VAR resistors
04 April 2011
Vishay introduces 60 W, 0.05 ppm/°C TCR resistors for high-end audio applications.
Vishay Precision Group, has announced that its Vishay Foil Resistors division is expanding its offering for high-end audio applications with the release of the enhanced ultra-high-precision Z-Foil VAR, in addition to a new generation of foil power current sensing resistors.
Based on Bulk Metal Foil technology, the new devices have been specifically designed to minimise the self-inductance and capacitance inherent in all resistors, offering improvements in sound quality by reducing audible distortion. With power ratings as high as 60 W, the devices are optimised for use in loudspeaker crossover assemblies, audio power amplifiers, and instrumentation amplifiers.
The company has enhanced the ultra-high-precision Z-Foil VAR resistor to offer low TCR down to 0.05 ppm/°C typical from 0°C to +60°C, +25°C ref, tolerances down to ±0.005%, and load-life stability to ±0.005% (50 ppm) at +70°C for 2,000 hours at rated power. The device has been bolstered with a thermal stabilisation time of <1 s (nominal value achieved within 10 ppm of steady state value) and can operate at higher frequencies.
The VAR provides a combination of low noise, low inductance/capacitance, and improved thermal response. Its special ‘naked Z-foil resistor’ design, without moulding or encapsulation, adds an additional dimension for reducing signal distortion and increasing clarity in signal processing.
The VAR has a low thermal EMF, which is directly attributable to the special design of the device. Thermal EMF results from the thermocouple effect that exists between dissimilar metals. Under normal conditions when both leads have the same temperature, the voltage generated in the contact between resistive element and one lead is cancelled by that of an equal level and opposite polarity in the contact between resistive element and other lead. When a temperature differential exists between the two leads, a voltage difference will occur between them.
While the VAR resistor is designed for signal processing, the same bulk metal foil resistor has been further expanded to provide high-current, high-power models for other audio applications. These are available from Vishay Foil Resistors as custom tailored products. For precise current measuring in audio applications, the new generation of foil power current sensing resistors combines very low noise with an absolute TCR of less than ±0.2 ppm/°C (0°C to +60°C or +20°C to +70°C), power TCR of less than 1 ppm/W with an absolute tolerance of ±0.01%, and a load-life stability of ±0.005% for 2,000 hours in free air. The devices offer a special construction that reduces thermal distortion. The new devices are offered in a number of configurations and power levels that best match each customer's requirements.
The resistors feature a power rating of 20 W in free air (60 W on heat sink) over a resistance range from 0.5 mΩ to 500 Ω. The maximum current for resistance values 10 mΩ and up is 22 A. Higher current values are available per customer requirements if needed. Thermal resistance (element to heat sink) for the resistors is 2°C/ W typical.
The construction of the new resistors can be tailored to the customer's specifications. Options include the resistor size as well as the size and type of the heat sink (with or without mounting holes). A wide range of resistive elements may be used and different lead diameters are available. The devices may be mounted with screws or bonded with conductive epoxy and clamps.
Bulk Metal Foil resistors owe their low excess noise and high linearity to the type of material they are made of; a cold-rolled special metal alloy foil with a thickness of several microns. This construction preserves the bulk-metal nature of the resistive element, eliminating any current noise in the element itself and providing the linearity that preserves signal integrity with no clipping or roll-off at signal extremes. These are the same resistor elements used in military and space applications but the unencapsulated VAR version does not have the damping effect or the slowing effect of shunting capacitance that is unavoidable in encapsulated resistors.
Samples and production quantities of the new resistors are available now, with lead times of five days for samples, and five weeks for standard orders.
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