Input impedance of Aleph X

Peter Daniel · 24th October 2002, 04:28 PM

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?

Nelson Pass · 24th October 2002, 07:58 PM

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.

Peter Daniel · 30th October 2002, 04:06 PM

What happens when I use 220K resistor instead of 100K resistor in a feedback loop?

Fred Dieckmann · 30th October 2002, 05:09 PM

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

Fred

jam · 30th October 2002, 05:23 PM

Fred,

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

Jam

Peter Daniel · 30th October 2002, 05:28 PM

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.

24th October 2002, 04:28 PM	#1
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	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? __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

30th October 2002, 04:06 PM	#3
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	What happens when I use 220K resistor instead of 100K resistor in a feedback loop? __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

30th October 2002, 05:09 PM	#4
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	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

30th October 2002, 05:28 PM	#6
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	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. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

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24th October 2002, 07:58 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	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.

30th October 2002, 05:23 PM	#5
jam diyAudio Member Join Date: May 2001 Location: Charlotte,NC,USA	Fred, Would your bandwidth not also change, and what about distortion? Any thoughts? Jam

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