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In Reply to: RE: Wiring autoformer as HF attenuator posted by Randal P on July 31, 2016 at 18:37:17
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
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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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