Spectra 11, Output, bi-amping

Read more on the Acoustat Spectra 11 in our home audio section

Mike Ballinger2004-02-23 22:21

I recently came into posession of a pair of Antique Sound Labs SPM-25 25 watt per channel, Class A tube mono blocks and am now considering possibly bi-amping the Spectra 11\'s using the Antique Sound Labs monos for the electrostatic panels, and something considerably beefier for the woofers. Which brings me to my reason for contacting you. Could you please help me out by telling me if it is possible to bi-amp the Spectra 11\'s, and if so, how to go about doing it? Thanks in advance, Mike Ballinger

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Andy Szabo2004-02-23 22:21

Have you actually tried driving the Spectra 11 with these 25-watt amplifiers? In my experience, most people would not be satisfied with such a small amplifier driving an inefficient loudspeaker. However, if you have tried these amps on the Spectra 11\'s, and are satisfied with the results, then it is possible to modify the speakers for bi-amping.

When bi-amping conventional cone speakers, it is traditional to use the larger amplifier for the bass, and a smaller amp for the mids and highs. This is because the woofer is usually the least efficient driver in the system, and requires the larger share of power.

However, with a woofer / electrostatic hybrid, it is the electrostat that is the least efficient component. So great is this mismatch in efficiency that the Spectra 11 has additional resistance placed in series with the woofer to reduce it\'s output to match the electrostat. You may find it better to use a larger amplifier on the electrostat, and a smaller (or equal power) amp on the woofer. You\'ll need to experiment!

Refer to the schematic posted elsewhere on this site. The input signal to the speaker goes through the input cup, and then through the six-pin connector to the interface. At that point, the signal is split for the electrostat and the woofer. After going through the woofer\'s crossover, the signal travels back through the six-pin connector and down to the woofer.

It is probably easiest to add the second pair of binding posts on the rear of the interface chassis. You\'ll need to cut some traces on the printed circuit board - I can\'t say exactly where without looking at a board. Note where the black and red wires attach to the board, and trace out the schematic. Cut the traces at such a point as to disconnect the woofer\'s crossover, while leaving the connections to the electrostat. It might be necessary to add some jumpers on the board to accomplish this. Run wires from the new binding posts to the woofer\'s crossover, making sure to observe the original polarity (note that the woofer is intentionally connected out-of-phase with respect to the electrostat).

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