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Seventh Circle Audio
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 Microphone Preamp Rev 2.1
Pag e 1 o f 18
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Related Manuals for Seventh Circle Audio C84
Summary of Contents for Seventh Circle Audio C84
- Page 1 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: •...
- Page 2 C84 Assembly Instructions 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.
- Page 3 C84 Assembly Instructions Assembly 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 sales@seventhcircleaudio.com...
- Page 4 C84 Assembly Instructions 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.
- Page 5 C84 Assembly Instructions 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.
- Page 6 C84 Assembly Instructions Carefully position the part over the pads and re-flow the solder to tack it in place. close attention to the IC markings. Align the dot on the IC with the dot on the silkscreen outline! Flow solder onto the rest of the pads. Use a flux pen or liquid flux for best results.
- Page 7 C84 Assembly Instructions 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.
- Page 8 C84 Assembly Instructions 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.
- Page 9 C84 Assembly Instructions 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.
- Page 10 C84 Assembly Instructions 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.
- Page 11 C84 Assembly Instructions 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.
- Page 12 C84 Assembly Instructions 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...
- Page 13 C84 Assembly Instructions 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.
- Page 14 C84 Assembly Instructions 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.
- Page 15 C84 Assembly Instructions 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,...
- Page 16 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.
- Page 17 C84 Assembly Instructions 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.
- Page 18 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. Page 18 of 18...
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