Coffman Labs, LLC www.coffmanlabs.com
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You have just purchased the Coffman Labs’ G1-A, a limited-
edition artisanal piece of audio equipment. This pre-amplifier
is a heritage item manufactured with care in Portland, Oregon.
Your G1-A is _______ / 500, purchased on _______________.
Authenticated by Damon Coffman, Founder of Coffman Labs:
Coffman Labs, LLC www.coffmanlabs.com
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WARNING
***DO NOT OPEN THE AMPLIFIER
OR POWER SUPPLY!***
THIS WILL VOID YOUR WARRANTY
THERE ARE NO USER SERVICEABLE PARTS INSIDE.
OPENING THE AMPLIFIER OR POWER SUPPLY WILL
DAMAGE THE EM SHIELDING BUILT INTO THE CASES
AND DEGRADE THE AUDIO SIGNAL OUTPUT.
***SEVERE SHOCK HAZARD!***
ALWAYS connect the power cable that runs between the
power supply and the pre-amplifier BEFORE turning the unit
on. This insures proper operation and safety of all parts.
The G1-A does not contain any internal parts that
may modified or serviced by the user. Attempting to fix or
change any interior component of your amplifier will void
your warranty. If there is a problem with your unit, please
contact Coffman Labs for assistance.
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PHILOSOPHY
Coffman Labs was partly inspired by a century old RCA pre-amplifier
that still works perfectly today. In this same spirit, your G1-A was
designed as a purely analog amplifier and contains no components
that could degrade easily over time.
Digital switches and microcontrollers can make the designer’s job
easier, but they are fragile and more susceptible to aging and
voltage fluctuations. The G1-A contains no circuit boards and all
wiring is point-to-point by hand. The layout is three dimensional so
that wire runs are non-existent (with the exception of the input and
output connections which are of bare wire suspended in air to
eliminate all shield capacitance and insulation affects). This greatly
increases the quality of the audio signal. All of the components are
over-sized for voltage; switching is via military aircraft quality parts
rated for millions of operations; Paper-in-Oil capacitors are used in
signal paths, to name just a few of the areas of special care.
Over three years were invested in prototyping and perfecting the
design of the G1-A. We took great care in selecting each of the
specialized components. This is an exceptional piece of audio
equipment and we hope you will enjoy it for many decades.
Coffman Labs, LLC www.coffmanlabs.com
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SETUP (PLEASE READ!)
The G1-A is very simple to set up. There are just a few things to
remember as you begin:
1. Always connect the power cable that runs between the
power supply and preamplifier before turning the unit on.
2. The power supply is in a separate box to reduce hum. We
recommend that you place it 18” or more from the pre-
amplifier unit. You can place the power supply closer—it
will not affect operation. Let your ears be your judge.
3. Make sure to place the right tubes in the right sockets. The
proper tube type is engraved into the top plate between
each pair of sockets.
4. The tube shields provide safety from burning your fingers.
They also isolate the vacuum tubes from external hum. We
recommend that the shields always be installed, though
leaving them off will not affect operation.
5. Line output impedance is low. You can safely run your
power amp input cables a long distance. We recommend
you place the G1-A pre-amplifier close to the phonograph
and the power amplifier close to the speakers. This allows
you to use shorter speaker cables.
6. Make sure the bottom of the power supply is free to vent—
there are vent slots on the bottom through which cooling
air passes.
7. Turn on your pre-amplifier first, and turn it off last.
That’s it! Place your pre-amplifier and power supply where you
want them, install the tubes, hook up the cables and enjoy!
See the next pages for explanation of all inputs, outputs, and
switches. And please read the tips and notes for understanding
about how to use the G1-A most effectively.
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S3
S1
PhonoOn
Off
Volume
InputOutput
S2Mute
Hi
Out
Low
OPERATION
Volume: Controls both the left and right channels.
Tip: The Volume control’s position may be anywhere from 8AM to
6PM, depending on the sensitivity of your power amp. The G1-A will
safely run from low to full volume without distortion.
Input: Selects one of 3 Line inputs or Phono input. The ‘Mute’
position shorts the input connections.
Tip: Use the ‘Mute’ position to change input cables. This stops any
noise from making it to power amplifier—no need to power off.
Note: You may occasionally hear a small pop when changing inputs.
This is normal and due to the fact that there are no digital automatic
switches. We recommend turning the volume control to full off
when switching between inputs.
Output:
‘On’: Selects both Main 1 and Main 2 outputs on the rear panel.
‘Off’: Disconnects all outputs and shorts the selected input.
‘Hi’: Output to Headphone jack—use for 100 ohms & above.
‘Lo’: Output to Headphone jack—use for 75 ohms & below.
‘Out’: Direct Line Stage output to Headphone jack (only).
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Tips for output control:
1. Do not change the output control if recording. Like the input
control, there may be occasional small pops when changing
the control. This really depends on the nature of your
power amp input and the types of sources connected.
2. When one of the headphone positions is selected, the rear
outputs are disconnected. If you don’t hear any sound, or
the sound is incorrect, check this control.
3. The ‘Out’ position gives a separate front panel output for TV
connection, computer connection, etc. This gives you the
flexibility of another output without going to the rear panel.
Headphone Jack: Use for headphone listening, or direct output
based on position of the Output control (page 5).
Tip: When a headphone jack is inserted into the headphone
connecter, the rear panel outputs are shorted to ground. This
insures that your speakers will not pick up any extraneous signal. If
you change the Output control to the ‘On’ position, there will still be
no sound to your speakers until the headphone jack is removed. If
you stay in the ‘Out’ position and remove the headphone jack, you
may hear very low volume music through your speakers. Be careful
not to turn volume to full, wondering where the sound is! Instead,
turn the selector switch to the ‘On’ position.
Tip: The direct ‘Out’ position was added to allow you to quickly
connect or disconnect a temporary piece of equipment like a
camera, TV, computer, etc. You can use this setting to send the
output through the Headphone jack to another device without
having to reach around back and fiddle with connections. It will give
you the same signal that comes out the rear panel with no loss of
fidelity.
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Note: The Headphone outputs use a separate gain stage and
transformer . They are not intended for use as a driver signal for
another amplifier. Use the ‘Out’ position for that purpose.
MC
S1
REC
Out
S2 S3
R
R
L
L
Main
Out 2
Main
Out 1
R L
001/500Coffman Labs G-1A
MM
47K 70K
95K
Main Outputs: There are two outputs, resistively buffered from
each other. Use ‘Main Out 1’ for longer cables.
REC Output: This functions as both a pass-through connection for
Line inputs and a Phono Stage output when in Phono mode.
Tip: Use ‘REC Out’ as a permanently connected video pass through
for your theatre system—or as an output for digitizing records.
Inputs S1,S2, S3: Connect your Line output devices like FM tuner,
CD, DVD, etc. here.
Tip: Use S1 and S2 for your best sources. S3 is configured with a
lower impedance to reduce noise from computer or video sources.
Phono Inputs MM/MC: Connect your phono player to the proper
input, depending on your cartridge. Your G1-A may have two MM
inputs or an MM and an MC. The lower MM input allows
47K/70K/95K input impedance selection to match your cartridge. If
you have a dual MM/MM configuration, the upper MM input is set
to 95K ohms.
Note: For very low output cartridges, a separate external step-up
transformer may be connected directly to the MM input. We do not
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recommend external step-up transformers of greater than30 dB of
gain.
Note: The center Selector Switch allows you to change between
upper and lower phono inputs. (This design feature avoids internal
cable runs for sensitive phono.)
PHONO GROUND CONNECTION: (red circle on the rear panel
diagram) Provided because many turntables have a separate ground
wire; most of the time this will not be required. The G1-A runs on
isolated power and a separate ground will not usually improve the
connection.
Tip: Try the ground wire if you are have any hum issues. Also, see
Notes below on ground loops.
Always leave the EXTERNAL POWER CONNECTION connected!
NOTES ON GROUND LOOP HUM: (Read only if you have a problem
with hum coming out of your main speakers.)
The G1-A should be very quiet at full volume on inputs S1-S3.
When the ‘Phono’ position is selected there may be a small amount
of hum and tube noise at very high volumes. (This will not be
audible while playing music, because the needle noise on the record
is several times larger.) Most systems will not require the volume
past the top (12 o’clock) in Phono mode.
If you are hearing a large amount of hum you have a ground loop
problem. Ground loops appear when more than one device is
connected via more than one wire back to the house AC ground.
This can happen with older gear or older houses with different
wiring. You can isolate the offending gear by removing the input
cables one at a time.
Coffman Labs, LLC www.coffmanlabs.com
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RECOMMENDATIONS
The G1-A will burn in over the first week or so, becoming more
mellifluous in the process. After that, you should not notice much
change.
We recommend that you play with various tubes to match your
personal taste and system. To assist you, here are some points to
consider, in order of importance:
1. 12AU7s provide the highest level of tube customization in
the phono stage. 1960’s and 1970’s USA production
12AU7s are a large improvement over current production
tubes. However, vintage Euro or old Black-Plate USA tubes
are most highly recommended.
2. 5AR4s can be changed for Euro GZ-34s to very good effect.
The main difference will be in improved tightness of the
bass.
3. 12AX7 phono stage, like the 12AU7, works very well with
vintage Black-Plate USA or Euro tubes.
4. 5687s are the last tube we recommend changing in your
“tube rolling” endeavors. TungSol Black-Plates are
considered premium by many, but the RCA Bronze-Plates
are also very good—though they tend to be a bit more
sensitive. Raytheon tubes are also very linear. The best
5687 tubes are the Sylvania Gold Brand, gold pin—however
these are very rare and also slightly microphonic.
Replacement tubes can be bought from a number of reputable tube
dealers, all of which can be found online. Coffman Labs has a large
stock of vintage Euro and USA tubes as well, and you may contact us
with any tube-related questions. We specifically screen all of our
appropriate tube types for use in the G1-A, so you may buy from us
with confidence at www.coffmanlabs.com.
Coffman Labs, LLC www.coffmanlabs.com
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SPECIFICATIONS
Inputs: 5-position Selector Switch:
Phono, Mute, S1, S2, or S3 selected at Front Panel
Phono: Rear Panel Switch selects type:
MM1 selectable impedance 47K / 70K / 95K
MM2 (95k) or MC based on purchase option
Mute: Shorts all inputs plus REC Out
S1 & S2: 100k input impedance
S3: 10K input impedance
Outputs: 5-position Selector Switch:
Out: (inverted) Direct output via Headphone jack
Note: Rear Panel Main outputs are disabled.
Low: (non-inverted) Selects Headphone jack (*32-100 Ω)
Hi: (non-inverted) Selects Headphone jack (*100-300 Ω)
Off: Shorts all outputs
On: (inverted) Selects Rear Panel ‘Main’ output jacks
-Output impedance: 600 Ω ‘Out 1’ and 1000 Ω ‘Out 2’
[Rear Panel REC Out (non-inverted) always present]
Gain:
Phono to REC OUT jack: MM=36 dB : MC = 52 dB
Max Phono Gain (Line out): MM=60.5 dB : MC = 76 dB
Line level output maximum undistorted: +14 dB
Max Headphone power: ½ watt at 32 Ω, ¼ watt at 300 Ω
Tube Complement:
2 X 12AX7 / ECC83 (RIAA Section)
2 X 12AU7 / ECC82 (Phono Boost)
2 X 5687 (Line Stage Output)
1 X 5AR4 (Power Supply)
* Let your ears be the judge—try both settings.
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Equalization: [measured into a 5 ohm load on MC input]
Passive RIAA, Paper-in-Oil (PIO) capacitors used for filters.
+0.5/- 0.2 dB 20Hz-20KHz (typical) 1Khz standard.
7 Hz rumble filter inherent in Phono Section
Frequency Response:
Line Stage: 20Hz–100KHZ +0.2/-0.5dB; 15Hz–150KHz + /-1.0dB
Headphone: -1dB at 15Hz; -2dB at 20KHz [Sennheiser 650]
Signal to Noise: (partially dependant on specific tubes used)
Phono: approximately 85+ dB
Line Stage: approximately 100+ dB
Power Supply filtering:
Separate external AC line filter not normally required
Line/Boost section supply hum down over 130 db
Phono section supply hum down over 150 db
Type: zero feedback multi-pole choke input
Heater Supply type: 7 pole filter. ~80 dB ripple reduction
Chassis Configuration:
Separate power supply on 4 foot cable.
110/120 or 220/240: Factory set per order
Purchase Options:
Moving Coil: Replaces one MM input with MC input.
Note: MC step-up is 15.5 dB, 0.3mV output
or higher recommended.
120/240 Voltage: Specify at the time of purchase.
Dimensions:
Amplifier: 12 ½” D x 7 ¾” H x 9 ½” W
Power Supply: 12 ½” D (with 5AR4) x 5 ½” H x 6 ½” W
Separate power supply on 4-foot cable
110/120 or 220/240: Factory set per order
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CONTACT & WARRANTY
Warranty:
3 years full coverage with normal use.
Tubes not covered by warranty.
No field serviceable elements.
Warranty is voided if the amplifier or power supply have
been opened.
Please contact us prior to returning unit for instructions.
Contact Information:
info@coffmanlabs.com
+1 503 223 2292 (Echo Audio)
+1 503 509 0390 (Coffman Labs)
Mailing Address:
Coffman Labs
c/o Echo Audio
1015 SW Washington St.
Portland, OR 97205
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