So what exactly was so special about the Intersonic Servodrive?

Probably played hell with the PA subs.

 

Here is a nice blurb from one of the pro audio blogs that kind of summarizes other stuff I've read about these. Basically, it explains why it disappeared and is not longer needed. Not sure about the exact 'mechanical motor' deal used to produce the sound, but these days, the powerful subwoofer drivers available seem superior.


'Tom Danley's Intersonic servodrive subwoofers had an Xmax of 12.5 to 16 mm depending on the unit.

At a time when most woofers had an Xmax of around 4mm, the advantages where huge, and with four units of the BT7 having a sensitivity of 110 dB 1 watt one meter, half space, 28- 125 Hz, they were as efficient as cone speakers. There simply were no cone speakers with the BL product or Xmax anywhere near what the servodrive units had at the time.'

 

+1 Makes little sense to have that sort of sub-bass coming out of a backline system (at least for an artist who ALWAYS would have front of house support).

 

Look at what Tom Danley now makes:

http://www.danleysoundlabs.com/

The Tapped Horn subs beat the ServoDrive ones on all fronts. Wouldn't want any of them in backline though!

 

Today there are neo drivers (i.e., standard heavy duty speakers) that have incredible specs for subwoofer systems. I'm just an interesting layman that reads about this stuff. Maybe Alex can chime in, since he posted above.

 

Servodrive was a way of getting high sensitivity and high excursion from a subwoofer driver, although with very limited upper bandwidth. With modern neodymium magnet motors you can achieve similar results. Then do something clever liked the tapped horn (which makes a driver appear to change parameters by driving the higher low frequencies with both sides of the cone in phase and then the lower low frequencies with just one side of the cone) to fully leverage a huge excursion, huge voice coil, high moving mass driver.

 

http://www.danleysoundlabs.com/products/subwoofers/tapped-horns/

 

I don't know about how the horn works, but one reason for pointing the speaker "backwards" is to suppress direct radiation from the dust cap.

 

I've seen Billy perform years before he used the Intersonics, as well as the year or 18 months he actually used one, and the many years after he stopped using them. He didn't sound remarkably better when he had them. They were designed and conceptualized to amplify sub-lows. They did that well but most of what they excelled at isn't of optimum usefulness for bass guitar. If they were, Billy and most everybody else would be using them still, and they aren't.

 

On a forum where many users balk at carrying an 8 lb. head when they can carry a 6 lb. head, I don't think many people will be buying a 412 sub that weighs 357 lbs

 

An astute observation.

 

The reason the speakers are backwards is that there isn't any rear chamber, so they wouldn't fit facing the other direction.

That's not right. A tapped horn and a bass reflex both use both sides of the cone. A tapped horn is more efficient than a bass reflex because it's a horn. There's nothing about a tapped horn that's magical, it's a simple variation of a rear loaded horn that's been around since the 1950s. It's only advantage over a regular horn is that it's smaller, because it doesn't have a big rear chamber for the speaker. It's disadvantage is that it needs a longer excursion higher power speaker than a regular horn, so they cost more.

 

https://en.wikipedia.org/wiki/Subwoofer

"Servo subwoofers

Some active subwoofers use a servo feedback mechanism based on cone movement which modifies the signal sent to the voice coil. The servo feedback signal is derived from a comparison of the input signal to the amplifier versus the actual motion of the cone. The usual source of the feedback signal is a few turns of voice coil attached to the cone or a microchip-based accelerometer placed on the cone itself.[29][30] An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible.[31] The primary disadvantages are cost and complexity.[32]

Servo controlled subwoofers are not the same as Servodrive subwoofers whose primary mechanism of sound reproduction avoids the normal voice coil and magnet combination in favor of a high-speed belt-driven servomotor. The Servodrive design increases output power, reduces harmonic distortion and virtually eliminates the loss of loudspeaker output that results from an increase in voice coil impedance due to overheating of the voice coil (called power compression.) This feature allows high power operation for extended periods of time.[33][34][35] Intersonics was nominated for a TEC Award for its Servo Drive Loudspeaker (SDL) design in 1986 and for the Bass Tech 7 model in 1990.[36][37]"

 

why hasn't anybody mentioned that for my stack? the 412 puts out 140db!