ServoDrive Subwoofer??!?

[planet10]

You'll need to go find the bass list archives... that is where all the original ServoDrive discussion took place.

dave

[Zero Cool]

Originally posted by sumsound


Servodrives work great up to 80Hz+/- I'd never run them up to 100-120.

I wouldn't run them at 20Hz either 28Hz is their -3dB in groups of 4.

I would never describe them as sloppy sounding. Though I own the BT7's perhaps you owned the SDL-5's which were maintenance nightmares. I think perhaps you never had them combining well with the upper boxes.

My BT7s have no breakers, just an inductor resistor , and a filter for the fans bridge rectifier. The Servo also has a ballist resistor.

The big problem most people have with the BT7 (Mostly DJ's and people not in the know) is that running the BT7s amps into clipping caused too much HF energy to make it to the Motor, causing the belts to slip and overheat, and the motor to burn up.

I'm not sure why you think they can't handle enough power, remember that 4 of them is ~110dB efficient with just 1kW they put out 140dB 142dB at rated power, and 145dB peak.

It has been 12+ years since i have dealt with these. a night club i did service calls to had a pair on the dance floor. I they could have been the SDL's? I replaced cones at least every other week. they ran them right to the edge. they really should have had four. whatever model they were, they had breakers in them. and when you would overpower them the breaker would open and the fan would kick in high with a big whiirrrrrrr then when the cooled back down the breaker would kick back on again. they dj's would turn it up until the bass kicked out then back it down a notch. and 5 hours of that bass heavy dance music just beat the crap out of them.

My comment about the power handling was for each. yes they are quite efficient in groups and very low power compression etc. I have known some to try and use them singly when they really needed more.

I have heard comments about them sounding sloppy by many live sound guys. not necessarily my own opinion.

[sumsound]

Zero did they have the timing belts with teeth??

[dangus]

I'm not an authority, but I did buy two of the parts kits and built 'em. They're impressive, but they do suffer from "grumbling" distortion at low powers, so the right way to use them may be to a) cross them over very low, and use normal subwoofers for the next octave and b) noise gate the Contra amp so they only respond to big rumbles.

If you're still interested...

The ghost of the Contrabass Corner at Archive.org

To save a lot of fiddling with making your own, get a used Contrabass or horn like the BT-7. Used and/or broken, they can sell for less than we paid for the parts kits.

I suspect the grumbling is caused by the rubber pads taking a set, because I could feel a "notch" when I turned the rotary-to-linear-converter by hand, without the motor or cones connected. Or it could be the motor.

The belts in the kit were flat and smooth, smelled like neoprene, and were probably reinforced with kevlar.

[sumsound]

Originally posted by dangus
I'm not an authority, but I did buy two of the parts kits and built 'em. They're impressive, but they do suffer from "grumbling" distortion at low powers, so the right way to use them may be to a) cross them over very low, and use normal subwoofers for the next octave and b) noise gate the Contra amp so they only respond to big rumbles.

If you're still interested...

The ghost of the Contrabass Corner at Archive.org

To save a lot of fiddling with making your own, get a used Contrabass or horn like the BT-7. Used and/or broken, they can sell for less than we paid for the parts kits.

I suspect the grumbling is caused by the rubber pads taking a set, because I could feel a "notch" when I turned the rotary-to-linear-converter by hand, without the motor or cones connected. Or it could be the motor.

The belts in the kit were flat and smooth, smelled like neoprene, and were probably reinforced with kevlar.

As far as I understand it there is a bit of reluctance on the part of the motor to start moving a low voltages. I've heard it refered to as Stiction, but I don't think thats a real word?

[GM]

It is, it's a blend of 'static friction': http://www.reference.com/search?q=Stiction

Anyway, the belt system has a strong centering torque (the 'notch'), so at very low power the motor may not have enough torque to be linear through its zero crossing stroke. I never noticed it with my two, but then I had them in the corners behind baffle boards to create large BPs, so there was fiberglass insulation to quell any turbulence, cavity/whatever generated noise.

BTW, did you stuff the dust caps with wadding? IIRC I read where some folks that didn't do this heard mechanical noise radiated by them, especially if it wasn't XO'd properly to keep from over-driving them with excessive HF. Then there was at least one person that misaligned the belt system WRT the radiators, causing it to bind at low power.

Gees, if used ones can be had that cheap, then maybe a SD TH might be a viable house wrecker.

GM

[dangus]

No, I never even installed the dust caps.

I did wonder if my assembly was at fault, but reviews of the real thing mentioned similar noises. It can't help that I'm using them in a relatively small room, with a sub only 10 feet from the listening position.

As for used stuff.... maybe talking to local sound businesses (sales, rentals, installers) will turn up some leads. Or patiently searching & watching online ads and brokers.

Does anyone have a service manual for the motors?

[GM]

Yeah, these weren't designed for small rooms. There was one owner that had a tiny HT with a pair up against the rear wall with the seating right in front and he wasn't overly impressed, so sold them and built an IB.

All I've ever seen is brush replacement instructions in the service manual.

GM

[Bry195]

I know this is an old post but i cant resist posting even if im the only reader. I have been in the servo motor, drive, control, and amplifier business for about 17 years. I have always been a self proclaimed audio enthusiast and dreamt of building a science project. A servo driven subwoofer. Hey what do you know a quick search on the internet and of course some one has done it. I cant help but think of ways to do it better which are pretty apparent to me considering i have seen the solutions to these problems evolve since the id 90's.

Servo 101-
How do you select a driver. Inexperienced engineers determine how much force is required to move an object. Usually works but not when you need that magic ratio that gets you the highest response out of the motor to the load. The inertia of the motor should match the inertia of the load. Use your imagination to apply this to speakers. I can have a big voice coil applying an ounce of force or a little one applying an ounce of force. Which will most quickly reverse when the commanded motion tells it to reverse?

Dc motors have a brush that delivers electricity to the rotating part of the motor. We will refer to this as the rotor henceforth. Its not easy to make an electrical connection to something that is spinning so the brushes are slowly wearing away. The brushes are designed to do this and thats where you get the dust. Cmpounding the trouble is it takes 3 times the current to to start a motor from 0 speed. Speaker motion is a series of high frequency starts and stops. So 3 times the current continuously rubbing the brush on 45 degrees of the rotor things go wrong quick.

So whats the answer. First, get rid of the brushed motor. Use an ac servo motor. More control smotther motion, no wear, less heat. You still have losses from friction and a mechanical device that changes the rotary motion to linear. New high end servo amplifiers pulse the motor at 20 thousand hertz. Sounds like the electrical bandwidth is there but can the mechanics keep up with that. Reality is a rotary motor will fall on its face at about 4000 hz because of physics. Ah but a linear motor. 50g of acceleration and 20000 newtons of liquid cooled low inertia bliss. We will call it the hadron speaker collider. A black hole opening near you soon.
Im kidding but in reality this would be a really cool science project. I would stick with the ac rotary servo motor though. Everyone failed to mention that a servo motor is a closed loop system that allows you to do some very high end math to make the woofer (mechanical motion) represent the electrical command exactly compensating for any changes in environment. Like temperature or even wear. That last statement was only meant for reasonable people but the point is the sound generated is more truely representative over a longer period of time.

One thing that baffles me is that everyone is saying the servo system has more spl for every given watt of input power. A voice coil is given an exact amount of current and you get an exact amount of movement with close to no loss in motion efficiency because you dont have belts and frictions and so on. Linear motor are the exact same thing as a voicecoil and they are much more efficient than rotary. Find a belt, rack and pinion, or screw assembly that doesnt steal 5 to 50 percent of your efficiency.

[dangus]

I'm still using the ContraBass on a daily basis, so I'm interested in alternative motor options. IIRC one of the advantages of the servomotor approach was that for long excursions, the coil never runs out of the gap. Long-excursion woofers usually have a lot of voice coil outside the gap, and the portion outside the gap just heats up. Tom may have experimented with a closed-loop version, but I guess the open-loop behaviour is similar enough to a conventional voice coil subwoofer to be modeled accurately. And implementing feedback control makes it more complicated to install and use. But, maybe feedback could compensate for non-linear effects like the motor stiction or notch in the rubber. DSP hardware is a bit cheaper now than it was 10 or 20 years ago. On the other hand, conventional woofers have improved as well, so for the same cost you could build an array of 12" or 15" long-excursion woofers and not worry about grumbling noises or brush wear or awkward impedances.

Here's a better mirror of the ContraBass Corner:
http://phy6.net/downloads/electronic...es/contra.html