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Old 3rd February 2009, 11:48 PM   #1
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Default Increasing ESP P3A input impedance

Hi all,
having a tube preamp with an output impedance of 5k or so, I just wanted to ask what do you think about increasing the P3A input impedance (~24K) to ~49K by increasing R2 to 47K. Obviously R5 would become 47K too, and R4 would be raised to 2k2 to mantain the same gain. C2 would be changed accordingly.

Would this be too detrimental for the amp's noise perfomance?

Thank you all,
Christian
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Old 3rd February 2009, 11:57 PM   #2
AKSA is offline AKSA  Australia
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Christian,

Might make 0.5dB difference. All my AKSAs, Lifeforce and Soraya power amps have 47K nominal impedance.

Insignificant in the scheme of things. Try it.

Hugh
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Old 6th March 2010, 10:43 AM   #3
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Quote:
Originally Posted by christian.lenci View Post
Hi all,
having a tube preamp with an output impedance of 5k or so, I just wanted to ask what do you think about increasing the P3A input impedance (~24K) to ~49K by increasing R2 to 47K. Obviously R5 would become 47K too, and R4 would be raised to 2k2 to mantain the same gain. C2 would be changed accordingly.

Would this be too detrimental for the amp's noise perfomance?

Thank you all,
Christian
Hi!

I would like to do same thing on my P3A, but something isn't clear to me...

To achieve an input impedance of ~49k I would have to:
- increase R2 to 47k
- increase R5 to 47k
- increase R4 to 2,2k

Nothing more than that?

What does "C2 would be changed accordingly" mean?

What is the general disadvantage of a higher input impedance, e.g. why is the input impedance of most power amps around 20k and not around 50k, which would obviously be much better suited for source equipment with BOTH low and high output impedances, and of course it would make the use of passive pre's much easier.

Thanks a lot!!

Martin
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Old 6th March 2010, 11:23 AM   #4
AKSA is offline AKSA  Australia
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Not sure what C2 is; if it's the input cap, you can halve it; if it's the feedback shunt cap, same again......

Hope this helps,

Hugh
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Old 6th March 2010, 03:52 PM   #5
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Hello Hugh,

thanks for your reply!

C2 is a small ceramic from input to ground (parallel to R2) and it's 100pF in the original schematic.
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Old 6th March 2010, 04:38 PM   #6
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C2 should remain untouched, it is C1 and C3 which need attention. However, you may find leaving them untouched is fine as it will just result in lower cut-off.
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Old 6th March 2010, 05:45 PM   #7
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Personally, i think with it's CFP output, making any change is asking for trouble
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Old 6th March 2010, 10:36 PM   #8
AKSA is offline AKSA  Australia
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Changes on the fb and input side should not affect the CFP output stage operation at all.

Hugh
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