Do servo subwoofers actually lower distortion?

[Joseph Crowe]

I was curious if anybody has measurements that show that a servo control loop on a subwoofer actually lowers distortion at frequencies in the 20Hz-50Hz region. I'm sure it would need to be the same test setup where the loop is turned off and on to show the difference. There is a lot of marketing push surrounding this technology and I thought it would be insightful to see some actual measurements.

 

[DonH56]

Contact Brian Ding at Rythmik Audio. He is a PhD EE (SerDes designer, similar day job to mine) and has researched them pretty thoroughly.

I designed a servo sub ca. 1980 to go with my Maggies. I measured it at that time but have no hope of finding them now. The servo circuit did reduce distortion (it was part of a college project so I had to write it up and show the results). I do not remember exactly how much but it was significant. I also measured a Velodyne (which used a different sensor, an accelerometer, where I sensed using a second voice coil) and IIRC it reduced distortion but not as much as my version and would not compensate some effects (e.g. voice coil heating) as well. But I was biased... Rythmik uses a modern version of the voice-coil sensing scheme that solves some of the problems I had.

 

[Xulonn]

Contact Brian Ding at Rythmik Audio

Interesting that Jim Salk of Salk Audio builds custom Rythmik subs [based on the $980 Rythmik F12] that match his gorgeous veneer-finished loudspeakers.

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However, $1,595 for the Salk Sub is a significant price increase for an "upgraded design" 12-inch Rythmik-based subwoofer. For budget audiophiles, there is an even less expensive entry-level Rythmik Model L12 (below) that sells for $559, including US48 shipping. (Rythmik also sells components with construction plans [LINK] to DIY'ers who want to build their own subwoofers.)

(Edit: Correction to show the Rythmik F12 as the basis for the Salk subwoofer.)

 

[pozz]

Contact Brian Ding at Rythmik Audio. He is a PhD EE (SerDes designer, similar day job to mine) and has researched them pretty thoroughly.

He has a few scant and scattered measurements on the Rythmik website.

@Joseph Crowe if Brian sends more your way please ask him for permission to post them here.

 

[dreite]

In the past, Brian Ding refused to divulge or discuss any of the techniques utilized in his products. I don't know if he still holds to that.
The basic concept is he uses a separate coil on the woofers for voltage feedback and also a current monitor on the driven coil. Those two parameters are inputs to the servo mechanism that's adapted into the amplifiers.

But, as mentioned by pozz, he provides next to nothing regards any objective data for his products.

If you want to get into a discussion where you ultimately throw your hands in the air......he's your guy.

Dave.

 

[LDKTA]

https://data-bass.com/

It can correct for some driver nonlinearities to some extent (dependent on the quality of the servo feedback design). When done properly, it certainly serves its purpose... people should understand that it isn’t some panacea though. I’d prefer a well designed enclosure with a far more robust driver.

Look to JTR. If not, consider going the DIY route and look to Stereo Integrity. Assuming you’re looking for extremely low distortion and extremely high output capabilities.

 

[Ron Texas]

L12 is back ordered until the end of this month, at least. I hope this coronavirus mess isn't the cause.

 

[DonH56]

Jim Salk builds custom versions of the F12, not L12. Jim's changes mainly involve more internal bracing and a custom exterior. He makes some beautiful speakers that reputedly sound very well too (I have not heard a pair).

Since my servo design was similar, as are our day jobs, Brian and I talked for a bit about what he was doing. There is not much on the website. I am not sure the status of his patent application. Rythmik does offer kits.

 

[DS23MAN]

https://rmsacoustics.nl/

 

[pozz]

https://rmsacoustics.nl/

Nice find. Apparently this company designed the Grimm servo subs.

They have a very short paper on the subject: https://rmsacoustics.nl/activecontrol.html

Edit: A few more white papers here: https://rmsacoustics.nl/audiodesign.html

 

[DS23MAN]

Actually I am working with Rob:

http://zelfbouwaudio.nl/forum/viewtopic.php?f=4&t=27161&p=1905153&hilit=piggyback#p1905153

http://zelfbouwaudio.nl/forum/viewtopic.php?f=3&t=27407&hilit=567

 

[q3cpma]

It might be the good thead to ask: where can you buy sealed servo subwoofers (with XLR out would be nice) at a reasonable price in Europe? Rythmik only sells kits without the cabinet to overseas customers.

 

[pozz]

Actually I am working with Rob:

http://zelfbouwaudio.nl/forum/viewtopic.php?f=4&t=27161&p=1905153&hilit=piggyback#p1905153

http://zelfbouwaudio.nl/forum/viewtopic.php?f=3&t=27407&hilit=567

I hope you can share some knowledge and measurements.

Which aspect of design and models did you work on?

 

[Xulonn]

Jim Salk builds custom versions of the F12, not L12.

Thanks for the correction, Don. I edited and updated my post.

 

[Winkleswizard]

It might be the good thead to ask: where can you buy sealed servo subwoofers (with XLR out would be nice) at a reasonable price in Europe? Rythmik only sells kits without the cabinet to overseas customers.

As for the OP, the short answer is yes, but would look into Velodyne reviews. Naturally, given the impact of the room to distortion at low frequencies, the real question is the extra cost of servo-based implementation. The owner of my local repair shop swears they are worth it!

As Velodyne was acquired recently by a European firm, would expect that they will be easier to find in the European market in the near future.

 

[Purité Audio]

I have the GRIMM version, but I don’t have the older pre motional feedback version to compare.
Keith

 

[HammerSandwich]

Data-bass measured differences with the Velodyne DD18's adjustable servo settings here. I wouldn't say the differences are huge, but they seem to be real. At least below 40Hz or so...

 

[pozz]

The most interesting data concerns FR linearity rather than distortion. Subs are incredibly lumpy. Servos seem to be the main way around that through control of diaphragm movement.

 

[dshreter]

I think there are two questions to ask. One is to what extent a servo reduces distortion and the other is under what conditions is this necessary. So the servo approach likely has merit, and there can be other ways to reduce distortion.

As far as I know, none of the big full range speakers have a servo as part of their designs, so it’s definitely an interesting question if they can address in some other fashion for passive speakers where it’s simply not possible.

 

[stren]

My unscientifically backed uncited opinion would say that a servo helps when..

- you don't have any room correction (i.e. in order to extend or flatten the operating range which room correction could also do)
- you are operating within the "normal" range of the subwoofer, i.e. not significantly distorting. Potentially servoing can help reduce distortion at higher outputs by "pulling back" certain frequencies etc.

If you have an appropriately sized sub or subs and room correction for them then personally I don't believe servos are necessary.