Thanks a lot for the added details.
So the motor is simply driven straight by the audio signal?
Amazing.
Would love to know its DC resistance, although its impedance must be much higher, not only because it's inductive by itself, but because as a highly efficient transducer , the (considerable) mechanical impedance offered by cone, suspensions + mass of additional moving parts (including coupling belts and pulleys plus rotor inertia) *appears* as an equivalent electrical impedance at the motor terminals.
Yes, I agree, there's a lot of careful research and design behind it.
Personally I'm just intrigued by a novel idea and would love to make "something", even if it's flat only between 18 and 24Hz
, simply as "proof of concept".
I am also amazed that he used rubber, because a narrow strip provides excellent extension force transmission, but very poor on compression ; I had imagined it was some kind of properly heat treated spring steel strip.
Probably the rubber strip is pre-tensioned, so it always works on traction, both ways.
Thanks again.
So the motor is simply driven straight by the audio signal?
Amazing.
Would love to know its DC resistance, although its impedance must be much higher, not only because it's inductive by itself, but because as a highly efficient transducer , the (considerable) mechanical impedance offered by cone, suspensions + mass of additional moving parts (including coupling belts and pulleys plus rotor inertia) *appears* as an equivalent electrical impedance at the motor terminals.
Yes, I agree, there's a lot of careful research and design behind it.
Personally I'm just intrigued by a novel idea and would love to make "something", even if it's flat only between 18 and 24Hz
, simply as "proof of concept".I am also amazed that he used rubber, because a narrow strip provides excellent extension force transmission, but very poor on compression ; I had imagined it was some kind of properly heat treated spring steel strip.
Probably the rubber strip is pre-tensioned, so it always works on traction, both ways.
Thanks again.
Yes, the servo motor is driven directly by the audio signal. As for the rubber belts, they are quite rigid, surprisingly so. The mechanism is built in such a way that there is no noticable issue with them stretching. Keep in mind that the suspension on the drivers is extremely stiff. It appears to be so stiff that I could actually hit one of the driving rod ends with a hammer and still only move the cones about 1/4". I hit it pretty hard with my palm and the cones moved only about 1/8".
Mmmhhhh, interesting, thanks a lot for taking the time to check.
Have you ever measured the resonant frequency of those speakers?
I wouldn't be difficult, just an oscillator, a series 10 to 100 ohms resistor and the original amp.
We are not trying to measure full TS parameters here, not even close, just find a voltage peak .
Thanks again.
PS: and yes, the idea behind those speakers is fascinating: any decent "rotary motor" is typically around 90% efficient, while typical "linear motor" (VC+magnet) is in the area of 1%.
Well, I am a bit hamstrung by the fact that this beast is now tucked in against the wall and I cannot really move it easily because it weighs about 230 lbs. However, I was able to get the DC resistance from the amplifier side - 3.5 ohms. Given that I have a 50 foot 12 ga. cable and the internal wiring in the mix, I suspect it is slightly lower than that. I will try to report back on the resonant frequency soon. I will have to make up a cable to be able to do that so it may be a day or 3.
Hey, thanks a lot, don't worry
Anyway I won't clone one, just make an experiment.
Already have most of what's needed, including empty frames, cones, suspensions, etc.
Plus a well equipped production shop and access to many others.
I saw a home version which I found interesting, with free radiating (no horns involved) drivers, but most interesting was that they used regular fiber cones (cellulose/kevlar?, I have both) impregnated in epoxy .
Cool, because that was the original path I was going to follow.
Way Down Deep II ServoDrive Contrabass | Home Theater
Anyway I won't clone one, just make an experiment.
Already have most of what's needed, including empty frames, cones, suspensions, etc.
Plus a well equipped production shop and access to many others.
I saw a home version which I found interesting, with free radiating (no horns involved) drivers, but most interesting was that they used regular fiber cones (cellulose/kevlar?, I have both) impregnated in epoxy .
Cool, because that was the original path I was going to follow.
Way Down Deep II ServoDrive Contrabass | Home Theater
Ok, now that I have had this setup for a couple of weeks, I have some initial impressions both bad and good.
First thing is the mechanical sound of the ServODrive. While many people have commented on this (well most everybody), I have a different take. As they commented it is most noticable at low volumes, but I say it is most noticable at low volumes on certain songs. Other songs sound fine at low volume. Obviously, it is frequency related but I have yet to quantify that. That being said, this is a sub that was designed for the outdoor PA environment, not as a sub for your home audio/theater setup.
Second note: I am sure that everybody here already knows this, but I want to spell it out. There is a reason that a setup like this is only used in a PA system. That reason is that the speakers are incredibly harsh at close range (near field) but sound extremely good at long range. My listening room is 25' by 15' and they are still harsh at times.
First thing is the mechanical sound of the ServODrive. While many people have commented on this (well most everybody), I have a different take. As they commented it is most noticable at low volumes, but I say it is most noticable at low volumes on certain songs. Other songs sound fine at low volume. Obviously, it is frequency related but I have yet to quantify that. That being said, this is a sub that was designed for the outdoor PA environment, not as a sub for your home audio/theater setup.
Second note: I am sure that everybody here already knows this, but I want to spell it out. There is a reason that a setup like this is only used in a PA system. That reason is that the speakers are incredibly harsh at close range (near field) but sound extremely good at long range. My listening room is 25' by 15' and they are still harsh at times.
Thanks for posting your real world review.
I think that harshness you hear must be similar to what happens with music digitized with very few steps (as in 8 bit audio).
The mechanism would be that at low volume you run out of "steps", the carbons/brushes either touch a certain pair of rotor contacts ... or the next pair, ..... , there's nothing in between.
Some music program may mask this, some other not so well.
At low volume, sound becomes "grainy" .
But the concept is very good, and of course, at high power nothing can beat it.
Thanks for posting.
I think that harshness you hear must be similar to what happens with music digitized with very few steps (as in 8 bit audio).
The mechanism would be that at low volume you run out of "steps", the carbons/brushes either touch a certain pair of rotor contacts ... or the next pair, ..... , there's nothing in between.
Some music program may mask this, some other not so well.
At low volume, sound becomes "grainy" .
But the concept is very good, and of course, at high power nothing can beat it.
Thanks for posting.
I should be more clear. The harshness exists not only with the ServODrive but also with my mains - the Peavey PR12's. Especially in the near field. That makes perfect sense because these speakers were not designed to play at low volumes nor to be listened to in the near field. Overall, I have no regrets (zippy, zero, zilch) in the spending the money to assemble the system because it sounds amazingly good at the intended volumes (loud).
That being said, I think I may need to augment the PR12's with another set of mains on tripods. Does anybody have a suggestion or should I stick with another set of PR12's?
That being said, I think I may need to augment the PR12's with another set of mains on tripods. Does anybody have a suggestion or should I stick with another set of PR12's?
The mains sound harsh beacuse they don't follow the general design guidelines that home speakers do regarding driver spacing and directivity match vs. frequency. That's what you get - position-dependent peaks and dips in the response.The further you get away the less it matters because the path lengths become equal. Higher end PA mains (and larger studio monitors) have big 2" compression drivers that can cross over at 500 hz and horn lenses with a more equal H vs. V pattern. That type of speaker will work nearfield.
Bass horns themselves have peaks and dips in their response. Look at the response of a single box in 2pi in Hornresp. It's bumpy - very bumpy. Those even out when boundary loaded (indoors) or in groups (outdoors). Of course you need to be far enough away to see those effects - and at bass frequencies the wavelength is longer so it takes more distance to do it. Even when you have a group of four of them that measures nice and smooth at 100 feet, put your head inside one of the horn mouths and have a listen. It sounds wonky, highly resonant and totally unlistenable.
Bass horns themselves have peaks and dips in their response. Look at the response of a single box in 2pi in Hornresp. It's bumpy - very bumpy. Those even out when boundary loaded (indoors) or in groups (outdoors). Of course you need to be far enough away to see those effects - and at bass frequencies the wavelength is longer so it takes more distance to do it. Even when you have a group of four of them that measures nice and smooth at 100 feet, put your head inside one of the horn mouths and have a listen. It sounds wonky, highly resonant and totally unlistenable.
You are absolutely correct. Can you recommend some higher end mains that will not break the bank? My mixer is quite powerful at 1600 watts so that will not be an issue.
I built a pair of the ServoDrive ContraBass subs when parts kits were available. After assembling the rotary to linear converters, I noticed that the pads take a set which gives a notchy feel when working the mechanism back and forth. That may contribute to the distortion. I suspect the belts are neoprene reinforced with Kevlar, based on the smell.
Servodrive used specially ordered motors with a higher resistance; the surplus servo motors are usually 0.9 ohms. But, you can use a 2:1 power transformer to convert that to 4 ohms. I went with the surplus motors, and use a big Hammond power transformer.
Servodrive used specially ordered motors with a higher resistance; the surplus servo motors are usually 0.9 ohms. But, you can use a 2:1 power transformer to convert that to 4 ohms. I went with the surplus motors, and use a big Hammond power transformer.
I think you may be right. There is what I thought was an iron core inductor on the back of the panel. It actually may be an autoformer. Like I said before, now that I have it tucked into place, it is virtually impossible for me to move by myself. I will try to move it when a friend is over to help and get some pics of the panel.
That being said, I found that there are 2 of them for sale for a huge bunch of cash:
Intersonics BassTech 7 ServoDrive Subwoofer Speaker Cabinet
If they actually bring that much $$$ on the market, I have a really good deal on my hands as I have about 1/3 of that amount in mine and it is in excellent condition.
Actually, I was able to move the beast so that I could get the back panel off. The DC resistance of the wires going to the motor was 2.9 ohms. And that is an inductor since it has only two wires going to it and is in series with the motor. I am attaching a pic of the back panel. The black and white wires are the ones going to motor. The brown and blue ones are going to the fan motor. Again, I am stumped by why they used a series inductor, full wave rectifier, and a capacitor in the circuit because that motor is rated for 120VAC.
Attachments
dangus is the man...
I know the Patent is long out. Might be Copyright on circuit-boards; but that is not really a problem.
I was going to build a couple of these some many years ago but who has time?
I even had a couple of HUGE IBM tape drives. Don't know if they had an appropriate motor -- a buddy heard me talking about the project and just went out and found a couple of the things at State Surplus Sale w/o doing any research... or even telling me. (Thought having them would somehow (?) magically deliver me time to do it.
I note that FairfaxStu mentions having contact w/ the "current supporters" of the project.
Could you pass such info along to us please, Stu?
Regards,
mds
STILL need time... but that should be coming up in about a year.
I know the Patent is long out. Might be Copyright on circuit-boards; but that is not really a problem.
I was going to build a couple of these some many years ago but who has time?
I even had a couple of HUGE IBM tape drives. Don't know if they had an appropriate motor -- a buddy heard me talking about the project and just went out and found a couple of the things at State Surplus Sale w/o doing any research... or even telling me. (Thought having them would somehow (?) magically deliver me time to do it.
I note that FairfaxStu mentions having contact w/ the "current supporters" of the project.
Could you pass such info along to us please, Stu?
Regards,
mds
STILL need time... but that should be coming up in about a year.
The cooling circuit was designed to make the fan run faster when more power was being used while still presenting a reasonable impedance, previous versions were not "amp friendly".
The belts use aramid fibers, good luck finding replacements
.The SDL 7 low frequency output was impressive for it's time, but now the DSL TH-118 (and similar tapped horns) can outperform it in output using current conventional loudspeakers (the B&C18SW115-4) and are less than half the size.
I don't recall any "harshness" when comparing SDL 7s to my own subs in the early 1990's, sounds like a room problem.
As far as support and resale value, big problems. I have had success with retrofitting SDL cabinets with conventional speakers though. There are not many working SDL modules left, most had hard lives.
Art
Attachments
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The cooling circuit was designed to make the fan run faster when more power was being used while still presenting a reasonable impedance, previous versions were not "amp friendly".
The belts use aramid fibers, good luck finding replacements
The SDL 7 low frequency output was impressive for it's time, but now the DSL TH-118 (and similar tapped horns) can outperform it in output using current conventional loudspeakers (the B&C18SW115-4) and are less than half the size.
I don't recall any "harshness" when comparing SDL 7s to my own subs in the early 1990's, sounds like a room problem.
As far as support and resale value, big problems. I have had success with retrofitting SDL cabinets with conventional speakers though. There are not many working SDL modules left, most had hard lives.
Art
Well, in that case, I am very much in luck because I have a cabinet in near mint condition and a module that works perfectly. I am very impressed by the sound and can't wait to get it to my first real PA gig. I think you are very much correct that room acoustics affect this piece tremendously. I also think I "could" get this module rebuilt if necessary as I made decent headway with the powers that be in support of this beast.
-Stu
Indeed, he said the motor inductance was much lower than a moving-coil driver, and if you clipped the amp driving it the sharp corners in the waveform caused huge accelerations in the system which shredded the belts and cones. The inductor brought the bandwidth down to something more sensible.
The motor was a high-speed DC servomotor from Pacific Scientific, similar ones now cost thousands of dollars secondhand and can handle less power than modern cone drivers.
The cooling circuit was designed to make the fan run faster when more power was being used while still presenting a reasonable impedance, previous versions were not "amp friendly".
The belts use aramid fibers, good luck finding replacements
The SDL 7 low frequency output was impressive for it's time, but now the DSL TH-118 (and similar tapped horns) can outperform it in output using current conventional loudspeakers (the B&C18SW115-4) and are less than half the size.
I don't recall any "harshness" when comparing SDL 7s to my own subs in the early 1990's, sounds like a room problem.
As far as support and resale value, big problems. I have had success with retrofitting SDL cabinets with conventional speakers though. There are not many working SDL modules left, most had hard lives.
Art
BTW, the LabSub is basically the BT7 horn with a modern drive system. Back when these monsters were built, we didn't have high BL, big Xmax drivers like we do today. The Servo mechanism was the best way to squeeze that much air volume into a horn.
LAB Sub Project Documentation
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