Help! Pi amplified speaker - reducing interference/noise

[jewelie]

Hi all.

I'm looking for some help.   

I'm trying to make a basic powered speaker for my Pi, to output voice messages about status (network down, network back, rebooting, etc.)   

Requirements:

• It should be basically noise free, silent, when not outputing any sound, even right up on top of the Pi with WiFi etc running.
• Using components I already have, if possible, if there's a neat way to achieve it (I've accumulated quite a stack of other inductors, capacitors, transistors (BJT and MOSFETs), etc.)
• Powered by the Pi
• Safe to leave running 24/7.

Acceptable compromises:

• Distortion; quality isn't important, it's only for outputing a computer generated voice.
• It doesn't have to be particularly loud.

The old powered speaker I was using (1st image below) is far too susceptible to interference from the Pi when up close, which it needs to be (WiFi, USB 5V noise etc.)  I've experimented with it, such as trying to improve it's noise isolation with the filter setup below, but nothing I did really helped much.


So, I've built a little amp instead (2nd and 3rd images below) which does seem to work (even if it shouldn't):

• Basic mono LM386 amp.
• Lowish input impedance (to try to reduce interference on the incoming audio signal.)
• Dodgy attempt at filtering the 5V USB power from the Pi (which seems to be pretty noisy.)

Concerns:

• I'm definitely out of my depth with the power filtering (which was done from vague memory of how to do this sort of thing, maths/graphs by ChatGPT, "using the force", and experimentation.)
  
• Plus excessive resistance from the inductor worries me a bit in case it gets hot, but it shouldn't really be carrying much current often, plus presumably it'll introduce distortion (which hopefully doesn't matter much as it's only for mono computer generated voices!)

Before I build this and put it in the case of one of the old speakers...

Q1: Is this okay?

Q2: Are there easy ways I could and should improve this?

Q3: Should I add a fuse, given it'll be running 24/7 (I think I've got some low enough current glass type ones to be relevant)?

- Julie


P.S. One thing that really did make a huge impact was moving our landline DECT basestation away.  In fact, I kept having to move it, wherever I put it it started to interere with other things too a bit.  DECT really is as evil as is made out.  Hoping we can let go of the landline soon anyway.

[radiolistener]

just buy this cheap amplifier, it will do what you want:
https://www.aliexpress.com/item/1005004704851444.html

It uses TDA7266 inside

[Terry Bites]

A capacitance multiplier circuit can provide good noise reduction. You might get away with BC547 or 3904 at low power.
www.electronics-notes.com/articles/analogue_circuits/transistor/capacitance-multiplier-circuit.php
An LC filter may be overkill.
Wiring matters. Poor wiring can lead to power supply noise and amplifier instablity.
Star grounds and such.

[Ian.M]

Use an amplifier with a Mute pin.  Have the Pi mute it when *NOT* making an announcement!