>> Given PWM of a high enough frequency (20 kHz plus frame rate) you should
>> be able to simply gate the audio signal with the PWM OR ground it if the
>> supplying impedance is high enough.

> Actually, the frequencies coming out of the speaker rarely exceed
> 2Khz, so I could even use a lower PWM freq and add a lowpass filter.
> But I'm not sure how fast I can push the PWM, since I don't want it
> taking too much software time.

> Now all I really need to do is replace the pot
> with another transistor, something like this:

That's exactly what I meant - just needs some "fine tuning".

Input bias needs work.

If you use a resistor load for the output transistor you can AC couple the 
speaker and avoid the DC in the speaker coil which does no good to speaker 
or transistor.

> +12V --------------------+------------------+-------
>                  OPTO   |                  |
>   +------------+        |                  |   _ /|
>   |           _|_     |/C                  +--| | |
> INPUT         /_\ --> |               --------|_| |
>   |            |      |\E           |/C          \|
>   +--\/\/\/----+        --/\/\/\-+--|
>                                  |  |\E
>                                |/C    |
>     FILTERED PWM INPUT >-------|      |
>                                |\E    |
> GND ------------------------------+----+------------
>

> But, of course, I either need a filtered mostly clean voltage, or a
> very high rate PWM (which I'm not sure I can afford).

PWM does not need to be too many bits effectively probably. I'd say 4 bits / 
16 steps would do depending on application. If you ran this at say 10 kHz 
with filtering the bit rate = 1/10kHz x 1/16 =~ 6 uS. A 6 uS IRQ would allow 
simple PWM in software. This may be easy or crippling depending on 
processorspeed.

> How clean can
> you get with some R-C filters?

Filters provide 20 dB/decade/pole roll off. Pure RC filters can not have 
enough Q to be universally useful. You can make a 2 or 3 poe low pass with a 
single transistor in an emitter follower configuration. 3 poles gives 60 dB 
at 1 decade. For 2 kHz audio you'd need 20 kHz PWM for 60 dB filtering (or 
is that 40 - no backs of envelopes to hand :-) ). Two stages of filtergives 
4 or 5 poles - sensible upper limit for this technology.

An easier approach may be a voltage controlled attenuator where slow speed 
PWM is turned into a DC control voltage.

If you have never met a diode switch/attenuator you're in for some fun. Two 
diodes say cathode to cathode with positive feed via R's to each anode and a 
common R from centrepoint to ground. When diodes are DC on AC will pass 
through the two in series (magic ;-) ). As the R to ground is increased 
attentuation rises. provide this bias voltage with PWM.

Google on diode switch.


        RM





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