[Elecraft] Harmonic Distortion and Audio Intermodulation in K3'sLIN OUT Port

[Jim Brown]

On Sun, 07 Sep 2008 11:27:39 -0400, Jack Smith wrote:

>Question: You are given a  transformer with no specifications. How do 
>you determine the transformer's rated operating impedance? The answer to 
>this question should suggest that there is no inherent operating 
impedance.

Only for Alice in Wonderland. It could also suggest that the manufacturer 
doesn't know any better and hopes you won't either. A transformer is a 
passive circuit element with a rather complex equivalent circuit that 
includes resistance in each winding, stray capacitance in each winding and 
between windings, and leakage inductance. There may also be stray 
capacitance to a frame or enclosure. The magnetic material contributes 
more resistance and non-linearity, which means that some values in the 
equivalent circuit vary with the applied signal. 

Transformer performance can be optimized for a given circuit configuration 
and set of operating conditions by many design decisions made by the 
transformer manufacturer. What core material, how much of it, what form? 
How many turns, how are they wound, to what degree of uniformity and 
balance? Is there a magnetic shield? Is there a Faraday Shield? Some 
manufacturers are far more aware than others of these parameters and how 
they can be optimized for a given application. If you look at the Jensen 
catalog (www.jensen-transformers.com) you will see transformers for use as 
output transformers and for use as input transformers. You will see 
transformers rated for a range of levels and frequency response. Ya pays 
yer money and ya takes yer choice. 

Fundamental circuit analysis dictates that for optimum performance, the 
complex equivalent circuit and its non-linear properties must be 
considered in the design of circuit. Deane Jensen's 1978 applications 
note, referenced below, addresses the function of a build-out resistor for 
a modern op amp. In essence, the op amp needs to be protected from 
instability that can occur with a capacitive load. Most pro products use a 
value on the order of 80-100 ohms. As Jack observes, there is already 
series resistance in the equivalent circuit, and this may be sufficient. 

http://www.jensen-transformers.com/an/an001.pdf 

600 ohm build-out and termination has not been used in pro audio for at 
least 40 years. Among other things, it wastes 6 dB of headroom, and as it 
is used here, inhibits damping of transformer non-linearity by the output 
stage.

73,

Jim Brown K9YC