Seventh Circle Audio C84 User manual

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Seventh Circle Audio C84 Microphone Preamp Rev 2.1
Based on the innovative “Double Balanced Microphone Amplifier” circuit published by Graham
John Cohen in 1984, the C84 microphone preamp offers exceptional performance in any
application where extremely low distortion and neutral sonic characteristics are desired.
Who Should Build This Kit?
The C84 is not difficult to build, but it is not intended for beginners. If you’ve never built an
electronic project before, this is definitely not the one to start with. To guarantee success, make
sure you have:
•The ability to make basic voltage and resistance measurements using a digital
multimeter (DMM).
•At least a rudimentary understanding of voltage, current, and resistance.
•Some experience soldering on printed circuit boards.
•The patience to follow instructions precisely and work carefully.
Essential Tools
Fine tipped, 20-30 watt soldering iron w/ cleaning sponge (Hakko 936 or similar)
Eutectic (63/37) rosin core or “no clean” solder (.025” diameter or smaller is best)
Good-quality DMM
Small needle nose pliers
Small diagonal cutters
Phillips screwdriver (#1)
Precision straight blade screwdriver (for adjusting potentiometers)
Highly Recommended Tools
Lead bender (Mouser 5166-801)
T-Handle wrench and 4-40 tap (Hanson 12001 and 8012)
MOLEX crimp tool (Waldom W-HT1919 or equivalent)
Magnifying glass
Optional Tools
Panavise w/ circuit board head
Oscilloscope
Signal generator

C84 Assembly Instructions
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Work Area
Find a clean, flat, stable, well-lit surface on which to work. An anti-static mat is recommended
for this project. If you’re in a dry, static-prone environment, it’s essential. The importance of
good lighting can’t be overstated. Component markings are tiny, and you’ll be deciphering a lot
of them.
Soldering Technique
Make sure your irons tip is tinned properly and keep it clean! The trick to making perfect solder
joints is to heat the joint quickly and thoroughly before applying the solder, and a properly
tinned and clean tip is essential for this. Apply enough solder to form a "fillet" between the lead
and the pad, a little mound of solder that smoothly transitions from the plane of the board up to
the lead, but don’t use too much. The finished joint should be smooth and shiny, not rough or
gritty looking.
If you've never soldered a board with plated-through holes, you might be surprised to discover
how difficult it can be to remove a component once you've soldered it in place. If you're using
solder wick to correct a mistake, be very careful not to overheat the pads, since they will
eventually delaminate and "lift". It's often better to sacrifice the component and remove its
leads individually and start over with a new part. If for some reason you need to unsolder a
multipin component (like a rotary switch or integrated circuit), remove as much solder as you
can with solder wick or a solder sucker, and then use a small heat gun to heat all the leads
simultaneously. With care, you can remove the component without damaging the board.
Instruction Conventions
Text in orange indicates a step where extra care needs to be taken. Doing it wrong isn’t a
disaster, but it’ll need to be corrected.
Text in red indicates a step that must be done correctly. Doing it wrong will guarantee
improper operation, and probably damage components and/or the circuit board.

C84 Assembly Instructions
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Assembly
1. Before you begin, carefully unpack the kit and examine the parts. Check the contents of
each small bag against the BOM to make sure all the parts have been included. If you
think something’s missing, please e-mail the details to [email protected] and
we’ll ship replacement parts ASAP.
2. Generally, the idea when "stuffing" or “populating” a circuit board by hand is to start with
the lowest profile parts, such as the resistors, and work your way up to the taller
components. In each step below, insert the components, flip the board onto your work
surface component-side down, and carefully solder and trim the leads. Use a piece of stiff
cardboard to hold the parts in place while you flip the board. First, orient the board as
shown.
3. Before installing the resistors, prepare the leads using small needle nose pliers or a lead-
forming tool as shown below. Whatever you do, don’t bend the leads at the resistor
body and force them into the board.This not only results in an ugly job, it can also
damage the parts.

C84 Assembly Instructions
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4. Insert the 1/4-watt resistors. Check the Bill of Materials (BOM) for help in reading the
resistor color bands. It's also a good idea to actually measure each resistor with your DMM
as you place it on the board, just in case you've decoded it incorrectly. Don't rely on the
photos for component placement. THE PCB PICTURES ARE NOT NECCESARILY THE
CURRENT REVISION! If the resistor value silk-screened on the board doesn't agree with
the value on the schematic or parts list, follow the schematic.
5. Next, add the protection diodes D1 through D19. Diodes are polarized and must be
installed the right way around! The colored band on the diode matches the white band
on the silkscreen.

C84 Assembly Instructions
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6. Add the small ceramic capacitors. These capacitors are not polarized and can be installed
in either direction, but pay close attention to the capacitor markings! These parts all
look alike, but they are not interchangeable. Putting one in the wrong spot will not prevent
the preamp from passing signal, but it can seriously impair its performance.
7. Install Q1 and Q2 now. Start by flowing a small amount of solder on two pads only as
shown.

C84 Assembly Instructions
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8. Carefully position the part over the pads and re-flow the solder to tack it in place. Pay
close attention to the IC markings. Align the dot on the IC with the dot on the
silkscreen outline!
9. Flow solder onto the rest of the pads. Use a flux pen or liquid flux for best results.

C84 Assembly Instructions
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10. Install U1, U2, and U3 now. Pay close attention to the IC markings. Align the notch
on the IC with the notch on the silkscreen outline! IC sockets are not included in the
kit and are not recommended. Socketing the ICs will interfere with the ability of the PC
board to conduct heat away from the parts.
11. Install the ferrite filters now. These parts are not polarized and can be installed in either
direction.

C84 Assembly Instructions
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12.
Unless you are installing this into a PC01 One Shot, jumper P1+ to M+ and P2- to M-
using pieces of trimmed leads. Solder into place. THE INPUT CIRCUIT IS NOT
COMPLETE WITHOUT FOLLOWING THIS STEP!!!
13. Install the 1uF film caps at C10 and C11. These parts are not polarized and can be
installed in either direction.

C84 Assembly Instructions
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14. Add small transistors Q3, Q4, Q7, and Q8. These parts are not the same and are not
interchangeable. Align the flat side of the transistors with the flat side of the
silkscreen outline.
15. Add Q11 and Q12. Again, these parts are not the same, and are not interchangeable.
Align the flat side of the transistors with the flat side of the silkscreen outline.

C84 Assembly Instructions
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16. Add the trim pots R64 and R68. R64 and R68 are used to adjust the on-board voltage
regulators as described later.
17. Add tantalum capacitors C34 and C38. Tantalum electrolytic capacitors are polarized
and must be installed the right way around! Be absolutely sure to observe the correct
polarity when installing these parts. The positive leads of the tantalum caps are marked
with a small "+" sign. The positive pads on the circuit board are marked with a small "+"
sign.

C84 Assembly Instructions
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18. Add the short electrolytic capacitors now. C23 and C30 are not polarized, but the rest are
and must be installed the right way around! Be absolutely sure to observe the correct
polarity when installing these parts. The negative leads of the electrolytic caps are
marked with a colored stripe. The positive pads on the circuit board are marked with a
small "+" sign.
19. Add transistors Q5, Q6, Q9, and Q10. Make sure to install the transistors correctly!
These parts are not the same and are not interchangeable. The heavy line on the
silkscreen outline indicates the back of the transistor, the side without markings. Make
absolutely sure you don’t insert them backwards.
20. Carefully mount the toggle switches SW2 and SW3. Be sure they're seated flat on the
board before soldering all the pins. You may find it easier to solder the first pin with the
board component side up.

C84 Assembly Instructions
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21. Add J3, the MOLEX power connector. Be sure to orient it as shown, with the locking tab
away from the edge of the board.
22. Add electrolytic capacitors C4, C7, C39 and C40. Again, electrolytic capacitors are
polarized and must be installed the right way around! Be absolutely sure to observe
the correct polarity when installing these parts. Add polypropylene capacitors C5 and C6
as well.

C84 Assembly Instructions
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23. Add large capacitors C5 and C6. Be sure to not overheat the leads, as these parts are
susceptible to damage from excess soldering heat.
24. Using the hardware supplied, attach heat sinks to U5 and U6 and solder them in place.
Make sure to install the regulators correctly! These parts are not the same and are not
interchangeable.

C84 Assembly Instructions
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25. Install rotary switch, SW1. Make sure the switch is fully seated and solder it to the board.
Try to make your solder joints as neat as possible, and don’t use too much solder.
26. Attach gain trim control R37. Make sure the control is seated flat to the board before
soldering the leads. You may want to add a small dab of silicone adhesive to the bottom
of the control to hold it more securely.

C84 Assembly Instructions
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27. Add CONN1 and CONN2 to the board. Make sure they’re fully seated before soldering.
28. Install the bulk filter capacitors C31 and C35. Push them in firmly until they are fully
seated against the board. Again, electrolytic capacitors are polarized and must be
installed the right way around! Be absolutely sure to observe the correct polarity when
installing these parts.

C84 Assembly Instructions
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29. Before going on to initial power-up, carefully check your work. Make sure you haven't
created any solder bridges between pads, or between a pad and the ground plane.
Initial Power-Up and Testing.
30. Again, carefully check your work. Make sure you've got the right resistors in the right
locations. Make absolutely sure you've got all the transistors, diodes, and capacitors
soldered in the right way around! Double check to make sure you haven’t inadvertently
swapped a transistor or voltage regulator. Check for poor solder joints and solder bridges,
and make sure you fix any problems before continuing.
31. Just to make sure you haven't created any blatant shorts, measure the resistance between
pins 1 and 2 of J3. Do the same for pins 3 and 2. If you measure a steady resistance of
less than 100 ohms, don't apply power. Carefully check your work until you find that short.
32. Turn R64 and R68 counter-clockwise 25 full turns, or until you hear a soft click with every
turn. This sets the regulators to their lowest voltage, about +/-14V.
33. Connect the PS03 to J3 on the C84 using a WH01 wire harness or similar. Verify that
there are no crossed wires or loose crimps in the harness. Be sure the locking ramps
engage.
34. Set your DMM to measure DC voltages of 20V or greater and turn on the power. Clip the
negative meter probe to the PGND test point at the bottom of R67 and measure the
voltage at the +B test point at C34. You should see about +14V.
35. With the negative probe still connected to PGND, measure the –B test point at C38. You
should see about -14V.

C84 Assembly Instructions
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36. If the voltages measured in the two previous steps are off by more than a volt or so, you
have problems. Possible things to check are incorrectly installed diodes D13-D18,
backwards caps C34, C38, C31, and C35, or shorts around U5, and U6.
37. With your DMM still set to read DC voltages of 20V or greater, adjust R64 and R68 for
+18V at +B and –18V at -B. If you need more headroom, the OPA2604s will withstand as
much as +/-24V.
38. Verify that the common mode servo is working by measuring the voltage at the junction of
R29 and R30 as shown. You should see a voltage within a few millivolts of 0V.
39. Verify that the differential mode servo is working by measuring the voltage between R35
and R38 as shown. You should see a DC voltage below 50mV.

C84 Assembly Instructions
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40. Verify that the output buffer is working by measuring the voltage between R50 and R62 as
shown. The voltage should be similar to what you measured in the last step.
41. Congratulations! You've got a working C84 preamp.