[Elecraft] K3 Audio Low Pass Filter

[Alexander Ponomarenko]

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

Paul, of course you are right if we talk about Phone-Out,
also on these plots you can see the graph from Line-Out.
This out have to matching ideally with a sound cards.
If we don't have needed LF from Line-Out, where it LF take for Phone-Out?

73! Alex