Danish Audio ConnecT. Technical info. SMD (Surface Mount Device) resistors versus conventional leaded resistors. DACT use Surface Mount resistors for all audio in-signal-path applications, where possible at all. For instance, all DACT stepped audio attenuators and balance controls use resistor networks based on SMD resistors. There are numerous benefits from using SMD resistors. For instance, the bandwidth is larger because of lower series inductance and lower stray capacitance compared to any kind of leaded resistor. 

Technical information from DACT
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Fig. 1. Simplified electrical equivalent circuit for a leaded resistor


A resistor is a more complicated component than most imagine. A simplified equivalent circuit for a leaded resistor is shown in Fig. 1. It should be noted that the equivalent circuit may be drawn in serveral other ways, and that it depends on the design of the resistor. Fig. 1 is a typical equivalent circuit.
 The resistor called Rinternal is what it is all about.  

Usually Rinternal is the purely resistive (ideal) ohmic value that is referred to as the resistor’s value. So for a 10kOhm resistor, Rinternal equals 10kOhm. This is true for both leaded resistors and for Surface Mount resistors. If a resistor consisted of Rinternal only, it’s specifications would be ideal: No distortion, no noise and a frequency range reaching infinitely high frequencies. 

Within the resistor body itself, there are then a few unwanted elements. There is a resonator circuit consisting of a capacitor C internal and an inductor Linternal., where Cinternal usually has a series resistor of significant size. Both of these two reactive elements of course introduce unwanted disturbances like non-flat frequency response. Besides the frequency dependent components, real-life resistors contains a noise source, where the level of the noise depends on the design of the resistor and of the materials used. High noise voltages are generated in cabonfilm resistors, while metal film resistor are more quiet.

 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.

So when Surface Mount resistor already has an edge over leaded resistors with the resistor body itself, the SM resistor advantages becomes real obvious when the leads are taking into account. Reason: a Surface Mount has practically no leads.
Both resistor leads have their own series resistance (Rlead1 and Rlead2) and own series inductance (Llead1 and Llead2).

Besides, there is a stray capacitance between the two leads (C leads). While R lead1 and Rlead2 usually do not disturb much, the three reactive components Llead1, Llead2 and Cleads may very much limit the bandwidth of the resistor. Finally, the larger physical size of leaded resistors and the presence of the leads, makes leaded resistors more likely to pick up noise radiated from the surrounds, or even influencing each other by radiating into each other.   

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