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WAV Trigger fix for DC offset on speaker output

The initial run of the WAV Trigger is missing a capacitor on the input to the on-board speaker amplifier, causing a DC offset on the speaker output connector. At a minimum, this causes a click on enabling the amp and clipping for high volume output signals, and in some cases interferes with operation of the WAV Trigger when certain speakers are connected. The board design has already been updated and the fix will make it’s way into the production pipeline soon. In the meantime, for those of you utilizing the on-board speaker amp, here’s the fix. You do not need to do this if you aren’t planning to enable the on-board amplifier (it’s disabled by default.)

You’ll need to add a series 0.39uF non-polarized capacitor to the to the input of the LM4990 amp, between LOUTL and R9. The easiest way to do this is to cut a single trace on the back of the board, scrape away the solder mask from the ends of the trace near the cut, and solder the cap across the resulting gap. See photos below. I apologize for the inconsistent PCB color (green) in the second photo – I wanted to post this ASAP.

First, a photo showing the location of the trace on a production board:

wtpatch01

 
And here’s a photo of the capacitor, in this case a ceramic SMT cap, soldered across the gap in the trace. If you happen to have a through-hole cap available, you can solder the leads to the vias at either end of the trace (after you cut it.)

wtpatch02

Adding this capacitor completely fixes the issue, eliminating any popping when the amplifier is enabled or disabled, and allowing you to get the full audio volume range from your speaker.

jamie@robertsonics.com

  • reply Rob ,

    Hi,
    i bought one of you’re boards 9wav trigger) and its working perfect with line out. However i’d like to use the onboard amplifier.
    Can I also use a 0,33 Uf capicitor?

    Thanx in advance

    • reply jamie@robertsonics.com ,

      Yes – you should add the capacitor before enabling the on-board amp and connecting a speaker. The value isn’t terribly critical – if you don’t have a 0.39uF cap, either a more common 0.33uF or 0.47uF will work.

      I also recommend that you power the WAV Trigger with 9V or less, to keep the 5V regulator from getting too hot. The on-board amp can draw significant current at high volumes. I use a 6V supply when using the amp.

    • reply Cindy ,

      Hi,

      I actually tried to amplify the sound before I saw the message and now the wav trigger is kind of out of order. It works for the first two or three sounds and then can’t make any sound at all. When I re-plug in the battery again it starts the same pattern– plays three sounds and then fail to play. Any advice how to fix this?

      Thanks!

      • reply John ,

        How do you know if you need to do this? You mention that this is a problem with the initial run of the board. How do I tell if the board I bought is part of that run? Is there a later run that does not have this issue?

        • reply jamie@robertsonics.com ,

          SparkFun has not started shipping the revised design. Unfortunately, their first run was rather large, and it’s taking much longer than I had hoped to sell through. Please feel free to contact them directly about this – it can only help.

        • reply Mark ,

          It looks like SparkFun have a version of your board now with the fix: https://www.sparkfun.com/products/12897
          They are still selling the older one which needs the capacitor inserted for the audio amp between LOUTL and R9: https://www.sparkfun.com/products/12000 but at least the older one is a few dollars less. The schematics they post for each appear to confirm the fix has been made in the newer model.

          • reply jamie@robertsonics.com ,

            This is correct. The older one is a great deal if you don’t plan to use the on-board amp. The new one has the required cap as well as better heat dissipation for the 5V regulator.

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