I found this article by Jeff Markwart.
Enter the H model
In early 2004 I acquired a pair of 604-8Hs, the last ALNICO magnet duplex model Altec produced before they switched to ferrite magnets. It was also the first Altec duplex model equipped with the Tangerine phase plug, Mantaray constant directivity high frequency horn, and a crossover that utilized dual equalization of the compression driver output. I swapped out my 604Bs for these Hs using the same cabinets, and was immediately impressed by the differences in output - well controlled upper mids, significantly more high frequency extension, improved imaging detail and more apparent bass & mid bass. What was in that Dual EQ crossover? Could it be adapted to older 604s as well?
At the basic level the 604-8H filter is a parallel 3rd order HP and 2nd order LP filter, with L pad shelving of all horn frequencies when the EQ button on the front panel is in the released position. Pretty much like a stock 604-8G filter. However, when the EQ button is clicked inward to the depressed position, the similarity ends. The older style shelving circuit is disabled, an adjustable, frequency dependent, contour circuit is pulled in for the upper mids, a high frequency trim circuit becomes active, and a driver-side series resistor further shapes the rising HF slope. At the recommended dial settings the upper mids are appropriately depressed relative to the high frequencies, and results similar to the famous Mastering Labs and UREI 604 filter implementations are achieved. Unfortunately, this Altec filter is not time domain corrected like the others - so I naturally wondered just how would it sound if applied to my phase corrected 604B filters
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