[Freo-1]

After conducting a lot of research about subwoofers, decided that the best approach is to get one with a Servo control setup.  The distortion characteristics are much lower (up to ten times lower), and overall performance seems to be better than a no Servo setup.  Seems like just the ticket for floor standing speakers whose 3 db down point is at 38 Hz or so. 
 
 
Right now, looking at a Genesis G 928 Subwoofer, this appears to be an excellent performing unit.  Makes me wonder why there are not more Servo Controlled units available?

[Rclark]

 You see a lot of reviews on forums like this one, people who have had servo subs, rythmiks, etc, that say modern subwoofers without servos can be as good or better (sounding). But yeah, otherwise it seems obvious, and all subs would seem to want to be servo'd. But doesn't seem to be the case.

 I've read plenty of reviews from people who've owned rythmiks, for example, but stated other subs sounded or performed better.

[Freo-1]

I’ve heard a lot of Servo and non Servo type of subwoofers.  I have never heard a non servo unit for a given size/driver combination that equaled a Servo unit. 
 
My issue with subwoofers in general was getting them integrated correctly.  With additional knowledge, and working with sealed dynamic vs. dipoles main speakers, found out that while still not all that easy, it’s much more achievable to get proper integration.

[Rclark]

Nice shot of the B-17 btw.

[randytsuch]

You see a lot of reviews on forums like this one, people who have had servo subs, rythmiks, etc, that say modern subwoofers without servos can be as good or better (sounding). But yeah, otherwise it seems obvious, and all subs would seem to want to be servo'd. But doesn't seem to be the case.

 I've read plenty of reviews from people who've owned rythmiks, for example, but stated other subs sounded or performed better.

I did a quick search, and couldn't find any reviews like you describe?
I did find a review at AVS where they compared some subs in the around $1000 range, from internet direct companies, and they had a HSU slightly edging a Rythmik 15.

Most of the Rythmik reviews I saw were quite positive.

I do own a 12" Rythmik which I use for my HT, and that I like a lot, so I am probably bias.  But, I have never compared it directly to anything.  It replaced a lower end Energy 12" sub, and it blew it away for HT, but not a fair comparison.

Randy

Randy

[gooberdude]

I'm a bit confused too.  The latest GR/Rhythmik ob servo set-up continually garnishes amazing praise at audio shows & from owners, State of the Art is often the term used.

I own this system, and also 15" Hawthorne Auggies.  Tough to go wrong either way, just can't say i've ever heard of people trying the A370 servo amp + GR ob servo subs and getting rid of them due to sound quality issues   

[cujobob]

Servo technology has improved over the years, I've heard of some issues with older servo stuff. I love my GR rythmik!

[medium jim]

Servo technology has improved over the years, I've heard of some issues with older servo stuff. I love my GR rythmik!

Subwoofers across the board have gotten better over the years.

Jim

[Early B.]

I recently replaced my MFW-15 sub with a GR Research open baffle servo sub on my two-channel system, and prior to that, I've owned lots of subs. The servo sub is more musical and has the ability to extract the nuances from the lower bass notes. Since bass represents the foundation of the music, the mids and highs are also improved, perhaps due to greater clarity from the absence of one-note (i.e., boomy) bass.

IMO, the value of the servo sub really depends on the type of music you prefer. Jazz lovers will appreciate a servo sub, for example. For use in home theater applications, it probably doesn't matter so much.

[walkern]

To some extent it may depend on the listener's priorities whether or not he or she prefers a servo.  I have a good friend with an expensive Rel sub who LOVES organ music.  When I listen to his system I always think 'WOW... listen to how good that sub sounds'.  When I go home and listen to the same music in my system (GR servo) I always think 'Wow... listen to how good that organ sounds'.  My servo doesn't play as loud as other subs I've owned, nor does it call much attention to itself.  It sounds realistic to me.  Bass instruments sound more like they do in live musical events.  And as was noted above, it is easier to blend with a variety of small to large main speakers.  If you want a sub to knock you around the room and that sounds BIG and impressive, there are other options that will give you more of that.  If you want musical realism, speed, control, low distortion, clarity, etc, I think it is hard to beat a servo.

[Freo-1]

To some extent it may depend on the listener's priorities whether or not he or she prefers a servo.  I have a good friend with an expensive Rel sub who LOVES organ music.  When I listen to his system I always think 'WOW... listen to how good that sub sounds'.  When I go home and listen to the same music in my system (GR servo) I always think 'Wow... listen to how good that organ sounds'.  My servo doesn't play as loud as other subs I've owned, nor does it call much attention to itself.  It sounds realistic to me.  Bass instruments sound more like they do in live musical events.  And as was noted above, it is easier to blend with a variety of small to large main speakers.  If you want a sub to knock you around the room and that sounds BIG and impressive, there are other options that will give you more of that.  If you want musical realism, speed, control, low distortion, clarity, etc, I think it is hard to beat a servo.

 
I agree whole heartily.  What I find puzzling is why there are not more Servo subwoofer options out in the market.  (Maybe because subs do not need to be low distortion to in order to reproduce explosions).    
 
 
Decided to snag a Genesis 928 Sub to go with the Cary Signature Oak Model One speakers.  Can hardly wait to get this and put it through its paces. 

[randytsuch]

My guess to your question is that it's harder to make a servo controlled sub.

You have to sense the driver position, and adjust the output based on the position.

It's much easier just to send a signal to the driver.

Harder = more money for engineering development => charging more money for your end product.  More money means lower volume, the more expensive your product is, the fewer you will sell, at least that's what econ theory says.

These days, I think HT is driving most sub sales, and most HT guys want volume and impact for the lowest cost.  They want to shake the house, but cheaply.
These requirements don't really match up with a servo sub.

So, servo's end up as a nitch market.

All this is IMHO.

BTW, if I had to buy a servo sub from a "normal" company, probably would not have one.  I bought the amp and driver from Rythmik, box from Parts express, added my own bracing, and put it together.  So, I was able to build it at a reasonable price, especially for the SQ I get.

Randy

[Freo-1]

Well, I would reckon that audiophiles may embrace Servo subs if they were plentiful (especially to go with monitors or limited deep range floor standing units).  Outside of the DIY, some Velodyne, and Genesis, there are not many out there.  Let’s face it, the entire high end audio market is a niche market to begin with, so why not have more Servo subs as well.  I do think they are a bit more involved to make correctly, and Genesis is a leader in this field.  Arnie Nudell has been working this since the late 60’s.

[Davey]

I got into a conversation a few years ago with a fella who swore by servo-controlled subwoofers and would not be swayed. 
However, I thought it was interesting that he was also hard over when it came to the use (non use actually) of negative feedback in amplifiers.    He would not permit any amplifiers in his system that used negative feedback! 
Just an interesting tidbit there.

Anyways, I'm not sure why you can't find a non-servo controlled subwoofer that performs perfectly acceptably.  Much cheaper.

I don't believe that 'equivalent' servo and non-servo subwoofer systems (all other things being equal) would exhibit differing distortion characteristics by a factor of ten.

Cheers,

Dave.

[Freo-1]

I got into a conversation a few years ago with a fella who swore by servo-controlled subwoofers and would not be swayed. 
However, I thought it was interesting that he was also hard over when it came to the use (non use actually) of negative feedback in amplifiers.    He would not permit any amplifiers in his system that used negative feedback! 
Just an interesting tidbit there.

Anyways, I'm not sure why you can't find a non-servo controlled subwoofer that performs perfectly acceptably.  Much cheaper.

I don't believe that 'equivalent' servo and non-servo subwoofer systems (all other things being equal) would exhibit differing distortion characteristics by a factor of ten.

Cheers,

Dave.

 
The advantages of Servo Subwoofers are pretty obvious, as pointed out below: (From the Genesis manual)
 
“The Servo Advantage:

Very few loudspeakers use servo drive, either because most designers think that it is too difficult to design, too expensive, or because of the extraordinary demands a servo system makes on the amplifier and the transducer. However, the Chief Scientist of Genesis, Arnie Nudell, first  introduced the servo system in the legendary Infinity Servo Statik One in 1968(!) – So we know how to design and build servo systems.
The concept of our servo bass system is an easy one to understand: It employs, an accelerometer as a sensor, to constantly monitor the movement of the woofer cone and instantaneously compares it to the input signal. This comparison circuit instantly identifies any deviation from the ideal and instantaneously applies a corrective signal to compensate for any deviation, resulting in the virtual elimination of the inherent distortion of the woofer.

As an example, when you have a high-impact, low-bass signal that starts and stops suddenly (for example a tympani), the inertia of the woofer cone makes it slow to start moving, and then after it is moving, the momentum of the cone makes it continue to move after the signal has stopped. The sonic result is overhang, bloat, lack of tautness and definition, and a blurring of dynamic impact. With the servo system, the circuit senses that the woofer is not moving as fast as it should, and it applies more current to make it move faster. When the signal stops, it detects that the woofer is still moving when it shouldn’t and applies a counter-signal to stop the woofer faster and more effectively than normal circuitry would.  Thus, the servo-drive reduces distortion and improves transient response by making the woofer seem massless. Typical non-servo woofer systems have distortion levels that exceed 10% at even moderate levels. The Genesis servo bass system reduces this distortion to below one percent at almost any output level. Moreover, it drives the woofer to constant acceleration which makes its frequency response perfectly flat! ”

Still wonder why Servo is a better solution?

[cujobob]

There are a lot of factors to consider when it comes to subs... the GR/Rhythmik sub many users have is the OB version (like in the Super Vs). They sound fantastic but output is limited and they can seem to sound better than other ported subs simply because they're easier to get good in-room responses from. It's not impossible with a ported sub but it takes a bit of work. The ear really doesn't get bothered by distortion (to a certain degree) at low frequencies so in-room response and output are the main factors in sound quality. That can be achieved through a number of means. Myself, I use an OB sub with one sealed and one MFW-15 which is ported. It gives me headroom, smooth response and any time I have a guest over I can tell them I have an open baffle servo sub biatchhh. I like to think it helps me get laid.

[Davey]

It's Genesis advertising copy.  Of course they would tout their superior performance and claims. 

You may want to re-read my sentence about the X10 reduction.  I said "all other things being equal."  That means if say, two identical woofers were utilized...you stayed within Xmax....same box....etc, etc, one with a properly implemented servo-control system and one without were used, then a X10 reduction in distortion would not be possible.  I don't believe that's an untrue statement.
Maybe you can find/quote some other advertising copy that disproves that?

Anyways, if it's so much better a solution, then your original query is very apt.  Why do you think there aren't more available?
I believe the question has already been answered in this thread.  They're too complicated and too expensive for the relative performance gain.  Just my opinion.

Cheers,

Dave.

[Rclark]

I did a quick search, and couldn't find any reviews like you describe?

Randy

  Probably won't in a quick search, but they're definitely out there. This was months and months of slogging through various subwoofer forums. I was surprised when I came across such talk, I just always automatically assumed that servo subs were king, bottom line, end of story. But it's just not the case. It seems a lot of modern subs are indeed competitive in sq, without servos.

[JohnR]

Outside of the DIY, some Velodyne, and Genesis, there are not many out there.

Rhythmik sells completed servo subs:

   http://rythmikaudio.com/products1.html

[JohnR]

You may want to re-read my sentence about the X10 reduction.  I said "all other things being equal."  That means if say, two identical woofers were utilized...you stayed within Xmax....same box....etc, etc, one with a properly implemented servo-control system and one without were used, then a X10 reduction in distortion would not be possible.  I don't believe that's an untrue statement.

I'm wondering what would limit the reduction. Is it the accuracy of the sensor? Excess power requirements on the amp?