PicoMite: driving a small speaker


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Turbo46

Guru

Joined: 24/12/2017
Location: Australia
Posts: 1601
Posted: 02:38am 25 Mar 2023      

Hiss
That appears to be the main problem at the moment. I suspect that it is coming from the Pico (all of those little switches inside going 19 to the dozen). From memory, Tom has taken no great effort to beef up the power supply circuitry on the bread board and that is where a well designed PCB would help as would some more bypass capacitors.

I would like to suggest some tests to find out for sure where it is coming from:

1. Disconnect the amplifier input at the input to the volume control pot. Is the hiss still there? If so, the noise on the power supply is probably affecting the amplifier. I would try a decoupling circuit, say a 10R resistor followed by a 100u electro and an 0.1u capacitor. Only if necessary. Hopefully that should help.

2. Is the hiss coming from the supply (as I suspect)?. With the amplifier still disconnected as above put two back to back diodes across the input to the volume control pot, just to protect the amplifier from any spikes. You can remove them later. Connect the volume control input to the positive supply via a small capacitor (0.047uF or there about). If you hear the hiss now then the supply is the culprit and surely would be getting to the PWM signal.

If that is so then I would try the circuit previously suggested by mozzie. Feed it from your positive supply (no 5v available) omit the 4K7 (R3), 1K will be OK for R2, 10uF will probably do for C1. Connect the output (the transistor collector) to what ever filter you are using now and from there to the volume control pot. Don't worry about any rise and fall times of the transistor, you don't need HiFi, it is of no consequence

The theory behind that is that if there is noise (hiss) on the power supply then it will get through to the PWM signal and eventually to the speaker. Using the decoupled supply and the extra transistor should remove that noise and give you a clean PWM signal.

Hope that helps.

Bill