What's the best 'active servo' sub on the market for the money? (1 Viewer)

[Bruce Hall]

Servo is just a fancy name for a "limiter" ,though it's a bit more complicated but more or less that's what it does.

Sorry, Lewis, I have to disagree with you there. A true servo sub uses an accelerometer mounted on the cone to report to a comparitor circuit the acceleration of the cone in real time. This info is then compared to the input signal and the sub amp output compensated for. If you can do this with enough loop gain (i.e. enough times per second to do any good), you get a system that delivers output much closer to the reality of the input signal than conventional designs.

As an example, if your sub is trying to play 20 Hz, and the cone is traveling inward and compressing the air in the box (assuming a sealed box design), the air being compressed is resisting the cone. The smaller the box, the more the resistance. Without the servo, the output wave would be distorted due to this air resistance. With the servo, the sub knows that more power is needed to push the cone in and resist the air, and delivers it, thus creating a wave much closer to the intended signal.

A servo loop sub might send more or less power to the cone, depending on what's needed to perfect the output signal.

A limiter, on the other hand, generally senses excessive input signal and limits it. Thus it does not allow the cone to overshoot its travel limits, or otherwise excessively distort or mangle the output.

Both these systems have their place in subwoofer design, but they are very different animals.

Bruce

 

[Lewis Besze]

Sorry, Lewis, I have to disagree with you there. A true servo sub uses an accelerometer mounted on the cone to report to a comparitor circuit the acceleration of the cone in real time. This info is then compared to the input signal and the sub amp output compensated for. If you can do this with enough loop gain (i.e. enough times per second to do any good), you get a system that delivers output much closer to the reality of the input signal than conventional designs

Yes I know what Velodyne try to do.
I was merely "simplifying" the process.Your goal isn't protection per se but low distorsion,however at high volumes your servo technology could limit the driver's excursion in order to meet the distorsion limit[1% if I remember correctly],at that time it becomes a "limiter".It also no longer represents the input signal,as it doesn't reproduce it faithfully.

 

[Jaime B]

Bruce:

Can I get a free HGSII-10 after I recommended it to Kyle?
Ok, just call Mike and Harry at Precision Audio and tell them to have a shinny brand new HGS-10 for me. You'll have to ship it first, since they don't have any in stock (as of last week).

Thanks in advance for your incredible kindness,

JaimeB

 

[Matt Sprouls]

Kyle,

Sounds like I was looking for the same thing you were.

I listened to the Paradigm Servo-15, and PW-2200.
I also listened to the Martin Logan Depth and Grotto.

I went with the ML Depth. It blends with my mains great.
The size is nice also.

The Servo-15 will go lower and louder.

I would have been happy with either. The way it blended and the look/size of the Depth is why I went with it.

The James Sub looks promising also, but I have not heard it.

Regards,
Matt

 

[Kevin C Brown]

I really like the technology in the James, but ... Only has a 0/180 deg phase switch. I want a variably-adjustable phase knob.

Man, I can't believe y'all trashing Velodyne. Insinuating that they use servos to get away with using sub-par components? I have had Velodyne HGS subs in my system. I liked them a lot. One of the better built components I've ever had. Remember, the servo is there to lower distortion at low freqs and high volumes. Not to prevent the sub from reaching it's full potential. It's the servo that *allows* the sub to reach its full potential, without bottoming out and with less than ~3% distortion. Go read some of the reviews that Tom Nousaine, D.B. Keele, and others have done with measurements *and* listening tests.

There *are* other ways to get loud, low freq, low distortion, good sound quality bass. But Velodyne's and Paradigm's servo technology have both been on the market for years and have certainly proven themselves with audiophiles everywhere.

 

[Jack Gilvey]

Remember, the servo is there to lower distortion at low freqs and high volumes.

Those two things are typically mutually exclusive, so the deep bass gets rolled off to lower excursion/distortion. They are much more idiot-proof than something like the limiter-less SVS or Hsu subs, so are a better choice for many.

 

[Bruce Hall]

I was merely "simplifying" the process.Your goal isn't protection per se but low distortion,however at high volumes your servo technology could limit the driver's excursion in order to meet the distortion limit[1% if I remember correctly],at that time it becomes a "limiter".It also no longer represents the input signal,as it doesn't reproduce it faithfully.

Lewis,
What you said is generally true, but let me add to it a bit. Yes, our goal is low distortion - a precision sub that is as accurate as possible, and at most playback levels we are well under 1%. Sometimes accuracy and loudness are mutually exclusive - distortion products can make 30 Hz sound quite a bit louder since the harmonics are easier to hear than the intended note.

By the time the input signal is so large that it has exceeded the physical limits of the sub (either clipping the amp or causing over excursion of the driver - a phenomenon that happens at the same time if the amp power matches the sub correctly), we do have a "limiter" (we call it a gain compressor) that limits the output signal. This is the circuit that is limiting the output, not the servo loop. Even the gain compressor can get a bit overwhelmed sometimes, and that will drive our subs above 1% distortion. This is at pretty high playback however.

Jaime,
I'd like to help, I really would.
However, the HGS-10 is soon to become even rarer since we are now shipping the next generation of HGS called Digital Drive (DD).

Bruce

 

[Sihan Goi]

Intriguing discussion here

So is the limiter actually built into the Servo system? Do Velodyne and Paradigm implement Servo systems the same way that Bruce described it, i.e. with an accelerometer that provides feedback to the amp? What about Yamaha? Does Servo in subwoofer terms only mean this, or are there other forms of "Servo" systems. Also, what's "Active-Servo"? Is there a "Passive Servo"?

Sorry for the newbie questions

 

[Brett DiMichele]

I guess any feedback loop/processor could be construed as
active under the Velo and Digm useage of "servo" technology.


Servo-Drive's subs are very "active" in that they use a
precision servo motor to actualy drive the cones (no normal
motor structure at all...) interesting stuff from all sides!

 

[Bruce Hall]

Do Velodyne and Paradigm implement Servo systems the same way that Bruce described it, i.e. with an accelerometer that provides feedback to the amp? What about Yamaha? Does Servo in subwoofer terms only mean this, or are there other forms of "Servo" systems. Also, what's "Active-Servo"? Is there a "Passive Servo"?

Sihan,
There are many uses (and a few abuses) of the word servo in the market. Let me tell you about two prominent servo systems that I know about.

The first one is where there is an accelerometer mounted on the cone - we call this our High Gain Servo (HGS) system and we named a line of subs after this acronym. We have patented this method, and found that it provides the most benefit in that it provides the most loop gain (that is, corrects the cone movement the most times per second).

BTW with Digital Drive we've gone to a digital accelerometer that has enabled us to increase the loop gain from about 3,500 times per second to about 15,000 times per second.

Other transducers have been used to detect cone movement and report the result to a comparitor circuit. There was a "positional" servo that reports the cone's actual postion. We tried this approach (I beleive Infinity had a unit like this a while back) but could not get enough loop gain to do much good. A microphone mounted inside the cabinet has also been used with some success (measuring Sound Pressure Level, not actual cone movement), but we found that, while beneicial, time delay made this approach not as good as the accelerometer method.

There is another servo approach called current sensing. This works by sensing the change in resistance of the voice coil (a.k.a. back-emf) which changes depending on how hard the coil is trying to reproduce a piece of music. A circuit in the amp is created to sense this change and react accordingly. This approach is not as effective as transducer-based servo approaches, but it does add benefit and is relatively inexpensive to implement. We use this technology in our CHT-15 product, and I believe this is how the Yamaha servo works.

These are the two main approaches to servos that I know about. I hope you find this useful - sorry for the long post.

Bruce

 

[Lewis Besze]

distortion products can make 30 Hz sound quite a bit louder since the harmonics are easier to hear than the intended note.

True.But there are other type of subs then "distorsion products",and Velodyne on the market as well.

 

[Sihan Goi]

Thanks Bruce, that was really enlightening...you mention the "positional" servo system, but how does the system know where the cone is in the first place? How is the cone position fed back to the system? What mechanism is it using?

Also, do you, or does anyone know what sort of Servo system the Paradigm Servo-15 is using?

Thanks again!

 

[ChrisHeflen]

Bruce, whaddya mean sorry for the long post? What an honor for you to even be posting here. Thank you, you big famous person!

I think Paradigm calls their servo system "closed loop".
I have no idea what that means, but if Bruce is allowed, maybe he could tell us what it is and what the differences between what they're doing and what Velo is doing.
Also Bruce, lower the prices on the new DD subs would ya?

 

[Bruce Hall]

What an honor for you to even be posting here. Thank you, you big famous person!

Well, according to my bathroom scale, the big part applies anyway!

Bruce

 

[ChrisHeflen]

Whoa!!!
I can't wait to tell all my friends that The Bruce Hall responded to one of my posts!

 

[Kevin C Brown]

I just realized that too. Wow, thanks for dropping in Bruce!

 

[Kevin*Harley]

Bruce,
I'm jumping on this thread a little late but just read your post about the DD and HGSX series.

I have a HGS15II and love it. Besides the increase in loop gain with the DD series. What additional motivation will I have to upgrade? Where can I find info.?

 

[Bruce Hall]

Hi Kevin,
DD will have quite a few improvements, let me try to list some of the key ones.

On-board room equalization. The unit comes with a microphone and has audio and video out. You hook up the video and audio out to your system using the supplied cables, and the unit sweeps out a 15 - 200 Hz tone that is played by both the sub and your speakers. The mic picks it up and the frequency response of the room is displayed on a graph on your TV. Below the graph are 8 graphic (i.e. fixed frequency) or parametric (i.e. frequency and Q are adjustable) equalizers that can be used to flatten out the response, just like with a Rane or similar unit.

Presets. There are 6 presets (action/adventure, movies, rock/pop, jazz/classical, custom, and EQ defeat). Each preset is preprogrammed but can be customized to taste. You can adjust volume, and theater/music (see below), and each preset has one extra EQ to contour the response.

Theater/music. Lets you choose between a "musical" or a "theatrical" sub. This adjustment affects the loop gain of the servo. The less the gain, the louder and less clean the output (for action adventure), the more gain the tighter it is (for jazz/classical, etc.). At the max. musical setting the servo is correcting the cone movement about 16,00 times per second. Adjustable for each preset.

Other on screen controls. There are on screen controls for subsonic and low pass crossovers, phase, polarity, volume, night mode, and auto on/off.

RS232. There are RS232 in and out jacks for control via universal remotes (e.g. Crestron), daisy chaining, and for programming updates.

Cabinets. They have been restyled, with black gloss, maple, and cherry finishes. They have an Electro-luminescent light on the front.

The whole unit is computer controlled via a Texas Instruments DSP microprocessor. We did the programming here.

Re information, we've been so focused on getting them out that we're now just finishing the materials. Look for them soon on the web site.

Hope this is informative.

Bruce

 

[Kevin*Harley]

Very informative.

Thanks!

Last question....................... $ ?

 

[Bruce Hall]

Suggested Retail:

DD-10: $2,499
DD-12: $2,999
DD-15: $3,999
DD-18: $4,999

Bruce