DIY Servo subs. (1 Viewer)

[Thomas M.B.]

I am looking at building my sub and have read about servo subs which use a servo motor powered by an amp to drive the sub instead of a traditional magnet-vc setup. Does anyone know anything more about these? Are they accurate. I know you can get a kit for $412 which comes with a 15, two 18 passives, and the rotary to linear converter for a servo motor. I've also heard of paradigm's servo sub but I don't think they meant the same thing. Maybe someone can enlighten me Thanks.

 

[AlexKunec]

I believe that servo subs just use some kind of a servo to detect how the driver is actually moving and comparing it to the input signal. It then alters the signal so the sub is producing what it should. Just my 2 cents, not sure if im 100% correct.

 

[Dustin B]

I think Thomas is talking about something like the Contrabass. If so there are a few people here who can help you out. Mark Seaton being one of them I believe.

 

[Mark Seaton]

Hi guys,
As noted, our ServoDrive mechanism shares only the term "servo" with the servo-feedback systems of many other manufacturers which employ accellorometers or other driver monitoring mechanisms. In our case, there is NO feedback or correction system, it isn't needed. The rotary motor we use is termed a Brush DC Servomotor within it's more common industrial use. These motors are used in many control applications, and also in some of the huge computer tape drive storage systems among many other uses. The word "servo" is appropriate in the motor's normal use as a feedback device is commonly used to monitor speed, position, or whatever might be of interest for the driving electronics or the application. In our application, there is no need for monitoring or feedback.
Knowing the evolution of our patent and the idea might help with the general understanding. Tom Danley, our designer/inventor, while working on acoustic consulting projects with Intersonics, needed higher output sound sources for a variety of applications, and he has always been and audio fanatic. His first idea along this path can be found in his patent for a commutated voice coil. The general concept is to use brushes to only drive the part of the voice coil within the magnetic gap, making the system much more linear and efficient. While he did get a prototype to work, he quickly made the realization that a brush commutated motor works in the identical manner, but with multiple "voice coils" and magnets which spin rather than moving in and out. The rotary to linear converter made this an easier and more flexible solution than the first patent, and this was pursued. In the end, a servodrive system is just another driver with unique specifications and capabilities. Just like a midrange differs from a woofer, the servodrive mechanism has it's own unique set of parameters.
As for the kit, this came purely from Tom Danley's generosity in making an offer to a DIY community known as the BASS List (now DIY Speaker list). Nathan Sargent did quite a job of collecting information from this project on the ContraBass Corner. I presume this is where the above prices came from. Note that this offer was years ago. Since then, the site was never taken down as it had some very interesting information, but I will have to have Nathan kill the prices page. The cost of the replacement parts for the cones and passives alone well exceed the listed $412 price. This was done at little or no profit "back in the day." Since it frustrates myself and others at ServoDrive to see Tom not driving his kids around in a properly running car from the last decade, we do need to make some profit to keep things going and to be able to bring to market the pile of ideas still waiting.
More recently Mike Early of Early-M.com was selling kits. We had planned to terminate this offer as it wasn't being well supported, and wasn't doing much benefit to anyone. I haven't been able to pull up early-m's page for a month or so, and I haven't had time to address the issue... this is why. I believe Early-M was selling the kit you mention above for about $700 or so. This was arranged before me, and again, has little profit or incentive to support it. As you can imagine, making the kit more expensive makes it less appealing to the bargain hunting DIY crowd. What many should also realize is that while you can certainly find surplus motors, they range from 0.5-1 Ohm DCR, where our custom motors are closer to 3-4 Ohms. Surplus motors with "close enough" specs run $80-120 where ours cost US 4-5 times that.
I am debating making a final offer for some kits directly through us, but in the end, the kit offer is going away. With drivers like Adire's HT XBL^2 driver seen in this forum, the cost benefit for DIYers is much less than before. Furthermore, we are a small company and don't have the personel to support such an offer, and I haven't seen anyone capable of supporting the kit and a business around it. I am hoping that as we grow in the custom installation market, and we develop next generations, we will eventually be able to deliver ServoDrive performance at more attainable prices(watch out Velodyne ).
If there are some very ambitous DIYers who aren't afraid to take on a complex project, feel free to post here and e-mail me. I can probably be convinced to make a few parts available if you're nice. For the record, our service parts carry the following prices:
15" Cone Assembly (ContraBass) $130 ea.
18" ContraBass Passive Radiator $140 ea.
Belt Arm Assembly (ContraBass) $165
There are a few other parts needed such as the mounting for the motor and such which is a machined aluminum plate with mountings. Adding up the numbers, I don't see us selling a kit for much under $1k, without a motor.
To some I knwo the high efficiency, low distortion and power compression would be well worthwhile if they have the space to hide the decorator-unfriendly box, but I also know the interest will be limited.
Regards,