Spectra 1100 Woofer Replacement Problem:

Both of my Acoustat Spectra 1100 dynamic 8" woofer driver cones have become separated from the rubber surrounds. So now I'm trying to find proper replacement drivers. Having the speakers out in my unheated sun porch may have been a factor in their failure. 

But the search is not so easy. The drivers were custom made for Acoustat, and the specs seem to have disappeared. There is some information on the Electrostatic Circuit, now the Audio Circuit. Unfortunately it's a frame based site, so you have to navigate through the very deep site, on every visit, and I can't give you specific links. But still highly recommended.

But no one, including ex Acoustat and Rockford employees, have really come up with a definitive solution. 1100 Woofer

After the first one died, a friend of mine measured the remaining good driver and you can see the results in the table below. I'm not so sure what's going on with the large differences in the numbers ?!!?  So, still no warm fuzzy, 'This is the One' feeling...

None-the-less, here's the information I've been able to gather, and my money is on Twinstatic. That is, when I can find a little extra to afford thier drivers !

An alternative to replacement is to fix the driver by re-gluing the surrounds back onto the cones. A suitable adhesive is a contact cement, 3M's Super 77 (try a Michaels store). Spray some into a cup and apply with a small brush. The trick is aligning the cone properly to maintain the coil gap and prevent any rubbing. Easier said then done !

Spectra 1100 woofer Specs:

The Spectra 1100 woofer was custom designed and manufactured by Rockford's Carbonneau division (now called RAD, or Rockford Acoustic Design). Two slightly different woofers were used during the 1100's production. Despite slight differences in appearance, they are considered to be sonically identical.

The original woofers were 8 inch diameter, 4-ohm, drivers with 23 oz. magnets, rubber surrounds, black paper cones, and oversized voice-coil caps. They have a very low resonant frequency (f3), about 28-30 Hz. A suitable replacement should have a compliance suitable for a well-controlled behavior in the 1100's woofer box volume (about 1.3 cu. ft.). 

The 1100's crossover begins to roll-off at 6 dB per octave around 150 Hz, and then steepens to 12 dB per octave above 250 Hz. The woofers are capable of credible bass response down to at least 40 Hz. And they were actually designed for rather low efficiency, to match the low efficiency of the electrostatic panels.


Thiel-Small Parameters:

Acoustat 1100 Woofers - Thiel-Small Parameter Comparison Table

Measure My good 1100 woofer 1100 woofer - CLIO Twinstatic replacement woofer - CLIO Peerless 220WR / 831709
D 8" 170mm 170mm 8"
Re (ohms) 3.38 3.4 3.4  
fs (Hz) 44 58 41.8 25
Qms 1.89 5.39 6.61 2.65
Qes 0.647 0.86 0.56 0.54
Qts 0.482 0.75 0.52 0.45
Mms (g)   34.45 22.63 33.4
Cms (mm/N)   0.22 0.64 1.21
Vas (ltrs) 2.16 15.72 46.08 86.9
B1   (N/ft 3)   7.03 6 6.9
L1K (mH)   2.18 1.11  
L10K (mH)   0.81 0.48  
Ro 3.92      
F1 (Hz) 73.2
F2   (Hz) 27      
Fct (Hz) 96      


Twinstatic Audio Replacement Driver:

This is the reply to an inquiry I made to Twinstatic Audio regarding 1100 replacement woofers:

"We measured the original woofer and found a replacement driver made in Germany. There is no audible difference between the original and the new one, tested with mono music signal.
We can ship you one or two for $ 30,- /pieceTransport: $ 25,- for max 2 units (max 5 kg. weight)

J. van Huizen
Twinstatic Audio
Zandvoorterweg 84
2111 GZ Aerdenhout
The Netherlands
Chamber of commerce nr .:
Bank account nr.: - BIC code: ABNANL2A
VAT nr.: 069.782.568.B01

They were also kind enough to send me some info on the drivers:

Acoustat woofer scan     replacement woofer scan


Here is another mans solution:

"Using the Bullock Box design software, I seem to have found a good replacement. It is a Peerless 220WR (Specs PDF) and in a 1.36 sealed box, it is down -3dB at 40 Hz. It can also be found for $38.00 at www.madisound.com."
Chris Behrens September 1, 2001

For other Acoustat parts and service:

'Sounds Like New' 
"While I closed my Sounds Like New service center years ago..... I still offer restorations, improvements and upgrades for Acoustat Interfaces, Power amps and Preamps, on a part-time, by appointment basis."
Acoustat Service / Roy A. Esposito / Florida
contact : soundslikenew@prodigy.net
Boynton Beach Fl. 
Phone: (561)736-0041

'Analogue Associates' 
Servo Amp Upgrades
Michael Savuto

  (Services Info PDF)


For replacement bias supply transformers:

All Electronics Corporation

Part number: ACTX-154 
Input: 120VAC, 60hz
Output: 15VAC, 400MA, with a 3.5mm termination plug
Price: $3 ea.