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In Reply to: RE: Wiring autoformer as HF attenuator posted by freddyi on July 31, 2016 at 16:32:59
It is shown in parallel with the autoformer's primary winding (taps 0 and 15). The tweeter is connected to two secondary taps, so I suppose it is in parallel with the tweeter as well.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?
Edits: 07/31/16 07/31/16 07/31/16Follow Ups:
without the parallel resistance, your crossover frequency would drop. 1uF would get the crossover down to 3KHz at higher attenuation levels.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
Edits: 07/31/16 07/31/16
This is in 1 dB steps as well (custom order). I figured with 802's going for $500 a pair it was worth the extra cash.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.
Edits: 07/31/16
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
That is interesting. Using the v-cap crossover calculator, a 3KHz first order with a 0.44uF capacitor woould require a reflected driver impedance of 120 ohms. Could that possibly be right?
Bob Crites shows a chart that gives you the turns ratio for different attenuation values. The reflected impedence is proportional to the square of the turns ratio. For 12dB the turns ratio is 4. Assuming an 8 ohm speaker, the reflected impedence is 8*4*4=128 ohms. Thats what the cap would see if in the circuit befor the attenuator. You can change the impedence value by paralleling a resistor across the attenuator inputs.
Or, you can move your cap between the autoformer and the driver and the crossover point will never change with different attenuation levels. The cap will only see the impedence of the driver.
Interesting - If I want to bring my 802 in at 8khz, with about 4 dB attenuation, I could use a 1uF Cap. That same cap would bring the crossover frequency down to 6khz at 5 dB attenuation, and 5khz at 6 dB.
With the 802G, this 1uF cap in series could easily work in a 3dB range.
you could also put a resistor directly in parallel across the driver - that would swamp some of the driver/horn impedance peaks, and not use the input shunt resistance. Sometimes if a horn/driver/speaker has a large impedance peak, it will crossover much lower than whats expected if the speaker was a pure resistance
oh - here's a Klipsch network with some reflected values noted
Karlson Evangelist
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