My New PA Setup w/ ServODrive

[dangus]

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.

[FairfaxStu]

Originally Posted by dangus

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.

[FairfaxStu]

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.

[its1110]

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.

[weltersys]

Originally Posted by FairfaxStu

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.

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

[FairfaxStu]

Originally Posted by weltersys

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

[David_Web]

IIRC Danley said that the inductor was there to limit current into the servo motor as it would otherwise tear off belts and cone with time. The motor apparently had a little too good HF response and oompf and had to be tamed to prevent damage, or at least minimize it.

[iand]

Originally Posted by David_Web

IIRC Danley said that the inductor was there to limit current into the servo motor as it would otherwise tear off belts and cone with time. The motor apparently had a little too good HF response and oompf and had to be tamed to prevent damage, or at least minimize it.

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.