Spectra 11, Interfaces, two sets

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Dan Brummitt2004-02-23 22:21

HI Andy .. Have a pair of spectra 11\'s which are stunning ... bought \'em used a few years ago and have enjoyed them tremendously ... have them bi amped with sub woofers so am largely circumventing the mid/bass driver ... a weakness in my opinion.

I fiddled around many years ago trying to fabricate my own esl\'s with no fantastic success ... but did manage to fiddle a few different prototypes into existance... one drove panels directly from the plates of push pull output tubes

... noted in the midst of fiddling that they don\'t seem to take much current .... but like the voltage ....wondering therefore if it would be possible to locate another pair of S11\'s and drive them (both sets) with the same set of electronics/voltage sources etc. This should extend the bass response somewhat and possibly provide slightly more sound volume and possible also expand the rather infinitesimal sweet spot somewhat .... Presumably it would also mess up impedances, frequency response ... and for all I know fry everything in sight...

I would be interested in your opinion. Thanks for helping bring these wonderful instruments into the world and regards. Sincerely, Dan Brummitt

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Andy Szabo2004-02-23 22:21

It certainly is possible to drive two ESL panels from one Spectra 11 interface. In fact, this is something I wanted to try years ago, and had Acoustat continued, it might have even been a commercial product.

There are two ways to approach this: a vertical stack, and a side-by-side arrangement. The two approaches differ by more than physical arrangement. Each arrangement requires different electrical considerations. Neither arrangement will increase the width of the listening window, because the highs will still radiate from only a half-panel width (true of all Spectra\'s). The Spectra 11 actually has a pretty forgiving ”sweet spot”. If you\'ve ever listened to a 1+1 (a pre-Spectra model), you\'ll know just how narrow a sweet spot can be!

Either arrangement will increase efficiency. You will still need a woofer system: the Spectra 11 interface is not designed for full-range operation. Resist the temptation to remove the high-pass crossover feeding the transformer: doing so will result in distortion and/or damage.

My preference would be to stack the two panels vertically, somewhat like a Model 1+1. This arrangement will greatly improve vertical imaging: standing up or sitting down, the speaker will sound the same. You can simply hook up the two panels in parallel to the interface, and see how you like the results. Theoretically, two resistors in the interface should be changed in value to maintain the same RC roll-off point for the Spectra sectoring. Because the mid-bass panel area is now doubled (and hence doubled the capacitance), the 330K resistors should be changed to approximately 150K.

But try simply paralleling the panels first before making this resistor change: you may be satisfied with the stock values. If you do change the resistors, make sure they are rated for at least 5 watts and 2500 volts. Or, you could borrow the resistors from your extra interfaces, and parallel them with the existing resistors. (i.e. two 330K in parallel equals 165K.)

Stacking the panels side-by-side can also be done: you would end up with a Spectra 22-like speaker, but not full range. This arrangement also has the advantage that the second panels don\'t need to be sectored Spectra panels. You could use older pre-Spectra panels. Wiring for this arrangement is a little more complicated: the left speaker will be discussed. The right speaker is mirror image. The original panel goes on the right, and is wired normally to the interface. The second panel goes on the left, and both halves are wired only to the mid-bass output of the interface. Since the mid-bass panel area is now tripled, those 330K resistors should be reduced to about 100K.

Finally, depending on the woofer system used, you may want to adjust the crossover values for both the ESL and woofer. However, don\'t cross over any lower than 100 Hz. The transformer isn\'t designed to operate any lower (and perhaps not even that low).

The information I\'ve provided is pretty sketchy. If this generates more questions, please feel free to write again. If you complete this project, please let us know how you did it, and how it turned out!

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