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In Reply to: I have some questions posted by Tre' on January 18, 2012 at 08:40:18:
"The value of the series resistor plus the value of the shunt resistor equals the output impedance of Lightspeed Attenuator. So as the series resistor increases or decreases in value we want the shunt resistor to do exactly the opposite. "First, this is wrong.
Second, this is no different than any other resistive attenuator.
The output impedance of a resistive attenuator is the value of the two (series and shunt) resistors in parallel, not "plus". (This assumes a zero source impedance. 100 ohms, as suggested is close enough)
The worst case happens at the -6db point when the series resistance and the shunt resistance are equal. For a 10K attenuator this would be at 2500 ohms. The output impedance will be lower at any other point of attenuation, up or down.
This would be true of a pot, a stepped attenuator or a LDR based attenuator.
I ask again, what's the difference between this and a pot or stepped attenuator?
A autoformer or transformer volume control is fundamentally different in that it is not a voltage divider.
No current is "thrown away".
Voltage is traded for current.
When a lower tap is selected the output impedance goes down (a good thing)
and the reflected impedance back to the source goes up (a good thing).Attenuators are all about attenuating. :-) Throwing away voltage.
A AVC or TVC does this without increasing the output impedance (in fact the output impedance is decreased) or decreasing the impedance seen by the source (in fact that impedance is increased).
At -6db the output impedance is 1/4 of the source and the impedance seen by the source is 4 times the input impedance of the amp.
A -12db the output impedance is 1/16 of the source and the impedance seen by the source is 16 times the input impedance of the amp.
At my normal listening level, my AVC is at -16db!
As with any other transformer, the trick is to keep the inductive reactance high enough at the lowest frequency of interest and the capacitive reactance high enough at the highest frequency of interest.
We want high inductance with low capacitance.
A good winder, like Dave, can do this and once done an AVC or TVC is kind of like a free lunch.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/18/12 01/18/12 01/18/12 01/20/12
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Follow Ups
- More - Tre' 09:06:09 01/18/12 (5)
- RE: More - Paul Joppa 10:39:37 01/18/12 (2)
- That is a big part of it...(nt) - clio09 20:08:24 01/19/12 (0)
- RE: More - Tre' 10:43:35 01/18/12 (0)
- RE: More - Bob_C 09:59:59 01/18/12 (1)
- RE: More - Tre' 10:43:46 01/18/12 (0)
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