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Old 27th January 2014, 09:59 AM   #1
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Default Back to 100K input impedance on the F5

Hi
After ordering the F5 pcb I notice that the input impedance are change to 47K.
Now if I understand its increase the gain.
My system is cd player and preamp so I donít need more gain from the F5 (15db is ok).
Its ok to back again to 100K resistor?
thanks
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Old 27th January 2014, 10:01 AM   #2
Zen Mod is offline Zen Mod  Serbia
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stay at 47k

lesser interconnect capacitance influence

NB that value of that resistor is not having anything with gain
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Old 27th January 2014, 10:04 AM   #3
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thanks ZM
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Old 27th January 2014, 10:31 AM   #4
AndrewT is offline AndrewT  Scotland
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The Zin resistance does not affect the frequency response of the cable connection.
You normally have a Source connected. That Source has a Source Impedance.
The amplifier (or receiver) sees the Zin resistor and the Source impedance as parallel loads. The source impedance being ~ 10 to 100times lower dominates what the Receiver sees. You can choose either 10k, or 22k, or 47k, or 100k, or 220k.

When you disconnect the Source with the Receiver still turned on, the receiver now sees the Zin resistor as it's sole load. This now works as an interference aerial. You will hear and/or measure a higher output noise level. If this is a concern during switching of inputs using a selector switch that opens and leaves the "hot" signal line disconnected, then a lower Zin can help reduce this momentary increase in interference.

Try it with your switching and listen to hear if it is a concern.
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Old 27th January 2014, 12:07 PM   #5
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thanks, good explanation!
so the reason for this Zin is for the amplifier keep wake.
Thanks
in my case i have cap bettwen the source impedance and amp Zin.
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Old 28th January 2014, 10:21 AM   #6
AndrewT is offline AndrewT  Scotland
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Most input stages have an input offset current.
This current can be into the input stage, or out of the input stage.
It is normal to use a resistor connected to Signal ground to allow this offset current to flow.

Rin/Zin provides this route.
The input stage MUST have some way to source/sink input offset current.

And it provides a Maximum Source impedance, if the real Source is disconnected even momentarily.
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