Spectra 11, Crossover question

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

David Taber2009-05-06 01:43

HI all. Just bought a pair of Acoustats, and looked at the circuit diagram for the xover. Given what sounds to me very thin high end performance, I have a question:

Across the primary of the step-up transformer for the panels is a big fat (40w) 10-ohm resistor. This is sure to swallow a lot of power, and lower the impedance of the high-end speaker quite a bit. So...what\'s the problem with removing this resistor, or replacing it with a 20 ohm?

The woofers in my pair were replaced with oddballs that are way too efficient, so I\'ve got the choice of lowering their output or increasing the output of the electrostats. I can imagine totally removing the 10-ohm resistor could cause some impedance-matching issues or maybe even some ringing of the xformer.

Enlighten me, please...thanks!


Andy Szabo2009-05-06 19:36

The 10-ohm resistor is part of a resistor-capacitor network that forms the high-pass crossover for the ESL portion. This is a vital component for the speaker\'s operation, and I do not recommend its removal. Removing it will not significantly increase the volume of the ESL, but it will adversely affect the frequency response.

Until the woofer is properly matched to the rest of the speaker, you cannot properly judge the overall performance. Many woofers are more efficient than the original ones used in the Spectra 11, so you will probably need to reduce their level to match the electrostatic portion. It is not possible to increase the efficiency of the ESL (otherwise Acoustat would have already done so). The woofer must be matched to it.

There have been some other posts to this website regarding replacing the woofers in the Spectra 11. Look around, you will probably find information on replacmeent woofers that may work better than the ones you have.

Ryan Benitez2012-12-24 04:17

My plan is to replace the crossover network with a band pass filter crossing over at 15 to 16 kHZ to a tweeter at about 7.5 to 8 ohm at the given crossover point. I also plan to crossover to the woofer at about 250 Hz. What I would like to know is what parts I can remove from the current crossover and what is vital to the panel operation and go be left in place or replaced with a higher quality part. The goal of added tweeters is to flatten then high frequency while increasing the seen impedance my the amplifier. Any help would be greatly appreciated.


David Taber2009-05-06 01:43

Ryan - Assuming that your bandpass filter will adequately protect the audio transformer from low frequencies, then all crossover components on the low-voltage, primary side of the transformer may be removed. All components on the high-voltage, secondary side of the transformer must be kept intact for proper operation of the speaker.

You did not say if you intend to use a passive or active bandpass filter. If active, I would recommend installing a 0.5 to 1-ohm resistor in series between amplifier and transformer. This will protect the amplifier from the very low impedance at low frequencies. This can be a problem for some amplifiers, even if you are not driving low frequencies. If the filter is passive, located between the amplifier and the audio transformer, then you probably don’t need the resistor.

Ryan Benitez2012-12-29 19:44

Thanks David, I still do not fully understand the 1 ohm and 10 ohm resistors. I suspect the capacitive value of the panel may be what I am missing. Also unless the impedance is much different than I am thinking (4 ohm) the 100 uF cap should be 6db at about 390 Hz. This is not close to what I was expecting. I am using passive filtering but may go active to the woofer in the future.


Acoustat Answer Man2012-12-29 21:33

Ryan – I’m not sure why that response was attributed to David – I wrote it. Anyway, on to your questions.

The purpose of the 10-ohm resistor is to equalize (to a certain extent) the impedance that the capacitor is working against for the high-pass filter, since the impedance of the transformer by itself varies widely with frequency. Calculations of crossover frequencies based on ‘4-ohm’ or ‘8-ohm’ will only get you so far, because no speaker has a constant impedance across the audio band. There’s always a good chance that a speaker’s impedance at the crossover point is not the same as its nominal specification. Final tweaking of crossover component values should always be done by measurement and/or by listening.

The 1-ohm resistor is in there to keep the high-frequency impedance above a certain value. It’s probably a good idea to leave it in there, especially if your amplifier is uncomfortable with impedances below 4-ohms.

Ryan Benitez2012-12-29 23:44

Thank you acoustat answer man. I based my impedance guess on the impedance chart of the spectra 1100. It makes sense that a real measurement at the given point in the circuit should be taken. I think I will leave the 10 ohm resistor in place and take measurements both with and without the 1 ohm. I am ok with dipping a little below 4 ohms but under 2 ohms is uncomfortable.


Acoustat Answer Man2012-12-30 00:25

Now that I think about it, leaving the 10-ohm in place makes sense if you are using a passive crossover.

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