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Seventh Circle Audio
Based on the BA183 amplifier circuit used in the 1066, 1073, 1272, and other Neve console
modules, the N72 microphone preamp will deliver the same immediately recognizable
transformer-colored character to your recordings.
Who Should Build This Kit?
The N72 is not difficult to build, but it is not intended for beginners. You should have built at
least one project on a printed circuit board (PCB) before trying the N72. Sorry, but soldering
cables doesn't count. If you've never built an electronic project of any kind, this is probably 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 multi-
meter (DMM).
• At least a rudimentary understanding of Ohm's Law and the relationship between
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 is usually best)
Good-quality DMM
Small needle nose pliers
Small diagonal cutters
Wire stripper
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
1/4" nut driver
5/16" nut driver
Oscilloscope
Signal generator
Page 1 of 19
N72 Microphone
Preamp
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Related Manuals for Seventh Circle Audio N72
Summary of Contents for Seventh Circle Audio N72
- Page 1 Who Should Build This Kit? The N72 is not difficult to build, but it is not intended for beginners. You should have built at least one project on a printed circuit board (PCB) before trying the N72. Sorry, but soldering cables doesn’t count.
- Page 2 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 highly recommended. The importance of good lighting can’t be overstated. Component markings are tiny, and you’ll be deciphering a lot of them.
- Page 3 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 we’ll ship replacement parts ASAP.
- Page 4 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 damage the parts.
- Page 5 Putting one in the wrong spot may not prevent the N72 from passing signal, but it could seriously impair its performance. Add the large resistors at R40 and R41. Add blue, 3-pin filters (For Rev 3.4) Add electrolytic capacitor C10.
- Page 6 Add the tantalum capacitors C6, C12, C14 and C16. Tantalum electrolytic capacitors are polarized and must be installed the right way round! 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 "+"...
- Page 7 12. Add Q7. This part is not the same and are not interchangeable with the BC184C transistors. You must read the part numbers! Align the flat side of the transistor with the flat side of the silkscreen outline. Do not push the transistors too far into the board. 13.
- Page 8 15. Install tantalum capacitor C26. Again, tantalum capacitors are polarized and must be installed the right way round! Be absolutely sure to observe the correct polarity when installing these parts. The positive leads of the tantalum caps are marked with a small "+"...
- Page 9 17. Install C11. Again, electrolytic capacitors are polarized and must be installed the right way round! 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 "+"...
- Page 10 19. Add C3, C29 and C30. Again, electrolytic capacitors are polarized and must be installed the right way round! 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 11 25. Add CONN1 and CONN2 to the board. Make sure they’re fully seated before soldering. Use the BLACK ¼” screws with the XLR’s!!! 26. Using a #4-40 x ¼” screw and a #4 Keps nut, attach the small heat sink to voltage regulator U1.
- Page 12 27. Install Q6 and large heat sink. Place the transistor into the heat sink first, and then maneuver the leads into the PCB. Note that the holes in the heat sink are offset to match the leads on the 2N3055. Don’t mount the heat sink backwards.
- Page 13 30. Solder the leads as shown. The color code is screened onto the PC board. BROWN, VIOLET, RED, GREEN, BLUE, ORANGE, BLACK, YELLOW 31. Install filter capacitor C23. Push it in firmly until fully seated against the board. Again, electrolytic capacitors are polarized and must be installed the right way round! Be absolutely sure to...
- Page 14 32. Bend the tabs on the input transformer mounting bracket as shown. 33. Attach the input transformer to the bracket using one of the existing screws on the front… Page 14 of 19...
- Page 15 34. and two #4-48 screws on the back. 35. Add J5, J6 and Conn 4 to the small PCB. 36. Place the PCB onto the pins of the blue input transformer, making sure that pins 2-6 are on top and 7-11 on the bottom. Solder the pins to the PCB.
- Page 16 41. Maneuver the leads into their respective holes on the PCB. 42. Bend the mounting tabs over until the bracket is secure. Solder the tabs and transformer leads as shown. Page 16 of 19...
- Page 17 46. Connect the output of your PS03 to J1. Wire the power supply connectors together in a 1:1 fashion. That is, PS03 J2, pin 1 to N72 J1, pin 1, pin 2 to pin 2, etc. Apply power while keeping your finger on R40. If R40 gets too hot to touch or starts to smoke, disconnect the power immediately and check for mistakes.
- Page 18 N72 Assembly Instructions Fine Bias Adjustment (if you have an oscilloscope). This step is optional. With a signal generator, apply a 0.5Vpp, 1KHz sine wave to pins 2 and 3 of CONN1, the XLR input connector. Rotate S1 fully CCW (minimum gain), then click back 5 positions CW.
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Page 19: Jumper Settings
Jumper Settings 50. J2 gives the option to connect a 604-ohm load resistor across the output. Unless you’re connecting the N72 to a piece of older gear with 600 ohm input impedance, install a jumper at J2. 51. J5 and J6 allow input impedance to be switched from 300 ohms to 1200 ohms. Use a single jumper across J5 to J6 (P2+ to P1-) for 1200 ohms (recommended).
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