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I'm still learning some of this stuff. This is the first time I'm really using an oscilloscope in a few decades. I picked up a small bluetooth only audio amplifier board. Puts out a whole lot of power, 40 watts/channel to 4 ohm speakers. Uses a TPA3116 Audio Amp Chip. The board requires 18 to 24vDC input.

BlueTooth Audio Amp

Unfortunately, I hear a lot of unwanted noise at low volume levels, or when ever there is a pause in a song. It's pretty irritating. Its more than simple white noise.

So I'm trying to figure out if my power supply is causing the issue. I'm using an old Compaq laptop switching power supply, circa 2000. The power supply puts out 19v, 3amps. Here's a trace of the power from that supply, under load with the audio board displayed.

Oscilloscope Trace of Input Power

On that trace, the time division is set at 1 micro second. The vertical division is set at 0.5v / div.

I'm not sure the best way to test for this. Build a filter and see what happens?

If I wanted to filter out that ripple, what type of filter design would I use? Any hint on how to size the components to handle the appropriate current / power? Many thanks.

Update: So I borrowed a friends bench top power supply. It was of pretty nice quality, and included a current display (as well as current limiter) I initially set it to 20volts DC, then hooked it up to the bluetooth amp. Uh-oh... I got nearly an identical display as before. The peak to peak wasn't quite as high (only 0.5v) but the shape of the curve and the time division was identical. Oh, and I did note that the amp meter registered 0.1 amp during normal audio steady state. There was no change to the undesired noise at low volumes.

As I played a bit with the audio volume, I was surprised to see the trace expand in height during heavy beats or loud portion of music. This got me to thinking.. with the oscilloscope, I'm just taking the measurement of a system. Who's to say that the trace I'm seeing doesn't represent the source minus the load of the audio amp. (I know those aren't exactly the correct words, but I think you get my meaning...)

So I went back to my original power supply. I added a 1/4 watt 3.28K ohm resistor across the output (6 mAmps) The output trace of that looks nearly like a sine wave, with a peak to peak voltage height of 0.040 volts, and a cycle at 285 KHz. That seems much more reasonable as the DC output for a switching power supply circa 2000CY.

power supply / resistor load

So my bottom line now is: I don't think the power supply is inducing the unwanted audio noise at all. I believe that is coming out of the blue tooth signal processing in the unit. I suspect mediocre board design. I do see nearly identical boards for sale, with additional capacitors on them (design improvement, phase 2?) ...Sigh.

update, 15 Jan 2016 @ruminant1 you are my hero. Much improved. Not perfect, but much improved. Many thanks. @ruminant1 filter fix

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  • \$\begingroup\$ I recommend you watch the eevblog videos to measure power supply noise \$\endgroup\$
    – PlasmaHH
    Nov 28, 2015 at 21:09
  • \$\begingroup\$ Being a class-D amp, it could well be the amp itself making the noise. See irf.com/technical-info/appnotes/an-1071.pdf for more that Wikipedia. \$\endgroup\$
    – Fizz
    Nov 28, 2015 at 21:19
  • \$\begingroup\$ You could try to filter the supply, but the scope image does not look like smps noise. If you wanted to filter, build a LC filter with a large cap and small inductor to give low output impedance, make the cut off freqency low like 1khz. \$\endgroup\$
    – MadHatter
    Nov 28, 2015 at 21:29
  • \$\begingroup\$ @MadHatter can you justify what you say about class d amps not being sensitive to noise on the power rails? \$\endgroup\$
    – Andy aka
    Nov 28, 2015 at 21:31
  • \$\begingroup\$ @Andy aka no I can not, I thought a pwm square wave output would be noise intolorant for the most part... But your response makes sense so I'm going to assume your correct. \$\endgroup\$
    – MadHatter
    Nov 28, 2015 at 21:34

3 Answers 3

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I was having the same problem. I tried a few grounding strategies to no avail. I then looked at the application notes for the Texas Instruments TPA3116 amp and noticed that they recommend decoupling capacitors of at least 100uF near the power pins on both ends of the chip. These caps are absent on this board.

Looking around eBay, I noticed one that doesn't have the big caps on the TPA3116 either, but it has a 10uF cap on the power pin of the op amp. I had one lying around so I soldered it between the resistor it sits next to and the ground plane and now I consider the amp usable.

I used a 10uF 16V cap because that is what I had. The board on ebay uses a 50V cap. Because the op amp runs at 5V, that seems like overkill.

I will probably add the recommended caps on the TPA3116 to see if I can reduce the little remaining noise and maybe help the low end a bit.

For clarity, here is a pic of the cap I added: pic of added cap

The wires are my failed attempt to solve the problem by directly connecting the op amp and the bluetooth module grounds to the TPA3116 ground.

Others have noted the same issue, even when powered by batteries.

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  • \$\begingroup\$ BTW, there is a youtube video of a guy demonstrating the same noise problem using batteries (youtube.com/watch?v=VcBvq4gztns), so the problem doesn't originate in the power supply. I am powering mine with an old laptop supply. \$\endgroup\$
    – ruminant1
    Jan 10, 2016 at 18:03
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    \$\begingroup\$ Tidied up the formatting, and added the links for you. Once you get more rep you won't have the issue. Nice find on the missing caps. \$\endgroup\$
    – Passerby
    Jan 10, 2016 at 18:57
  • \$\begingroup\$ Based on the two boards, the one with the cap is clearly a new revision to fix an issue like this. Notice the extra ground vias for better heat management, the beefier DPAK regulator on the right instead of a tiny sot-89. It's chock full of "We underestimated the first time". \$\endgroup\$
    – Passerby
    Jan 10, 2016 at 19:03
  • \$\begingroup\$ Thanks for the cleanup. I noticed the extra ground vias (but not the beefier regulator). When I found the other board, I remarked that I should have bought that one in the first place because it was only a few bucks more than the one I had. I have no regrets though because I learned quite a bit in the journey. \$\endgroup\$
    – ruminant1
    Jan 10, 2016 at 19:14
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    \$\begingroup\$ Because the north end of resistor four is grounded, I suppose you could just piggy back the cap on the resistor, with the negative end to the north. \$\endgroup\$
    – ruminant1
    Jan 11, 2016 at 19:09
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Class D amps are notorious at having really poor power supply rejection. Basically the FETs in the output stage "bang" close against the power rails and anything on those power rails get superimposed on the square wave output AND, to make things worse, the PWM switching waveform actually modulates the power supply noise down to baseband and hey presto, noise at 100 kHz to 10 MHz (or wherever) suddenly appears thru your speakers. It's called synchronous rectification but basically it's a switching mixer like a ring modulator. Here's a picture that may help you understand: -

enter image description here

Imagine the green trace is power supply noise and the red trace is the PWM. The blue trace (significantly magnified) is what results due to switching-mixing and, you should be able to see that it has a significantly lower frequency than either the PWM or power supply noise. Eventually, as power supply "ripple" frequency and PWM frequency start to coincide you get an audio beat frequency. But given there's an awful lot of bandwidth that can be down-mixed to audio there's something always whistling through at audio frequencies.

A series inductor with appropriate current rating in parallel with a 47 ohm resistor and a capacitor down to ground usually stops a lot of this. But to do the job properly needs more info about your power supply and a better idea about the noise on your scope shots.

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  • \$\begingroup\$ OK who is the wise guy down voting me? \$\endgroup\$
    – Andy aka
    Nov 28, 2015 at 22:09
  • \$\begingroup\$ Is it @user65652? \$\endgroup\$
    – Andy aka
    Nov 28, 2015 at 22:11
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    \$\begingroup\$ obviously a class D amp fanboi \$\endgroup\$
    – PlasmaHH
    Nov 28, 2015 at 22:27
  • \$\begingroup\$ Hee hee @PlasmaHH \$\endgroup\$
    – Andy aka
    Nov 28, 2015 at 22:29
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The best way to filter a 15-year old SMPS is to buy a new one. And if you actually plan to use it for audio pay attention to its datasheet. Like ripple noise and so forth. See Can an audio circuit be powered by a switched-mode power supply?

And yes, you'd want to first figure out what component is the main culprit for this chirpy noise. I don't know if you need car batteries just for this... if the noise is audible a low power levels you don't need to crank the amp all the way up. If you have some linear supply around (LM317 etc.) it might be enough to figure this out.

And regarding ripple, 0.7V is a lot! I bet that SMPS wasn't like that when new. Laptop power supplies run pretty hot so caps dry out substantially. 15 years is almost enough for 85C-rated caps to dry out of spec even without use, i.e. at room temp. You could try repairing it by replacing its caps. It becomes a project on its own.

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  • \$\begingroup\$ Isn't that the same suggestion as hooking up two car batteries in series to perform the test? \$\endgroup\$
    – zipzit
    Nov 28, 2015 at 21:53
  • \$\begingroup\$ Look at the vertical scale above. That ripple has a height of 0.7 Volts. Probably okay for a year 2000 laptop driven via battery, not ideal for an audio circuit. That LM317 chip looks pretty awesome, I just don't have any here. Frankly the two battery thing looks like an easy test. \$\endgroup\$
    – zipzit
    Nov 28, 2015 at 22:14
  • \$\begingroup\$ @zipzit: for someone having an oscope on hand it's pretty strange not have a lab supply (or at least a makeshift variant) on hand. But if you have the batteries (even UPS ones would have plenty of amps) by all means that's one way to eliminate a variable/unknown. \$\endgroup\$
    – Fizz
    Nov 28, 2015 at 22:17
  • \$\begingroup\$ I just noticed in the photograph above, there is a LM317k on the amp board. Doesn't that imply that using a LM317 between power supply and amp board will not be fruitful, that the noise in question is coming from somewhere else? \$\endgroup\$
    – zipzit
    Nov 28, 2015 at 22:31
  • \$\begingroup\$ @zipzit: I don't think they power the amp through that. It would not get enough current. Also your photo is too fuzzy for me read any chips. Oh yeah, I see it's marked on the silkscreen. That's probably good for for half an amp maybe. Try tracing the circuitry and see if the LM317 powers the TPA or just the BLE module. \$\endgroup\$
    – Fizz
    Nov 28, 2015 at 22:32

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