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I built something... similar. It was a load box, with an out for silent recording, rather than an attenuator but the core is basically the same (I've been meaning to write it up for the forum... maybe this is a good kickstart). @klaasvakie helped with the inductor calcs and sourcing the components. I know he was looking at this circuit but I'm not sure how far he is.

The biggest trick is winding the inductors, which I found very intimidating but turned out to be relatively easy. Honestly, once you have those, the resistors and switches are simple.

I'm not planning on building one myself any time soon I'm afraid, but for someone of your skill I doubt it would be more than an afternoon's work, especially if you simplify the design.

Yeti
Look forward to your load box information.

I can wind inductors, if I can find wire, but I shall need assistance with design calculations. I see many formulas for shapes and sizes out there. I still think cutting up old scrap speakers should be fine.

      7 days later

      modulator Hey @modulator

      I have a half-built version of that exact circuit. It's been half-built since December, and I don't think I'll get to it soon, but I'm happy to share the components I actually ordered. I had to change some of the values around to stuff I could get easily, but I did plug it into spice to match the impedance curves here.

      My plan is to built this circuit, as well as the silent recorder that @Yeti has into a single spot at the back of my amp.
      Here was roughly laying things out:
      Left is the silent recorder, right is the reactive load.

      I'll go dig out the component values if you are interested.

          Here is my mantech parts list for this - first column is mantect part number

          352M0569 ENAMEL COPPER WIRE 0.71 API - PRICE PER 250G
          82M0872 POT S/T LOG P/M 24MM 1K
          14M8002 RES 1W RND M/F 1% 10K0
          65M0009-H RES 50W ALU W/W 5% 68E
          72M1509 RES 50W ALU W/W 5% 47E
          14M0322 RES 25W ALU W/W 5% 12E
          14M0413 RES 25W ALU W/W 5% 68E
          72M2156 RES 10W SQR W/W 33E
          72M1505 RES 25W ALU W/W 5% 10E
          15M9664 RES 25W ALU W/W 5% 22E
          82M0248 RES 25W ALU W/W 5% 33E
          14M6675 RES 50W ALU W/W 5% 0E22
          65M2465-H RES 50W ALU W/W 5% 15E
          72M1390 RES 10W SQR M/O 56E
          14M5879 RES 50W ALU W/W 5% 8E2
          14M6566 RES 50W ALU W/W 5% 22E
          65M2465-H RES 50W ALU W/W 5% 15E
          65M3119-H CORE TOROID 125u 28x14x12
          82M1485 CORE TOROID 125u 23.6x47.1x18.5
          14M0418 RES 25W ALU W/W 5% 2E2
          82M0248 RES 25W ALU W/W 5% 33E

            Thanks, I hope to get to this reasonalbly "soon", meaning it may take a few years...

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