression levels. It's the ideal choice for tracking
or tight program compression. Use it whenever you don't want
to hear the OCL-2 working.
The PRESETS mode consists of six attack/release settings which
are ideal for program compression. These presets are identical
to those found on the classic Fairchild 670 Limiter. The first
four presets offer a selection of fixed attack and release times,
with the release times indicated on the dial markings. Presets
5 and 6 are program-dependent release times, where there is initially
a quick release, followed by a longer decay time to zero gain
reduction. Think of these settings as a 'gated' release time,
where the compressor operates more rapidly at the average program
level, but takes much longer to return back to zero gain reduction.
In other words, the compressor does not immediately 'suck back
to zero' when there is a brief pause in the program, e.g. between
words in a vocal track.
The MANUAL mode offers total control over the attack and release
times. The range of the attack and release controls is 1ms to
40ms and 0.1s to 2s respectively. Manual control is particularly
useful for creative compression effects or for processing bass
guitar. Quite often, a greater level of compression can be applied
to program material if a very long attack time and a short release
time are used. In this instance, the compressor is responding
to the average program level, does not 'over-compress' on short
peaks, and recovers quickly. In other words, it doesn't 'pump'
On the rear panel are two TRS 1/4" jacks (one for each channel)
for inserting an outboard EQ into the sidechain detector circuit.
This is useful for frequency-dependent compression or de-essing.
The Tip is Send, and Ring is Return.
When this switch is set to SPLIT, the two channels of the OCL-2
operate independently. In LINK, the sidechain circuits for the
two channels are coupled for stereo operation. The threshold
and dynamics processing (fast, preset and manual modes) are controlled
entirely by Channel 1. The ratio and output controls for the
two channels should be set to the same values. NOTE: Although
the optical elements for the two channels are matched, as they
age some shift in the stereo image may occur at high compression
and ratio settings. To bring the channels back into balance,
simply adjust one of the ratio controls to center the image in
the stereo field.
Although linking the two channels will keep the stereo image
centered, it may tend to collapse the stereo spread a bit too
much on some program material. In this case, run the two channels
of the OCL-2 split, and set all the controls for both channels
to the same values. Listen carefully to verify that the degree
of image shift that occurs is acceptable.
These are 'hard' bypass switches for comparing the processed
sound directly with the original source. IN connects the output
of the OCL-2 tube line amp directly to the output connectors.
BYPASS connects the input source directly to the output.
There are two adjustments to the OCL-2 that are only accessible
by removing the top cover:
· Calibration of the meter when measuring gain reduction
· Stereo balance of the optical elements
These adjustments are required only if one of the optical
elements are replaced, or if they drift out of calibration with
age. Since these adjustments must be made with the ac power on,
and potentially lethal voltages are present inside the chassis,
we recommend that they be made only by qualified service personnel
who are familiar with working around high voltage tube circuitry.
For your safety, we strongly recommend that you contact us
for servicing. On request, detailed instructions for performing
these procedures will be provided to qualified service personnel.
There OCL-2 is available with transformer-balanced inputs
and/or outputs using high-quality Jensen transformers. However,
we do not recommend this unless absolutely necessary, since the
transformers will color the sound. If at all possible, try to
make it work without them. See the section entitled 'Preventing
Ground Loop Hum' for more information.
Replacing the Tubes
All vacuum tubes have a limited life due to reduced electron
emission from the oxide coating on the cathode and/or a buildup
of impurity gases is the bulb. The life of the preamp tubes in
the OCL-2 is estimated to be several years. If you notice the
sound quality deteriorating - higher distortion, muddiness, or
microphonic behavior - it's time to change the tubes. We recommend
changing all the tubes at once. If you are uncomfortable with
replacing the tubes yourself, please have it done by qualified
service personnel. Replacement tubes are available directly from
1. Unplug the OCL-2 and wait at least 30 minutes for the high
voltage in the unit to discharge and for the tubes to cool to
2. Remove the top cover by removing the nine #6-32 Phillips-head
DO NOT remove the bottom cover.
3. Note the position of the four tubes (V1-V4) in the porcelain
· The input tubes are 6072A/12AY7 (V1 and V3)
· The output tubes are 6922/6DJ8 (V2 and V4)
4. Remove each tube and replace with the same type removed
from the each socket. Do not mix up the tube positions.
5. Reinstall the top cover and screws.
There are a large variety of 6072As available. Each type has
slightly different internal structure and design. Consequently,
each type has its own sonic signature. Sometimes the differences
are subtle - sometime not. You are encouraged to sample the different
varieties and pick the one that sounds the best to you.
The 6922 is a rugged, military style 6DJ8. Since it is used
as a high current output driver, we recommend replacing it with
the same type and rating. The 6922 used in this fashion has much
less influence on the sound of the OCL-2 than the 6072A input
Circuit Type: optical input attenuator and class A
gain stage with transformerless output
Freq. Response:-1.0dB 8Hz and 145kHz with 10K output load
-1.0dB 20Hz and 115kHz with 600 output load
Noise: less than -78dB below +4dBu output level
with Output control at 0dB
less than -75dB below +4dBu output level
with Output control at +20dB
Distortion: less than 0.05% THD+N, 20Hz to 10kHz,
measured with +4 dBu input, +10dBu output, 600 load
Output:+35dBu into 10k load
+24dBu into 600 load
Max. Gain Reduction:27dB
Threshold: off to -20dB
Ratio:1.5:1 to 15:1
Output:off to +20dB
Modes: fast (peak-averaged), preset or manual operation
Presets: 1ms/0.1s, 1ms/0.3s, 2ms/1.0s, 4ms/2s,
2ms/1 to 4s and 1ms/0.5 to 20s (program dependent)
Manual:attack time varaible from 1.0ms to 40ms
release time varaible from 0.1s to 2s
Meter: selection of input level, gain reduction, or output
input and output levels are referenced to +4dBu = 0VU
In/Bypass:hard bypass of input signal to output connector
Link/Split:links channel 1 and channel 2 for stereo operation
Polarity:input and output XLR connectors are pin 2 hot
Vacuum Tubes: (2) 6072A, (2) 6922
Power: 120Vac, 30W (240Vac optional)
Power Supplies:+300Vdc, +12.6Vdc and ±15Vdc, fully
with soft-start warm-up and output muting
Dimensions:2U enclosure, 19" x 3.5" x 12.5"
(48.2 x 8.8 x 31.8 cm)
Pendulum Audio, Inc. warrants to the first purchaser of a
new Pendulum OCL-2 Vacuum Tube Compressor/Limiter that the unit
is free of manufacturing defects in materials and workmanship
for a period of one (1) year from the date of purchase. Pendulum
Audio, Inc.'s sole obligation under this warranty shall be to
provide, without charge, parts and labor necessary to remedy
defects, if any, which appear within one (1) year from the date
of purchase. All warranties expressed or implied made by Pendulum
Audio, Inc., including warranties of merchantability and fitness,
are limited to the period of this warranty. Pendulum Audio, Inc.
is not responsible for indirect, incidental or consequential
damages arising from the use or failure of this product, including
injury to persons or property.
This warranty does not cover damage due to: misuse, abuse,
modification, accident or negligence. The warranty does not apply
if the unit is repaired or altered by persons unauthorized by
Pendulum Audio, Inc. in such a manner as to injure, in Pendulum's
sole judgment, the performance, stability or reliability of the
unit. The warranty does not apply if the unit is connected, installed
or used otherwise than in accordance with the instructions furnished
by Pendulum Audio, Inc. There is no warranty on vacuum tubes.
If the equipment requires warranty repair, return authorization
must be obtained from Pendulum Audio, Inc. prior to shipment.
Equipment should not be shipped to Pendulum Audio, Inc. until
return authorization and the proper shipping address is obtained
from us. The equipment (with all its components parts and connecting
cables) must be suitably packaged, including a note with the
owner's name, address, telephone number and a description of
the reason for return. The owner pays two-way shipping (we recommend
UPS), and we suggest that the shipment be insured for its full
This limited warranty is in lieu of all other warranties,
expressed or implied, and no representative or person is authorized
to represent or assume for us any liability in connection with
the sale of our products than set forth herein. This limited
warranty gives you specific legal rights, and you may also have
other rights which vary from state to state.
Pendulum Audio, Inc. · P.O. Box 339, Gillette, NJ 07933
· (908) 665-9333 · www.pendulumaudio.com