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Original Message

RE: Electro-Voice Regal lll

Posted by Paul Eizik on January 12, 2017 at 10:41:57:

Craig

The Kbapps calc is better than the other ones we started with here for the job at hand. While it also warns about avoiding a less than 3 octave spread, you can just enter the numbers and ignore the warning which comes up by just closing it. The concern for avoiding a less than 3 octave spread in the crossover is due to the effect of rising voice coil inductance in the driver as frequencies rise, which will show itself as a rising driver impedance as the frequencies go higher, with the possibility that the low and high inductors will interact with this and/or each other. While rising voice coil inductance could be a concern with the woofer in the Regal (and there are ways to address this), the mid and tweeter are horn loaded where the rules are different, and the acoustic resistance of the horn will dominate the driver impedance.

So punching in a 3500 Hz high pass for the 8 Ohm T35B, the calc gives the familiar 5.68 uF cap. With the 1000 Hz high pass on the 8 Ohm mid it gives an 18.41 cap, which is not far from the math derived 19.89 uF I came up with, but also note that the calc changes the higher frequency to 1080 Hz and the lower one to 3240 Hz. which explains the different results.

So what to do?

I would start by checking the midrange presence L-pad with your DMM to see that it's appropriate for an 8 Ohm driver, with attention to see if there is a series resistor somewhere there which would justify the rather low cap value in the mid. If there is, then this would explain the mystery, but I would junk the original crossover (and L=pad too if inappropriate) and build a new crossover. I originally suggested a 2nd order crossover because EV later used them (likely mainly for driver protection with the encroaching higher powered transistor amps), but I would go with a 1st order crossover like the originals seeing as you are using a tube amp.

From the previous math: C= 159,155/F (Z) we get C1 as 5.68 uF, and this is where F=3500 Hz and Z= 8 Ohms. For C2 we get 19.89 uF for a 1000 Hz high pass with the 8 Ohm mid.

For the inductors (L is the symbol) we use: L=159.155(Z)/F. This is where L= the inductor value in milliHenrys, Z= driver impedance in Ohms, and F= the crossover frequency. So for the low pass with the 4 Ohm woofer we get 0.63 mH which agrees with the calc. With the 3500 Hz low pass for the 8 Ohm mid we get 0.36 mH which is not far from the 0.39 from the calc result.

If you decide to "roll your own" crossovers use only air core inductor coils. and definitely not iron core ones. The original ones are air cores. Also be sure to orient them at a 90 degree angle to each other like the originals, but don't crowd them like the originals, as you'll likely just mount them on a wood board and you will have plenty of room.

Wth a first order crossover you have the option of configuring the drivers as a parallel crossover )like the diagram in the calc), or a series crossover, as the parts values will be the same. See Rod Elliot's excellent study of the parallel vs. series crossovers on his website, and I would go series.

Note that the 159 constant has a comma following in the C version, but a decimal following in the L version.The wrong parts could cost a lot of money.

Keep us posted on the project either way you decide to go

Paul