OAL Inductors
As far as I know, OAL sold 2 different inductor sets for several different EQ kits, but also sold the inductors individually.
You can still find full OAL EQ kits here and there on Reverb or eBay now and then. Most of them I’ve seen are Model 103/104 kits described above that were made for Model 201 channel strips (see schematic above). The more desirable Model 109 kit is really rare, on the other hand. Unless you find kits, and given the difficulty/impossibility of obtaining the original inductors by themselves, you’re probably better off winding your own/getting some made if you want to try to recreate any of these circuits. But I did the research, so I’ll share what I know.
87/8709 series:
Some documentation refers to “87” series inductors, while other docs refer to “8709” series inductors. From the documentation I've found, these were probably the same parts with different numbers. From the schematics, the frequency bands associated with each tap of the 8709 series inductors are close enough to the 87 series that this could be the case (see Model 109 EQ above and the chart of values for the 87 series below), but if it’s true then the nominal frequencies are off in some of the bands by a fair amount.
These are the inductors found in the Model 109 and the Model 105P/106P set, above. They have large, 1 inch diameter cores. The series consisted of 3 multi-tapped, Siemens ferrite inductors.
The model 87-x inductors were still available to buy from OAL up until last summer. I bought 10 sets of 3 then, at $10 per inductor. I was told that that was the last of the ones that they don’t have to dig for. (Sorry, not interested in selling)
. I was going to deconstruct a set so that I could share the specific ferrite cores used, wire gauge and turns/tap so anyone who wanted to could get some cores and wind their own. But the cores are these old Siemens ferrites from the Pleistocene, and I can’t even find the data for them in the 1983 Siemens ferrites catalog I’ve got. I also can’t find any of the numbers in any of the old TDK/Epcos catalogs, which seem to use the same nomenclature Siemens did. We could probably calculate AL for each core, but I was hoping it would be a simple matter of looking them up. Maybe someone who knows more about ferrites than I do could suggest equivalents. If CJ wants to tear them down and tell us how to build them, I’m happy to send him a set.
8083-x series:
These were smaller inductors (half the size of the 87 series) and are the ones used in most of the vintage consoles and kits I’ve seen online. These are the ones used in the Model 103/104 described above. I have one set of these that’s part of a completed EQ kit from a Model 201 channel strip (Models 103 and 104 EQ kits). I bought them already built, but I haven’t tried the kit out yet. The smaller size of these makes me wonder how quickly they’d saturate. But they do conveniently fit onto a small board, making for a space saving design, as in the Model 103/104. I’ll eventually get around to experimenting with the 103/104 kit that I have, and when I do I’ll post what I find out.
That's it.
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