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Minimalist LM3875 +  PLLXO = what input impedance?
Minimalist LM3875 +  PLLXO = what input impedance?
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Old 5th November 2012, 12:48 AM   #1
uncle_leon is offline uncle_leon  Poland
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Default Minimalist LM3875 + PLLXO = what input impedance?

Hello, I have a question that is probably trivial, but I have not been able to find a simple answer, and all this impedance business has always rather baffled me... I am trying to find out what is the input impedance of an LM3875 chip?

The amp I am using at the moment is in the attached picture. It is a complete schematic of everything that sits between the DAC output and the input of my speakers. As you can see, it is just a Gainclone with a passive attenuator, only stripped of all parts that are not necessary for stable operation: there are no input caps, no input resistors, no smoothing caps in the PSU etc. (not even fuses ).
The 300R resistor is swappable, and serves two purposes: firstly to regulate volume (I change it for a different value when I need to adjust volume), and secondly to tie the input to the ground.

I have recently been considering a "passive line-level crossover". The idea seems sound, and what's more, the values of components required seem to fall into the sort of range I could make myself.

So I did a bit of reading, and I found this article:
and also this "calculator":
TLS.org | Passive Line-Level Crossover

The equations are simple, but they need "input impedance". As I would (obviously) like to insert the PLLXO after my volume control resistors, the question is, what is my input impedance at that point? The datasheet does not mention this, so I am guessing it is something obvious, like zero... I just want to make sure.
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