Effect of 440kHz switching frequency residuals on tweeter distortion?

[pma]

Effect of 440kHz garbage on tweeter distortion?

Following my measurements on Hypex NC400 modules, I was curious if permanent output residuals of class D switching frequency, which I measured 943mVpp at 446kHz, would have any effect on measurable tweeter distortion. Especially taking into account that level of useful audio signal during listening at moderate or low volume would be well below the HF switching frequency residual.

So I prepared a test, where the power amp is driven from a sum of two signals, one comes from the sound card in audio band, the second one from sine generator GAG-810. The level of HF signal was set to get about 960mVpp/440kHz at power amplifier output, and audio signal level was set to get 220mVpp in 1st round and 440mVpp in the 2nd round of tests. The 440kHz HF frequency was switched on and off and measurements taken in both cases. The output was sent to Beyma T2030 tweeter and measured by Behringer ECM8000 microphone, in near field.

Conclusions

No significant effect of 440kHz HF frequency with 940mVpp amplitude was observed in acoustical measurements. Neither in frequency response, nor in distortion plots. My previous suspicion was not confirmed by measurements. Very small differences in harmonics plots are at the limit of repeatability of acoustical measurement.

As I have the setup still connected, please feel free to tell me your suggestions what more you would like to measure re this topic, if you have any.

Images

Measurement setup

Tweeter + microphone

Frequency responses from 5 measurements. Lower, at 220mVpp signal level, HF on/off. Higher, at 440mVpp signal level, HF on/off. HF on/off curves overlap in both cases. Y-axis in dB, but level not calibrated.

Distortion, signal level 440mVpp, HF off.

Distortion, signal level 440mVpp, HF on.

 

[Soniclife]

How much do you have to lower the frequency before it shows up? Some class D use much lower frequencies.

 

[JohnYang1997]

How much do you have to lower the frequency before it shows up? Some class D use much lower frequencies.

This is a great suggestion.

 

[pma]

How much do you have to lower the frequency before it shows up? Some class D use much lower frequencies.

This is a good question, thank you for that. I made a new set of measurements with HF frequency from 200kHz decreasing down to 30kHz. Please see it below. Though higher frequencies seem to change H3 a bit, but leaving overall THD unaffected, 30kHz and 40kHz show clear intermodulations with the audio sine sweep. This is a new set of measurements and my microphone has moved a bit from #1 post, so please the 1st plot in this post (HF = off) as a reference (repeatable - tested).

 

[Soniclife]

Weird how the 3rd harmonic drops lower as the frequency lowers, before it all goes wrong.

So simple conclusion, good class D good, bad class D bad.

 

[pma]

Weird how the 3rd harmonic drops lower as the frequency lowers, before it all goes wrong.

So simple conclusion, good class D good, bad class D bad.

Yeah it is interesting, with the H3 "smoothing". The amp output voltage distortion remains unchanged with HF injection (I checked it for all frequencies used), so it seems something is happening in the driver when additional HF power is added, and it seems to linearize symmetrical distortion. However at inaudible level. Maybe some speaker/driver expert might help us with an answer?

 

[solderdude]

The H2 distortion is (with an almost 100% certainty) caused by the used condenser microphone.

 

[pma]

The H2 distortion is (with an almost 100% certainty) caused by the used condenser microphone.

This is highly questionable. Vout(rms) is only 150mV, SPL on acoustical side is low even in the proximity and I have the microphone measured to have distortion of -70dB and better at that low SPL. Another check is to go more distant, at the expense of higher noise, so the higher harmonics become unmeasurable. The setup chosen is quite optimal and distortion is that of the tweeter. As a conclusion, I do not agree with your statement.

 

[SIY]

Again, my thanks for you showing test setups in detail so that they can be replicated and/or critiqued.

I wouldn't necessarily discount the mike as a source of the 2nd, especially with the nearfield placement. It's a significant mechanical perturbation to the acoustic area. I use a smaller diameter (1/8") mike there, but that might even be insufficient- how do you see the 2HD vary with mike distance?

 

[mjwin]

[...] it seems something is happening in the driver when additional HF power is added, and it seems to linearize symmetrical distortion. However at inaudible level. Maybe some speaker/driver expert might help us with an answer?

I'm not an "expert", but my hunch would be that the increased linearity has something to do with biasing the magnetic core (as is done intentionally with the ultrasonic bias on magnetic audio tape). Magnetic materials can be terribly non-linear and, although in a speaker, they are present in the form of a permanent magnet, the material is still within the magnetic circuit of the voice coil.

Interesting tests though. I think the real problems occur when the sum & difference intermodulation tones approach audio frequencies - as in the early class D amps & switching power supplies which ran at frequencies only just above audibility.

 

[JohnYang1997]

Hmm. If one want to dig out distortion, get a proper piston type. You can get one pretty cheap from China. Less than 200 bucks. Without calibration tho.