Spectra 2, Cross-over, low pass filter

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Scott Olive2004-02-23 22:21

Hey Andy, my name is Scott Olive, You may remember me, we worked together at Hafler some 11 years ago. I have a pair of acoustat spectra 2\'s and am in need of the value of the coil for the subwoofer 100hz low pass filter. I had one go bad and the coil it self is browned from years of listening. I cannot make out the value of the coil. I\'m pretty sure the low pass filter is a 12db per octave low pass, the caps in the circuit are 2 220uf\'s in parallel, do you remember the value of the coil? I looked at the specta 2200 schematic and it does not show the crossover. Doing the math I come up with a 12mh coil and that just does not seem right. If you can help I should would appreciate it. Thanks, Scott Olive.

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Andy Szabo2004-02-23 22:21

Well hello Scott! Yes, I do remember you. Unfortunately, my memory is not as good when it comes to the crossover coil in the Spectra 2 and 3. Unfortunately, I do not have any documentation on those products. I am not sure if there ever was a published schematic.

Your calculated value of 12-mH is correct, if one assumes that the filter is a traditionally aligned two-pole filter. However, I don\'t think this assumption is valid.

I did the math too, and I believe this is how you arrived at that value: working backwards from the known 4-ohm woofer and 400-uF capacitor, the capacitor equation for a -12 dB/oct filter yields a crossover point of 70-Hz. Then, plugging 70-Hz into the inductor equation yields a coil of 12-mH.

All the crossovers I did for Acoustat (Medallion Model 1, Spectra 2, 3, 11 and 1100), sounded best when the two poles of the woofer crossover were staggered: this technique seemed to best match the roll-off of the esl. That is, the woofer\'s crossover begins to roll-off at -6 dB/oct, but then goes to -12 dB/oct at a higher frequency.

Since the design-point for the crossover was 100 Hz, I suspect that the coil was a smaller value than 12-mH. A 100 Hz crossover would yield a coil value of 9-mH. So, I think the value was probably more like 9 or 10-mH. You can probably find a decent replacement coil from one of the companies that sell crossover parts. It may be necessary to add a few ohms in series with the coil to get the level correct. Most commercial crossover coils will have a lower resistance than was used in the Spectra.

You might also try to measure the value of the old coil. Unless it is burned so badly as to have shorted turns, it will still measure the correct inductance.

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