this is my logitech headphone with mic bertus sir i want know why this headphones mic did not given that RFI noise problem when i used with my preamp with laptop charging mode when i had used it worked like butter can u tell me the reason .i know that the previous problem was because of unshielded cable but why thic mic cable not picks any unwanted frequency

 

All my replies to this simple old preamp are not here. Maybe on another website.
This circuit was designed using "typical" hFE values for the transistors, probably by using a simulator or by using trial and error resistor values that work only with the transistors that are used. But the simulator and trial and error way of designing does not work when the transistors have minimum or maximum hFE values.

I think the original circuit used a much higher supply voltage then many resistor values must be changed for the 5V supply used here.

The value of the volume control R8 loads down the level from Q1 and cuts all bass sounds with the small value for C6. I recommended a 100k log volume control and a higher value for C6. Of course I recommended adding a coupling capacitor between R8 and Q2.

I laugh when students over there call us Knights, SIR!

 

I posted these on the other website:

 

The circuit you built with the single 3904 transistor works
very well and is as simple as it gets. no hiss- no hum- no buzz.
Just keep soldering till you get it right !

http://hackaweek.com/hacks/the-single-npn-transistor-audio-preamp/

 

See