The Mini³ portable stereo headphone amplifier

Before you start

This section assumes that you have pre-drilled your front and rear panels (for the jacks, volume pot and LEDs). If you haven't done so, see the board and panels section for details and work on that first.

You can print out an image of the circuit board silkscreen layer (PNG format | PDF format), to use as a guide for installing components.

If you're building a Mini³ for use with the χ1 battery management board and LiPo batteries, some of the items listed in the parts list should be omitted. See the χ1 website for details. The instructions below assume a standard Mini³ with NiMH 9V battery.

Do not remove the opamps from their sealed packaging until you're ready to solder them on the board. This is to protect them from electrostatic discharge and moisture.

While you look at the board layout, please also take the time to look at the schematic diagram and associate each part with their location in the circuit. While this is not normally required to build a working amplifier, one of the opportunities of DIY is to learn about how the circuit works. Try to determine what each part does and why the particular part or value is chosen. There are many web resources to help you with this, including the AMB DIY audio forum. You will find the overall DIY experience more rewarding as a result.

Due to Mini³'s small size and close approximity of parts, as well as the two surface-mount (SMT) opamps, you should have the following tools and supplies to help you work on the board and case:
  • A good soldering iron with fine tip, preferably with adjustable temperature. For example, a Weller WLC100 with ST6 tip. A more deluxe soldering station such as the Weller Weller WES51 or Hakko FX888 is nice, but not necessary.
  • Liquid flux or flux pen for electronics soldering/rework.
  • Thin gauge solder, such as 0.025". Your choice of 60/40 or 63/37 tin/lead. Avoid silver solder as it requires high heat. Lead-free solders also require higher heat and their durability is still in question.
  • Fine tweezer with sharp points, such as those from Wiha
  • Desoldering braid
  • Chip Quik® SMD desoldering kit (if needed).
  • Vision aid, such as magnifier lamp, loupe, magnifying eyewear (e.g., Fisherman Eyewear Flip-n-focus), etc.
  • Needle nose pliers
  • Diagonal cutter
  • Cotton Q-tips
  • 99% isopropyl alcohol or specialty electronics flux remover spray
  • Emery board or thin file (for black or blue Hammond case only)
  • Imperial #6-32 or metric M3.5 tap (optional, if not using stock Hammond screws)
  • Multimeter with sharp probes

Circuit board assembly instructions

Clean both sides of the blank Mini³ board with paper towel and isopropyl alcohol or electronics flux remover, then solder the components to the board, beginning with the opamps for maximum access. If you're new at soldering surface-mount devices, please view the following video, an excellent tutorial. The opamps on Mini³ are easier than the examples in the video.

Surface Mount Soldering 101
by Curious Inventor

Do the U5 opamp first, as described below.

Apply some liquid flux to the opamp pads. Optionally, apply a very small dab of heatsink thermal compound below the opamp (this is recommended especially for the AD8397ARDZ opamp). Place the opamp over the pads, use the tweezers to pick and nudge the opamp until it is centered and aligned perfectly over the pads. Be sure the thermal compound does not ooze out the sides and the pin orientation is correct. The side of the opamp with pin 1 is beveled, and usually has a dot denoting pin 1. Viewing the board with the "Mini³" logo proper side up, the pin 1 pad of both U4 and U5 are at the bottom left side.

Press the tweezer tip on the top of the opamp to keep it from shifting while soldering. Apply only a tiny amount of solder to the tip of your iron, and tack down one corner pin of the opamp. I find that it helps to use a "wiping" motion of the tip on the pin and pad. If necessary, make small adjustments while heating that pad and pin again. If all is well, do the pin on the diagonally-opposite side. Then, do the remaining pins one at a time, reflow any pin that needs a bit of touch-up. If necessary, use the desoldering braid to remove any excess solder, and be sure there are no solder bridges between the pins.

When done, your solder joints should look something like this:


Use your multimeter and check the resistance between each pin of the opamp and the pad to make sure you don't have a cold solder joint (i.e., your meter should read close to zero ohms). Also check continuity between adjacent pins for solder bridges (meter should read infinity).

Repeat the above procedure for the U4 opamp. When you are done with the opamps, use a Q-tip and isopropyl alcohol, or specialized electronics flux remover to clean off the flux.

Now mount and solder the rest of the parts, starting with the lowest profile parts and work up, in the following order (see details below):
  • Resistors
  • Diodes
  • Multilayer ceramic capacitors
  • Voltage Regulators U1, U2
  • Rail-splitter chip U3
  • Ferrite beads L1L, L1R, L1G
  • Tantalum capacitor C2
  • Electrolytic capacitor C5+, C5-
  • Volume control potentiometer
  • DC power jack J1
  • 3.5mm stereo mini phone jacks J2, J3
  • Electrolytic capacitor C3a, C3b
  • LEDs
  • Battery contacts B+, B-

Set the volume control potentiometer to the power off position (rotate to fully counterclowise position until you feel a click) before you install it on the board.

Since the Mini³ circuit board holes are plated through, with the exception of the SMT opamps (or if you chose to use the SMT schottky diodes for D2 and D3), you only need to solder the parts from the bottom of the board. Do not drill or enlarge the holes because that would damage the through-plating.

Make sure the correct part goes into each position on the circuit board. Measure each resistor with your multimeter to ensure it's the proper value before installing it. Pay attention to the polarity of electrolytic and tantalum capacitors, diodes, voltage regulators and rail splitter chip. For electrolytic and tantalum capacitors, the positive lead should be the longer one.

The ferrite beads should be mounted elevated from the board surface, allowing at least 1/8" (3.2mm) clearance, as shown in the following picture:

The U2 voltage regulator should have its pins bent 90° downward for insertion into the board. You must check that the mounting hole is aligned properly while the pins are bent to fit their pads. Apply a small amount of heatsink compound to the bottom of the regulator to help it transfer heat to the board. No isolation pad or TO-220 mounting kit is needed. The imperial #4-40 or metric M3 mounting screw should have a low-profile pan head (so as not to touch the case when the board is installed), or you could use a nylon screw. The screw should be inserted from the bottom up. Secure the regulator using a hex nut from above (see photo above) before soldering the regulator's pins.

When soldering the DC power jack and 3.5mm stereo mini jacks, you should heat the solder tabs and pads sufficiently to allow solder to fill the entire hole. Remove heat immediately and cool the joint by blowing air at it as soon as you achieve a good fill. This assures that the jacks will be mechanically-secure.

For LED1, bend the LED leads 90°, paying attention to the polarity and which way it will face through the rear panel. Since the LED will be inline with the center of the DC power jack, insert the LED into the board until the LED is aligned properly. Solder one LED pin, then heat the pad and make small adjustments if needed. Then, solder the other pin.

For LED2, also bend the pins 90° (noting the polarity and direction), then install the front panel temporarily by placing it over the 3.5mm mini phone jacks and volume pot. Insert the LED into the board until the lens is aligned to the hole in the panel, and push it into the hole. Solder one LED pin, and make any adjustments as necessary. Solder the other pin.

The easiest way to solder in the battery contacts is to first attach them to a 9V battery (preferably a dead one), making sure that the two contacts do not touch, and then carefully insert the contacts into the board. Be sure the battery positive and negative contacts go into the correct locations! They are marked clearly on the board, and should match the labels on the battery (Also see the photo above).

Before soldering in the battery contacts, you should check the fitment into the case. When the battery is snapped into the contacts, there will usually be about 1mm to 1.5mm gap between the battery and the top surface of the board. If the battery is too thick, the whole assembly may not fit into the case. In that event, you can file away a portion of the bottom of the battery contacts to eliminate the gap and obtain additional clearance. See the following diagram.

When soldering the battery contacts, it will help to first secure the battery and contacts to the board by wrapping several turns of rubberband around the whole assembly. As with the jacks, you should fill each hole with solder.

Please be aware that even a dead battery may have enough voltage to cause component damage if you accidently short circuit either battery contact to other points on the board, such as with DMM probes, pliers, or some other metallic tool. Handle the board carefully. After you're done soldering the battery contacts, remove the battery.

Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush. Using your diagonal cutter, trim all protruding pins, leads, solder tabs, etc., on the bottom side as close to the board as possible to prevent a short circuit to the case after installation.

Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. Correct any mistakes before moving on to the next phase.

Proceed to the initial check section to test the amplifier before connecting any headphones. When you are done with testing, continue to the section below for preparing and mounting the board in the case.

Preparing the case

This section assumes that you have already done the initial checks of your assembled board.

You may use the panel screws provided with your Hammond case, but they aren't as attractive as socket cap screws. Also, the Hammond screws are self-tapping, and are not designed to be removed and installed for many cycles. Eventually the screw threads would wear out.

For this reason imperial #6-32 or metric M3.5 flat-head socket cap screws are recommended. These screws should be at least 3/8" (9.5mm) long but no longer than 3/4" (19mm). Flat-head screws are required for the front panel to avoid interfering with the volume knob or a large headphone plug.

Optionally, you could use thumb screws of similar specifications for the rear panel to make removal easier.

If you use #6-32 or M3.5 screws, you must first tap the case to match. Taps for your portable electric drill or drill press could be purchased from a tools and hardware store.

If you use the screws provided by Hammond, installing and then removing the screws before the final assembly keeps you from getting pieces of aluminum all over the board.

The circuit board has a hole beneath the battery, near the version number marking. You can optionally tie a ribbon in a knot to the bottom side, and run it through this hole and over the top of the battery, from between the contacts toward the rear of the case. Once installed in the case, pulling on the ribbon will pop the battery out easily (after the rear panel is removed).

If you have a black or blue anodized Hammond case, you should grind away some of the anodizing from the board slot, in order for the exposed ground plane strips on the bottom of the board to make contact with the case. This provides RFI shielding. You do not need to do this with the clear anodized version.

The board will go into the bottom slot of the case. Using the emery board or thin file, carefully grind along the bottom surface of that slot on each side, until you could see bare aluminum showing through. Also, you should use the emery board or file to deburr the sharp inside edge of the case to prevent it from scraping the top of the battery as you slide the board into the case. When you're done, clean the case of all metal dust and shavings.

Insert the board into the slot (with a good battery installed), Be careful while doing this to prevent any part of the circuit from touching the case, especially the battery contacts and any protruding leads on the bottom of the board.

Use your multimeter to check the continuity between the sleeve ("input ground") of the inner 3.5mm stereo mini jack and a point where you could make good contact with the bare metal of the case (such as at the panel screw threads). You should get a low ohms reading. If not, remove the board, grind the slots further and repeat.

Install the front and rear panels (taking care to guide the LEDs into their respective holes), fasten the panel screws, install the volume knob, and you're done.

Connect your source and headphones, and enjoy the music!

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