[Elecraft] Re: K3 Audio

[Jim Brown]

On Wed, 6 Aug 2008 11:14:51 -0600, Bob Cunnings wrote:

>It's a 600 ohm transformer, rated for 7dBm max (about 1.7 Vrms 
>across 600 ohms).

This datasheet does not show a graph of distortion vs level vs 
frequency, but it is specified for 300Hz - 3.5 kHz, and its 
frequency response is specified at 0 dBm (0.78 vrms). 

It is well known that saturation in transformers is a function of 
the core, and increases rapidly at lower frequencies below the 
design limits of the core. As audio transformers go, these are 
small transformers. They are specifically designed for telecom, 
where levels are generally well controlled and there's no energy 
below 300 Hz. They're fine at low levels, but if you overdrive 
them or feed them LF, the DO saturate, and both IM and harmonic 
distortion can get nasty. They ARE big enough for the intended use 
-- driving a sound card to decode RTTY and PSK -- but you've got 
to keep the level down in the range where they're designed to 
operate. 

See  http://audiosystemsgroup.com/FilterTestNotes.pdf  for my 
measurements with a high quality professional FFT analysis system. 

BTW -- Jensen Transformers are pretty much the accepted standard 
for professional audio transformers, and their data sheets are 
quite detailed. You can learn a lot about how transformers behave 
by studying them.  http://www.jensen-transformers.com/

73,

Jim Brown K9YC