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Date:	Sun, 03 Jun 2012 13:02:35 -0700
From:	Phil Taylor <phil AT plastitar DOT com>
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To:	geda-user AT delorie DOT com
Subject:	Re: [geda-user] Power to a speaker OT
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On 6/3/2012 12:26 PM, Rob Butts wrote:
> I am using a microchip microcontroller to read in a 8ks 16 bit .wav file
> and output it through the dac.  I have a small 109 dB speaker 32 ohm
> impedance, nominal input power of 12mW and max input power of 20mW for 1
> minute.

12mW = Vrms^2 / zspkr

so ... that's 1.8 Vpp continuous or 2.3Vpp for shorter times.  The dac 
rails are high enough to hit the power level most likely (even on 3V). 
The current provided by the DAC is probably not high enough though to 
drive the speaker directly.  you'd need 30mA before output current 
limiting kicks in, a power op amp, or some discretes inside an opamp 
feedback loop to drive that speaker.  (compute max current at waveform 
peaks, not with average or rms voltages.)

we need to know what kind of audio you are playing back.  if it's a sine 
wave, you won't need any gain ... to hit your power level.  But if it's 
music you will need some gain to get it loud.

As for other ways to limit the power it would be possible to pre-process 
your sound file, design a limiter circuit, or clip the analog going into 
the speaker.  Each of these options would have its own tradeoffs.

Some compression may be needed to get the audio to feel loud, as will 
some gain, though it sounds counterintuitive.

Phil

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