APPLICATION NOTE 3171

Do Passive Components Degrade Audio Quality in Your Portable Device?

Jun 16, 2004

 

 

Abstract: In an audio circuit, passive components define the gain, provide biasing and power-supply rejection, and establish DC-blocking from one stage to the next. Portable audio, for which space, height, and cost are usually at a premium, forces the use of passives with small footprints, low profiles, and low cost.

The audible effect of these devices is worthy of some examination, because poor component choice can significantly degrade the measured performance. Some designers assume that resistors and capacitors have no measurable effect on audio quality, but the nonlinear characteristics of many common passives used in the audio signal path can seriously degrade total harmonic distortion (THD). In some cases, the nonlinear contribution of passives exceeds that of active devices such as amplifiers and DACs, which are assumed by many designers to be the limiting factor in audio performance.

Sources of Nonlinearity

 

Il valore di resistenza e capacità dei componenti passivi dipende dalla tensione applicata ai loro morsetti. (voltage coefficient). Per esempio un resistore da 100K può diventare da 101K con 10 V applicati.

Alcuni costruttori mostrano questa caratteristica nelle specifiche.

I moderni resistori a strato metallico non presentano problemi. I condensatori invece ne hanno molti.

 

 

 

Tutti questi difetti sono rilevabili attraverso le normali misure di Distorsione.

Test Description

 

In questo filtro passa alto:

ad alta frequenza l’impedenza del condensatore è piccola rispetto al resistore ma le non linearità si sommano all’uscita.

La distorsione tende ad un picco attorno alla frequenza –3dB

 

 

 

Figure 2. THD+N in funzione della frequenza per un filtro passa alto a 1000 Hz realizzato con un condensatore al polyestere

 

 

Si nota l’incremento di Distorsione a bassa frequenza.


 


 

 

 

 

Value

Case Size L x W (mm)

Voltage Rating (V)

1µF

A (3.2 x 1.6)

25

1µF

B (3.5 x 2.8)

35

1µF

C (6.0 x 3.2)

50

 

Condensatori al tantalio

Figure 3. Confronto della  THD+N in funzione della frequenza per diversi condensatori al tantalio (filtro passa lato a 1 kHz).

Table 1. Comparison of SM tantalum capacitors tested in Figure 3.

 

 


 

Value

Case Size

Voltage Rating (V)

Dielectric Type

1µF

0603

10

X5R

1µF

0805

16

X7R

1µF

1206

16

X7R

 

Condensatore ceramico

Figure 4. Confronto della  THD+N in funzione della frequenza per diversi condensatori ceramici (filtro passa lato a 1 kHz).

Table 2. SM ceramic capacitors tested in Figure 4.



 

Figure 8. THD+N vs. frequency for large-valued, 100µF capacitors driving a 16Ω load. Both types (tantalum and aluminum electrolytic) contribute heavily to THD at the nominal -3dB point of 100Hz. No such output-coupling capacitors are required with Maxim's DirectDrive headphone amplifiers.

 

Condensatore elettrolitico da 100 uF



 

How to Avoid Capacitor Voltage-Coefficient Effects

 

Figure 5 shows a line-input topology whose novel AC-coupling configuration allows a much lower valued input capacitor than that of a traditional configuration. The input capacitor in this example (C1) is 0.047µF, which can be specified as a ceramic with C0G dielectric in a 1206 case size—a configuration that minimizes the THD contribution from voltage-coefficient effects. DC feedback for the op amp (which should be a device with low input-bias current, such as the MAX4490) is provided by the two 100kΩ resistors. The effect of the DC-feedback path at audio frequencies is attenuated by C2 and R5, so the majority of the feedback is from R1 and R2 through C1. With the values shown, the -3dB point is set at 5Hz.



Figure 5. This novel line-input stage reduces degradation due to voltage-coefficient effects. Including the traditional AC-coupling capacitor inside the amplifier's error path lowers the value of that capacitor, and enables the use of C0G capacitors in portable designs.

Figure 6. Frequency response for the circuit in Figure 5 shows a smooth rolloff below 10Hz with the -3dB point at 5Hz. The ultimate rolloff rate with decreasing frequency is 20dB/decade.



This type of compound feedback ultimately has a first-order LF rolloff, but can be tuned for a 2nd-order response around the highpass cutoff frequency. Consequently, pay careful attention to overshoot and peaking when adjusting the component values from those shown in Figure 5. Values in the example approximate a maximally flat highpass function. This circuit principle can easily be adapted to quasidifferential (ground-sensing) and fully
differential input stages.

 


 

 



 

Summary

Passive components can add real, measurable degradation to an analog audio path. Those effects can be easily examined and assessed using standard audio test equipment. Of the capacitor types tested, aluminum-electrolytic and polyester capacitors give the lowest THD. X5R ceramics give the poorest THD.

When choosing active components, take care to minimize the number of AC-coupling capacitors in analog audio stages. For example, use differential signal paths or DirectDrive components for headphone feeds (e.g., MAX4410). When possible, design audio circuitry with low capacitance values in which C0G or PPS dielectrics can be used. To reduce voltage-coefficient effects in AC-coupled audio stages, restrict potential problems to the subaudio frequencies by lowering the -3dB point much more than necessary, by 10x, for example.

DirectDrive is a registered trademark and registered service mark of Maxim Integrated Products, Inc.