F5 input impedance

[Das Car]

The production F5 has an input impedance of 101k ohms where the v3 diy version is 47.5K ohms. What is the reason for the change?

Apologies if this has already been discussed please direct me to the thread.

 

[rayma]

The lower the better, if your preamp can properly drive the load, including the effect
of any coupling capacitors that it may have.

 

[Nelson Pass]

Like a lot of things, it's pretty arbitrary.

 

[McQuaide]

Nelson at one point someone complained about the "unrelenting hardness" of the F5, and you recommended trying 100K here. That's how I built mine. Not sure if I hear a difference though.

 

[Das Car]

Can the input impedance be optimized for the preamp? If you have a low output preamp would it be better to increase the input inpedance of the amp?
I ask because I'm considering a Rougue RP-1 preamp which only has 7dB of gain on the line stage.

 

[Nelson Pass]

The input impedance doesn't have a significant effect on the gain of the amplifier,
and it would be an odd preamp that had issues with the input impedance range we
are talking about. The Rogue would appear to want something at 22 Kohm or
greater based on Stereophile's measurements.

 

[Das Car]

Nelson- thanks for your feedback on this and also thanks for supporting and sharing your knowledge with the diy community.

I just finshed the F5 a few weeks ago and it sounds fantastic!
I'm lining up a demo Rouge preamp so I'll report back on how it sounds with the F5.

 

[DougL]

The input resistor is bound by 3 things:

1 The input capacitance of the power amp. You want the input resistor to be 10x lower than the capacitive resistance at the highest frequency you are interested in.

2. The output impedance of your preamp / source. You would like the resister to be 10x higher than the output Z.

3. Tradition. Tube amps are typically 100K ohm. Solid state range from 10k to 50k ohm.


There may be an opportunity to optimize noise, distortion, frequency response or something else.


HTH


Doug