Impact of passive components in portable devices on audio quality

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In audio circuit design, passive components are usually used to set the gain, provide current bias and current decoupling, and are used to separate relatively independent DC circuit modules. And for portable audio design, because of the space, Height and price constraints must use passive components in small packages, low heights and low prices.

1 source of nonlinearity

Both capacitors and resistors have a voltage coefficient, which means that their physical parameters change if different voltages are applied across them. For example, a resistor with a precision value of 1.00kΩ at zero voltage, if a terminal voltage of 10V is applied, its resistance will become 1.01kΩ. The degree of influence of the voltage coefficient depends on the type, structure and chemical composition of the component (for capacitors). Some manufacturers will provide a graph of the voltage coefficient of the component, giving a plot of the nominal voltage percentage versus the nominal capacitor percentage. The new generation of thin film resistors has a very good voltage coefficient and it is difficult to measure the error under laboratory conditions. Capacitors are different and will limit audio performance in the following ways.

● Voltage coefficient.

• Dielectric Absorption (DA): A capacitor that appears to be completely discharged will still have a very small amount of charge remaining.

• Equivalent Series Resistance (ESR): This is a frequency-dependent parameter, a low-impedance earphone or loudspeaker driven by a series-coupled capacitor that limits the maximum output power due to the ESR of the coupling capacitor.

● Howling effect: Some capacitors have a significant piezoelectric effect, but when they are bent by external pressure, they will produce corresponding voltage outputs at both ends.

figure 2

    ●Tolerance: For most large-capacity capacitors (several microfarads or higher), tolerance values ​​are rarely indicated. The tolerance of the resistor is generally 1%~2%.

The following describes a test method that also includes a simple test circuit. From the results of the audio test equipment display, it is necessary to clearly quantify the effect of the capacitance of the audio signal circuit on the audio quality. Our aim is to alert the reader to this phenomenon, to carefully observe this representative result, and to provide an effective test and comparison method.

2 test methods

The nonlinear AC effect of the capacitor is relatively easy to find. If divided by the frequency response of the analog audio circuit, the most basic filters include high pass, low pass and band pass. The nonlinear characteristics of these filters are real and quantifiable.

Consider a simple high speed RC filter (see Figure 1). When the input signal frequency is above its -3db cutoff frequency, the capacitor has a very low impedance relative to the resistor. Such a high frequency AC signal will produce a very small voltage difference across the capacitor, so the effect of the capacitor voltage coefficient can be ignored. However, the product of the equivalent series resistance (ESR) of the capacitor and the input signal current will produce a corresponding voltage drop across the capacitor. It must be noted that the nonlinearity of the ESR increases the total harmonic distortion (THD) of the circuit.

    When the signal frequency is accepted or equal to the total harmonic distortion (THD) of the -3db cutoff frequency, this test highlights the effect of the nonlinearity of the capacitor's voltage coefficient on THD. The test circuit is based on a high-pass RC filter with a -3 db cutoff frequency of 1 kHz. When we choose different structures, different materials and different types of capacitors, observe the changes in THD on the audio analyzer. We selected several types of 1μF capacitors for testing. With a 150Ω load resistor, it forms a headphone filter with a nominal cutoff frequency equal to 1kHz. Note that there is no additional DC offset across the capacitor and the input/output has the same DC potential.

3 measurement results of different capacitors

Figure 2 shows the THD+N vs. frequency curve for the above circuit. Figure (a) uses a polyester capacitor. The through-hole polyester capacitor with a rated voltage of 25V is not suitable for portable devices. The effect of the capacitor voltage coefficient on the total harmonic distortion THD can be clearly seen from this figure. Note that the polyester capacitor will cause an increase in THD below 1 kHz and the actual output signal will decrease. In addition, we noticed that the effect of the polyester capacitor after the frequency is higher than 1 kHz is very small, and the TND+N index is only slightly higher than the reference value.

Tantalum capacitors are used extensively in portable devices, and the DC blocking capacitors of headphone amplifiers are typically above a few μF. Figure (b)

Is another THD+N vs. frequency curve, which contains a traditional pass-and-tantalum capacitor test curve and three common

Surface-mount tantalum capacitor test curve. All capacitors have a capacitance of 1μF, the only difference being the physical size and rating

Voltage (refer to Table 1). Note that no DC bias voltage is applied during the test.

Ceramic capacitors are often used as AC coupling elements in audio circuits, and are also widely used in low frequency boosting and filtering circuits. The test curve shown in Figure 2(C) is similar to Figure 2(b) except that the three ceramic capacitors given in Table 2 were used for testing.

Table 1 Parameters of three surface-mount tantalum capacitors

Table 2 Parameters of three surface mount ceramic capacitors

Figure 2(c) also shows a test curve for a randomly selected hole ceramic capacitor. As seen from the figure, for the X5R ceramic capacitor, the worst THD+N value near the -3db cutoff frequency (1kHz point) is 0.2%, which is equivalent to -54db distortion. Most 16-bit audio DACs and codecs (CODEC) have better THD metrics than this. Here, we need to pay attention to COG dielectric capacitors have a very low voltage coefficient, but its maximum capacitance is limited, usually only a maximum of 0.047μF. The above test used a 1μF capacitor, so it did not include a COG capacitor.

4 How to avoid the influence of capacitor voltage coefficient

The audio amplifier shown in Figure 3 uses a novel AC coupling method that requires only a very small coupling capacitor compared to a conventional coupling circuit configuration. The input capacitor (C1) in the figure has a capacity of only 0.047 μF. Therefore, we can use a COG/1206 ceramic capacitor with a very low voltage coefficient to minimize the effect of the voltage coefficient. The DC feedback of an op amp (an amplifier that must have a low bias current, such as the MAX4490) consists of two 100kΩ resistors (R3 and R4) that attenuate the audio frequency of the DC feedback loop. The main audio feedback components are three passive components R1, R2 and C1. The -3db cutoff frequency of the circuit is set at 5 Hz according to the component values ​​shown in the figure.

    Figure 3 shows a novel input-coupled configuration audio amplifier that allows the use of COG/1206 ceramic capacitors with smaller capacitance values ​​as input coupling capacitors to minimize the effects of voltage coefficients for portable audio amplifiers. The composite feedback loop basically has a first-order low-frequency attenuation response, but it can be adjusted to a two-step response high-pass filter. Pay attention to the overload response of the amplifier circuit and the peaks associated with it when adjusting the associated passive components in Figure 3. The illustrated circuit has an approximate maximum flatness high pass response. This circuit can be easily applied to pseudo-differential and fully differential input stage amplifier circuits.

Figure 4 shows the frequency response curve of the audio amplifier of Figure 3. The circuit has an attenuation of -20 db/10 octaves at frequencies below 10 Hz, and its -3 db cutoff frequency is around 5 Hz. The MAX4410, a stereo headphone amplifier shown in Figure 5, features an innovative patented DirectDrive. Although it operates from a single supply, its output DC level is set to 0V, so the amplifier output can be directly coupled to the headphones using DC coupling. DirectDrive technology has the following advantages:

● It is not necessary to use a large-capacity (100μF~470μF typical value) DC blocking capacitor to avoid deterioration of the output audio THD index caused by the voltage coefficient of the capacitor.

● The circuit shown in Figure 5 has a very low -3db cutoff frequency. The cutoff frequency is calculated to be 1.6Hz based on the input capacitance and input resistance. If we consider a standard AC-coupled 16Ω headphone amplifier with the same 1.6Hz -3db cutoff frequency point, then the required coupling capacitor value is 6200μF. Therefore the low frequency response of the amplifier is almost independent of the load.

● Saving large capacity AC coupling capacitors also saves board area. At the same time, the large-capacity coupling capacitors are also expensive compared to the 1μF and 2.2μF small ceramic capacitors required by the MAX4410.

• This output architecture supports sink current and source (relative to ground-referenced load) load current. The MAX4410 amplifier integrates a charge pump that produces a negative supply (Pvss) that is opposite in polarity to the input positive supply (Vdd). The amplifier's output voltage swing will be close to 2Vdd, which is twice the output swing of a conventional single-supply ac-coupled headphone amplifier.

Figure 5

    Figure 5 shows a typical stereo headphone amplifier application circuit for the MAX4410. Setting the input capacitance Cin equal to 10μF limits the effect of the capacitor voltage coefficient to the sub-audio frequency, which omits the large-capacity output coupling capacitor.

In this example, we only need to select a suitable input coupling capacitor (including capacity and media type) to minimize the effects of the voltage coefficient. If a 10kΩ input resistor and a 10μF input ceramic capacitor Cin are selected, the -3db cutoff frequency of the circuit is equal to 1.6Hz.

Regarding the large-capacity capacitor, Figure 6 shows the THD+N versus frequency for a passive high-pass filter composed of two 100μF capacitors and 16 resistors. At 100Hz, -3db cutoff frequency, the voltage coefficient of both capacitors will cause the THD indicator to deteriorate. The 100μF tantalum capacitor has a THD+N index of 0.2% at the -3db cutoff frequency. If Maxim's proprietary DirectDrive amplifier is used, the low frequency and audio quality can be greatly improved by omitting this large easy-to-output coupling capacitor. In the curve shown in Figure 6, the MAX4410 test curve is approximately equal to the reference value.

Figure 6 shows two different types of 100μF capacitors (tantalum capacitors and aluminum electrolytic capacitors) that drive a 16Ω load with a -3db cutoff frequency equal to 100Hz. Maxim's proprietary DirectDrive amplifier omits this large-capacity output-coupling capacitor.

5 Summary

Passive components in analog audio circuits can adversely affect audio quality. We can easily evaluate and check for this adverse effect using standard audio test equipment. Observing the test results of the above different types of capacitors, it is found that tantalum electrolytic capacitors and polyester capacitors have extremely low THD, while X5R ceramic capacitors have the worst THD test results. When we choose active devices, the number of AC-coupling capacitors should be minimized in the audio channel. For example, for a headphone amplifier, choose a differential signal path or a DirectDrive amplifier. If possible, use low-capacitance capacitors such as COG or PPS dielectric capacitors when designing audio circuits to reduce the adverse effects of the capacitor's voltage coefficient and set the -3db cutoff frequency to the sub-audio range.

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