RE: Wiring autoformer as HF attenuator

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Posted by Joe Roberts (M) on August 2, 2016 at 06:31:07

In Reply to: Wiring autoformer as HF attenuator posted by Randal P on July 30, 2016 at 14:21:42:

So you're digging the full range 414 setup? Some do, some don't.

I started messing around with autoformers but haven't finished the experiments. Those German autoformers look very nice. Tempting. I've been using Peerless and old Triads to mess around with.

The way I see it, there are two directions to go, given that the autoformer will step up the impedance the cap looks into. This, of course, will change the crossover frequency for a given cap.

1) One could install a resistor across the primary of the autoformer to give the cap a more stable impedance to work into.

The transformed impedance of the driver will appear in parallel with this resistor Rl, so the impedance will not be perfectly stable with different taps but more so than without it. If/when you settle on a tap, you can calculate the resistor to give exactly the load resistance you need.

2) Let the autoformer transform the impedance to a higher value, measure it with DATS/Woofer Tester, and use a small, high grade cap at that high impedance.

Measuring this, I find maybe 75-125 ohms reflected at potentially useful settings of the autoformer, peaking like crazy with rising frequency. For that reason it might be useful to put a resistor on the secondary side of the autoformer to limit the rise.

The multiplicative effect of the impedance transformation enhances the rising tendency. It won't rise above the value of that added resistor though.

The phase diagram of an impedance sweep on this #2 setup looks quite strange. Not sure what to make of it.

Sounds good this way but I went with #1 because it was more straightforward.

In any case, the challenge is to control the reflected impedance that the cap sees when you change taps.

I need to get back to that project. Got sidelined. Among other things, I would like to measure the acoustic output of the various schemes.

Dave Slagle knows a lot about this, or at least he has been messing with it for a while, so maybe he'll pop in with his observations.

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Follow Ups Full Thread

Topic - Wiring autoformer as HF attenuator - Randal P 14:21:42 07/30/16 (14)

RE: Wiring autoformer as HF attenuator - Joe Roberts 06:31:07 08/02/16 (2)

RE: Wiring autoformer as HF attenuator - Randal P 17:40:59 08/02/16 (1)

RE: Wiring autoformer as HF attenuator - Joe Roberts 18:08:11 08/02/16 (0)

RE: Wiring autoformer as HF attenuator - freddyi 21:56:38 07/30/16 (10)

RE: Wiring autoformer as HF attenuator - Randal P 16:03:32 07/31/16 (9)

RE: Wiring autoformer as HF attenuator - freddyi 16:32:59 07/31/16 (8)

RE: Wiring autoformer as HF attenuator - Randal P 16:51:55 07/31/16 (7)

RE: Wiring autoformer as HF attenuator - freddyi 17:41:42 07/31/16 (6)

RE: Wiring autoformer as HF attenuator - Randal P 18:37:17 07/31/16 (5)

RE: Wiring autoformer as HF attenuator - freddyi 21:32:03 07/31/16 (4)

RE: Wiring autoformer as HF attenuator - Randal P 09:03:54 08/01/16 (3)

RE: Wiring autoformer as HF attenuator - D Mike 09:22:48 08/01/16 (2)

RE: Wiring autoformer as HF attenuator - Randal P 12:53:49 08/01/16 (1)

RE: Wiring autoformer as HF attenuator - freddyi 15:28:05 08/01/16 (0)

Follow Ups

RE: Wiring autoformer as HF attenuator - Joe Roberts 06:31:07 08/02/16 (2)

RE: Wiring autoformer as HF attenuator - Randal P 17:40:59 08/02/16 (1)

RE: Wiring autoformer as HF attenuator - Joe Roberts 18:08:11 08/02/16 (0)

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