Foil Resistors in High-End Audio Applications
High-end analog audio applications require low intrinsic noise, high linearity of amplification, and minimal dynamic distortion. The typical audio amplifier consists of a voltage preamplifier (preamp) and power amplifier (final driver). The voltage preamplifier deals with low-level signals. That is why its intrinsic noise level is critical. Resistors are among the principal noise sources in the amplifiers.
The main requirements for the audio power amplifier are high linearity of amplification and minimal dynamic distortion. Foil resistors are characterized by very low intrinsic non-linearity of the resistive element, which is made from bulk metal. Wirewound resistors and some metal film resistors have similar non-linearity characteristics, but in the real world the intrinsic linearity of the resistors is not sufficient to ensure linearity of amplification.
Audio power amplifiers are often based on a circuit design that is similar to an operational amplifier (Fig. 3, below).
k = 1 + R2 / R1
depends on the R2/R1 resistance ratio in the negative feedback voltage divider. The resistors R1 and R2 dissipate power
P1 = [VO / (R1 + R2)]2 R1
P2 = [VO / (R1 + R2)]2 R2
P2 / P1 = R2 / R1
Commonly k > 2 and consequently R2 > R1. This means that the power dissipation and temperature rise in resistor R2 will always exceed the power dissipation and temperature rise in resistor R1.
Even if both resistors have the same TCR (ideal case) the gain of the amplifier will vary because the R2/R1 ratio will depend on output voltage VO. Thus spikes and pulses that are typical for sound signals may result in transient changes of amplifier gain. The result is a nonlinearity of dependence between input voltage VI and output voltage VO (Fig. 4, below).
This phenomenon is called temperature-induced nonlinearity of amplifier . It is caused by resistor self-heating which is characterized quantitatively by power coefficient of resistance (PCR) . The way to abate PCR is to choose R1 and R2 resistors having minimal absolute TCR values.
For example, a load of 0.3 W was applied simultaneously to three 1206-size chip resistors manufactured using different technologies:
- Thick-film chip resistor with TCR of about + 42ppm/°C
- Thin-film chip resistor with TCR of about + 4 ppm/°C
- Bulk Metal Foil based on the Z-Foil technology with TCR of about -0.1 ppm/°C.
The results are shown in the graph below.
Figure 5. Effect on resistance of self-heating (power coefficient of resistance) on three resistor types, measured over 9 seconds under 0.3-W power load .
The stabilized values of relative resistance changes are the following:
- Thick-film chip resistor: +2000 ppm;
- Thin-film chip resistor: -140 ppm;
- Bulk Metal Z-Foil: +5 ppm.
The Bulk Metal foil resistor is the preferable choice for audio amplifiers when high linearity of amplification and minimal dynamic distortion are required. In particular, Vishay Precision Group recommends the VAR, Z201, S102C, Z203, VSHZ, VSMP Series (0603-2018), VFCP Series, SMRXDZ Series resistors for noise-free performance in audio applications (www.foilresistors.com).
The authors wish to acknowledge the invaluable assistance of Joseph Szwarc in the preparation of this article.
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