Servodrive

Well, I'm at it again...

I need a project to keep me busy and happen to have two 15" passive radiators somewhere (I know I have them, I know they're special somewhat, but where and what?)

So I thought...what uses two 15" pistons?

Servodrive!

Now how do I go about thinking about this one - [img]smileys/smiley5.gif[/img]

Here's a few pics of the real thing I found...

I looked at this design (motor) a few years ago, to me it's a bit 'Antique'and not as efficient as it could be

to me , the more gears and arms you have the less efficient and responsive it would be

look at the first picture! tooo much going on

what i would have put in there is a piston that moves forwards and backwards + -

like a bike pump thats shifting air both ends

Edited by: mykey

You'd be surprised how efficient (400W and can smoke most VC based driver systems on efficiency in a similar sized cab...), responsive, quiet, and simple it is in real life Mykey...the linkage is much simpler than it looks, the minimum for translating the turning movement to linear movement.

It actually functions much like what you are talking about regardless of the appearance, a pair of pistons with the surfaces in push-push.

If you had a plain piston, you'd have to get a way to push it back and forth - servomotor with simple linkage is a very good way imo - I really don't want to fool around with linear motors...which leads me to voice coils don't it?

Does anyone have an idea of what specs in a servo drive I'd be looking at?

I've got my own sort of system in mind...

One of the biggest downfalls about that design is the expense of the belts when they break. The distortion is very low and, without hearing any distortion many snapped the belts because they never knew they were overdriving the woofers.

By the way, the Lab Sub is/was the Servo Drive Bass tech 7. Tom Danley probably made some minor adjustments for the Lab 12s but the overall concept is the same.


Best Regards,

I realised that already, but thanks anyway.

Have had a few friends who warned of this if I ever got into servo madness - no telltale distortion before the break.

Differences between the Lab and BT7 i am aware of. Both use same folding but:
BT7 has 15" pistons
BT7 has that expansion in another plane bit near the throat

True, folding is the same as well as push-push loading.

The frequency response is pretty much the same as well. I remember when the whole project began and was pretty surprised that it followed the same pattern as the Bass tech 7 once completed.

I would imagine that the design patent expired and, he decided to give it away than have someone steal it and sell it for a profit.

It could be why under the terms and conditions of building the Lab Sub, it is ilegal to sell a completed design to the public.

Best Regards,

the rotary to linear converter is a marvel and will be hard to replicate without fighter jet engineering quality. why not look at replicating a rotary servo sub? like the phoenix gold 'cyclone'. i have visions of a horn assisted cyclone cabinet. the fs on the cyclone is 19hz, but you can determine the fs yourself by changing the tension on the centering device.

this is why i thought the tas sub might be a rotary due to the lack of fs spike.

for suitable servos you should look at pacific scientific servos. you need low inertia types. i remember reading info on a website called the 'contrabass corner'. i thik they managed to replicate a spl contrabass.

That website has now disappeared, I remember it too.

I gave up on this a while ago (am more busy in work than I have been for a while) - i have three diy projects on the bench, and an ac at work that needs some designing. I reckon I could do the 'fighter jet assembly' thing, but it now looks like too much work. It doesn't look very complex - metal and rubber bands. The materials would be challenge, if did it.

AFAIK, it was danley behind the phoenix gold cyclone. I recall that there's a quote of his saying how their implementation was vastly inferior to his original design (IB or OB oriented I think) as they had tried to optimise it for too many different box designs. I don't think a fan can work in a horn - think about the basic theory. Pressure at the throat -> velocity at the mouth. The fan can't build up pressure, thus no horn. Besides, you would need a massive horn to make it worthwhile. It's pretty useless in pro sound terms, but would be insane in a cinema.

There is now a topic on PSW showing how the original servodrive motor assembly is matched by todays driver tech, I think it mentioned the TH-115 or something.

Found the quote from Mark Seaton:

The Cyclone is based on a revolutionary new technology that uses the rotational force of an unconventional moving_magnet system to drive a radiator directly. This new system converts low frequency signals into acoustical output without the physical limitations of conventional voice coils, cones, spiders, and surrounds. The result of this innovation is a subwoofer with ultra low distortion and 3 times the displacement of a conventional 12_inch subwoofer.

The Cyclone looks like a modified ducted fan _ a 12" outer cylindrical housing with what appear to be two simple fan blades attached to a central shaft, but with the addition of a pair of vanes. The motor uses a moving neodymium magnet within the rotor, with the VC anchored to the stator. This gives it excellent power handling (300 W, continuous).

Like any other driver (even the Contra's), the Cyclone also produces a back wave and so the same enclosure decisions have to made when designing with it as with a more conventional driver. This is a little difficult because normal Thiele_Small parameters are largely meaningless for a driver of this type. (It is worth noting that Fs is quoted _ at 11 Hz!) As guidance, Phoenix Gold recommends its use in a 3 cu.ft. sealed or 3.5 cu.ft ported enclosure. Due to the extreme excursions possible with this design (125 cubic inches/2 liters displaced), to use it in a passive radiator (PR) design, you'll probably need two 15" PR's.

However, according to Danley, the real key to using this driver well (he has them installed in his own listening room) is to provide sufficient air volume. Continuing the comparison of a Cyclone to a fan, one thing every engineer knows about fans vs. pumps (conventional drivers act as pumps) is that fans can move more air, but they can't generate much pressure. The vanes in the Cyclone prevent it from working exactly like a fan, but neither does it work exactly like a pump either. Still, like a fan, it works most efficiently when working into large volumes. Therefore, to effectively use a Cyclone, you don't want it working into a small contained volume. It would, however, make an excellent driver for a dipole woofer.

"It appears that the product is discontinued. I wonder why. They couldn't tell me. I tought it was rather fun to buy one and test it, but alas... You might find it interesting that our own Tom Danley designed the Cyclone and ServoDrive liscenced it to Phoenix Gold. Unfortunately Phoenix took too many design liberites before final production and all the quality aluminum parts in all prototypes became cheaply molded plastics with poor tolerances and rigidity. Also, the original design was for an IB use, and they then tried to change it to work in many applicaitons and ended up with something which wasn't quite optimal for any alignments at all. For what it's worth,

Tom had a similar pair of these units loaded into a crawl space which had response down to 11_13Hz (I forget). The final Cyclone ended up having some reliabliity problems along with odd parameters for the car market, and ultimately died. Interestingly, I believe one of the Hi_Fi rags had noted a Cyclone in a sealed box to be what he thought was a reference subwoofer and one of the top innovations on the market... and this was a while after the product was out.

Thanks to Mark Seaton from Sound Physics Labs Inc. for the above comments about the Cyclone.

So it really needs an IB...