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On reflection, you may not need an amplifer per se.
If the speaker signal swing is 5 volts (it may not be) then that is 
about 2v RMS and power into a 8 ohm speaker would be about 0.5 watts 
IF the source impedance was low. Half a watt is ample for most such 
applications. Quite possibly either the source impedance is higher or 
there is less than 5v swing.

Either way, try something like the attached.

R!/C1 convert PWM to DC - as before
Q1 is an emitter follower which buffers DC level - effectively a power 
amplifier (not voltage).
R2 and Q2 as before. Input to Q2 must gard switch it as shown here.

For extra simplicity remove R2/C2 and put speaker where R2 was. BUT 
this places DC in speaker and gets bad as voltage rises.

The emitter follower is the magic bullet here (which may even work 
:-) ) as it turns the smoothed PWM voltage into a stiff source which 
improves max available power.

A square wave is notionally Vpk/2 RMS equivalent. Regardless of V+ max 
PWM-DC is Vin - 0.6 or about 4.4v or 2.2v RMS. Still V^2/R = 0.6 watt 
in 8 ohms. R2 needs adjusting to suit notional peak power (play).

Another variant has no C2 and speaker in series with R2. Reduced 
power - limits max volume and PWM take over from there.

Let me know if/how it works.


        RM




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