Art PRO CHANNEL II User manual

PRO CHANNEL II
TUBE PREAMP - OPTO COMPRESSOR - EQ
SERVICE MANUAL

II
IMPORTANT SAFETY INSTRUCTIONS – READ FIRST
This symbol, wherever it appears, This symbol, wherever it appears, alerts
alerts you to the presence of uninsulated you to important operating and maintenance
dangerous voltage inside the enclosure. Voltage instructions in the accompanying literature.
that may be sufficient to constitute a risk of shock. Please read manual.
Read instructions:
Retain these safety and operating instructions for future reference. Heed all warnings printed here and on the equipment.
Follow the operating instructions printed in this user guide.
Do not open:
Aside from one vacuum tube, there are no user serviceable parts inside. Refer any service work to qualified technical
personnel only.
Power sources:
Only connect the unit to mains power of the type marked on the rear panel. The power source must provide a good
ground connection.
Power cord:
Use the power cord with sealed mains plug appropriate for your local mains supply as provided with the equipment. If the
provided plug does not fit into your outlet consult your service agent. Route the power cord so that it is not likely to be
walked on, stretched or pinched by items placed upon or against.
Grounding:
Do not defeat the grounding and polarization means of the power cord plug. Do not remove or tamper with the ground
connection on the power cord.
Ventilation:
Do not obstruct the ventilation slots or position the unit where the air required for ventilation is impeded. If the unit is to be
operated in a rack, case or other furniture, ensure that it is constructed to allow adequate ventilation.
Moisture:
To reduce the risk of fire or electrical shock do not expose the unit to rain, moisture or use in damp or wet conditions. Do
not place a container of liquid on it, which may spill into any openings.
Heat:
Do not locate the unit in a place close to excessive heat or direct sunlight, as this could be a fire hazard. Locate the unit
away from any equipment, which produces heat such as: power supplies, power amplifiers and heaters.
Environment:
Protect from excessive dirt, dust, heat, and vibration when operating and storing. Avoid tobacco ash, drink spillage and
smoke, especially that associated with smoke machines.
Handling:
To prevent damage to the controls and cosmetics avoid rough handling and excessive vibration. Protect the controls from
damage during transit. Use adequate padding if you need to ship the unit. To avoid injury to yourself or damage to the
equipment take care when lifting, moving or carrying the unit.
Servicing:
Switch off the equipment and unplug the power cord immediately if it is exposed to moisture, spilled liquid, objects fallen
into opening, or the power cord or plug becomes damaged during a lightning storm or if smoke odor or noise is noted.
Refer servicing to qualified technical personnel only.
Installation:
Install the unit in accordance with the instructions printed in the user guide.

III
PRO CHANNEL II
TUBE PREAMP - OPTO COMPRESSOR - EQ
IMPORTANT SAFETY INSTRUCTIONS – READ FIRST......................................................................................................II
INTRODUCTION.....................................................................................................................................................................1
INSTALLATION......................................................................................................................................................................2
AC Power Hookup..................................................................................................................................................................................2
Analog Audio Connections.....................................................................................................................................................................2
FRONT PANEL CONTROLS and JACKS.............................................................................................................................3
Mic Preamp Controls..............................................................................................................................................................................3
Instrument Input .....................................................................................................................................................................................3
Gain Control...........................................................................................................................................................................................3
Impedance Control.................................................................................................................................................................................3
GAIN Switch...........................................................................................................................................................................................4
Phantom Power Switch ..........................................................................................................................................................................4
Invert Switch...........................................................................................................................................................................................4
Low Cut Control......................................................................................................................................................................................4
Preamp Output Control...........................................................................................................................................................................4
Tube Voltage Switch ..............................................................................................................................................................................5
Compressor Controls .............................................................................................................................................................................6
Threshold Control...................................................................................................................................................................................6
Ratio Control ..........................................................................................................................................................................................6
Attack Control.........................................................................................................................................................................................7
Release Control......................................................................................................................................................................................7
Compressor Output Control....................................................................................................................................................................7
Gain Reduction LED Meter.....................................................................................................................................................................8
Semi-Parametric EQ ..............................................................................................................................................................................8
EQ Bypass Switch..................................................................................................................................................................................9
CLIP LED ...............................................................................................................................................................................................9
Master Output Control............................................................................................................................................................................9
Output Meters.......................................................................................................................................................................................10
VU Meter Switches...............................................................................................................................................................................10
REAR PANEL CONNECTIONS...........................................................................................................................................11
INPUT Jack..........................................................................................................................................................................................11
PREAMP OUT Jack .............................................................................................................................................................................11
COMP IN Jack......................................................................................................................................................................................12
COMP OUT Jack..................................................................................................................................................................................12
EQ IN Jack...........................................................................................................................................................................................12
OUTPUT Jacks ....................................................................................................................................................................................12
OUTPUT LEVEL Switch.......................................................................................................................................................................12
APPLICATIONS....................................................................................................................................................................13
Bypassing Components Of The Pro Channel II....................................................................................................................................13
Optimizing The Preamp For Lowest Noise...........................................................................................................................................13
WARRANTY INFORMATION...............................................................................................................................................14
SERVICE...............................................................................................................................................................................27
SPECIFICATIONS................................................................................................................................................................28
LIST OF FIGURES
FIGURE 1 – Preamp section...................................................................................................................................................5
FIGURE 2 – Compressor Section...........................................................................................................................................6
FIGURE 3 – Equalizer Section................................................................................................................................................8
FIGURE 4 – Signal Flow Block Diagram ................................................................................................................................9
FIGURE 5 – Output Section..................................................................................................................................................10
FIGURE 6 – Rear Jacks........................................................................................................................................................11

1
INTRODUCTION
The ART Pro Channel II is the answer to your recording and computer audio interface needs. Our
second-generation discrete Class-A microphone preamp provides clean quiet gain while maintaining
incredible transparency. A powerful dynamics processor subtly controls transients and noise of the
most demanding sources. The ART Pro Channel II’s semi-parametric EQ offers wide tune-ability and
can be patched before OR after the dynamics processor. Separate insertion jacks allow you to use
your favorite external signal processing gear immediately after the Mic preamp and before the EQ
and dynamics processor. Both analog and LED meters provide a detailed indication of audio levels.
Key Features Include:
• Fully Featured discrete class "A" Tube Microphone Preamplifier
• ¼-inch High Z input on the front panel
• Selectable plate Voltage
• Variable Input Impedance
• Smooth musical sounding Optical Compressor
• Separate Attack and Release controls
• Auto/ Manual Attack/ Release switch
• Parametric EQ
• Large VU and LED meters
• Multi section clip monitor LED
• Selectable Source switches for VU signal monitoring of any section of the unit
• Two insert loops
• +4/-10 dB switch for optimal signal output levels
• Solid case
• Toroidal power transformer for low noise
• Vents for the 12AX7a and 12AT7 tubes.

2
INSTALLATION
The ART Pro Channel II may be used in a wide variety of applications and environments. In a rack-
mountable, all-steel enclosure, the unit is designed for continuous professional use. Mounting location
is not critical, however for greater performance reliability we recommend that you not place the unit
on top of power amps, or other sources of heat and/or strong magnetic fields. The tube circuitry
needs about a minute to “warm up” and stabilize from a cold power up.
AC Power Hookup
The ART Pro Channel II has an internal power supply. Only connect the unit to mains power of the
type marked on the rear panel. The power source must provide a good ground connection, and the
ground pin on the mains plug should never be defeated.
Analog Audio Connections
Audio connections to and from the Pro Channel II are:
Front panel Instrument input: [1/4-inch] Tip = Hot (+), Sleeve = Ground
Rear panel balanced input: [XLR] Pin 2 = Hot (+), Pin 3 = Cold (-), Pin 1 = Ground
Rear panel balanced output: [XLR] Pin 2 = Hot (+), Pin 3 = Cold (-), Pin 1 = Ground
Rear panel 1/4-inch output: Tip = Hot (+), Sleeve = Ground
Rear panel 1/4-inch insert inputs: Tip = Hot (+), Sleeve = Ground
Insert Outputs: Tip = Hot (+), Sleeve = Ground

3
FRONT PANEL CONTROLS and JACKS
Mic Preamp Controls
The Pro Channel II input consists of a discrete Class-A differential preamp. The circuit is optimized
for low impedance microphones as well as line level signals. Up to 56dB of gain is available from this
stage. The output can be inverted using the INVERT switch.
The impedance of the rear XLR input is continuously variable for fine-tuning the preamp to a wide
variety of mics. Phantom power is available on the XLR input as well.
A selectable low-cut filter removes rumble, wind noise, and pops, thereby increasing clarity.
Instrument Input
The 1/4-inch T/S jack on the front panel provides a high impedance unbalanced input, and when
used, automatically switches off the mic pre-amp. (The rear XLR balanced input is lower impedance
and is part of the mic pre-amp. The rear jack is not intended to be used with high impedance
microphones or instruments.)
Gain Control
This control adjusts both the mic pre-amp gain as well as the instrument input gain. The gain
marked applies to the mic pre-amp without the GAIN switch depressed. Refer to the APPLICATIONS
section to learn how to optimize the gain control for low noise operation.
Impedance Control
This knob sets the load impedance at the rear panel XLR input of the Pro Channel II. Use the
IMPEDANCE CONTROL to subtly tune the sound of your microphone. Various microphones will
change their sound at differing load impedances. The correct setting is subjective. Adjust this control
to personal taste.

4
GAIN Switch
This switch increases the tube gain by 20dB to support low output mics and instruments. It should
only be used when the GAIN control cannot provide enough gain.
Phantom Power Switch
The switch safely applies +48Volt phantom power to the XLR input. Use phantom power only when
the microphone that you are using requires it. Doing so will extend the life of the Pro Channel II as
well as reducing the possibility of shock hazard.
Invert Switch
This switch selects the output phase of the Pro Channel II. There is a 180-degree phase shift
through the Pro Channel II preamp section when lit.
Low Cut Control
This control allows adjustment of a 6dB/Oct high pass filter from 10Hz to 250 Hz. The filter is
designed to remove rumble, pops, and wind noise, yet still sound natural.
Preamp Output Control
This control adjusts the output of the Preamp section allowing up to 10dB of gain or allowing you to
mute the output when fully CCW.

5
Tube Voltage Switch
The vacuum tube preamp section can be adjusted to run at two different plate voltages. Refer to
Figure 1 for the location of the switch.
Choose the “NORMAL” setting for adding warmth to the input signal. This setting has an increased
amount of tube saturation at higher signal levels.
Choose the “HIGH” setting to increase overall gain, headroom, and bandwidth.
NOTE: The change between tube voltage modes is gradual, taking 10 to 20 seconds to be fully activated.
FIGURE 1 – Preamp section

6
Compressor Controls
The ART Pro Channel II Compressor section consists of an above threshold Optical
Compressor/Limiter with adjustable slope. The attack and release controls allow a wide range of
adjustment while the complex detector assures fast response without distortion.
Threshold Control
This control sets the level, above which the Compressor in the Pro Channel II starts to act on the
input signal. As the control is turned clockwise, more input signal is required to begin reducing gain.
The compression action can be seen in the Gain Reduction LED meter.
Ratio Control
The RATIO control sets the amount of gain reduction that takes place based on how far the input
signal is over the threshold level (set by the THRESHOLD control). A good starting point for vocals is
2.5:1.
To have the unit act as a limiter, set the RATIO control to ∝:1
FIGURE 2 – Compressor Section

7
Attack Control
The ATTACK control sets the time it takes the Compressor/Limiter to respond to increases in signal
level (by reducing gain). You can use this control to shape the “front end” of the dynamics envelope.
One example is to listen to a snare hit and adjust the attack control. A short attack makes the snare
sound “thin”. As the attacks go longer (and the knob is turned clockwise) you should hear more of the
thump in the compressed snare. The downside is that this creates an overshoot, (a large transient),
the length of which is the time set by the ATTACK control.
Overshoots less than 1 msec are very hard to hear even when they are clipped. If the attack is set
too fast, the gain may be reduced too much and thereby create a “pumping” sound
1
. One way to
eliminate this is to use the LOW CUT filter to remove plosive sounds in vocals that can make the
detector overreact.
Release Control
The RELEASE control sets the time the Compressor/Limiter takes to increase the gain after the
input level drops.
Longer settings maintain the dynamics of the input signal, while shorter settings reduce the
dynamics. Shorter settings will also increase the apparent reverberation, and at extreme gain
reduction settings, lead to “breathing” artifacts
2
.
Compressor Output Control
The Output control can provide from10dB of gain recovery to fully muting the output. It adjusts the
output level provided to the COMP output jack.
1
“Pumping” in a Compressor/Limiter sounds like the processor over-reacts to the increase in level and has reduced the
volume too much.
2
“Breathing” is the sound of the Compressor/Limiter turning up the gain so quickly you can hear breathing noises between
words during vocal processing.