I'm putting together a two-way Altec 414c/802g/32a system. At the suggestion of Joe Roberts, I'm using a single 3uF oil cap on the 802 to bring it in at about 6-8kHz while letting the 414c run full range.

I need to attenuate the 802 to match the 414C. Rather than using a resistor-based L-pad, I wanted to get fancy with an autoformer made by Werner Jagusch custom made in 1dB steps (14 dB total range). Included is a connection diagram:



The autoformer has taps at 0-15, with the diagram indicating 0 (negative) and 15 (positive) to be connected at all times (they are bare wire). Tap 14 is in parallel with Tap 0 (negative terminal) and one of Taps 1-13 connected to the positive terminal.

Questions:

1. Can someone enlighten me as to how this works?
2. Is there a multi-position rotary switch (10W or so) that will be sonically transparent that will allow 12-position switching for the autoformer?
3. Has anyone had direct experience with Werner's autogformers?

Thanks,
Randy

 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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 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)

put the tap wires from the autoformer to one side of a screw terminal block, then move the wire with the spade lug which goes to your horn to the desired tap.

Lets say the striped red wires in the picture below are from your autoformer and the single aqua wire will go to your Altec 802/32a horn.

for a fixed crossover point, the capacitor value gets smaller as more attenuation is used as the reflected impedance to the "primary" is higher. you could also use an appropriate resistor in parallel with your 802 so the reflected load would be the same to the highpass cap and not have to change the capacitor.

Karlson Evangelist

It looks like the attached image (at least for the HF portion). Werner indicated on this (and my) diagram that the variable resistor was for rolloff. From what you are saying without it the value of my 3uF cap (6,500hz) may be seen as smaller (7,000-8,000hz). In my case, it is not critical because of the slow first order rolloff and the gradual natural rolloff of the Altec 414c, but being able to use the variable resistor to tweak the crossover frequency would be a nice feature. Any idea of the range it would vary?

if you have a multimeter or VOM, temporarily disconnect the L-pad and measure and record its values at the center, high, and low positions. That will give an idea of the impedance available seen by the capacitor for different taps.

looking at an old wirewound 8 ohm pad as as resistor, I can get around 43 ohms on the high range and below 1 ohm on the low end. Half way might be around 25 ohms.

If you didn't have the parallel input resistor, then 6dB attenuation would reflect 64 ohms impedance for a 16 ohm driver (assuming it was a pure resistance) and 0.33uF might give ~8KHz crossover with a 16 ohm load, or 0.68uF with an 8 ohm load.

those are very high-end looking autoformers - I've been happy with Bob Crite's model which allows 1dB steps.
Karlson Evangelist

Is there a way to measure/calculate the impedance at each attenuation level? Does the autoformer as as a series resistor?

With the added impedance of the autoformer, I may be able to experiment with a smaller capacitor (i.e., a nicer capacitor since the value will be smaller). If instead of 8 ohm impedance,the total impedance is now 28 ohms, I can go from a 3uF to 0.82uF capacitor.

Am I completely off base?

EDIT: Dave Slagle in another thread stated "another interesting thing with the autoformers is you can do the crossover network at higher impedances making the cap values smaller."

Using an online l-pad calculator, if I need 6dB attenuation, R1 will be 4 ohms and R2 will be 8 ohms. Plugging this into a first order calculator gives me a 2uF capacitor (12 ohms) with an 8 ohm damping resistor.

if you had a woofer tester or similar, you could directly measure the reflected impedance

here's Bob Crite's two way "first order" network for Eminence's Beta8cx with a Selenium D210ti compression driver. The required 12dB attenuation dropped the needed capacitor value to ~0.44uF for a nominal 3KHz crossover. That's nice as exotic caps would be more affordable. IIRC those autoformers used to sell for around $35US each and have 1dB steps - I think they have enough power handling for most "Klipsch style" situations.
Karlson Evangelist

• http://www.critesspeakers.com/3636atz.pdf

I have another potential alternative: I have a Dennis Had SE KT-88 amp that can use anything from a 6V6 to a KT-150 to give 5-12wpc. I also have another amp on order from him, this one a parallel SE amp that will output 15wpc with KT-88's. Depending on tubes use and the amount of attenuation needed on the 802, I may be able to run the PSE amp on the 414 and the SE amp on the 802, with a 3-5 dB difference in power between the two.