AudioChiemgau motion feedback amplifier with Doppler compensation

[Keith_W]

I haven't seen this discussed in ASR, nor am I aware of any speakers that use this technology.

ASR members might have heard of AudioChiemgau because they manufacture the Mode Compensator which allows two microphones to measure bass frequencies, removing the effect of the room - as discussed in this ASR thread.

They also manufacture these modules:

- AC-AAZ02 analog audio processor which needs to be placed upstream of their power amplifier module,
- AC-PAR75 power amplifier module with integrated motion feedback sensor module

The way the components are hooked up looks like this:

In use, a sensor is placed on the voice coil of the speaker and connected to the Motion Feedback (MFB) sensor. In their words:

An analog processer generates a signal representing continuously the speaker membrane position. This signal controls a phase modulator which exactly equalizes the phase modulation caused by the variable membrane position. The AC-AAZ generates in other words a virtually steady speaker membrane – or more accurately described, the acoustic phase center is kept at a fixed location.

They also claim:

A sensor measures the travel, velocity or acceleration of the speaker membrane and a closed control loop ensures, that any deviation from the ideal value is immediately corrected. In an ideal case – and we at AudioChiemgau can approximate that ideal case extremely well – only the non-linearity of the sensor and its frequency response determine the overall characteristics of the MFB speaker. Especially the advances in the MEMS-technology allow now to build nearly perfect speakers. The amplifier module AC-PAR75 uses that new technology. The measured acceleration signal of the speaker membrane is compared with the audio input signal and closely controlled. This innovative technology allows on one hand to extend the SPL frequency response of a speaker far below its original resonance frequency (16 Hz are achievable even for small speakers) and on the other hand, it drastically reduces the non-linearity of the speaker (harmonic distortions between 0.1 and 0.03 % for the third order component are achievable). Even small high-end speakers with very low corner frequency become such feasible. Our AudioChiemgau caveat: The MFB control loop would keep the SPL constant even at very low and at infra-sound frequencies.

As mentioned, I am not aware of any speaker that uses this technology. It would seem as if this could be retrofitted to many suitable drivers, the only caveat is that you would have to make the amplifier yourself using their modules.

It seems as if the main benefit would be to reduce nonlinearities, improve LF extension, and provide Doppler compensation.

When I read all this I thought it sounded marvellous but then my immediate question was - where is the evidence? That website has a nice description of how the technology works, along with some data sheets, but I do not see any measurements proving that the intervention works. Nevertheless, it does sound very exciting. In fact, any new technology that improves loudspeaker performance is exciting for me.

My questions to ASR:

- are you aware of anybody using this technology at all? Any measurements?
- what do you think of the claims of the manufacturer?
- anybody keen on purchasing some modules to test?

 

[voodooless]

MFB isn’t exactly new Philips used a lot of MFB speaker back in the good old days.

It works. I’m not sure about the Doppler distortion though. Would be interesting to know more about that.

 

[eboleyn]

Maybe I'm commenting a bit too "drive-by" and not reading the detail well enough, but how is this different from say what Rythmik Audio calls "Direct Servo" they have used in their subwoofer amps for years?

I.e. using a sensing coil in the speaker driver unit that feed into the amplifier control to decrease distortion and make it drive the cone more correctly to the specified signal.

 

[Keith_W]

I know it isn't new. I have a pair of Rythmik subwoofers with MFB. But a module that you can retrofit to a speaker is certainly something new for me.

 

[voodooless]

I have a pair of Rythmik subwoofers with MFB.

That is not exactly MFB. It uses a different mechanism and the effectiveness is not as good.

But a module that you can retrofit to a speaker is certainly something new for me.

It’s not MFB kits have been available for a while, for instance:

Motional Feedback Audio.

All things servo. StarBass drivers & accelerometers, loop electronics and cash & carry Little/One and Grown/Up sales via servolabs.nl

piratelogic.nl

There is a whole scene around the vintage MFB stuff as well.

 

[eboleyn]

That is not exactly MFB. It uses a different mechanism and the effectiveness is not as good.

Hmm. How is it different? From the short descriptions the only difference I'm seeing is that the sensors might be different. I.e. Rythmik using a sensing coil in the speaker unit, while the MFB thing above says it's using a MEMS-based physical detector separate from the speaker coil.

 

[voodooless]

Hmm. How is it different? From the short descriptions the only difference I'm seeing is that the sensors might be different. I.e. Rythmik using a sensing coil in the speaker unit, while the MFB thing above says it's using a MEMS-based physical detector separate from the speaker coil.

Exactly! One actually measure acceleration, the other measures changes in magnetic field and then translates that to pseudo-acceleration for compensation. Guess which one will give you more accurate data?

 

[kemmler3D]

This is really interesting but AFAIK the problem with motion feedback is it only works at low frequency because of the delay in feedback. Using an analog processor is basically a must for this reason, but the highest frequency this is usable for is unclear.

As for high SPL at 16hz using a small driver... well, I'm sure it's all dandy until you hit Xmech? I don't think a feedback sensor can help with that...

That aside it's really appealing in the basic principle. Adjust the amplifier's output in real time to force the cone to reproduce the input signal, instead of using various tweaks to get it closer to reproducing the input signal. Sure, why not?

 

[voodooless]

but the highest frequency this is usable for is unclear.

If you look at the documentation of the Dutch MFB kit, you’ll see that all of this is documented quite well.

 

[test1223]

There is a whole bunch of things which can be compensated (at least in theory) if you have the signal of a sensor available.

- power compression

- none linear motor plus suspension behavior which also can be split up in different subparts

- temperature dependent resonance frequency shift and its according frequency response deviation

- doppler distortion

For every compensation an algorithm /technique with some feedback loop is needed which will also determine how good the compensation works.

Therefore there are many different solutions on the market which are only sometimes identical.

 

[Keith_W]

That is not exactly MFB. It uses a different mechanism and the effectiveness is not as good.

My understanding is that large drivers can tilt when they accelerate and the motion is not always linear, so accelerometers may provide incorrect feedback. Is this wrong?

It’s not MFB kits have been available for a while, for instance:

Motional Feedback Audio.

All things servo. StarBass drivers & accelerometers, loop electronics and cash & carry Little/One and Grown/Up sales via servolabs.nl

piratelogic.nl

There is a whole scene around the vintage MFB stuff as well.

I see, so the only new offering with this design is the Doppler compensation.

 

[voodooless]

My understanding is that large drivers can tilt when they accelerate and the motion is not always linear, so accelerometers may provide incorrect feedback. Is this wrong?

Tilt cannot be very much, otherwise the voice coil would rub. What could happen is that the cone may flex? Either way, a coil pickup will not work in these cases either.

I see, so the only new offering with this design is the Doppler compensation.

I don’t know, maybe it’s an inherent feature of the technology?

 

[Blumlein 88]

Without getting into how they use this, I don't see how motional feedback will make a driver perform worse unless the feedback is incorrect or of inadequate bandwidth. It may not fix all issues with a driver making it give perfect distortion-less output, but seems it would be a good thing that might matter a little, might not matter.

 

[ErVikingo]

Similar in concept to the Servo Controlled woofers on my Infinities and on the Rythmic and other subwoofers.

 

[kemmler3D]

I think what's actually new here is the doppler compensation. I don't have an 100% intuitive sense of how phase shift can correct it, but it kinda makes sense.

I imagine if you used this with Purifi drivers that already have minimized IMD and other types of distortion as much as they can, you'd have a really clean system on your hands.

Anyone up for a beyond-SOTA DIY job?

 

[eboleyn]

I think what's actually new here is the doppler compensation. I don't have an 100% intuitive sense of how phase shift can correct it, but it kinda makes sense.

I imagine if you used this with Purifi drivers that already have minimized IMD and other types of distortion as much as they can, you'd have a really clean system on your hands.

Anyone up for a beyond-SOTA DIY job?

What would particularly interest me is how the correction works at large volume/power output. Natural distortion of multiple kinds generally increases with higher volume, so it would be most illuminating if that more gross error could be corrected.