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AR sliding crossover frequencies


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For instance, the AR3a tweeter circuits use a 4 ohm nominal impedance driver, and a 15 ohm pot, with a 6mfd capacitor. When the pot is at full on, you are basicly 15 ohms pure resistance in parallel with a 4 ohm impedance (which will be calculated as resistance to simplify things, although this might disprove this question) 

Using fc= 1/(2piRC) for crossover frequency:

with tweeter pot full on, the load would be calculated to 3.16 ohms, and with a 6mFd cap, cover frequency would calculate to 8.4KHz


 With the tweeter pot at mid point, you would parallel 7.5 ohms with your 4 ohm tweeter, then add the remaining 7.5 ohms of the pot, so this works out to be a10 ohm load. Now the  crossover frequency  works out to be 2.7KHz. 

Am I on the right track here?  Would this be the "magic" behind  the potentiometers versus L pads, which offer a consistant load,  differing from the variable load as described above.

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I think you are correct in your analysis.  Depending on the position of the pot, both the apparent crossover frequency as well as the sound level will change.

The actual slope of the tweeter is affected by the raw response as well. A 6 db/oct electrical high pass (such as a single capacitor in the case of AR3a tweeter) can become a much steeper 18 db/oct. 


According to the  graph below, there is a substantial overlap between the midrange and tweeter. The crossover over point for the mid and tweeter in Ar3a is difficult to call:)




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