• The Build and The Fail

    Todd06/09/2017 at 20:12 6 comments

    Now that I had all my parts chosen, and working on my desk with a slew of alligator clips, it was time to assemble it all together. In rough order:

    • I removed the old voltage and signal test leads.
    • Superglued the pot to the bottom edge, after bending the pins/supports in a useful way. I had originally tried hot glue here but it wasn't strong enough (or at least I was too impatient to let it fully cure).
    • Soldered in the resistors between the signal lines and the pot.
    • Hot glued the amp board on. While there's no real exposed connectors, the hot glue also forms a "pad"/air-gap ensuring nothing's shorting.
    • I cut to length and soldered the wires between power, the pot, and the amp.
    • Hot glued the speaker on, and cut and soldered it's wires.

    Everything looked better than I expected.

    Then I turned it on...

    And nothing.

    I tried:

    • unsoldering power and signal lines
    • checking for sloppy shorting soldering
    • checked for burned out components
    • charging the battery/trying a different one
      • at best I can get only a quick flash of LED1
    • tried USB power
      • LED1 blinks repeatedly, but no screen
    • I broke out the VOM and measure:
      • With battery: 3V3 = 0V and 5V = ~0.5V
      • With USB: 3V3 = 0V and 5V = ~4.8V

    So I suspect something's wrong with the power regulator circuit.

    At this point all the easy troubleshooting didn't resolve it.... not sure if/when I'll get back to this.

  • Speaker Selection

    Todd06/09/2017 at 07:51 0 comments
  • It's too hot, so pot

    Todd06/08/2017 at 04:18 2 comments

    Galaga (which happens to be one of my faves from my youth) turned out to be the loudest.

    So I just threw my super convenient Xminilab Portable scope (affiliate link) on it, and wired in a 100K potentiometer to manually find the the point where the signal into the amp wouldn't exceed the clipping voltage (see prior post). If you don't know, you don't just wire the pot as a resistor, you connect both the signal wire and signal ground, to create a voltage divider. Sometimes a voltage divider in an audio usage is referred to an L-pad (because of the shape of the schematic) but often that's only used for speaker-level control, it seems.

    This worked: R1 = 65.5K, and R2 = 27.2K, for a R2 / (R1 + R2) ratio of 0.293. I suspect the amp's impedance was having a significant effect.

    That worked out well but I found a weird effect, if I turned down the volume too low (R1 = 88K and R2 = 5K) I would get a weird "machine gun" effect over the audio. Listen here:

    This is where I admit I'm not so good with AC circuits....

    So my next thought was if R2 was too low so I tried putting a number of different resistors in series with R2, but while that seemed to work, it also prevented it from getting very quiet. So I increased R1, which again led to "machine gunning" at low levels, so I increased R2 again, and so forth. Some EE is probably laughing at me right now... but after a bunch of experiments I realized I couldn't both minimize the volume on one end *and* maximize the volume on the other end.

    To complicate matters I didn't want to use the full-size potentiometer I was experimenting with so I popped over to All Electronics and found something suitable (see main completed image) but not exactly what I wanted. What I wanted was something like the volume control you'd see on an old transistor radio.... but I didn't know until later that's called a Thumbwheel Pot.

    That pot turned out to be 150K (actual 144K).

    For best loudness without distortion at low end:

    • R1S (in Series with R1) = 150K (146.7K actual) and R2S = 18K (17.49K)

    For best softness although giving up max loudness:

    • R1S = 500K and R2S = 4.3K (No actual values recorded)

    So I went with the later... next to choose which speaker.

  • The Amp

    Todd06/08/2017 at 00:53 0 comments

    I had on hand some of these 10Pcs XPT8871 5V Mono Channel Digital Audio Lithium Amplifier Board for Arduino (affiliate link) and they were the right size so I'd give them a try.

    Since I was aiming for 1W 8ohm speakers, I was wondering what the peak voltage I might see would be:

    V = (P * R) ^ 0.5, or (8) ^ 0.5, or 2.83V peak

    On the badge there's a choice of 3.3V or 5.0V.

    Amp Specs:

    Specs from eBay:

    • Chip: XPT8871
    • Input: DC 5V 1A
    • Output power: 5W
    • (also a schematic of the board)

    Doing some more research I came across this link which corrects and expands on that:

    • Input: 2.0V - 5.5V
    • 5W (2Ω load) and 3.5W (3Ω load), 3W (4Ω load)
    • AB / D switching (Judging by the image of the board, it looks like "Mode" is connected to GND so it's in A/B mode)
    • Compatible with LM4871 (although this seems suspect)

    There's also a link to the datasheet but it's mostly in Chinese so it's only so helpful.

    While the charts in the PDF are in English, they're not numbered, but here's some choice notes:

    • For VDD = 3.3V, RL = 8 ohms, THD+N shoots up past 0.4W
    • For VDD = 5V, RL = 8 ohms, THD+N shoots up past 1W

    If one is to believe it's compatible with the LM4871, the datasheet offers more information, particularly:

    • Figure 22 - clipping voltage for 5V is ~0.3V, and for 3.3V is ~0.2V


    At first I tried the 5V source but it was way too noisy and had drop outs - I thought about filtering and adding a cap to get past the drop-outs, but I decided to try the 3V3 source first. I think the issue with 5V is because of the MP3401 Boost circuit, see the description and schematic towards the end of this post.


    This worked much better but I could tell I was over-driving the input to the amp and getting distortion. I'll tackle that in the next post.

  • The Speakers

    Todd06/06/2017 at 04:44 0 comments

    I happen to have a number of small low-power speakers "in stock". From Left to Right:

    I connected a couple of them up directly to the "line out" connections - it was barely audible. An amp will be needed... in the next post.

  • Identifying the parts, and an Easter Egg

    Todd06/06/2017 at 03:21 0 comments

    The Connections

    Not that you can't mostly figure these connections out without flipping it over ... but I've also identified the line out signal and GND.

    The Easter Egg

    When talking to one of the creators at LayerOne he let me know there's an unpopulated LC circuit for a low-pass filter on the board, to keep down excessive treble. I haven't seen a schematic from theme, but it seems likely it's this: