On 12/29/2010 12:49 PM, Don Pearce wrote:
On Wed, 29 Dec 2010 12:45:49 -0500, Randy
wrote:
On 12/29/2010 12:44 PM, Don Pearce wrote:
On Wed, 29 Dec 2010 12:05:12 -0500, Randy
wrote:
An open-circuit, class D amplifier output has extremely sharp
edges due to the switching of the bridge. The expectation is that,
somehow, the output is averaged over at least one cycle of the switching
period to form a continuous, analog voltage.
If this output is connected to a speaker with a mechanical lowpass
characteristic, then the speaker's mass will effectively average
the "digital" output into a continous position.
Thus my question is, when connecting a class D output to a speaker,
why do we need filtering at all? That is, why wouldn't the mechanical
lowpass filtering of the speaker be sufficient?
Because there are EMI regulations that seriously frown upon high power
transmitters that broadcast a signal every 40kHz from DC to light.
d
Apart from EMI, is there a reason?
Yes. That high frequency square wave would dissipate maximum power
into any tweeter connected to it - even without any audio signal
present. It has to be suppressed.
d
Are you saying that a tweeter will have a significant mechanical
response at some 2 MHz (e.g., the SSM2301)? Otherwise, the current
and voltage will be 90 degrees out of phase and no power (or very
little) will be dissipated.
--
Randy Yates % "My Shangri-la has gone away, fading like
Digital Signal Labs % the Beatles on 'Hey Jude'"
%
http://www.digitalsignallabs.com % 'Shangri-La', *A New World Record*, ELO