Polk RT3 speaker mod.

This is a mod for the Polk RT 3 speaker.  It uses this woofer.  This is a pdf. of its specification sheet.

It uses this tweeter.   Here is the spec. sheet.

This is the woofer alignment.

This is the crossover pdf and simulations.

The crossover is at 3 KHz.  However the component values have been significantly modified. There is 2 the 3 db of diffraction compensation and the response irregularities of the woofer between 200 and 2000 Hz have been significantly blunted.

L1 and C1 form the high pass filter.  This starts to attenuate first order at around 6.5 KHz and switches to second order around 1200 Hz.  The tweeter is down 33db at resonance.  This should mitigate against any tweeter roughness.  Rp1 and Rp2 pad the tweeter output.

L2 and C2 form the low pass filter.  This starts as a first order filter around 400 Hz and starts to perform diffraction compensation.  It changes to second order just before crossover at 3 KHz.   Req and Ce perfrom impedance compensation.  Without it there is significant rise in response above crossover.

Note that providing diffraction compensation has sacrificed about 3 to 4 db of driver sensitivity.  Although the sensitivity is around 87db 1 watt 1 meter which is good for a small speaker like this.

There are impedance peaks of 50 and 40 ohms as a result of reflex loading at 25 and 100Hz respectively.  Because of the equalization of diffraction and  smoothing of the response the impedance is 6 ohms over most of the operating range.  There is a rise to 12 ohms at crossover.

The phase response with the polarity of the tweeter reversed is excellent. with the drivers having a maximum pahse angle of 45 degrees in the crossover region.  Driver offset of the front baffle will modify this a little.  As  result group delay is only significant on the driver tuning range.

The existing vents should be made 7.62 inches each.

The crossover can be mounted in and out of the cabinet.

This mod to the RT 3 should be able to be accomplished with minimal cabinet modification.

If a bigger box were to be built, then F3 could be lowered to 59 Hz.

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