500 series stereo pultec eq?

[canidoit]

Has anyone ever made a kit for a 500 series stereo pultec style eq?

Thank you.

 

[yuka42]

I only know of the DIYRE eqp5 kit. It’s mono though.

https://www.diyrecordingequipment.com/collections/500-series/products/eqp5-passive-eq-kit

 

[johnchantler]

AML, also mono:
http://www.audiomaintenance.com/acatalog/aml-17-037_extended_info.html

 

[canidoit]

I cannot find a single DIY 500 series stereo eq out there. I wonder if there would be a market for atleast one stereo EQ DIY?

 

[Mworthin]

You just make two of them and use one on one channel and one on the other.

 

[gyraf]

..very hard to get decent stereo tracking, so two monos are much easier..

 

[TwentyTrees]

I cannot find a single DIY 500 series stereo eq out there. I wonder if there would be a market for atleast one stereo EQ DIY?

I'm sure there would be a market, though perhaps not enough of one to make it worthwhile for a designer. To do stereo properly isn't easy, particularly in the constraints of the 500 series. I've given this a bit of thought, as I'm sure have many others.

Depending on the topology of your EQ, you'd be wanting a number of two or four pole controls. Multi-gang pots generally don't match closely enough* - 10% isn't really good enough for proper stereo work - so we'd be looking at rotary switches.

Good rotary switches are generally not cheap, and things get mechanically complicated if you're trying to use PCB-mounted switches rather than lots of off-board wiring, for which there isn't a lot of space in a dual 500-series module. Lorlins are out (not enough poles), so we're looking at either 90° switches like the Grayhill 71, or radial switches like the Blore Edwards Type 72. Each has its own set of mechanical challenges. If using 90° switches, we effectively need half of each channel's EQ section on each PCB - not impossible, but would need a lot of thought, as we need each channel's I/O stages on their respective PCBs. If using radial switches, we're looking at a number of daughter board PCBs which eat up space and make the mechanical design much more complex. Either way, the increased number of board interconnections makes for additional complexity and potential points of failure, and eats up space.

On that note, the switches themselves are often pretty big, too - not a lot of space left over for transformers, inductors and DOAs in a Pultec-style EQ, never mind everything else required. An active EQ would be easier in this regard.

Is it impossible? Not at all. But it wouldn't be easy to design, and it wouldn't necessarily be any cheaper than two mono single-slot units - possibly more expensive, depending on the switches. With that in mind, there's perhaps not much of a commercial incentive to design a stereo 500-series EQ, which might be why there aren't any.



*Sidebar - I had an Elysia xfilter500 a few years ago. They make a big virtue in their marketing of "laser-trimmed" wafers to improve stereo tracking in their detented potentiometers. However, when I measured it, it was still between 0.5 and 1dB out at every step on the gain and at least 5% out on the frequency controls. Fine for some work, but not tight enough for what I was looking for.