The Mini³ Portable Stereo Headphone Amplifier

Initial check

Warning: Handle the assembled board with utmost care, especially when the battery is attached. When making measurements, do not allow the test probes, soldering iron tip or any other objects to cause a short circuit between the battery contacts and any other part of the circuit, or between circuit traces, even if the power if turned off. Treat the board as one with live voltages on it, because it is. Even a brief short circuit could damage the opamps. Also, take care not to allow sharp meter probes from piercing through the circuit board soldermask and shorting to the ground plane when making measurements.

The steps below applies to a standard Mini³ with 9V NiMH battery. If you are building a Mini³ for use with the χ1 battery management board and LiPo batteries, please refer to the alternative initial check procedures found at the χ1 website.

Keep the Mini³'s power turned off until called for. While testing, be very careful not to short-circuit any part of the board to each other, especially the battery contacts. Do not connect any sources or headphones to the input and output jacks.

You should use a well-charged battery for these tests. A 7-cell "8.4V" battery should measure at least 8.8V DC across its terminals, and an 8-cell "9.6V" battery should show at least 10V.

If, at any point you do not pass the check, do not proceed to the next step until the problem is first corrected.

Sanity check

  1. Remove the battery from the Mini³ board, then turn on the power. You may see the power LED illuminate briefly and then fade out. This allows the electrolytic capacitors to discharge. Wait a few seconds and then turn the power switch to the off position. You should do this anytime you must check any part of the circuit with the power off (i.e., test a resistor or check continuity of a solder pad or trace).
  2. With your multimeter set to Ohms mode (use the lowest resistance range on a manual-ranging meter), check for short circuits between the points listed below. If you read less than a few Ohms for any of these, then there is a short circuit. Carefully locate the short (such as a solder bridge) and fix it.

    • Battery + and - contacts
    • U4 pins 7 and 4
    • U4 pin 7 and IG
    • U4 pin 4 and IG
    • U5 pins 8 and 4
    • U5 pin 8 and IG
    • U5 pin 4 and IG
    • Output jack tip (OL) to sleeve (OG)
    • Output jack ring (OR) to sleeve (OG)
    • Output jack tip (OL) to IG
    • Output jack ring (OR) to IG
    • Output jack sleeve (OG) to IG


    You can use the input jack sleeve contact as IG.

Quiescent current check


  1. Clip the battery's positive terminal to the Mini³ board positive battery contact, but leave the negative battery terminal unconnected (see photo above).
  2. Set your multimeter to DC mA mode (for a manual ranging meter, set to 200mA range), and connect the meter across the battery's negative terminal and the board's negative battery contact as shown. On most meters, the red probe lead must be connected to a different jack on the meter to measure current. Be sure that the meter's protection fuse is intact.
  3. Turn on the power, but keep the volume at its minimum position. The power-on LED should illuminate. Be very careful with this step if your multimeter probes are held by your two hands. A slip of the meter probe could cause a short circuit and damage an opamp.
  4. Observe the meter's readout for the amplifier's total quiescent current draw. For the Mini³ high performance edition, the typical current is about 25mA. The extended runtime edition is typically 10mA. Your results may vary slightly due to device tolerance. If your measurement is far below or above the typical figures, then your amplifier is not functioning correctly. Do not proceed any further and check your work carefully.
  5. Turn off the power. If the quiescent current reading was correct then you may install the battery to the board in the "normal" manner.
  6. Re-connect the red probe lead to the proper multimeter jack for voltage measurement (if applicable) for the next step.

Rail splitter check

This section checks the operation of the TLE2426 rail splitter chip.
  1. Turn on the power.
  2. With your multimeter set to DC V mode (a manual ranging meter should be set to 20V range), place the black probe on V- and the red probe on V+ (see the test points photo above). Remember what the reading was.
  3. Keep the meter black probe on V-, and put the the red probe on IG. You should read about half the voltage from previous step.
  4. If your results are correct, turn off the power and then continue to the next step.

Output DC offset check

Install the battery, and turn on the amplifier, but set the volume control to its minimum position. Set your multimeter to measure DC mV.
  1. With the meter's black probe on IG, check the voltages at OG, OL and OR. Use the output jack's sleeve, tip and ring solder pads.
  2. The measured voltages should typically be less than 5mV in each of these locations. The Mini³ high performance edition has lower DC offset. The extended runtime edition may exhibit somewhat higher output DC offset.
  3. Turn off the power, remove the battery, and discharge the rail capacitors by turning the power on again and wait a few seconds. The power LED may illuminate momentarily and then fade out.
  4. If you have high DC offset on any channel, check your soldering for cold joints and/or bridges. If the problem cannot be corrected by reflowing the solder joints, then the opamp associated with the channel with the high DC offset is most likely damaged. See notes below about how to desolder and replace an opamp.

Battery charging circuit check

This section checks the battery charging circuit for correct operation. The battery should be removed from the board.
  1. Connect your AC adapter to the AC mains, but do not plug its output into the Mini³ yet.
  2. With your multimeter in DC V mode (for a manual ranging meter, set to 200V range), measure the output voltage of your adapter at its output plug. Be sure that it's between 15V DC and 24V DC, and that the center pin is positive. If not, do not use this adapter.
  3. Set your multimeter to measure DC mA (for a manual ranging meter, set to 200mA range). On most meters, the red probe lead must be connected to a different jack on the meter to measure current.
  4. Connect the red probe to the positive battery contact and the black probe to the negative battery contact.
  5. Connect the AC adapter output plug to Mini³'s DC jack. The charge LED should illuminate.
  6. If the R8 resistor is the default value (75Ω), then the the meter should read approximately 16mA. If you used a different R8 value, then you should read the current that you set the charging system to. See the Parts list section for details.
  7. Switch the meter to measure DC V (for a manual ranging meter, set to 20V range). Reconnect the red probe lead on the meter to the proper jack for voltage measurement if necessary.
  8. Connect the red probe to the positive battery contact and the black probe to the negative battery contact.
  9. With the AC adapter still connected, you should read about 12V DC.
  10. Disconnect the AC adapter. If your measurements are OK, then you are done.

If you have completed and passed all the above checks, then your Mini³ is functioning properly. You can now proceed to the "Preparing the case" heading in the instructions section.

Notes

  • If you are going to be moving the board, desolder parts or otherwise work on the board after turning off the power, be sure to remove the battery. Then, turn the power on to discharge the rail capacitors. Battery power and stored charge in the capacitors could damage parts if board traces are short-circuited to the battery contacts or to each other by contacting other tools, objects or the soldering iron.
  • If a SMD chip becomes defective and must be replaced, and if you don't have specialized reworking equipment, the recommended method is to use Chip Quik® SMD removal kit. Their website has instructions on how to use it. Chip Quik is available directly from the manufacturer, as well as from Digi-Key (part number SMD1-ND).

    If you don't have Chip Quik, use a sharp knife to cut the leads off the bad chip near the chip body. Take care not to score the board. Then, remove the chip from the board. Heat each solder pad with your soldering iron and scrape away the residual pin. Then, remove any excess solder using your iron and a desoldering braid. Be careful with the iron so you don't melt any surrounding parts. Also, don't heat the board for prolonged periods of time to avoid delamination. Soldering in the new chip will be just like the first time. You need to be careful not to touch the other parts already in place with your iron.


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