[Elecraft] K3 Audio Low Pass Filter

[Paul Christensen]

>I think 100uF caps is enough value. Look these plots (with 100uF in 
>AF-amp),
> we see identifical plots for Line-out and for Phone-out.
>
> logarithmic view: http://forum.cqham.ru/download.php?id=43583
> linear view: http://forum.cqham.ru/download.php?id=43582
>
> 73! Alex

Alex,  The S/T FFT program you have allows the user to display in semi-log 
mode.  That's the preferred way of looking at audio response. Resolution of 
low-end response will show much more detail that way, especially the area of 
interest below 200 Hz.

Also, keep in mind that the choice in coupling cap value is wholly dependent 
on the load Z.  When making response tests of the headphone output with the 
FFT program and sound card, try loading the headphone output with various 
values of resistance in increments starting at 8-ohms, working your way up 
to say ...50 ohms.  You'll end up with an interesting family of curves that 
can then be saved in the S/T program.

Using a sound card without external input termination often results in 
termination Z onto the circuit of only about 2K-ohm or so, depending on the 
model of the sound card.  Pro sound cards (typically with XLR pig tails) 
have the ability to select various degrees of input Z.

Paul, W9AC