Archive for December, 2008

The Thor line with the SEAS Prestige CA18RNX and 27DTC Drivers

Friday, December 12th, 2008

The cost of the SEAS EXEL drivers is too great for some to bear in these economic times.  I have explored the possibility of using the above drivers as a less costly alternative.

The Thiel/Small parameter are an acceptable match.  Fs is 4 Hz higher, VAS is virtually identical, and Qts is slightly lower.  I consider the mismatch however to be acceptable.  The line as built is slightly long.  However, I think the line will function well.

The SEAS CA18RNX bass/mids are $75 from Madisound, so that would be $300 for the pair.

The SEAS 27DTC Tweeters are 39.91 from Madisound, around $80 for the pair, bringing the driver coast to an acceptable $380.

Here is the pdf for the woofers.

Here is the pdf for the tweeters.

I have designed a crossover for these drivers.

This crossover is elegant and simple.  Although the pdf states that these are second order Linkwitz/Riley filters, they are actually highly customized.

The low pass filter is actually first order from 1KHz to 5 KHz.  At5 Hz, the composite electrical and acoustic roll off becomes fourth order.  The woofers are down 18db an octave above the crossover frequency of 2.8 KHz.  Req and Ce provide impedance compensation to the woofers.

The high pass filter is also first order 8 KHz to 1.8 KHz and transition to fourth order after that.  The program does not let me show R1, which changes the Q of the filter.  Note that C2 shows a resistance of 2.005 Ohms.  Now no cap will have that resistance.  So a 10 watt 2 ohm resistor needs to be placed in series with C2.

This is an easy to build seven element crossover.  You can see that the filter is first order in the crossover region, as the phase angle is 90 degrees, or a quarter cycle in the crossover region.  Also this crossover puts the tweeter 48 db down at its resonance  of 550 Hz.  So there should be no tweeter harshness.  A standard first order filter would have put the tweeter down only 14db, at resonance.  Since this is an odd order filter at crossover, it will fulfill the lobing requirements of the d’Apollito configuration.

There is also around 3db of diffraction compensation with this crossover.

Since the phase and time response of this filter is well above average, transient response should be superior.

Good parts should be used for the crossovers.  Air cored inductors and polypropylene caps, such as Solen caps should be used.

I think where funds are a problem, these drivers and crossover should be given serious consideration.

I will work on a TL for these specific woofers.  I think this design carries the promise of unbeatable value for money.  The line will end up being a little smaller than the Thor line.  Power handling will be less, but a respectable 160 watts.