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Crossover cap value?

Basically - I want to copy what Magnepan did with their XO-1 electronic crossover box. This simply sends the full spectrum audio signal to the bass amp ( which is low pass filtered post amp ) and allows you to roll off the bass ( high pass)ahead of the input of the amps driving the mid/tweeter panels in a T-3A or T-4. Have set these up for a few folks over the years and would like to accomplish the same effect with my system. My scheme is as follows:
Paradym Studio 20-S2 are described as having a 3rd. order electro-acoustic crossover at 1.5kHz. These speakers have a dual binding post set up for bi-amping or bi-wiring. I would like to run my Hafler DH-120 into the bass speaker ( no problem low pass filtered by the existing passive crossover) and my Hafler SE-120 into the tweeter. What I'd like to do is roll the bass out of the input of the SE-120 tweeter amp (high pass filter). SE-120 's input impedance is 22kOhm.
Q-1: I'm wondering if I want to get close to 1.5kHz for a selected bass roll off/ high pass or should I go down a octave or two? Since a single cap gives me a 6db/octave filter( not exactly a infinite slope) - should I calculate the bass roll off/high pass for 1.5 kHz, 750 or 275 ?
Q-2: What is the formula to determine the cap value for various frequencies into a 22kOhm input impedance.
My goal in all this is to unload the tweeter amp from having to deal with the current hungry demands of deep bass - hopefully gaining a little more clarity at higher volume levels from what are admitidly small ( but sweet sounding) amps. And still use both sections of the excellent passive crossovers built into the Studio 20.
Or does this come under the catagory of "has inhaled too much solder flux" ?
audiopile
5 responses Add your response
eldartford
4,262 posts
 
Theaudiotweak...Have you calculated the phase shift at 1500 Hz caused by a 6 dB high pass filter at 275 Hz?

You ought to.

You might also like to compare this with the phase shift of the speaker's 1500 Hz crossover at this frequency.
jeffreybehr's avatar
jeffreybehr
805 posts
 
...the 'passive' devices in series with the signal and probably the parallel devices too. Most crossovers are easy to diagram, with the easiest high-pass being merely a series cap and maybe a parallel coil, while the simple low-pass filter is a series coil and maybe a parallel cap. (The parallel devices make the slopes 2nd-order = 12dB per octave. A third-order filter adds another series device after the parallel device.) Most manufacturers now use good-quality caps (instead of the 10-cent electrolytics of 20-and-more years ago), but you can still do better. I started my capacitor-replacement insanity over 20 years ago with WonderCaps and have used lots of them and Reliable's film caps and MultiCaps, but I think I'll try the auricaps in my Aerial CC3B, which uses 3rd- or 4th-order filters and Axon polypropylene-film caps. Next will be ANOTHER new crossover for my 17-year-old Kindel PLS-As.

I did say 'insanity', didn't I?
theaudiotweak
2,801 posts
 
I feel Jeff is correct when he says to let the speaker crossover do its job. I would not add any more caps to this scenario..you will be adding phase shift. Why don't you consider upgrading some of the internal passive components of the Studio20's, the resistors especially..and easily..Tom
eldartford
4,262 posts
 
Since your objective is just to get the LF signal out of the HF amp, a 6 dB filter set well BELOW the driver crossover frequency would be best. Such a filter will be "out of the picture" at the break frequency of the speaker system's built-in crossover, which you want to use.
Try 275 Hz. This will take most of the heavy lifting off the HF amp.
jeffreybehr's avatar
jeffreybehr
805 posts
 
1. The formula for calculating filter frequency in Hertz is always 159155/RC with R = resistance in ohms and C = capacitance in microFarads. So a 1uF cap into a 22000-ohm load = 7.2 Hz. If the load resistor in the SE-120's circuitry is first, calculate directly with the above formula. If it isn't, you'll probably need to add an input-load resistor to use with the filter cap.
2. When one is replacing a higher-order high-pass filter with a lower-order one, one needs to use a HIGHER, not lower, crossover frequency in order to block sufficient lower frequencies from the midrange/tweeter. Personally, I'd start with a 3KHz filter and see how it sounds, especially at the highest SPLs you expect to use. If you're not overdriving the tweeter, add capacitance gradually to decrease the filter point.

But why not just use the input terminals and filtering in the speaker's crossover? Unless you wire around it, you're using the crossover's filtering anyway, and adding another filter ahead of the amp multiplies capacitor colorations and phase errors. IOW, drive both amps full range and let the speaker's crossover do the job it was designed for.

Or am I missing something?
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