PicoMite: driving a small speaker


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Volhout
Guru

Joined: 05/03/2018
Location: Netherlands
Posts: 3701
Posted: 08:06pm 24 Mar 2023      

@Peter,

Yes, emitter follower is an option for 32 ohm headsets. But for Tom's problem (8 ohm mini speaker) that is not solution. Note that the emitter follower is essentially a class-A amplifier. Meaning that continuous power consumption is equal or greater that the amount of power you want to output. So it you want 0.5 watt maximum output power, the current consumption (even al low volume) is 0.5 watt or greater.

Also requirements may differ between people looking for a solution. Some are happy with 80's game sounds, some may look into decent audio quality.

Currently the PWM is running at 44.1kHz, and PWM is refreshed every 44kHz. Peripherals (PWM) are clocked at 62MHz(VGA)/66MHz(PicoMite). This gives a 10bit sound quality (10.5 bit), which is a good optimium between amplitude quality and phase quality.

Simpler filters however can be made when the PWM would run at 4 x 44kHz (and still refresh the PWM at 44kHz). You would gain 2 octaves in filter rejection, but loose 2 bits in quality (10bit -> 8bit).

I am not in favour of going that way, but for game sounds it would be okay. And honestly, even cats don't hear 180kHz....

Note that such a change will not make it easier to drive a loudspeaker. It would only save 4 or 6 components in the audio filter (2 or 3 per channel). At cost of sound quality. It is possible, but I prefer the current system (44kHz).

Volhout

P.S. the switchmode power supply: On the VGA mite, the picomite switchmode power supply automatically turns to PWM mode since the current needed to drive the monitor forces the switchmode IC to PWM mode. Not need for OPTION POWER PWM. But then still you would need a linear regulator or other solution to get rid of the ripple.
Edited 2023-03-25 06:17 by Volhout