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In Reply to: RE: What is the best chose for matching pre amplifier into SET posted by Bob_C on January 16, 2012 at 20:40:59
This is still just a resistive attenuator.What is the resistance range of the Silonex NSL32SR2?
From the data sheet 0 ohms (40 ohm max) to 5 meg ohms.
So what is the output impedance?
Why would this sound different than any other resistive attenuator?
What am I missing?
"It is worth mentioning that the Lightspeed Passive Attenuator is not for every audio system though. First off, it is a passive attenuator so there is no gain. Also, your source output impedance needs to be less than about 100 ohms which makes it suitable for use with several sources. The Lightspeed Attenuator works best with amplifiers that have input impedance greater than about 50k. If you would like to use the attenuator with a low input impedance amplifier best results are achieved by using a suitable buffer between the attenuator and amplifier. "
http://diyaudioprojects.com/Solid/DIYLightspeedPassiveAttenuator/
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/18/12
Worst case scenario is output impedance is 7k ohm. Input impedance is 10k ohm.
A resistive attenuator with an input impedance of 10K has a worst case scenario output impedance of 2.5k ohm.
This is simple math.
With a 10K ohm attenuator adjusted so that the series resistance is 5k ohms and the shunt resistance is 5k ohms, the attenuation level is 6db and there are two paths to ground from the output point.
One path back through the series resistor to the source (with a low source impedance we can go ahead and call this ground and the math will not be off by very much at all)
The second path is across the shunt resistor to ground.
These paths are in parallel.
5k ohms in parallel with 5k ohms is 2500 ohm and that is the output impedance of the attenuator.
If the attenuator is readjusted in either direction the output impedance will go down.
Let's say the volume is turned down a little so the series resistance goes up and the shunt resistance goes down.
6k ohms for the series resistor and 4k ohms for the shunt.
6k in parallel with 4K = 2400 ohms.
Here's the equation for parallel resistors =(R1*R2)/(R1+R2)
So if we turn the volume up so the attenuation is less than 6db and our series resistance is 4 k ohm and the shunt resistance 6k ohm the output impedance is 2400 ohm.
Now let's make the resistors 2k and 8k (it doesn't matter which is series and which is shunt) 2k ohm in parallel with 8k ohm is 1600 ohms.
No magic, just simple math.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm going by the information provided by the manufacturer. Specifically that frequency response is 0hz to near infinity, input impedance is approximately 10k ohm, and output impedance is approximately 7k ohm. Perhaps my stating 7k ohm was a worst case scenario was misleading or just wrong.
Bottom line is my Lightspeed is better than my Slagle AVC and another transformerbased passive I have. I have to give most of the credit to the fact that the switch and contacts are removed from the equation, as well as the short signal path and simple design (single input/output, no balance or input controls). According to the manufacturer due to the use of the LDRs the performance of the Lightspeed with a cheap Radio Shack pot will be indistinguishable sonically as with a much more expensive switch.
There are caveats that influence performance as with any other resistivebased passive, and even some transformerbased ones, although I admit they are much more forgiving. The impedance requirements stipulated by the manufacturer should be adhered to, but I will say I've used mine contrary to the input impedance recommendation and found the sound to still be quite good.
gootee has some theory and sims on LDR's at the DIYaudio thread linked below.
I have used all the usual suspects both active and passive and IMO this is by far the best. Transformer VCs soften the sound IMO. I have found that the impedance matching is not as much of an issue as stated. Or maybe I am just lucky. I am using George's LightSpeed...
regards
Bob
Bob, my question was a technical one.
What would make it sound better?
You have stated your opinion and I respect that.
Now, can you tell me the why?
Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Well there are a million posts on it and I was not trying to give a technical review. :)
But there is something less intrusive about it. Now I am fully aware that most of what I am listening to has been recorded many years ago with miles of wire, and tons of tubes... :) Beats me...
regards
Bob
"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
> > I ask again, what's the difference between this and a pot or stepped attenuator?
I expect it's that there is no switch or sliding contact.
nt
Fair enough.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Have you actually listened to an LDR?
regards
Bob
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