At last, not shown on Fig. 1, there is one or more non-linear elements inside the resistor body creating distortion. The level of the distortion depends on the design of the resistor and of the materials. A common source for creating distortion is the joint between the leads and the resistor body.
Principally, the equivalent circuit of the resistor body of leaded and Surface Mount resistors are the same. For both types, the audio qualities vary greatly with the design and choices of materials. Usually, however, the best Surface Mount resistors exhibit better audio qualities than their leaded counterpart as far as the resistor body is concerned.