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I'm trying to build a simple audio amplifier for an 8 Ω speaker. The schematic/design is from here (Wayback Machine link just in case that link breaks). I used a different transistor as I happened to not have the specified one, but other than that, the circuit is un-modified.

schematic

simulate this circuit – Schematic created using CircuitLab

A battery worked, but I wanted to use a wall power supply. I found a barrel-jack wall-wart power supply that said 9V @ 1A on the back. To be safe, I plugged it into the wall, and confirmed that there was indeed 9 V coming from the barrel jack. I disconnected the battery, and connected the 9 V power supply in its place using a barrel-jack to screw-terminals (in my case, 9 V and ground). I turned it on, and strange noises came out of the speaker, seemingly at random. I'd guess maybe every few seconds, and if I try to play audio, it sounds distorted (it sounds fine with the 9 V battery).

I also tried a 5 V power supply, but I still get strange sounds (noise?) from the speaker, albeit less loudly. Found an adjustable 2 A power bench power supply (labeled "Regulated DC Power Supply"), set it to 9 V, but it also made the speaker make strange noises.

If it helps, it isn't a constant hum/buzzing. It is much more random-sounding.

I've also heard of something called a "linear" power supply, but they're pricy and big, so I'm hesitant to get one. I could also get a 9 V voltage regulator and give the regulator 12 V noisy power, but I'm not sure where to approach this from.

Unfortunately, I do not have an oscilloscope to more precisely measure the power supplies.

What's a good way to get rid of this noise so I can use a wall power supply and not a battery? Ideally I'd like to be able to keep using a normal barrel-jack power supply, if possible. I'm open to adding capacitors, a voltage regulator, etc., but I'm not sure where to begin or what to change in the circuit.

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  • \$\begingroup\$ Try also using a pot for reducing input signal ... \$\endgroup\$
    – Antonio51
    Commented Nov 19, 2022 at 7:57
  • \$\begingroup\$ I can add a 9v regulator. For the cap, do I just do regulator out to ground…? What size capacitor should I get? I haven’t had to deal with filtering before. I can try adding a pot to the input tomorrow as well \$\endgroup\$
    – cocomac
    Commented Nov 19, 2022 at 8:00
  • \$\begingroup\$ See figure 5 in datasheet st.com/resource/en/datasheet/l78.pdf . Be aware that you need more DC input voltage. For L7809, you need ~ DC 12 V minimum already well-filtered (filtering capacitor ~ 1000 uF). If you don't have enough DC voltage, use a L7805. \$\endgroup\$
    – Antonio51
    Commented Nov 19, 2022 at 8:31
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    \$\begingroup\$ It will be more work to add things to current design to remove noise. It will be less work to start from a better design, there are even amplifier ICs which just need a few external parts to drive a speaker. \$\endgroup\$
    – Justme
    Commented Nov 19, 2022 at 8:52
  • \$\begingroup\$ That makes sense. If I got an amplifier IC (i.e., LM386) and used it with a filtering capacitor (and maybe a regulator), would it deal with the noise itself, or would it still have issues? \$\endgroup\$
    – cocomac
    Commented Nov 19, 2022 at 16:03

1 Answer 1

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Here is what you can get with a battery or power supply not well-filtered.

enter image description here

enter image description here

If the signal is too big, BJT could be off, generating "distortion".

enter image description here

How to fix noise from speaker when using wall power supply (but works with a battery

I'm open to adding capacitors, a voltage regulator, etc.,

Although simple, you need a "good" power supply, because of the way it polarizes the BJT.
More filtering (adding a capacitor) and a regulator are needed.

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