AR-9 XO Questions

[Guest cober]

I am recapping my AR-9s. After considering advice read in this forum, I will recap only the upper drivers using mylar film for the small and electrolytic for the mid-range values. I have these questions:

1) The sizes of the original Callins capacitors in the woofer's XO are enormous, especially my 470uF. See attached picture of the lower XO board - it is even larger than the 2500uF. I have seen other pictures of the AR-9 XOs where the 470uf is blue in color and only a little larger than the 30uF on the same board. What would make this capacitor so physically large? It is clearly marked 470uF, 50v. Currently in the process of recapping, one can see my diminutive 80uF new electrolytic piggy-backed onto the original. What has changed in 30 years allowing new electrolytic capacitors to be so much smaller? No need to restrain in technical explanation - I am an EE.

2) There is an extra inductor in the physical XO but not shown on the library schematic. This has been pointed out before. In other words, 8 inductors are found in my XO but the schematic shows 7. The extra inductor is seen in the picture of the upper XO board, on a black spool. (My new caps are in place on this board.) I believe this inductor is 1.37mH, connected to the node of the 24uF capacitor / 0.2mH inductor of the upper mid-range filter AND to common ground This is the filter topology used in the AR-90. Why isn't it shown in the AR-9 schematic? Was there a model run change?

Thanks for the great help.

[xmas111]

1) The sizes of the original Callins capacitors in the woofer's XO are enormous, especially my 470uF. See attached picture of the lower XO board - it is even larger than the 2500uF. I have seen other pictures of the AR-9 XOs where the 470uf is blue in color and only a little larger than the 30uF on the same board. What would make this capacitor so physically large? It is clearly marked 470uF, 50v. Currently in the process of recapping, one can see my diminutive 80uF new electrolytic piggy-backed onto the original. What has changed in 30 years allowing new electrolytic capacitors to be so much smaller? No need to restrain in technical explanation - I am an EE.

I'm also thinking about recapping my AR9's and have wondered the same thing about the physical size of the caps.

I'm guessing that technology has eliminated the need for those big old can type caps..... but I'm only guessing!

Someone will come along and give us the actual answer I'm sure.

John

[johnieo]

What has changed in 30 years allowing new electrolytic capacitors to be so much smaller? No need to restrain in technical explanation - I am an EE.

The oxide layers on early electrolytic capacitors were formed on smooth Al foil. The foil is run through an acid bath and anodized by a dc forming voltage. Then alternate layers are wound between porous spacer layers, stuffed in a can and filled with an ionically conducting liquid. By the 1940s or perhas a bit earlier, manufacturers imade these "dry" electrolytics with roughened aluminum foil. Roughening the foil surface increased its effective surface area from 3-6 times over that of smooth foil. The latest generation NPE and PE use highly corrogated and etched Al foil. Deep, narrow, closely-spaced grooves in these capacitors yield a capacitance-to-volume ratio of 5X that of standard etched capacitors. For example, the Tobias Jensen 2,700-uF, 63-Vdc capactior (may not be available now) is 66 mm long and 35 mm diam. The anodization constant, amorphous oxide, and maximum voltage rating have not changed; however, organic electrolytes with buffers are now standard. The dissipation factor of the corrogated devices is somewhat greater than ordinary roughened devices. Unfortunately, chemistry and mat. sci. had been or are about to become completely eliminated from the EE curriculum!