Seventh Circle Audio J99-500 User manual
Seventh Circle Audio
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J99-500 Microphone Preamp
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The J99-500 is a kit version of the Jensen Twin Servo circuit, the basis for some very well
known and highly regarded microphone preamps. The J99-500 provides exceptional
performance with high gain, extremely low noise and distortion, plus the benefits of transformer
coupling at both input and output.
Who Should Build This Kit?
The J99 is not difficult to build, but it is not intended for absolute beginners. If you’ve never built
an electronic project before, 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 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
Oscilloscope
Signal generator
Page 2 of 16
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.
Soldering Technique
Make sure your iron's 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.
Page 3 of 16
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
damage the parts.
Page 4 of 16
4. Add the protection diodes D1 –D11. Diodes are polarized and must be installed the
right way around! The colored band on the diode matches the white band on the
silkscreen.
5. Insert the 1/4-watt resistors. Check the Bill of Materials (BOM) for help in reading the
resistor color bands. It's a good idea to actually measure each resistor with your DMM
as you place it on the board, just in case you've read it incorrectly. Don't rely on the
photos for component placement. If the resistor value silk-screened on the board
doesn't agree with the value on the schematic or parts list, follow the schematic.
Page 5 of 16
6. Add small, yellow ceramic capacitors These capacitors are not polarized and can be
installed in either direction, but pay close attention to the capacitor markings! Add
blue EMI filters L1 through L6. These parts are not polarized and can be installed in
either direction.
7. Add film capacitors. These capacitors are not polarized and can be installed in either
direction. They may be a different color and/or shape than in the picture.
Page 6 of 16
8. Add the 0.1" 2-pin and 3-pin headers now.
J5 connects a 604-ohm load resistor across the output. Unless you’ll be connecting the
J99 to a piece of older gear with 600 ohm input impedance, dont connect a jumper at
J5.
J6 and J7 –These headers allow the servos to be switched out of the circuit for
adjustment as described in the testing section below.
9. Add tantalum bypass capacitors C4, C5, C10, C11, C19, and C23. Tantalum
electrolytic capacitors are polarized and must be installed the right way around!
These parts can be damaged by static discharge and must be handled with care. It’s a
good idea to test them for shorts as you place them on the board. Be sure to observe
the correct polarity when installing tantalum capacitors. 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.
Page 7 of 16
10. Add power supply components: L101, C104, C105, C2, and U4.
11. Add trim pots R43 and R47. R43 and R47 are used in the bias current compensation
adjustment described later.
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12. Add Q7, U6 and U7. Be sure to install semiconductors correctly! These parts are
not the same and they are not interchangeable. Read the markings carefully and orient
the packages according to the silkscreen outlines. Don’t mix up the positive and
negative regulators and the transistor!
13. Add electrolytic capacitors C24, C25, C102, C108, and C109. Aluminum 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 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.
Page 9 of 16
14. The first components to place are the Mill-Max receptacles. Press them into the holes
using the point of a Philips screwdriver or similar tool. Be sure to support the board from
behind while applying pressure. Push the sockets in until they're flush with the
board, and solder them from the back.
15. Carefully mount the toggle switches SW2, SW3 and SW4. Be sure they're seated flat
on the board before soldering all of the pins. You may find it easier to solder the first pin
with the board component side up.
Page 10 of 16
16. Attach gain trim control R24. 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, but it isn’t necessary.
17. If it hasn’t been already, insert the stop pin in rotary switch SW1 at the position shown.
Push the pin in completely.
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