There are a few things I wonder about, The first 604 was 20 ohm and had a 2000hz crossover point, the "B" looked identical and it had a 1000hz crossover point, the "C" which has a slightly different horn had a 1600hz crossover point and the "E" which began using the accordion surround had a 1500 crossover point.
In the earlier days, the ohms ratings (impedance) alloted to drivers were very much without any fixed rules. The spec given was purely an assesment of the manufacturer alone. For example, Philips ratings were always the same as their DCR values. But later on the computed values (impedance) at 1KHz was accepted as the standard value.
Though in most cases this impedance value turns out to be 1.25 times the DCR value, this is not the same for all drivers, especially in the case of older and unusual drivers like the 604 types. Here the imedance values can be the double of the DCR value, as their f/imp slopes rise very fast, an indication of their very high efficiencies/sensitivities.
Regarding the varying crossover 'f' choices, I think it was part of the usual R&D stuff. A standard '15 inch' speaker has its toughest task while trying to reproduce the range between around 350Hz to 550Hz, b'coz this is the zone during which its cone's physical limitations start coming into play, and restrict the HF stretch severely beyond 1KHz. Now the HF horn in the 604 is perfect for 2KHz, but limits itself below 1KHz. So this way after trying out both the limits for various reasons, they finally settled for the middle path at 1500 Hz.
I'm pretty happy with how they sound they have a real 3dish type of sound. When Avatar is playing all those weird bug noises in the jungle are going off to the side of you and above you It's kinda freaky but cool. I am considering different cabinets again, not because I don't like these but the other day when I switched back to the altec receiver from the marantz I literally made the back panels of these things distort. (they're barely 3/4 thick particle board with no bracing at all,The baffle is the same way) the bass was so much more deep than the marantz that when I fired up the altec and Nady eq the back panels of the heathkits were shaking violently enough to sound like a distorted speaker. I've since tuned them in nicely but when I think of how much power they must be producing to jump the cabinet panels by better than a 1/4 inch I started thinking Butcher block thickness cabinets are in order.
Nice to hear that you are happy finally. I am curious to know....
What is your present xover choice ?
What adjustments did you need via the Nady equalizer in order to make the sound better ? B'coz the reverse of this adjustment is your speaker+xover+amp's response ! Let us know.
That chart is a great find Aditya, I've never come across it in my hunting adventures.
Thanks.
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