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Author Topic: Why no other "motor" driven designs ie BassTech7  (Read 8232 times)

Jeff Babcock

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Why no other "motor" driven designs ie BassTech7
« on: November 20, 2008, 05:17:07 pm »

Just wondering if there is any information why the motor design in the BassTech 7 was departed from?  Is it exclusive property of Servodrive?  Is there some other performance limitation?

Just curious as to why the departure from this design when it was so highly regarded when it came out.  Maybe there have been past threads about them, if so, I can't find them.


Duncan McLennan

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Re: Why no other "motor" driven designs ie BassTech7
« Reply #1 on: November 20, 2008, 05:51:52 pm »

I seem to remember Ivan saying that they were unreliable.
Waterloo & London, Ontario

John Halliburton

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Re: Why no other "motor" driven designs ie BassTech7
« Reply #2 on: November 20, 2008, 06:13:51 pm »

Duncan McLennan wrote on Thu, 20 November 2008 16:51

I seem to remember Ivan saying that they were unreliable.

As Ivan says, "It Depends".  The design is inherently robust, with very low distortion at rated power and beyond-this is where the problem usually originated, as the user would wind up with too few subs for the rig, someone would just flat out push the system until they heard the power compression or other distortion that told them they were now at the "right" power.
Yes, the system is different than a regular loudspeaker driver, but service was performed in the field instead of popping out the four 18" drivers someone blew at the gig the night before and you send them out to your reconer(or do it in house).
Another advantage is the ability to produce smaller finished designs with great performance-the Contrabass subwoofer is only around 8 cubic feet, yet has the ability to produce 114dbspl at 16hz with 200watts of input.
Head to head the Basstech 7 is still a viable subwoofer product when comparing performance, twenty odd years after it's predecessor hit the marketplace.
The original design was patented by Tom, and I believe ran out last year.  There were a couple of attempts at variations on the design, but they did not last.
What has happened in the years since the Servodrive subwoofer debuted?  Existing technology caught up-higher thermal and electrical capacity, better adhesives, suspension systems with greater linear capacity, among other things.  Tom couldn't have produced the LAB subwoofer design back in the early '80's as there weren't any suitable drivers made.
The hard part was getting acceptance in the marketplace and distribution/sales, which doesn't happen just because it's the greatest thing since sliced bread. ;>)

Best regards,


Ivan Beaver

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Re: BassTech7-Clarification/history
« Reply #3 on: November 21, 2008, 08:50:51 am »

Duncan McLennan wrote on Thu, 20 November 2008 17:51

I seem to remember Ivan saying that they were unreliable.

I wanted to clear up what might have been misunderstood.

Back in the mid 80's I purchased the SDL5's from Servodrive. I was the first one in the DC area to get them. Those were the predecessors to the Basstech 7.  A LOT of money for my mid sized company at the time.  Of all the money I have spent (good and bad) in this industry ,that was WITHOUT A DOUBT, the WORST investment I have made.  But that statement requires some clarification-so here goes.

When they were working (which was around 25% the time Mad ) they were quite awsome-once I figured out the importance of signal delay and aligned my tops to the long path in the cabinet. This was pretty much unheard of back then-at least on the level I was at. The bass was smeared-so to speak.

I can't tell you the number of times that I spent HUNDREDS and HUNDREDS of dollars getting repairs and overnight shipping.  Most of the time it was in the range of $600-900 a pop! Shocked   Remember that this was back in the mid 80's, so that was a lot of money. Mad

I only ever had one motor go back.  And that was due to a beer can getting wedged in the rotary to linear converter and locking it up. I think the motor was like $400 plus overnight shipping.

The rest of the time it was stripped "timing" belts and torn straps on the converter, torn cones-due to the belts becoming not aligned straight in the drive mechanism..

Several times I shipped the whole assembly back to Servo (the expensive repairs) and other times I did it myself, but one was not better than the other in terms of actually getting the cabinets to make it through to the end of the show.

I found out years later (after I sold them), that during the time I had them, the belts were not being made to the specifications that Tom had given, and were failing early (teeth getting stripped off the "timing belt" and converter belts "shreading".  I was using these in mostly Heavy Metal shows at the time-so they were taking a beating.  But I did try to "baby" them, knowing what it cost to repair.

For what it is worth, the Basstech 7 is a different design (both in cabinet and drive assembly) and got rid of some of the "problem" components that I was having issues with.  From what I hear, they are much more reliable.

The next issue was the amps being used.  At the time I purchased them I was using mostly Carver PM1.5's.  I asked Servo what amp I should purchase and they said the Crest 8001, that had just come out.  AGAIN, a lot of money for my company, but I did it anyway-trying to get the most out of it.  I was also the first person in the DC area to get an 8001.  Even with just one cabinet per side of the 8001, the amp would shut down due to thermal heating, and it was not being run anywhere near full output. Shocked  

So after talking to Servo and Crest I found out that the SDL5's, being an almost purely inductive load, was the worst thing you could put on an amp.  I had to come up with more cooling for the amps.  The solution was to put put the amps in a pressurized rack that had 2 fans in the back for every fan in the amp.  All the air leaks were sealed up.  This way the fans in the back force a lot more air through the amp-keeping it cooler.

So now that I could actually run the amps harder, the cabinets started failing.  Part of the problem could be that the amp was quite a bit more power than what the cabinets rated for-but it was the amp THEY suggested.

Later I changed amps to the lower powered Crown MA2400.  The difference in sound quality was AMAZING! Very Happy   It ran circles around the Crest 8001.  Yet with my 2x18's, the sound was really close (so as not to matter).  But with the SDL5's there was quite a difference.  Deeper, fuller, punchier etc.

I had the CLP card in the MA2400's to keep them out of clip, along with a limiter ahead of the amp, but still had failures, but less than with the 8001.

From what I understand from users who have used the BT7's for years with no problems, the MA1200 (one cab/side) is the best way to power them.  This gives the 400w @4 ohm load that the cabinet is happy with.

Sorry for the long story, but wanted to "throw it all out" there.  Thanks for reading
For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Jeff Babcock

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Re: BassTech7-Clarification/history
« Reply #4 on: November 21, 2008, 12:27:18 pm »

Thanks for all the info Ivan and John,

Sorry to hear about the SDL5 difficulties you had over the years Ivan.  That must have been very frustrating to see glimpses of great performance but being overshadowed with maintenance issues.  I never heard or saw the SDL5 but the BT7 was just nuts for its time and there seem to be quite a few still in service.

I guess the question is whether it is possible NOW in 2008 to design a motor-based system that is superior to the better driver designs of today.

I realize drivers have come a long way in 20 years, but should the technology to build a motor based system not also have improved?  I would think you could build a motor based system that had its own built in power source and avoid many of the overpowering issues that might have happened with previous designs like in Ivan's experience.  You might also be able to build in a power supply that was better suited for the motor than a standard issue power amp.

I'm sure there is some reason why this won't work or else I'd bet Tom would have been giving it some thought.


Art Welter

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Re: BassTech7-Clarification/history
« Reply #5 on: November 21, 2008, 01:25:38 pm »


As John Halliburton said, “existing technology caught up-higher thermal and electrical capacity, better adhesives, suspension systems with greater linear capacity, among other things”.

The rotary to linear conversion that Tom came up with is clever, but there are a lot of extra parts involved over the standard speaker motor.

Now that speaker companies are making magnet structures deep enough to keep the coil in the magnetic gap over long excursions, the servo motor design benefits would hardly offset the complexity disadvantages.

Motor technology advancements are mostly in the size, weight and heat dissipation areas, which would apply to both types of motors.

Even if you could design a servo motor based system that is superior to the better driver designs of today, the difference would be rather slight.

Selling a different technology was difficult enough when there were obvious advantages, pushing that rock uphill with only a small advantage in output, even if it was achievable, would be a hard sell.

An integrated approach as you suggest would be advantageous, but again, probably only incrementally, and would require a lot of tooling expenses for a technology that was not embraced very well on the first go round.

Art Welter


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Re: Why no other "motor" driven designs ie BassTech7
« Reply #6 on: November 21, 2008, 09:55:46 pm »


Whilst the idea was interesting, the execution was somewhat lacking. Servo were obviously let down by their belt suppliers, but that should have been spotted quite quickly with QC during assembly. The other issue was that the units needed proper maintenance and setting up. Whilst there was a Patent in America, a German claims prior art in Europe. (it's always Germans!) Using 16 in a block was an eye opener. Like Ivan said, a Macro 2400 per unit was a good match.

As others have said, technology has moved on, and how! I am looking at an OEM driver that has

Tom Danley

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Re: Why no other "motor" driven designs ie BassTech7
« Reply #7 on: November 24, 2008, 08:08:03 pm »

Hi Jeff, all

Sorry for the delay in replying, I ate a bunch of nuts a week ago Sunday and got a case of diverticulitis.  Note to self, if your going to eat a bunch of nuts watching the Bears loose a football game, follow it with a bunch of roughage.

I know I have written sort of a history of Servodrive, but I can’t find one to copy / paste.
I can’t promise I am 100% yet but I will explain the mechanics and history.

I wanted to make loudspeakers since I was a kid and had tried it a number of times.
In spite of a somewhat Blues Brothers like outlook on “The band, man”, I had given up on the band and my dream of making loudspeakers for a living and had resigned myself to a life going back to electronic repairs with audio as a hobby.  

At one of these jobs in 1979, a co-worker told me about a position at Intersonics and the idea of making experimental space hardware sounded a lot better than fixing the mechanical Klienschmidt Teletype machines and electronic paper punch / readers I was servicing.

Intersonics turned out to have 6 people at that time, so it wasn’t a big company.
Also, the premise, that high intensity sound could position a sample inside a high temperature furnace while in zero G, had yet to be proven.
My first task was to build a control panel for a sounding rocket experiment, yet I was fascinated with these weird “St. Clair” sound sources.  They had a water-cooled stationary voice coil (inductively coupled to an aluminum bar resonator) and water-cooled field coils.  Perhaps most boggling was a 400lb, 2000 Watt CML amplifier that used 12 little transmitting tubes and sounded like a turbine when you powered it up.

The president of the company, Roy Whymark, was an English Acoustician from WWII, he had worked at Mullard Labs and his team also developed a sonar transducer that was important (some BQS something or other). He had a fiery temper that made one think carefully before speaking, a very sharp but open mind and lots of funny stories from his past.
Anyway, he liked hifi and we slowly hit it off. I eventually was able to by pass the irritating “protection” in his Quad ESS speakers and that helped a bunch too.
Being self taught and having Roy and Dr. Rey (who was a physicist who taught at Notre Dame and worked at Fermi labs) to ask questions was wonderful, I would wave my arms and scribble, they would explain the physics I was talking about.  
They allowed me to try new things in addition to electronics and eventually I wore many hats.  
Perhaps my proudest moment there was getting my first patent or some years later when it dawned on me that I was building space flight hardware, was Principal investigator on an SBIR contract, was considering astronaut training and was directing a couple of Scientist’s and engineer’s efforts.
At that point I stopped feeling bad about my 5 years in high school and understood what Roy’s encouraging words meant about “anyone can have a good idea”.
He was the first successful person I had met who looked only at results and didn’t think that much about academia.

Anyway, I came up with a practical new levitation source and built hardware around it. On the way home from the numerous flights to MSFC Huntsville, I was paging through a surplus catalogue looking at DC servomotors.
I asked Charlie (Dr. Rey) about what a 10ms mechanical time constant meant for a motor.
The explanation sounded to me like they could respond fast enough to produce sound AND I remembered seeing two of the same motors at Harrison Supply, a local junk and surplus store my Dad started taking me to when I was little.

I fooled around with the motor, made sort of a crank throw to drive a flat foam disk and it actually worked.
After blowing up my receiver at home, I brought prototype number three into Intersonics and demonstrated it to Roy and Charlie.
That same day, Roy said if I wanted to pursue it there casually, I could as long as it didn’t interfere with my job and only cost floor space and lights.  Later, when it looked like it might fly as a real product, he allowed me to start the Speaker division.

The Servodrive patent, yes one of the prototypes was a square cube as shown. ;lpg=PP1&dq=%22commutated+voice+coil%22+danley&sourc e=bl&ots=gPFxym9kiM&sig=pw_oQCP-V7LMR9eiCVaF5HAip6A& amp;hl=en&sa=X&oi=book_result&resnum=3&ct=re sult#PPA1,M1

Motor description and picture; rview.html

In the 17 years following that we had proven container-less processing works we added  material sciences division and flew several sounding rocket and two shuttle flight payloads. The company grew to about 45 people at its peak, and the Servodrive speaker division grew as well.
It was truly a sad day when the Shuttle blew up, we were all in Arlene’s office watching it; it took some time before anyone was able to speak.
With the space station work on hold, shuttle flights on hold indefinitely, came the beginning of the end for Intersonics and so to the unique atmosphere that brought about the Servodrive speakers.
I think Greg Bottimer summed his experience up a year ago at an AES meeting when he said “The best job you’ve ever had isn’t supposed to be your first job”.

Howzat work?  
Was probably the most frequent question asked at trade shows.

A Voice coil is a DC motor, It has a limited linear motion capacity however.
Also an indicator of a motors strength is a figure of merit found by BL^2 / Rdc or BL/Sq root of Rdc which gives Newton’s of force per Watt of dissipation.
The polarity of the current governs which direction the force is produced.

It seemed clear to me, that while a long VC allowed more excursion, more of the wire was outside the gap and only functioned as a heater and series inductor.
The idea I had was to “commutate” the voice coil so that power only flowered through the coil where the magnetic field was.  At the time is was on that airplane flight thinking about servomotors, I had no way to get speaker parts modified to try this.
Later, I did get a chance to try it and it does work.
Commutated Voice coil, the idea that actually lead to the Servodrive woofer. bstract&zoom=4&dq=thomas+danley#PPA1,M1

Once you understand what this does, the Rotary DC servomotor is a mechanical arrangement, which allows a recirculation of the conductor wires, resulting in an actuator that can turn either direction indefinitely without changes in operation, while confining the current to the region where the work is done.
From the first prototype it was “simply” a matter of figuring out how to do something with it (which took several years).

Eventually the Servodrives were refined and in spite of a not having full support internally at Intersonics, they became fairly popular on large tours or for special effects.  

Part of this improvement came from a better understanding and ability to model things. At one point I had a mathematician named Dan Riordan who was assigned to help me with acoustic modeling and I challenged him to write a MathCAD program that could model an arbitrary acoustic passage with a thermal gradient.  Once that was done (no small task) we could sort of model what happened in the furnaces.    
Of course, one could kill the thermal gradient part altogether, substitute a real horn geometry and driver and viola, a predicted frequency response on a real horn.  
Being able to model a horn’s response before building was HUGE I thought.
At an AES convention I met and became friends with Dave Martin and Richard Long, a couple of speaker enthusiasts. Dave had his company in England while Richard made giant disco horns in NY.  They were fascinated with the Servodrive system and I was as interested in their approaches to Horn design. They were as different as night and day, both had strong ideas about what was needed and Richard was one of those guys who had a calculator in his head.  Anyway, sadly Dave and Richard are both gone now but if I were going to thank anyone for helping me see deeper into the horn loaded subwoofer, it was these guys and Dan’s programming.  
I concluded that as opposed to lore of the day, the proper driver usually was pretty different from the norm.
One needed a heavy strong driver with a compression ratio, not a lightweight, slightly loaded system as was normal then. Nowadays most bass horns use that heavy strong driver approach.

The first “big” Servodrive sale thus far that was a huge boost for the company was in conjunction with Clair Bros for one of the early Michael Jackson Tours. The Servo’s proved to be much more reliable than the dual 18”s they were using while producing more sound with fewer boxes.  
They were eventually used on a bunch of big tours, like U-2, Def Leopard, Garth Brooks etc, and hundreds were used at several Disney venues.
Intersonics eventually started getting a lot of oddball acoustics jobs too.
We built servodrive based sonic boom simulators for BBN and a larger valve speaker as well for the NASA space plane research. 8995_1994028995.pdf

Even a weird measurement trip with a TEF machine; ramid.php

Which was a result of this movie showing acoustic levitation back then.

Part of what made the Servodrives work was its motor, it lent itself to forced air cooling and it was possible to nearly eliminate power compression.
While most all of the Servodrives past the TPL-2 had this, it was also possible to apply this to a conventional driver with similar results. j+danley

Anyway, the thing with the motor was that it was developed as a computer tape drive capstan. It was an exotic motor that could accelerate from standing still to thousands of RPM in a few degrees of rotation.
Having an extraordinarily low series L, this meant that when presented with a clipped signal, the force produced followed the current while its velocity followed Voltage.
This proved to be hard on the mechanical parts and it became necessary to add a series L to decouple high frequencies. Keep in mind that the motors went up to 4-5KHz on their own.

In the case of the SDL-5, there was an additional issue in that a secondary belt was used to provide a better mechanical advantage. The belt supplier had been switching over to a new yellowish “wonder fiber” and at some point quietly switched over to the Kevlar based core material.
This might have been great for something’s but compared to the original polyester based cores these had a problem with stripping off the teeth when the amp clipped too much.
This was a heartbreaker for some in the field but just as much for the speaker guys at Intersonics who faced a lot of scrutiny and where we scrambled to figure out what made them die.
The solution was found in going back to a one step drive and using a simpler lighter “snail hell” cabinet like the older TPL-3.  This was the BT-7, the most powerful and successful of the Servodrives.

I mentioned the motor was developed as a capstan direct drive motor, as this primary application ‘went away”, the motors became much more expensive. They also had four big bars of alnico magnet weighing pounds (as in pounds of  $$) which held no “gosh” factor.

In the 20 odd years it took to be able to make conventional drivers with similar parameters, the Servodrive motors became ridiculously expensive.
One can get a feel where the BT-7 driver is when you consider that the LAB sub cabinet, having the same exterior size and low cutoff, has a very similar horn.
The lab Sub 12’s (heavy, strong 12” VC drivers) are suitable for that horn while the Servodrive was enough more that 2X15’s were about right, given its strength.

Anyway, at this point I have not hung up the idea of making a motor driven sub again; I simply don’t have a design problem at the moment that can be solved this way.
Also, radiators have there own problems, and it was possible to use a rotary system for that too.  Actually there are several driver approaches we might try in time but it is much easier / logical to find ways to use conventional components in ways that stop some of the existing issues before tooling up drivers there are no stock parts for.
I have to confess I do have a thing for rotary systems as they follow a different set of rules so far as the distribution of mass.
It is highly likely you will see an unspecified “rotary’ driver from us in time.

For those who haven’t nodded or have reached terminal boredom, there are some “olden day” pictures of some Intersonics space stuff at our Web site and some of the stuff I worked on. 2&sa=N&start=10

John Haliburton (Intersonics alumni) has a picture of the first sonic boom Valve speaker prototype posted. The system was eventually shown on “Beyond 2000” as the “speakers from hell”

Well that ended up running long, but I left out a bunch of stuff,  if that helps.
Funny thinking back, I did end up making speakers after all, well you never know I guess.

Tom Danley

Iain i can't think of what Germans or prior art you refer to.


Jeff Babcock

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Re: Why no other "motor" driven designs ie BassTech7
« Reply #8 on: November 27, 2008, 10:54:33 am »

Thanks so much for your response and all this great history Tom.

Much appreciated, a great read.  This is why I love this site.

John Roberts {JR}

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Re: Why no other "motor" driven designs ie BassTech7
« Reply #9 on: November 27, 2008, 12:30:38 pm »


perhaps Bink need to put this in a wiki since  TD is already a historical speaker guy IMO.


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