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Re: How can that work well?

12.9.147.132

Several related issues are at work here. First, not all sources are the same. The rated load impedance will vary, as will the degree to which the source relies on that value being resistive in order to maintain minimal phase shift and specified frequency response. For example, a source with a 10 ohm output impedance that is rated by the manufacturer for a 100 ohm load won't be much affected by a reactive load of 5,000 ohms. However, a source that derives its output from the anode of a 12AU7 will likely suffer audible anomalies driving such a load. Source devices are specified by manufacturers for a resistive load; failure to provide it places the integrity of the signal at risk.

Second, signals couple between the primary and secondary of a transformer by several means and with numerous modifiers. In addition to the desired magnetic coupling, transformers exhibit capacitive coupling between windings. The effect of this capacitance varies as a function of both the frequency and the load. As the load impedance is increased beyond the transformer manufacturer's design specification, the energy coupled by means of the capacitance becomes a larger percentage of the total energy at the output of the transformer. This can create very large peaks and nulls in amplitude across the spectrum, as well as phase shifts approaching 180 degrees. It also plays a role in the exaggerated ringing that occurs when unloaded transformers are tested with square waves.

You are correct regarding similarities to choke loading, but there are significant differences. The most obvious in my mind is the lack of primary-secondary coupling and capacitance. Then, there's the current flow. For a transformer to operate correctly, AC current must flow through the primary and magnetically induce a corresponding current in the secondary. Not only is that impossible when the secondary is unloaded, but a choke has no such requirement. In the case of the shunt choke, the goal is simply minimal AC current flow through the (single) winding. Distributed capacitance and other strays must be controlled, but the design process and in-circuit application are otherwise considerably simplified.


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