What's wrong with my C-notes?

[AdrianD]

I was playing with the xo in vituixcad using your FRD and the PE supplied ZMA files. Have you tried swapping C2 and C1 around? When i do that in my simulation it looks like the transform function of the filter creates a higher Q value and makes the rising response less dramatic. I'm not super versed with crossover design, i was just poking around seeing what would happen.


Filter function with C2 and C1 swapped

Filter function with your current crossover.
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[joshshetter]

So I think this is finally solved. A user at ASR shared some info on load dependency with certain class D amplifiers. I borrowed a class A/B amplifier and measured. Orange is Class A/B, green is Class D that I was using before.

[Steve Lee]

^ I'd like to hear more about this - its kind-of disturbing if true . . .

[JRT]

I am not familiar with the amplifiers mentioned above, and didn't bother to dig further. Maybe this does not apply.

Here is an oldie but goodie... see sect 7.1.4

https://www.ti.com/lit/pdf/slaa117

...which pertains to a type of class D amplifier that has a low pass PWM demodulation filter (aka reconstruction filter) affected by load impedance, does not have the PWM demodulation filter inside of a control loop, and so load impedance at high audio frequencies affects that filter, with linear distortion effects in frequency response at those high audio frequencies (top octave).

For an example different from that, Purifi modules are largely independent from load impedance within the constraints of maximum peak current output.

[a4eaudio]

I have generally heard not to use Class D amps for measurements. I was recently reading a thread from back in 2014 and the HDMI input on a Class A/B amp, even in Direct mode, was messing up the measurements. I think this just goes to show how important it is to really spell out the measurement process when posting, just in case someone recognizes a potential problem.

[joshshetter]

Here is the thread the ASR member linked me that prompted me to test different amplifiers.

https://www.audiosciencereview.com/forum/index.php?threads/comparison-of-power-amplifier-distortions-and-fr-into-complex-speaker-dummy-load.21682/

I don't really think it's class D in general that has this issue with these speakers, I have measured many speakers that use class D including genelec 8030 and there is no issue. I also have Amiga kit that does not exhibit this issue, and my current project that involves a DX25 + WG300 does not exhibit the issue with A07 amp, so I believe it's down to a complex interaction between the c notes xover and particular class D designs that lack post filter feedback as stated by the ASR member (I don't know what that is though, that is beyond me).

I guess at this point my options are to attempt to redesign the crossover for these drivers or buy a new amplifier. Both of those options kind of suck but it is what it is. I took several measurements just to make sure the results were consistent and they were, there was even a very audible difference between the two amps used (A07 and old audiosource amp 100). I hope this marks the end of my troubles.

[Steve Lee]

Thanks for the info, guys.

[joshshetter]

I sent my findings to PE but never got a response, don't think they really care too much. I gave my c-notes to a friend who owned the Audiosource amp I borrowed, he should be set. I wonder how many people out there are using cheap class d amps that have this issue and don't even know it.

[norman bates]

That is a very good statement.

[joshshetter]

Thanks? Man this forum is kinda weird lol.