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Old 24th October 2002, 04:28 PM   #1
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Default Input impedance of Aleph X

Since I'm very close to finishing an Aleph X, I got an occasional question as I go. This one is about input ground resistors.
Both Grey and rtirion used resistor's values of 68k and 100k respectively. Reading Balanced BOZ article one might learn that the input capacitance of the MOSFETs puts a natural upper limit on that value and for best performance values exceeding 25kohms are not recommended. Is it a case here too?

All the other Alephs ground resistors don't exceed 10kohm values. And what about the feedback resistor? What effect does changing from 100k to 200k brings, if any?
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Old 24th October 2002, 07:58 PM   #2
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The resistive value experiences is essentially the parallel
values of feedback, input+source, and ground resistors.

Resistors to ground are best implemented at the input
itself, so that all three resistors attach to the input node,
and this means that the load experienced by the circuit
driving the amp's input is the input resistance alone.

25K is close to the natural limit for the input resistor, but
this depends on the input capacitace of the devices and also
the gain. If you want to give up something else, you can
go to very high impedances using small Mosfets or even
Jfets on the inputs.
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Old 30th October 2002, 04:06 PM   #3
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What happens when I use 220K resistor instead of 100K resistor in a feedback loop?
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Old 30th October 2002, 05:09 PM   #4
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Default What happens when I use 220K resistor instead of 100K resistor in a feedback loop?

Your gain is 2.2 times as high all else being equal.

Fred
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Old 30th October 2002, 05:23 PM   #5
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Fred,

Would your bandwidth not also change, and what about distortion? Any thoughts?

Jam
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Old 30th October 2002, 05:28 PM   #6
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Thanks for enlightening me. It seems that I'm not the only one who lost it.

So I guess too much gain is not good and I better stick with 100K. What would be recommended value of resistance devider at the input. I was hoping for 10K series, 68K to ground, because this would work really well with my silver foil 0.022uF filter cap, providing cut off frequency around 120Hz (high pass). But isn't 68K a bit high for input ground resistor? I'm just putting it together BTW.
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