The Mini³ portable stereo headphone amplifier

The circuit board

The Mini³ circuit board is made of high quality FR-4 glass epoxy, double-layer with plated-through holes as well as silkscreen and solder mask. It is intended to slide into the bottom slot of a Hammond 1455C801 or 1455C802 extruded aluminum case. The board dimensions are 1.9875" x 3.1375" (50.48mm x 79.69mm) and is 0.062" (1.58mm) thick.

Photos of the top and bottom side of the circuit board are shown below. They are not in actual size.

The circuit board layout is shown below. The top layer is shown in red, the bottom layer is in blue, areas where there are traces in both the top and bottom layers are in lavender, and the top silkscreen is in light grey.

The following is a view of the silkscreen layer.

You may also view the following:

The front and rear panels

If you purchase the CNC-machined and engraved panel set from AMB, then you may skip this section and read the Notes below. The information here is relevant if you will be drilling your own panels.

Since all Mini³ panel components (DC power jack, input/output jacks, LED indicators and volume control potentiometer) are board-mounted, it is important that the holes on the front and rear panels be drilled accurately to match. The following diagrams and table shows the dimensions.

A2.1230" (53.924mm)
B0.9030" (22.936mm)
C0.1135" (2.883mm)
D0.4000" (10.160mm)
E0.6813" (17.305mm)
F0.9625" (24.448mm)
G1.6125" (40.958mm)
H0.0200" (0.508mm)
I0.2500" (6.350mm)
J0.3600" (9.144mm)
K0.6600" (16.764mm)
Hole diameters
Φa0.1600" (4.064mm), countersink cone diameter: 0.2500" (6.35mm)
Φb0.1142" (2.900mm)
Φc0.2550" (6.477mm)
Φd0.2800" (7.112mm)
Φe0.3300" (8.382mm)
Corner radius
r0.155" (3.937mm)
Panel thickness
t0.0591" (1.500mm) to 0.0787" (2.000mm)

The following PDF files may be printed and used as drilling templates, but their accuracy depends on the resolution of your printer. You should use a good, calibrated digital caliper to verify all measurements.

Notes about the front and rear panels

You may use the listed dimensions to drill the panels provided by Hammond, or fabricate custom panels. If you use a Hammond 1455C801 case (with metal end panels), it is recommended that you discard the plastic bezels provided, and mount the panel directly to the case. The bezel pads the panel about 0.065" (1.65mm) away from the edge of the circuit board, causing the front 3.5mm stereo mini jacks to become recessed, possibly preventing the plugs from seating properly. The plastic bezel also creates an unsightly gap around the rear DC power jack, and possibly preventing the DC power plug from making proper contact.

It is important that the panel screw holes be countersunk (see Φa in the diagrams above), so that flat-head screws could be used to secure the panel to the case. This assures that the screw head will not interfere with a large headphone plug. You may use the self-tapping flat-head screws provided with the Hammond case, or tap the case and use imperial #6-32 or metric M3.5 flat-head machine screws. A minimum screw length of 0.375" (9.5mm) is recommended.

In the Mini³, the case is connected to input ground via strips of exposed ground plane (along the bottom of the circuit board) against the case slots. If you have a metal front panel, you should use "isolated" 3.5mm stereo mini jacks as specified in the parts list. This type of jack's sleeve contact is electrically insulated from the front panel, and prevents a short circuit from the output ground to the input ground.

The DC power jack hole should be large enough to prevent the plug's outer conductor (which is V-) from touching the edge of the hole. Since the case is input ground, if the panel is metal and V- shorts to input ground (even for a short moment), the onboard TLE2426 rail splitter chip will be damaged.

You may omit the volume control potentiometer panel nut. Some knobs would rub against the nut unless mounted with a large gap to the panel. The potentiometer is securely attached to the board via eight solder joints and does not need additional reinforcement. Similarly, if you have a plastic front panel and are using non-isolated 3.5mm stereo jacks with a threaded collar, you may omit the panel nuts.

The indicator LEDs should be mounted with their leads bent 90° and soldered on the board. Measure the the amount of needed lead length carefully prior to soldering, in order for the LED to fit properly in their respective panel holes. Only T-1 (3mm) should be used for the hole sizes listed above.

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