The α24 fully-differential line amplifier

Initial check

After assembling your α24 boards, follow the procedure below to make sure that they are functioning properly. If, at any point you do not pass a check, do not proceed to the next step until the problem is first corrected.

Perform this procedure one α24 board at a time.

Make sure your regulated power supply is producing the correct output voltage. The recommended range is between ±8V and ±12V. Turn off the power supply and let all its capacitors discharge completely. Do not connect the power supply, input and outputs to the α24 board until the procedure calls for it.

Basic sanity check

  1. Do a visual once-over to make sure the correct part is installed in each location. Check that the opamps and the organic/conductive polymer capacitors are mounted in the proper orientation. Inspect all solder joints to make sure they are shiny and smooth. If in doubt, reflow any that doesn't look good.
  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.

    Use the J4 connector for these measurements. If you used Phoenix Contact MPT connectors, turn the screws fully clockwise and use the screw tops as the test points.

    • V+ and G
    • V- and G
    • V+ and V-

DC offset check

  1. Install a temporary wire jumper across IN+ and IN- at J1.
  2. Connect the power supply to the α24 board, making sure not to mix up the V+, G and V- wires.
  3. Turn on the power.
  4. With your multimeter set to DC mV mode, check the voltage between the following points listed below. Use the J2 connector for the first three measurements, and J3 for the fourth. If you used Phoenix Contact MPT connectors, turn the screws fully clockwise and use the screw tops as the test points. All of these should read less than a few mV DC (preferably less than ±5mV).

    • OUT+ and G
    • OUT- and G
    • OUT+ and OUT-
    • OUT and G

Noise check

  1. Keep the power turned on.
  2. Keep the wire jumper (installed in the previous step) in place on J1.
  3. Set your multimeter to AC mV mode and make the same measurements as in the DC offset check above. You meter should display a very low value mV AC (less than 0.1mV AC, or zero).

Finishing up

Repeat the above procedure for any additional boards. If this is the last board, then proceed with finishing up below.
  1. Turn off the power.
  2. Remove the temporary wire jumper from J1.
  3. Connect the proper inputs and outputs.
  4. Hook it up to your system and enjoy the music!

Notes

  1. Be sure your input source(s) do not have significant DC offset at their outputs. The α24 has no coupling capacitors and such DC offset will be amplified by the gain and appear at the output. For balanced input, a common mode DC offset (i.e., same DC offset voltage on IN+ and IN-, relative to G) is fine.
  2. 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 wait a moment to allow the capacitors to discharge. Stored charge in the capacitors could damage parts if board traces are short-circuited to each other by contacting other tools, objects or the soldering iron.
  3. 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. If you are trying to remove U2 and have polypropylene film capacitors around it, you may find it easier to remove one or more of these capacitors first to get better access to the opamp pins. 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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