[Elecraft] Harmonic Distortion and Audio Intermodulation inK3'sLIN OUT Port

[Paul Christensen]

> You'll have to address that question to the Elecraft design staff.
> My guess is that it was to protect the DAC's output amplifier from short 
> circuits a user might inadvertently apply across J22, the LIN OUT port, 
> but that can be done with much less  resistance.

I can't speak on behalf of the designer either, but more often that not, a 
series build-out resistor equal to the specified primary Z is oftentimes 
seen in professional audio applications when driving the transformer from a 
near zero Z source -- like an op-amp.  *Some* designers feel an absolute 
need to match the source Z with the transformer primary.

In reality, the best value for Rs is what produces the best compromise 
between distortion, transient response, protection to the driving source, 
noise, and secondary output level.

Speaking of transient response, I have ordered the TTC-108 and Triad TY-145P 
transformers and will be testing them against a high quality Jensen type. 
High quality transformers like those made by Jensen are generally uniform in 
phase and amplitude as a function of audio frequency.  A Tektronix TM series 
audio test set will be used compare square wave response under varying 
degrees of source and terminating Z across all three transformers.

A square-wave test through a transformer can be important in this 
application since an improper termination of the secondary can lead to 
severe overshoot and ringing into relatively Hi-Z bridging devices like a PC 
sound card.  While sine-wave level testing may appear to be acceptable, 
bizarre results can occur when trying to feed a demodulated CW signal (a 
square wave-like source) into a PC sound card for applications like CW 
Skimmer.

Paul, W9AC