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In Reply to: RE: PLLXO question related to LDR posted by JBen on February 26, 2012 at 10:47:28
The problem with having a passive pre (regardless if it is an LDR or not) is that the output impedance varies if you change the volume. This means the the fc of the filters and the xo point of the PLLXO will also vary everytime you change the setting on the passive. You could put an opamp buffer between the passive and the PLLXO but then you might as well go to an active crossover.
On the violin: "Heaven reward the man who first hit on the idea of sawing the innards of a cat with the tail of horse."
Edits: 02/26/12Follow Ups:
Neo, I may have failed to realize this. It could mean that my UBS DAC and the pre-amp have the same output impedance, which I don't know for a fact. I can plug into each separately and measure no change in xover points.
I never saw a spec call for it while designing the PLLXO filters and simply assumed that input impedances was all I had to worry about.
Well, time to start lining up the active stage ducks, then. It was in that "future projects limbo" for when a proper subwoofer integration became a need anyway.
You have to consider/include 'everything' when designing PLL's. :) Even interconnect capacitance might be an issue.
You can measure output resistance of a source fairly easily. Take a voltage measure unloaded and also with a 1k resistor attached.
Rout = 1000 times (Ropen-Rload)/Rload
You'd be surprised how high the source resistance is on many components.
Cheers,
Dave.
Shouldn't that be Z = 1000*(Vload-Vopen)/Vopen?
On the violin: "Heaven reward the man who first hit on the idea of sawing the innards of a cat with the tail of horse."
So is it's OK to place the pot after the PLLXO? I'm thinking of a dual deck (4 channel) stepped unit.
Edits: 02/26/12
Thanks Davey! Where do I find the impact relative to changes in Rout in a given configuration?
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