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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
