[Elecraft] Poor Low audio frequency response on JT65 mode with my K3 ?

[Jim Brown]

On 11/13/2012 4:04 PM, Clifford Hynds wrote:
> I am using line in/out on the K3 and DATA mode so no equalisation,

It isn't clear whether your problem is on TX or RX, so I'll respond to 
both. For RX, check the bandwidth settings of your K3  (front panel, can 
be used as width and shift or as high pass and low pass).  That will 
affect the RX bandwidth.

For TX,  the K3 DSP is programmed so that there's a high pass filter at 
about 200 Hz in all digital modes.  This in firmware, and cannot be 
changed.

Another possible cause if your problem is TX is that you might be 
transmitting through a filter that's too narrow.  You should be 
transmitting through either the 2.7 kHz of 2.8 kHz roofing filter.

Yet another possible cause of a TX problem is a relatively small value 
capacitor in series with the audio drive feeding the Line Input.  
Elecraft misguidedly chose to make their input and output impedances 600 
ohms, thinking that was the "pro" standard, which has not been true for 
nearly 50 years.  A small value cap in series would cause a low 
frequency rolloff.  Pro audio inputs are typically 5kOhms or so, and pro 
outputs are typically about 100 ohms.

And yet another possible cause of low frequency rolloff is an open 
shield connection on the audio cable.  Note also that the K3 audio 
inputs are unbalanced, even though they use transformers. So, for a 
3-circuit TRS plug, the tip is hot, the ring is hot for the other 
channel, and the sleeve is common. The line input has hot on tip, no 
connection on the ring, and common (shield) on the sleeve.

Hope this helps,

73, Jim K9YC