Spectra 1100, 1100 woofer replacement info available

Read more on the Acoustat Spectra 1100 in our home audio section

Dave Ayers2005-08-16 23:51

I have collected a lot of information on replacement woofer drivers for the Acoustat Spectra 1100 speaker. A lot of the info came from this site, but it\'s not always easy to find. Plus I have found what looks to be a pretty good source for a specific replacement driver. I have published all this info on a page in my web site to assist other 1100 owners. The URL is: http://www.daveayers.com/Acoustat_1100.htm Thanks for all your help in Acoustat matters ! Dave

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Liam2008-11-19 00:07

I think their is a mistake in the Thiel small value figures Dave quotes in his table. I think the column labelled \'1100 woofer CLIO\" are actually the measured figures for a Spectra 11 speaker hence the very different figures from the column labelled \"my good 1100 woofer\". When you calculate the box size with a Qts of .88 you get a volume of 1.47 cft (1.6 less 10% for the volume the driver takes up in the box) and a lower 3dB frequency cut off point of 57.2 hz. The figures in the \'1100 woofer CLIO\" column are better suited to the larger volume speaker enclosure of the Spectra 11 speakers.

Liam2008-11-24 22:06

Hi I am trying to understand the crossover networks for the Spectra 11 and Spectra 1100 to help in finding a high quality replacement 8\" woofer and making the required amendment to suit the different impedance of the the new drivers. It\'s been years since I studied electronics so I am struggling more than a little! So please excuse me in advance for the vast errors and assumptions I have made in the following. Hopefully Andy will make some comments and corrections on what follows. UPGRADING REPLACING THE DRIVERS The Spectra 11 used a 4 ohm driver and the spectra 1100 used an 8 ohm driver. I have not been able to locate a direct replacement for either of these two drivers (They seem to be fairly unique). You might get acceptable sound by just chucking in the nearest replacement you can find without changing any of the circuitry (you might even strike it lucky and get a sound that pleases you more). However if you want to be a bit more scientific there are two key things that should be looked at when swapping out the drivers in these speakers. 1. These speakers appear to use Zobel networks in the woofer circuits. Zobel networks consist of a capacitor and resistor in parallel with the driver and are designed to present a stable impedance to the crossover network over a wider frequency range (a very good thing). If the drivers are replaced in either of the speakers the Zobel networks will need to be modified so that with the new driver they will still present the same impedance to the crossover networks. If an 8 ohm replacement driver (instead of the original 4 ohm unit) is used in the Spectra 11 this will present an extra challenge. 2. Efficiency. Andy makes the point that the efficiency of the woofer drivers was deliberately chosen to be very low to match that of the electrostatic portion of the speaker. There seems to be a smaller range of 4 ohm drivers available than 8 ohm drivers. The good news is that I suspect that using an 8 ohm speaker will be less efficient than the original 4 ohm speaker used in the Spectra 1100. SPECTRA 11 In the Spectra 11 the 4 ohm resistor and 150uF capacitor in parallel with the driver appear to be a Zobel network designed to maintain the impedance presented to the single order (6dB) crossover filter network (This crossover is comprised of the 3.7mH inductor in series with the Zobel network. I assume in the case of the Specta 11 the impedance of the Zobel network is around 4 ohms (can\'t be sure without knowing the inductance of the driver) Assuming the Zobel network presents approximately a 4 ohm impedance to the crossover network in the Spectra 11 the 3dB cut off frequency would appear to be 170Hz. Calculated using this formula. F = XL/2 x pi x L. 172Hz =4 ohms/(2 x pi x 3.75mH). Andy has mentioned elsewhere that the actual crossover frequency is 6dB at 187 Hz steepening to 12dB at 240 Hz. The variation between my calculated 172Hz and the actual 187 Hz could be due to a variation in the actual impedance of the Zobel network. I don\'t understand how it steepens to 12dB at 240 Hz though? SPECTRA 1100 The Spectra 1100 uses a nominal 8 ohm driver and the values of the series inductor in the crossover and the capacitor and the resistor in the Zobel network are different. from that used in the Spectra 11. The spectra 1100 arrangement seems very weird. The Zobel network is made up of a 1 ohm resistor and a 200uF Capacitor in series with a nominal 8 ohm woofer driver. Normally in a Zobel network the value of the resistor would be chosen to be the same as Re (DC resistance of the driver) so 1 ohm in this case would seem way to low. I am guessing in the case of the Spectra 1100 the Zobel network is designed to drop the impedance presented to the single order (6dB) crossover network to a lower value (instead of presenting the 8 ohms of the driver). Why this was done I am not sure. My suspicion is that if you use the Zobel network to drop the impedance it will be at the cost of the stability of the impedance. However over the relatively small frequency range the driver is used over this may not be much of an issue.. Andy has advised that the crossover frequency of the Spectra 1100 was chosen to be 160Hz. Using this frequency I think that I can simply calculate the approximate impedance of the Zobel network as follows using this formula. Xl = 2 x pi x F x L. 2 ohms = 2x Pi x 3.75mH. I suspect (wild uneducated guess) the reason for presenting a much lower impedance to the crossover network is to compensate for the very different efficiency of the 4 ohm speaker used in the Spectra 1100 compared to the 4 ohm driver used in the Spectra 11. OTHER OPTIONS There is another simpler but more expensive approach to this exercise and that is to ditch the woofer low pass crossover network and replace the woofers with two active woofers with a tunable crossover and volume control. the volume control would be necessary to match the efficiencies of the woofers to the electrostatic speakers. In this arrangement I think you would need to keep the electrostatic portion of the crossover – maybe Andy can comment on this? A second option (perhaps the ultimate arrangement) also expensive would be to ditch the woofer low pass crossover, replace the woofer drivers with a high quality driver appropriate to the size of your cabinet, and bi-amp. You would need some method to control the volume of each amplifier. This could be done with an active crossover with volume controls. In this arrangement could also ditch the high pass crossover as well. Using active crossovers apparently offers many advantages. For an excellent article on this see http://www.sound.westhost.com/biamp-vs-passive.htm. A third option might be to ditch your Spectra 11 or Spectra 1100s and upgrade to some Acoustat full range Electrostatic speakers from ebay, etc. What a huge pity the original Acoustat is no longer in business think how good a range of speakers these brilliant engineers would be making by now!

Seth2019-04-29 06:18

Liam, did you ever find a decent replacement woofer for Spectra 11s?

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