I have a question concerning distortion or other issues with a class D amplifier driving a high impedance load. Most of the class D amps are spec'd on their ability to drive 2, 4, and 8 ohms. I have a speaker load that is 30 ohms or greater. The frequency response of the amplifier will be limited to less than 1KHz. I mostly need the ability to deliver high voltage swings under dynamic conditions.
Would anyone knowledgeable about class D amplifiers see any issue with driving a 30 ohm load that could get near the voltage rails under dynamic conditions. I would be looking at modules that could deliver 500-600W into an 8 ohm load (bridged module). Obviously, current would not be an issue.
The speaker is an electrostatic panel for bass.
I would like to thank you in advance for any responses you provide.
Best Regards,
gwho
Would anyone knowledgeable about class D amplifiers see any issue with driving a 30 ohm load that could get near the voltage rails under dynamic conditions. I would be looking at modules that could deliver 500-600W into an 8 ohm load (bridged module). Obviously, current would not be an issue.
The speaker is an electrostatic panel for bass.
I would like to thank you in advance for any responses you provide.
Best Regards,
gwho
Only issue I can think of is high frequency peaking in the output filter of a class D amp without post filter feedback, due to the higher load resistance. But if you're only using it for bass, shouldn't be an issue.
You'd likely have to redesign the output filter for 30 ohms and add real damping to get best results, if by 30 ohms you mean capacitive reactance at some frequency.
Yes it would be only used below 1khz. I was hoping it would be a non-issue thanks for the input gmarsh.
Regards,
gwho
Regards,
gwho
Andrew,
So you are saying that the output filter characteristics would change with a high impedance load?
I'm not sure that the impedance is capacitive. I believe the circuit is an inductor in series with a transformer. The output of the transformer generates a signal that drives grids/stators (+-) of the electrostatic panel which is biased at a high voltage.
Regards,
gwho
So you are saying that the output filter characteristics would change with a high impedance load?
I'm not sure that the impedance is capacitive. I believe the circuit is an inductor in series with a transformer. The output of the transformer generates a signal that drives grids/stators (+-) of the electrostatic panel which is biased at a high voltage.
Regards,
gwho
Yes, assuming everything else were to function normally (probably the case especially in amps with no post-filter feedback) you would still get maybe more than 12 dB of peaking before roll-off if the typical two pole filter designed for 4 ohms were only loaded at 30. What problems that would cause depends on the specific implementation. Assuming you're out to optimize the whole thing, the filter should be redesigned as a starting point. Most class D output filters have lots of focus on DCR when driving 4 ohm loads, but in your case the copper volume could easily be shifted toward more turns with thinner wire, though ideally you don't want any DCR in either case. By increasing the first output inductor you'd likely end up with reduced idle loss since the peak inductor currents at zero drive could be much reduced, an advantage you'd want to take. I'd recommend spicing the entire circuit to see what you're getting.
Andrew,
Thank you for the advice. I do not know a lot about class D amplifiers. I was hoping the answer was "it will work fine". I do see that it would have an affect if the load is designed in or a significant part of the filter. Changing the load value 10X probably would change the characteristics of the amplifier. I will read a little more about class D but I assume the filter is a low pass or bandpass filter? The modification is probably beyond what I could do. I will likely look at a standard class A/B amplifier.
Regards,
gwho
Thank you for the advice. I do not know a lot about class D amplifiers. I was hoping the answer was "it will work fine". I do see that it would have an affect if the load is designed in or a significant part of the filter. Changing the load value 10X probably would change the characteristics of the amplifier. I will read a little more about class D but I assume the filter is a low pass or bandpass filter? The modification is probably beyond what I could do. I will likely look at a standard class A/B amplifier.
Regards,
gwho
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