FEA OPTI-FET User manual

FEA
OPTI-FET
COMPRESSOR
www.fealabs.com
Battery replacement:
Remove the four screws on the bottom cover to access the battery
compartment. If storing the unit for long periods of time the battery
should be removed to prevent corrosion of the battery snap. Be sure
to place the battery wires along the side of the battery in the battery
pocket and use care while replacing the bottom cover.
Warranty:
The FEA OPTI-FET Compressor is fully covered for a period of 5
(five) years and the foot switches are covered for 1 (one) year against
defects in material and workmanship. Abuse and neglect are not
covered under the warranty. Any modifications or repairs not
authorized by FEA voids all warranties. The customer will be
responsible for shipping cost to and from FEA Labs for repairs.
Contact me before attempting to ship a unit for repair at:
[email protected].
All repairs made outside of the warranty period will be very
reasonable (usually only the cost of the parts)… your satisfaction is
priority one.
Contact FEA Labs at:
www.fealabs.com
[email protected]
REV 3.4

Technical Specifications:
ATTACK: 2mS to 150mS
RELEASE: 50mS to 1S
RATIO: 1.5:1 to 20:1
THRESHOLD: -47dBu to 
Make-up GAIN: -4 to 23dB
Side-chain EQ Freq: LOW 30Hz, MID 250Hz, HIGH 2KHz
Maximum Input: 13dBu
Maximum Output: 13dBu
Residual Output Noise: -52dBu from 10Hz-20kHz
(un-weighted) with no
compression and output gain set to
maximum. **This is the absolute
worst case noise scenario.**
Frequency Response: 20Hz –20KHz -2dB
Input Impedance: 1M ohm
Output Impedance: 3K ohm
Current Consumption: Approx. 30mA
Battery Life: Approx. 17 hours continuous use
Power adapter (optional): 9VDC 2.1mm negative center pin
About the design:
The FEA OPTI-FET Compressor is designed with both audiophile and
studio quality features that many musicians enjoy.
The new audio path has been redesigned with the FEA 3S output
amplifier circuit for warm tone and excellent noise rejection that
features a “class A” FET output driver. The 3S amplifier utilizes an
audiophile grade Texas Instruments™(formerly Burr-Brown™)
SoundPlus™ operational amplifier and a class-A Fairfield™FET
driver for outstanding performance. The operational and FET driver
amplifiers are integrated in a unique way to take advantage of the
favorable qualities of both types of amplification. This 3S amplifier
has selectable modes of operation for a clean or warm tone. The
warm tone mode was modeled to match the tone of the original OPTI-
FET circuit. The input stage is a high impedance CMOS, hi-fi amplifier
buffer with internal RFI protection. The input amplifier has been
designed to have minimal signal distortion and noise, with maximum
headroom for all input signal levels.
Another feature in the FEA OPTI-FET Compressor is the extensive
side-chain control which is not commonly found in most pedal type
compressors today. Like many quality studio rack compressors, the
OPTI-FET compressor has all of the common controls available to the
user to accurately shape his or her tone. Along with the full feature set
of side-chain controls, a three band equalizer is available. This EQ
only affects the amplitude of frequencies of the signal in the
compressor control circuits. This EQ allows the user to modify the
threshold setting by frequency band. The OPTI-FET compressor also
has a side-chain loop feature, similar to many rack compressors. This
allows the user to be more creative with the use of other outboard
pedals. Multi-band EQ, or buffered volume pedals are a few that can
make this pedal even more versatile when used in the side-chain
loop.
The Luna Optoelectronics™AOI’s (analog optical isolators) used in
the FEA OPTI-FET Compressor utilize a combination formula for the
photocell to overcome limitations found in other traditional optical
components affecting reaction and recovery times. This fast current
monitored AOI allows the side-chain in the compressor to accurately
control the dynamics of the compression. The purely resistive photo
element in the AOI exhibits less noise and distortion than most
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designs using a VCA (voltage controlled amplifier). A good number
of audiophiles advocate that only high-quality passive components
should be in utilized the audio signal path and this is one application
that I would have to agree with them.
The dual rail power supply in the OPTI-FET Compressor is built on
its own discrete circuit board to physically separate it from the audio
circuits. The power supply is over filtered two times to assure
exceptionally clean power for the signal circuitry. This power supply
provides 18Volts (+9V and -9V) to the compressor circuit board to
provide plenty of headroom for the signal. The power supply was
designed to provide separate power for all of the side-chain control
circuitry. This assures that any natural electrical noise that the
compressor side-chain control circuitry generates will not bleed into
the signal circuitry. I have not seen this approach to power
distribution in any of the other manufacturer’s guitar effects. Is it
overkill? Maybe…but I feel that it is absolutely crucial to eliminate
every bit of noise where possible.
Frank E. Appleton (FEA)
Block Diagram:
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audio signal coupling capacitors are tight tolerance, quiet,
multi-layer and metallized polyester film type. The
transistors utilized in the audio signal path are popular low
noise audiophile grade devices.
The power supplies onboard voltage charge pump allows
the circuitry to operate at 18volts (+9 and –9 volt rails) from
a single 9-volt battery or DC power adapter. This allows the
audio signal plenty of headroom from active electronic
guitars and aggressive playing techniques (i.e. pop and
slap).
The “switch on” power supply current is less than 1µA
(micro amp) on the signal ground at the INPUT jack. This is
approximately 45,000 times (-93dB) less than the commonly
used method of connecting the battery’s negative terminal
to ground via the sleeve of the plug inserted into the INPUT
jack. The “switch on” sensing method used in the FEA
OPTI-FET Compressor keeps nearly all of the circuit’s
generated white noise and transient currents out of the
INPUT stage signal ground. Extreme measures have been
taken to keep the power and signal paths as clean as
possible. NOTE: Unplug the cord from the INPUT jack when
not in use to prolong battery life.
Features:
THRESHOLD, RATIO, ATTACK, RELEASE and GAIN controls.
The control ranges for the ATTACK and RELEASE have been
optimized for use on both bass and guitar.
The compressor exhibits soft-knee compression at lower
RATIO settings and hard-knee at maximum RATIO settings.
The COMP LED indicates when the THRESHOLD has been
reached. This LED does not indicate the RELEASE
response.
The compressor side-chain utilizes precision full-wave
rectification of the audio signals in the THRESHOLD circuit
for accurate envelope tracking. This also reduces the
possibility of “pumping” with very low frequencies.
ACTIVATE foot switch places the compressor unit in the
signal chain or in Direct Bypass mode. In Direct Bypass
mode the signal at the input is directly connected to the
output and does not pass through any electronics.
EQUALIZER foot switch enables the three band equalizer
controls for the side-chain control circuit. The three band EQ
only affects the amplitude of frequencies of the signal in the
side-chain control circuit to modify threshold sensitivity by
frequency band.
LOOP SEND/RETURN jacks are for placing external
processors or EQ’s in the compressor side-chain control
circuit. These jacks are only active when the EQUALIZER
foot switch is enabled.
Internal DIP switches to select the output amplifier mode of
operation. The three modes are clean (SW 1&2 off), warm 1
(SW 1 on) and warm 2 (SW 2 on). The warm 2 mode adds a
subtle amount more of the dynamic harmonics of mode 1.
Internal DIP switch for a 12KHz filter for an ‘old school’ tone
or to reduce the hiss from pedals proceeding the
compressor.
The audio signal compression is accomplished with only a
resistive component (Light Dependent Resistor or Photocell).
All resistors in the circuit are low noise metal film type. The
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