UTA UnFairchild 670M II User manual

UnFairchild 670M II User Manual Version 1.3
UTA
UnderToneAudio
1. INTRODUCTION 2
2. SAFETY/ POWERING UP 3
3. REAR CONNECTIONS 6
4. FRONT PANEL CONTROLS 7 - 8
5. CALIBRATING PROCEDURES
Calibration Procedure 9
Balancing by Ear 11
6. USING THE SIDE CHAIN
Using the Loop 12
Side Chain Compression FX 12
7. FEEDBACK VS. FEEDFORWARD 13
8. TIME CONSTANT PRESETS 14
9. EXAMPLE SETTINGS
Drums 15
Bass 16
Guitar 16
Piano 17
Vocals 17
Mix Buss 17
10. MAINTENANCE 18
11. SPECIFICATIONS 18
1

1. INTRODUCTION
Congratulations on your purchase of the mythological UnFairchild! It does actually exist, a faithful, mod-
ern revitalization of the original Fairchild! The Undertone Audio team worked passionately to preserve
the character of the Fairchild sound while improving the reliability, functionality and aordability of this
legendary circuit. From our sound lab to yours, we sincerely hope you will nd the results as useful and
satisfying as we have.
What’s dierent about the UnFairchild 670M II from an original Fairchild?
We are using Cinemag recreations of the original transformers used in the audio path. They are
extremely close to the originals with only small adjustments made to accommodate a more desirable gain
structure. The side chain circuit has been simplied to improve reliability and aordability and because
the original design was totally insane. It’s still a tube circuit..it still preserves the important sonic character-
istics of the original...the only dierence is that it’s not insane.
The original Fairchild runs at a very high plate voltage. So high, in fact, that the original Fairchild
manual recommends that you should change the tubes every 6 months. We lowered the plate voltage
going to the 6386 tubes to help extend their life and reliability. 6386 tubes can cost anywhere from $120
-$500 each and the unit uses 8 of them! Best case you’re looking at $1,000 every 6 months to be sure an
original Fairchild is running up to spec. This is one of the reasons so many of them sound so dierent from
one another...and why some sound so bad. With the lower plate voltage and a good set of tubes, the Un-
Fairchild will run reliably for years.
MORE CONTROL: We’ve added a wider range of controls. As cool as the original Fairchild is to own, it’s
cooler to have that sound with modern exibility. The UnFairchild oers the legacy of the Fairchild; with
much more of what one would expect from a compressor built in 2018.
In addition to the original time constant presets there are:
• Independent control over the Attack and Release
• User control over the DC Oset (functions similar to a ratio control)
• Access to processing or EQing the side chain signal
• Fine-trim adjustments for precise matching of the L/R balance
• True stereo linking
• Multiple M/S processing modes
• Options for feed-forward compression
• Calibration for meter level acuracy and channel compression matching
• True bypass
• Output metering
2

SAFETYINSTRUCTIONS

WARNING:Toreducetheriskoffireorelectricshock,donotexposethis
producttorainormoisture.

CAUTION:RISKOFELECTRICSHOCK.DONOTOPEN.Therearenouserserviceablepartsinside
theproduct.Referservicingtoqualifiedservicepersonnel.
Inordertoensuresafeoperationofthedevice,followtheseguidelines:
1. Readtheinstructionmanualinitsentiretybeforeoperatingtheequipment.Retainthe
manualforfuturereference.
2. Observeallsafetyprecautions,warningsandinstructionsnotedinthismanual.
3. Alwaysunplugthisdevicefromthewallsocketbeforecleaning.Useonlydrycloth.Do
notuseaerosolsorsolvents.
4. Keepthisdeviceawayfromsourcesofwatersuchaspools,bathtubsandsinks,anddo
notexposeittorainorsplashesofwater.Donotplaceobjectsfilledwithfluidonthe
device.
5. Ventsareprovidedforheatdissipationonthesidesandtherearofthedevice.Maintain
atleast2”(5cm)spacearoundtheseventstoprovidesufficientventilation.
6. Keepthedeviceawayfromsourcesofheatandopenflamesuchasheaters,radiators,
stoves,litcandles,etc..
7. Makesurethepowercordisintactbeforepluggingitintothedevice.Donotusecords
withvisibledamagetotheinsulationorconnectors.
8. Thisdeviceisequippedwithasafetyfeaturethatrequirestheuseofathreepin
groundingpowerplug.Donotdefeatthesafetypurposeofthegroundingplug.Ifthe
providedplugdoesnotfityouroutlet,consultanelectriciantoreplaceyourobsolete
poweroutlet.
9. Useonlyaccessorieslistedinthismanualorotherwisespecifiedbythemanufacturer.
10. Donotinstallthisproductoncartsorothermovingobjects.
11. Whenthedeviceisinuse,routethepowercordinsuchawaythatwillpreventitfrom
beingsteppedon,trippedon,pinchedordamaged.
2. SAFETY/POWERING UP
3

12. Donotusethisdevicewithwallorceilingmountsnotspecifiedbythemanufacturer.
13. TocompletelydisconnectthedevicefromtheACMains,disconnectthepowercord
fromtheACreceptacle.Foradditionalprotection,unplugthedeviceduringelectrical
storms,orwhennotusedforlongperiodsoftime.
14. Nouserserviceablepartsinside.Referservicingtoqualifiedpersonnel.Iftheunitwas
exposedtoliquid,excessiveheatorfire,orsustainedmechanicaldamageofanykind,
donotattempttooperateit.Disconnecttheunitfromthewalloutletandconsult
qualifiedservicepersonnel.

INSTRUCTIONSFOR220240VACOPERATION
1. Beforeproceeding,disconnectthepowercordfromtheACinlet.
2. ThisdeviceisprovidedwithapowercordforNorthAmerican110120VACoperation.
Foroperationinotherlocationsand/orfromothervoltagesources,usethecorrecttype
ofpowercordforyourareaandvoltagesource.
3. ThisdeviceisprovidedwithT0.4AACMainsfusesinstalledfor110120VACoperation,
andincludesT0.2Aaccessoryfusesfor220240VACoperationpackedseparately.Open
thefusedrawerontheACreceptacleontherearofthedeviceandreplacetheT0.4A
fusewiththeT0.2Aaccessoryfuse.
4. Inthefusedrawer,changetheACMainsswitchfromthe110Vpositiontothe240V
position.
5. Assurethatthefusedrawerissecuredclosedbeforeattachingthepowercord.

INSTRUCTIONSPOUR220240VACFONCTIONNEMENT
1. Avantdecommencer,débranchezlecordond'alimentationdelaprisesecteur.
2. Cetappareilestfourniavecuncordond'alimentationpourl'AmériqueduNord110
120VAC60Hz.Pouruneutilisationdansd'autresendroitset/oud'autressourcesde
tension,utiliserlebontypedecordond'alimentationdevotrerégionetsourcede
tension.
3. CetappareilestfourniavecT0.4AACMainsfusiblesinstalléspouruneopérationde
110120Vca,etcomprendT0.2Aaccessoiresfusiblespourunfonctionnement220
240VACemballésséparément.Ouvrezletiroirdufusiblesurlaprisesecteurà
l'arrièredel'appareiletremplacerlefusiblede0.4ATavecl'accessoirefusibleT
0.2A.
4. Dansletiroirdefusible,changerlessecteurCApasserdelaposition110Vàla
position240V.
5. Assurezvousqueletiroiràfusiblesestfixéferméavantdefixerlecordon
d'alimentation.
4

2. RACK MOUNTING and POWERING UP
Before powering up, be sure you are plugging in to a properly grounded outlet. The standard
setup for the UnFairchild is for a 110V-120V 60hz AC power source. It can be setup for 220V-240V op-
eration upon request. You can switch to 220V-240V operation by ipping the voltage selector board
inside the power entry module on the back of the unit. Use a screwdriver to pull out the drawer that
houses the voltage selector board. ip the board so it displays the voltage you wish to use with the
device. Make sure the proper fuse is installed for the selected voltage.
PROPER VENTILATION IS ESSENTIAL TO ENSURE THE UNFAIRCHILD DOES NOT OVER-
HEAT!! There are important ventilation holes on the top and bottom of the chassis that help keep the
unit within the proper operating temperature. If the vent holes are covered up the unit WILL overheat,
possibly stressing components.
When sitting on a at surface:
Install the included rubber feet so there is a gap between the bottom of the unit and the surface it is
sitting on. Do not place any items directly on top of the unit without a spacer to provide an air gap.
When racking:
Please leave a 1U rack space or at least a 5/8”space for an air gap ABOVE AND BELOW the UnFair-
child.
If using a slanted rack:
Make sure that the unit is mounted no atter than a 25 degree incline.
5

3. REAR CONNECTIONS
AUDIO INPUTS/OUTPUTS
You are now ready to plug in cables to the rear of the unit. All
audio connections on the unit are XLR.
• Plug the source signals you want to be processed into
the INPUT LEFT and INPUT RIGHT XLR input jacks on the back
(see gure 1). Use the provided splitter cables as describbed on
page 13 if you would like to implement the Feed Forward option.
• Plug the OUTPUT LEFT and OUTPUT RIGHT signal from
the UnFairchild into the inputs of the device that will be either
recording or monitoring the processed signal.
SIDE CHAIN CONNECTIONS
• Connect the signal from the UnFairchild SC SEND - L and
SC SEND - R into the input of an external processing device (typi-
cally an equalizer).
• Connect the signal from an external source or process-
ing device into the SC RCV - L and SC RCV - R inputs on the back
of the UnFairchild (see gure 2).
(Find more information on applications for the side chain feature
see the “USING THE SIDE CHAIN” section on page 12)
6
INPUT
L
R
L
R
L
R
L
R
SC SEND
GND LIFT
SC RCV
CAUTION
!
RISK OF ELECTRIC SHOCK
DO NOT OPEN
WARNING: SHOCK HAZARD - DO NOT OPEN
AVIS: RISQUE DE CHOC ELECTRIQUE - NE PAS OUVRIR
6386
6386
6386
6386
6386 ECC99
ECC81/12AT7
ECC83/12AX7
ECC99
ECC81/12AT7
ECC83/12AX7
6386
6386
6386
Voltage select
in fuse drawer
110V - T2A
240V - T1A
MAINS FUSE:
Serial #
Model: 670M II
Designed and built in California by
UnderToneAudio
UTA
AC LINE
50/60Hz
150VA
OUTPUT
INPUT
L
R
L
R
L
R
L
R
SC SEND
GND LIFT
SC RCV
CAUTION
!
RISK OF ELECTRIC SHOCK
DO NOT OPEN
WARNING: SHOCK HAZARD - DO NOT OPEN
AVIS: RISQUE DE CHOC ELECTRIQUE - NE PAS OUVRIR
6386
6386
6386
6386
6386 ECC99
ECC81/12AT7
ECC83/12AX7
ECC99
ECC81/12AT7
ECC83/12AX7
6386
6386
6386
Voltage select
in fuse drawer
110V - T2A
240V - T1A
MAINS FUSE:
Serial #
Model: 670M II
Designed and built in California by
UnderToneAudio
UTA
AC LINE
50/60Hz
150VA
OUTPUT
INPUT
L
R
L
R
L
R
L
R
SC SEND
GND LIFT
SC RCV
CAUTION
!
RISK OF ELECTRIC SHOCK
DO NOT OPEN
WARNING: SHOCK HAZARD - DO NOT OPEN
AVIS: RISQUE DE CHOC ELECTRIQUE - NE PAS OUVRIR
6386
6386
6386
6386
6386 ECC99
ECC81/12AT7
ECC83/12AX7
ECC99
ECC81/12AT7
ECC83/12AX7
6386
6386
6386
Voltage select
in fuse drawer
110V - T2A
240V - T1A
MAINS FUSE:
Serial #
Model: 670M II
Designed and built in California by
UnderToneAudio
UTA
AC LINE
50/60Hz
150VA
OUTPUT
UNF V2.0, CMB REAR SILKSCREEN, RevB2
LAYER 1 BLACK
VIEW #1 COMPLETED PART
VIEW #2 COMPLETED PART INSTALLED
NOTES:
PART: UNF V2.0, CMB REAR SILKSCREEN, RevA
COMPOSITE VIEW (FOR REFERENCE ONLY):
SILK SCREEN LAYER 1 BLACK
Undertone Audio
323-208-1566
(gure 1)
(gure 2)

4. FRONT PANEL CONTROLS
UNFAIRCHILD
UNFAIRCHILD
210
1
3
6
89
45
711
12
13
14
DESCRIPTION OF FRONT PANEL CONTROLS
1. POWER SWITCH - Use this switch to turn the UnFairchild on and o. Push the switch UP to the “ON”position to
turn the unit on and push the switch down to power the unit o.
2. POWER LIGHT - This light will illuminate when the unit is powed up properly.
3. METERS - The meters can show three dierent things; output level, the amount of gain reduction or the balanc-
ing of the tubes.
4. GAIN TRIM - Fine adjust trim pots for matching left and right output level. These will also aect the character of
compression and harmonic distortion. (See “Calibrating the Audio Levels” under the “CALIBRATING PROCEDURES” section
on pg. 9/10 for more details.)
5. BALANCE TRIM POT - This trim pot is used to balance the circuit for optimum performance based on the particular
set of tubes installed. (See “balancing” under the “CALIBRATING PROCEDURES” section on pg. 9 for more details.)
6. GR ZERO - These trim pots are used to set the meters to 0dB on the meter scale when there is no compression
happening. This setting will drift for the rst 20 or 30 minutes of operation. If you adjust them properly after the
unit is fully warmed up, the calibration will appear to be owhen the unit is rst powered on (they will typically
be below the 0dB mark when rst powered up). Be Patient!! As the unit warms up, the calibration will typically fall
back into place and not need to be re-calibrated. 7

7. BYPASS/VU SWITCH - This 4-position switch allows you to bypass the active circuitry and control the meter
functionality.
8. INPUT GAIN CONTROL - These rotary switches control the amount of input gain. The scale (0 - 20) is showing how
much the signal is being padded before the active circuitry. The setting of 20 represents -20 dB of padding. Using
the gain trim pots, unity gain can be calibrated for a setting anywhere from -12 to -10.
9. THRESHOLD - The threshold knob controls the sensitivity of the gain reduction circuit. When turned all the way
counter-clockwise, the UnFairchild will not compress at all. As the threshold control is turned clockwise, the amount
of compression will increase.
10. CHANNEL MODE - This rotary switch controls how the two channels interact.
11. TIME CONSTANT - The TIME CONSTANT rotary switch provides the original six (1-6) Fairchild presets, and provides
four more settings (VAR1 - VAR4) for added exibility. With settings 1 - 6, the attack and release times are preset. VAR1
- VAR4 allow you to access the variable attack and release controls, located on the lower panel. (See page 14 for more
information on the variable attack /release controls.)
12. SIDE CHAIN INSERT - These toggle switches engage the SIDE CHAIN insert. When in the DOWN position, the Un-
Fairchild gain reduction circuit will respond to the signal connected to the input L/R jacks. When the switch is in the
UP position, the UnFairchild gain reduction circuit will respond to the signal connected to the SC RCV L/R jacks.
13. ATTACK AND RELEASE CONTROLS - These are the Attack and Release controls for any of the 4 “VAR” modes. The
lower numbered settings in the counter clockwise direction are faster and the higher numbered settings in the clock-
wise direction are the slower settings
14. DC THRESHOLD - This control does 2 things: It adjusts the range of volume that the THRESHOLD control is sen-
sitive to and it changes the ratio or ‘knee’ of the compression. In the fully counter-clockwise position the UnFairchild
will be sensitive to the lowest range of input levels and have the‘softest knee’ or ‘lowest possible ratio’. As the control
is turned clockwise, the ratio increases and the threshold becomes less sensitive and requires a higher setting to
maintain the same amount of compression.
• BYP: When set to the bypassed BYP mode, the signal plugged to the input jacks will be connected
directly to the output jacks; bypassing all of the active electronics.
• VU: When set to the VU mode, the input signal will be processed by the compression circuit and the
VU will show the resulting output level.
• GR: When set to the GR mode, the VU meters will show the amount of gain reduction.
• BAL: When set to the Balancing BAL mode, the unit is put into a special mode used for balancing the
circuitry for optimal performance with the particular set of 6386 tubes that are installed.
• IND: When set to “IND” mode, the two channels will behave like two individual mono compressors.
• LINK: When set to the LINK mode, the two channels will be linked together to make sure both chan-
nels compress the same when compressing a stereo signal.
• M-S: When set to the M-S mode, the UnFairchild will compress all of the mono information with chan-
nel A, and all of the stereo information with channel B. In this mode, the mono and stereo information
will be compressed dierently and consequently the stereo image may uctuate when dierences
occour.
• M-S LINK: When set to M-S LINK mode, the mono and stereo compression are linked together so the
stereo image will not uctuate. You can now use the INPUT GAIN controls to change the balance be-
tween the mono and stereo signals. This can be used widen or narrow the stereo image.
8

5. CALIBRATION PROCEDURES
9
Tools Needed:
1. #2 Phillips head screwdriver
2. Tweaker tool or small at head screwdriver
3. Signal generator (e.g. from DAW)
4. Level meters (e.g. DAW meters or console meters)
5. Multimeter (optional)
Prep UnFairchild:
1. With the UnFairchild powered o, make sure the front, back, and right side of the UnFairchild are accessible
•If you’re unracking the UnFairchild, make sure there is at least 5/8” air gap below. Do not cover the ventilation
holes on top of the UnFairchild.
2. Remove the back tube cage with a #2 Phillips head screwdriver.
•Tip: remove the top middle screw last
3. Set the UnFairchild’s controls as follows:
•Channel A & B Input Gain: 12
•Channel A & B Threshold: 0
•Link Mode: IND
•Channel A & B Time Constant: 1
•Channel A & B Side Chain Insert: OUT
•Channel A & B DC Threshold: 0
•Channel A & B Meter Mode: BAL
4. Turn the UnFairchild on
5. With Channel A & B Meter Mode in BAL, using a tweaker tool or a small at head screwdriver, rotate the BAL pot
on each channel so each UnFairchild VU meter is as far left as it goes
•The UnFairchild VU meters don’t need to match each other in BAL, each just needs to be at its leftmost
deection
6. Leave the unit on for 1-3 hours to let the tubes warm up
Set Up Tone and Metering:
1. Send a 1kHz sine wave to both of the Main Inputs of the UnFairchild. This tone will be left on for the entirety of
this calibration (The exact level of the 1kHz sine wave is inconsequential)
2. Send both of the Main Outputs of the UnFairchild to volume meters (e.g. to DAW or console meters). We’ll call
these the return meters.
3. Ensure that there’s nothing in the signal path between the tone generator, the UnFairchild, and the return meters
Unity Gain Calibration:
1. Put Channel A and Channel B Meter Mode in BYP
2. Note the level on each return meter (e.g. DAW or console meter)
3. Put Channel A and Channel B Meter Mode in VU
4. Rotate each channel’s Gain Trim pot so that the level on each RETURN METER (e.g. DAW or console meter) match-
es each level noted in step 2
•Note: don’t worry about what each UnFairchild VU meter is displaying while adjusting the Gain Trim here—
only look at each return meter. The UnFairchild VU meters will be calibrated in the next section

10
VU Meter Calibration:
1. Verify that the test tone is at +4dBu
•With the test tone still going, put a multimeter in AC Voltage mode,
disconnect each male XLR connector that’s plugged into each
Main Input of the UnFairchild, and measure between its pins 2 & 3.
Adjust the level of the signal generator until the multimeter reads
approximately 1.228v, and plug each XLR back into the UnFairchild.
2. Put Channel A and Channel B Meter Mode in VU
3. Make sure the INPUT GAIN setting for both channels A & B are set to 12
4. There are two pots on the right side of the UnFairchild chassis. Using a
tweaker or small screwdriver (at head or phillips) rotate each pot so
that each UnFairchild VU meter reads 0
5. Put Channel A and Channel B Meter Mode in GR
6. Rotate each channel’s GR Zero pot so that each Unfairchild VU meter reads 0
Gain Reduction Calibration:
1. Put Channel A & Channel B Meter Mode in GR, and rotate each
threshold control to 10
2. There are two pots on the back of the UnFairchild chassis labeled
SC Gain. Using a tweaker or small screwdriver (at head or phillips),
rotate each SC Gain pot so that each UnFairchild VU meter is as low
as it goes (full clockwise)
3. Adjust each channel’s threshold contol so each channel’s VU meter is
as close to -3 as possible
4. Note the value of each channel’s threshold control knob. If they are
dierent, raise the threshold control knob with the lower value to
match the higher value on the other channel. Both threshold control
knobs should match.
5. Note which channel’s VU meter has a lower reading. Rotate the
SC Gain pot (counter clockwise) on that channel so that it rises to
match the higher reading on the other channel
6. Put both Threshold controls back to 0
Threshold Symmetry:
You will be happy to know that the calibration settings for the threshold
symmetry are not hugely consequential. If you want to check it, the best
thing to do is put a low frequency through the unit (around 50hz) get a
good amount of compression going (about 10dB), and adjust the threshold
symmetry for a minimum of audible distortion (or maximum if you’re into
that sort of thing :) The main thing is to try and have the two sides sound
similar. Compare L/R channels and adjust the symmetry so the two chan-
nels are as close as can be and you’ll be good to go!!
Finalize:
1. Put the tube cage back onto the UnFairchild
•Tip: instal the top middle screw rst
2. There must be a gap above and below the UnFairchild when rack mounted. At least 5/8” of space above and
below the UnFairchild is needed. If the UnFairchild is not racked, please install the rubber feet included in the
packaging and ensure that nothing is on top of the UnFairchild covering the vent holes to restrict airow.
Channel A
Channel B
Channel B Channel A
Channel B Channel A
Figure 1: VU Meter calibration pots
Figure 3: Threshold Symmetry calibration pots
Figure 2: Gain Reduction calibration pots

11
BALANCING BY EAR
Balancing with the meter will work for the majority of applications, but there are circumstances
in which balancing by ear may achieve better results. The balance between a set of tubes can be
dierent based on the amount of compression. If you are using the UnFairchild to do some very ex-
treme compression on transient sound sources; such as drums, you may hear some sonic artifacts,
such as thumping. You can solve this simply.
1) Play the sound source with the compression setting where you’re hearing the thumping.
2) While the sound source is playing, adjust the BAL trimpot until the thumping goes away.
IMPORTANT! There is another aspect to the level gain trim adjustments; the gain trim control
adjusts the bias current of the 6386 tubes. It is a great way to adjust the level, but it will aect the
sonic character of the compression, and can also aect the lifespan of the 6386 tubes. This oers
another opportunity for sonic exibility!
Lower bias current settings will have a softer, thicker, more saturated sound. If you want the softer,
thicker more saturated sound, set the INPUT GAIN to 9 or 10. This will force you to set the GAIN
TRIM much lower to achieve unity gain with a +4dB output level. This can add some beautiful col-
oration when using the UnFairchild for tracking or mixing individual instruments.
The higher bias current calibration settings can shorten the life span of the 6386 tubes. If you want
to avoid the possibility of shortening the life of the tubes, we advise you not to calibrate to an IN-
PUT GAIN setting lower than 12.

6. USING THE SIDE CHAIN
There are many ways to make use of the side chain feature on the UnFairchild. In this manual, there are 2
common approaches that we will discuss.
USING THE LOOP
In this application, the SC SEND - L and SC SEND - R outputs on the back panel send signal to an ex-
ternal processing device (typically an equalizer). The signal can be equalized to emphasize or remove
portions of the frequency spectrum. The signal is then fed back in to the UnFairchild via the SC RCV - L
and SC RCV - R inputs on the back panel. The gain reduction circuit will respond to the equalized signal
when the SIDE CHAIN INSERT toggle switches are in the UP or ON position.
Equalizing the side chain signal gives you control over what frequencies the UnFairchild is compressing.
The most common application is to remove low frequencies with the equalizer on the side chain signal,
forcing the compressor to let more of the low frequencies through when you are compressing material
that has the full spectrum of frequencies (i.e. drum mix or nal stereo mix). If you have a drum mix that
you want to compress that has a lot of kick drum in the blend, heavy compression can cause the kick
drum to get turned down too much by the compression. By removing low frequencies with an equal-
izer in the side chain, you can compress the drum mix heavily without changing the balance of the kick
drum in the blend.
When using the UnFairchild on vocals, you can boost higher frequencies on the side chain signal and
make the gain reduction circuit more sensitive to the higher frequencies. This creates a de-esser type
result when compressing vocals.
These are just a few examples. There are many more applications for this frequency based manipula-
tion of the side chain circuit. It’s an extraordinarily powerful feature that has never been available on a
Fairchild or Fairchild-inspired circuit before...we invite you to experiment!
12
SIDE CHAIN COMPRESSION FX
A very common modern application for the side chain feature is to send a totally dierent instrument
to the side chain input than the instrument or sound being sent to the audio inputs/outputs. This is
the ‘pumping’ eect commonly heard in electronic dance music. In this application, the instruments
or sounds you want to apply the ‘pumping’ eect to are sent to the INPUT L/R jacks on the back panel
and returned to your mixer (or DAW) via the OUTPUT L/R jacks on the back panel. The instrument that
you want to have trigger the compression eect (typically a kick drum), is then sent to the SC - RCV L/R
jacks. Every time the kick hits, it triggers the compression circuit and turns down the instruments or
sounds being sent to the INPUT L/R jacks. There is something very appealing about applying this very
modern technique to a device that was originally conceived almost 60 years ago!

13
FEEDFORWARD COMPRESSION SETUP
The concept is pretty straight-forward: Run the input signal to both the INPUT of the compressor
and the SC - RCV. You can use a Y-splitter cable, or split the signal externally in a patchbay.
SPLITTER FEEDFORWARD SETUP
1) Use an XLR splitter consisting of a female XLR on one end and two male XLR connectors on the
other end.
2) The split left signals get connected to both the LEFT INPUT and the SC - RCV - L.
3) The split right signals get connected to both the RIGHT INPUT and the SC - RCV - R.
4) Switch the SIDE CHAIN INSERT toggle switches to the IN or UP position to hear the FEEDFOR
WARD compression.
PATCHBAY FEEDFORWARD SETUP
1) Send the signals that would normally be going to the input of the UnFairchild to a ‘mult’ in your
patchbay. Both the left and right signals would be mult’d.
2) The mult’d left signal gets connected to both the LEFT INPUT and the SC - RCV - L.
3) The mult’d right signal gets connected to both the RIGHT INPUT and the SC - RCV - R.
4) Switch the SIDE CHAIN INSERT toggle switches to the IN or UP position to hear the FEEDFOR
WARD compression.
THE BEST WAY TO DO IT: THE UNFA-1 ACCESSORY BOX
Feed Forward compression is a wonderfully aggressive sound but can be a bit unwieldy at times—
wildly over compressing to the point where the signal is being cut oentirely. The UNFa-1 UnFair-
child Accessory box has, in addition to many other features, a FB/FF blend control, so you can get
the full benets of applying Feed Forward (“FF”) compression to the UnFairchild circuit without
having the compression fall of a cliof over compressing. The UNFa-1 is truly the best way to do it!
7. FEEDBACK VS. FEEDFORWARD
One of the advantages of having a side chain insert on the Fairchild circuit, is that you can set up the com-
pressor to be FEEDFORWARD instead of its intended FEEDBACK design. The add-on expansion unit to the
UnFairchild, the UNFa-1 Accessory Box, allows you to blend between the two.
FEEDBACK compression means that the compressor is using the signal from the nal output of the device to
trigger the compression. Counterintuitively, when the compressor starts compressing, the signal triggering
the compression gets quieter. In a way, it gives the compressor an awareness of how much it is compressing.
If it starts to compress a lot, the UnFairchild will automatically ease oon the compression. This FEEDBACK
design is the one if the dening characteristics of the Fairchild sound. It is what makes it so smooth when
compressing vocals, bass, piano etc. Smooth is nice; but it’s not the only way! Extreme, insane, aggressive
compression is also awesome. That‘s where FEEDFORWARD comes in.
FEEDFORWARD compression simply uses the input signal to trigger the compression directly. This approach
is much more common in VCA type compressor designs (DBX 160 or similar) When set up for FEEDFOR-
WARD, the UnFairchild will sound more like a VCA compressor. It’s a very extreme, unforgiving compression
eect. It can be amazing on any source material where you want to REALLY hear the compression. The one
benet of doing this type of extreme compression with tubes instead of a VCA is that the end result is more
organic and musical sounding while still being very aggressive.

8. TIME CONSTANT PRESETS
PRESET # ATTACK RELEASE
1 .2 ms .3 Sec.
2 .2 ms .8 Sec.
3 .4 ms 2 Sec.
4 .8 ms 5 Sec.
5 .4 ms 2 sec. for individual peaks, 10 seconds for multiple
peaks
6 .2 ms .3 sec for individual peaks, 10 sec. for multiple peaks
and 25 sec. for consistently high program level
VAR1 .1ms - 4.5ms 30ms - 1.2sec
VAR2 .2ms - 9ms 50ms - 2.4sec
VAR3 .4ms - 18ms 110ms - 4.8sec
VAR4 .8ms - 36ms 200ms - 9.6sec
14
Presets 1 - 6 are identical to the original Fairchild. VAR1 - VAR4 allow you to access the variable
attack and realease controls on the lower panel. Here’s how it works: The timing of the attack and
release on a Fairchild circuit are derived two ways; capacitively and resistively. The capacitor aects
both the attack and release times. A resistor can be used to aect each the attack and release inde-
pendently. VAR1 is the smallest capacitor and consequently the fastest attack/release setting when
the attack/release controls on the lower panel are set to 1. You can then make either the attack or
release longer resistively with the attack/release controls on the lower panel. If you select VAR2,
then BOTH the attack and release will get slower. You can then make either of them even slower
with the attack/release controls on the lower panel. Both VAR3 and VAR4 will continue to increase
both the attack and release time.
We did it this way because it was the only way to get all of the exibility of the combinations of
capacitive and resistive timing controls with the UnFairchild. This is important because the charac-
ter of the attack and release is slightly dierent when it is increased either capacitively or resitively.
You will nd that slower attack/release settings derived capacitively sound more aggressive or
‘grabbier’ than the ones derived resistively. Ultimately, it is best to play with the various compina-
tions and just listen for the sound you want.
There are times when using one of the original presets, it would be nice to adjust it slightly. Maybe
shorten the release a bit or let through a little more transient. This is easy because the rst four
presets can be easily recreated with specic settings in the Variable modes. Use these settings to
recreate the presets and then adjust from there!
PRESET 1 = VAR 2 / ATTACK 1 / RELEASE 6
PRESET 2 = VAR 2 / ATTACK 1 / RELEASE 9
PRESET 3 = VAR 3 / ATTACK 1 / RELEASE 10
PRESET 4 = VAR 4 / ATTACK 1 / RELEASE 10

9. EXAMPLE SETTINGS
Here Eric Valentine provides some examples in which TIME CONSTANT presets have rendered the best re-
sults. These examples can be a good place to start, but compression settings in general can be very subjec-
tive, which is why you should experiment and nd settings that work for you!
There are a lot of cool applications for the UnFairchild with drums. The UnFairchild 670M II is liter-
ally the tube compressor I had been looking for... for 20 years. I had never found a tube compressor
that had the right combination of the smoothness, that is inherent in tube circuitry, with the ex-
ibility and aggressiveness I need for making modern records. The 670M II has become one of my
very favorite goto compressors for drums, dare I say, even more so than my beloved Distressors.
The variable attack/release times now make it possible to easily get the slower attack/faster release
type combinations that I LOVE for drums and many other things. Because they are variable, I can
nese the settings to match the tempo of the source material better. For Drums, I have been loving
using either VAR1 or VAR2 with a slower attack time setting (around 7-12) and a faster release time
setting (around 1-4). This gets that wonderful eect of enhancing the transient attack of the drums
while pulling the ambience forward for a thicker more explosive sound.
If you are looking for a more vintage Fairchild type sound, settings 1 or 2 are great for getting a
more Beatle-esque drum sound.
It is important to mention that, when I use the UnFairchild on a drum mix, it is almost always in con-
junction with the side chain feature. The addition of the side chain makes the UnFairchild innitely
more exible on drum mixes. By EQ’ing the side chain you can literally have individual control over
how much the kick, snare or cymbals are being compressed. Typically low end is
removed from the side chain to let more of the kick drum through or mid range is boosted to add
more compression to the snare relative to the kick drum. You can boost specic high frequencies
to help keep the cymbals from getting to loud when compressed. The side chain really makes a
huge dierence when compressing the whole drum mix.
For an extra aggressive sound, the FEEDFORWARD mode is incredible. This is sometimes best as a
parallel blend with the original signal.
DRUMS
15

Bass is one of the instruments that has always worked well with the original Fairchild. You will nd
that settings 1,2 or 3 feel very similar to an original 660 or 670. Settings 4 and 5 always had to slow
of a release for bass guitar in my opinion. The same is true on the UnFairchild. The VAR1 - VAR4 al-
low me to do my favorite slow attack/fast release settings for bass. It is really nice for acoustic bass
when you want to accentuate the “thump” of the nger pluck. For this I like VAR2 with an attack of
about 6 or 7 and a release of 1 or 2. It is a great punchy thick bass sound.
Again, this is another situation where the Side Chain feature can make a big dierence. If you
boost the 2k-5k range on the side chain, then the compressor will better grab the occasional “Clack”
sound that happens when the bass player bounces the string oof the fret board or pick up. You
can also remove sub frequencies in the side chain to help balance out a bass sound that is over
emphasizing the higher notes.
The UnFairchild has yielded great results for both electric and acoustic guitar.
For electric guitar, I like the slower attack times paired with a slower release time. VAR2 or VAR3 are
great starting points for this. You can get that cool punch when the distorted guitar rst comes
in and then it will hold in place after that. For this, sometimes the FEEDFORWARD mode is really
eective. It gets even more of that dbx 160 smack when the guitar rst comes in. For clean elec-
tric guitars there is a cool thing that happens with the really fast attack times. Sometimes I nd
clean electric guitars to be to pointed sounding and I like to use a limiter type setting to smooth
the attacks. For this, the attack needs to be REALLY fast. Fortunately, VAR1 can provide you with
this eect. VAR1 is with an attack setting on the lower panel of“1”. Is fast enough catch most any
transients and can totally smooth out an unwieldy percussive sound. It is really beautiful on clean
electric guitars.
The UnFairchild has been exceptionally good on Acoustic Guitar. When I am trying to get a really
aggressive acoustic guitar sound (ala Polythene Pam or Space Oddity). Not suprisingly, Settings
1 and 2 are really great for this because they have been used for acoustic guitars a lot over the
years. With the VAR2 setting I can get more punch by slowing the attack and keeping the release a
bit faster. An attack around 5 or 6 with a release around 1 or 2 is great for that. I can play with the
overall gain verses the threshold to get the right balance of compression verses grit/distortion. Be-
cause of the longer release time, VAR4 can be really good for some overall levelling without really
sounding compressed. All of my original Fairchild favorites for acoustic guitar (1,2,3 or 6) feel very
much the same on the UnFairchild.
BASS
GUITAR
16

For piano I have two types of recordings I typically do. Either a bigger more open natural sound or
a really aggressive aected sound.
For the big open sound VAR4 (ATK5/REL3) is great.
For the really aggressive sound I like to use the FEEDFORWARD mode. You can get this really cool
eect of the sound almost imploding on itself. It is a ne line because there is a point where the
attack of the piano hit almost totally disappears. I like this one when I am having someone play big
dramatic chord stabs.
My favorite setting on the UnFairchild for vocals is denitely good old #1. The fast attack and
release times even out the dynamics while pulling the vocal forward for a very ‘in your face’ vocal
sound. The really fast attack/release times also cause additional harmonic coloration on the vo-
cals. When you really push both level and compression with this setting the vocals get thicker and
fuzzier in really great way. It always seems to make the singers voice sound bigger in the track. The
UnFairchild 670 II has been especially attering when driven into heavy harmonic coloration. It can
actually puts the harmonic overtone structure more in balance. It can x an unpleasant high end
quality you sometimes get from a cheap condenser mic.
There are two ways to go with the UnFairchild on the mix buss. If all of the individual tracks in the
mix have already been compressed to taste then the slower attack times VAR2 (ATK5/REL2) have
been great. This setting is slow enough to not alter the attack on the drums in the mix while add-
ing some glue, additional pumping and presence to the overall mix. EQ’ing out low end in the side
chain can help minimize sensitivity to the low end and maintain a much bigger bottom end on the
mix.
The other approach is to compress the individual elements less or not at all and rely more on the
mix buss compression to achieve a compression eect on everything at once. In this context I like
using the faster attack settings preset 1 or 2 or VAR1 (ATK3/REL7). These settings can do an amaz-
ing job of gluing everything together, get some really cool pumping on the low end and have a
more vintage quality. This is denitely where the side chain feature can make a huge dierence.
It will allow you to control what elements in the mix are being compressed more or less. That can
be very important when your trying to get all of the compression on a mix from one device. If you
are ever in a situation where you need to get a more nished sounding mix really really fast and
don’t have time to address individual track with compression, these settings are a pretty incredible
instant ‘mix be good’ button.
PIANO
VOCALS
MIX BUSS
17

10. MAINTENANCE
There are only a couple topics for general maintenance with the UnFairchild. First, is the issue of when to
keep it ON and when to turn it OFF: There are diering opinions on this but my rule of thumb with tube gear
is to leave it on if I am going to be using it again the next day and turn it oif I won’t be using it again for a
couple days or more. The trade ohere is the stress on the tubes caused by powering up/down vs. leaving
the tubes on and racking up unnecessary hours on the tubes. We won’t know for sure what is better until
these units have been around long enough for people to start reporting back to us with their results either
way. UPDATE 2022: We have found that turning oyour UnFairchild every night or between sessions will
increase tube life.
The second issue is the eventual replacement of tubes. They should last a long time under normal circum-
stances but there is always the possibility of a tube getting smashed or unexpectedly failing prematurely.
Tubes can be user acquired and replaced. After replacement, please follow the calibration instructions in
section 5. The following is a list of the tubes used in the UnFairchild, please use these exact tubes when
acquiring replacements: JJ 6386-LGP, JJ ECC99, JJ ECC81, JJ ECC83-S.
Lastly, please retain the custom shipping boxes for future use.
11. SPECIFICATIONS
Input Impedance............................................. 2.5KΩ
Output Impedance......................................... 200Ω
Output Level..................................................... +4dB @ 0Vu (+28dBm clipping point)
Gain...................................................................... 20dB (in 1dB steps)
Frequency Response .................................... 20hz - 20Khz (+0/-.5dB)
Noise Level........................................................ -83dB
Harmonic Distortion..................................... Better 1% at +18dB
Power Consumption..................................... 150VA or W
Weight................................................................ 37.4 lbs (46 lbs with packing materials)
Dimensions (LxWxH)..................................... 12” x 19” x 14” (knobs add one inch to length)
Rack Height...................................................... 8U (10U with ventilation space)
18