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I am making an audio amplifier using an LM2879. I am using the schematic provided in the data sheet.

enter image description here

For a starting point, this design works pretty well, although the base cutoff point could be a bit lower. When I power this circuit on, I am greeted by quiet, high pitched static regardless of the volume of my device. When I play music, the quality of the music is good. I believe the static is coming from my power adapter because someone else in the reviews for this product said it was electrically noisy. Is there a way to filter out this noise at all? If you have any other ideas of where the static is coming from, I would appreciate the help.

Thank you!

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  • \$\begingroup\$ The noise might be coming due voltage drop across the internal impedance of the power supply. This drop acts as a feedback to the circuit so it nearly resembles an oscillator. To avoid this, try connecting a capacitor (somewhere around 10 pf) across the supply pin and the ground pin. Let me know if it works. \$\endgroup\$
    – Sâu
    May 23, 2016 at 6:57
  • \$\begingroup\$ Thank you for your suggestion! I tried this with a 100 pF capacitor and what I think is a 10 pF capacitor. Neither seemed to make a noticeable difference. \$\endgroup\$
    – Gigaxalus
    May 25, 2016 at 4:00
  • \$\begingroup\$ One other interesting thing I noticed was once I turned my music on, the static noise reduced significantly. Once I paused the music, the noise was still very soft for half a second, then increased in volume to how it is normally. Perhaps there is something wrong with my signal wires? \$\endgroup\$
    – Gigaxalus
    May 25, 2016 at 4:03
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    \$\begingroup\$ I tried grounding my signal wires, and the noise was much quieter, but still slightly persisted. I also noticed that the noise increase with the proximity of my hand to the IC. Can anyone think of why this might be? \$\endgroup\$
    – Gigaxalus
    May 25, 2016 at 4:10

3 Answers 3

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What you describe looks almost sure as an interference from your switching power supply to your amplifier. The design from previous reply does not help at all, sorry.

The origin of the problem is: the 24 V output of your PS (both wires!) is effectively "jumping" with respect to ground potential, most likely, at switching frequency, its harmonics and sub-harmonics.

The bad news are that it is very difficult to solve this problem. One (may be - easiest) solution is to use another 24 V DC source, based on traditional iron core 220 / 24 V transformer and rectifier.

If you wish to use this kind of switching supply, you can try:

  1. Install a common mode choke on both DC 24 V wires
  2. Use 2 common mode chokes on these wires and a pair of capacitors between them:

schematic

simulate this circuit – Schematic created using CircuitLab

In the second case, the mid-point of capacitors is connected to "true ground". You can try using the third line (0 or GND) on your Mains circuit, or some ground connection, if it is available.

Capacitors must be larger then 10 uF and rated for 50 V at least.

Chokes must be rated for 2 A DC current and their inductance must be no smaller than 1 mH, preferably much more.

And the last circuit to try is to add a differential mode LC filter. Hope it is not needed.

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As noted in the comments you will be able to reduce this issue by placing a decoupling capacitor across the power supply.

The wikipedia article on the subject provides a very good explanation of the technique. Place the capacitor as close as possible to the IC, and if this does not work you can use multiple capacitors of different types (e.g. electrolytic, tantalum) to gain the benefits of each type (e.g. high frequency performance).

If you are using a dual-rail design then ensure that you are using capacitors between each rail and ground, and also between both of the rails:

Opamp decoupling

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Having recently taken apart a 20V laptop power supply of a kind which is rather noisy (basically, as soon as you plug it in you get a terrible ground loop that sounds like frying bacon) I concluded that the solution is to find a different power supply. Consider a linear supply perhaps (transformer, bridge rectifier and big electrolytic caps) which although larger do not suffer the noise problem inherent in switched mode power supplies.

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