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#1
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Class D Output Filtering
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? -- 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 |
#2
Posted to rec.audio.tech
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Class D Output Filtering
On Wed, 29 Dec 2010 12:05:12 -0500, Randy Yates
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 |
#3
Posted to rec.audio.tech
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Class D Output Filtering
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? -- 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 |
#4
Posted to rec.audio.tech
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Class D Output Filtering
On Wed, 29 Dec 2010 12:45:49 -0500, Randy Yates
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 |
#5
Posted to rec.audio.tech
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Class D Output Filtering
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 |
#6
Posted to rec.audio.tech
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Class D Output Filtering
On 12/29/2010 01:08 PM, Dick Pierce wrote:
Randy Yates wrote: 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. 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. No, it won't. It will be more like 45 degrees for quite some way above 20 kHz. Another issue is whether distributed capacitance can have an effect at these frequencies. I'm not usre I have any better an answer for that than anyone else, but my suspicion is the effect is probably not significant. Thanks Don/Dick for this information. -- 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 |
#7
Posted to rec.audio.tech
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Class D Output Filtering
In article ,
Randy Yates 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? Self-contained systems, like cellphones, don't always have filtering on Class D amps. In those cases the speaker is guaranteed to have short wires, high impedance, low capacitance, and not be paired with any kind tonal or impedance compensation. -- I will not see posts or email from Google because I must filter them as spam |
#8
Posted to rec.audio.tech
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Class D Output Filtering
On Dec 29 2010, 12:05*pm, Randy Yates 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? -- 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 I was reading about the NXP mono amp on a FTM flyer. Suppose the output frequencies mixed and caused byproducts ?? They would need some delays. greg |
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