NATIONAL 3D ENHANCEMENT

The LM4846 features a stereo headphone, 3D audio en-

hancement effect that widens the perceived soundstage

from a stereo audio signal. The 3D audio enhancement

creates a perceived spatial effect optimized for stereo head-

phone listening. The LM4846 can be programmed for a

“narrow” or “wide” soundstage perception. The narrow

soundstage has a more focused approaching sound direc-

tion, while the wide soundstage has a spatial, theater-like

effect. Within each of these two modes, four discrete levels

of 3D effect that can be programmed: low, medium, high,

and maximum (Table 2), each level with an ever increasing

aural effect, respectively. The difference between each level

is 3dB.

The external capacitors, shown in Figure 6, are required to

enable the 3D effect. The value of the capacitors set the

cutoff frequency of the 3D effect, as shown by Equations 1

and 2. Note that the internal 20k

Ω resistor is nominal

f

π * 20kΩ *C

3DL

(1)

f

π * 20kΩ *C

3DR

(2)

Optional resistors R

7) to affect the -3dB frequency and 3D magnitude.

f

π * (20kΩ +R

(3)

f

π * 20kΩ +R

(4)

∆AV (change in AC gain)=1/1+M, where M represents

some ratio of the nominal internal resistor, 20k

Ω (see ex-

ample below).

f

π (1 + M)(20kΩ *C

(5)

C

(6)

TABLE 6. Pole Locations

R

Ω)

(optional)

C

M

∆AV (dB)

f-3dB (3D)

(Hz)

Value of C

3D

to keep same

pole location

(nF)

new Pole

Location

(Hz)

068

0

0

117

1

68

0.05

–0.4

111

64.8

117

5

68

0.25

–1.9

94

54.4

117

10

68

0.50

–3.5

78

45.3

117

20

68

1.00

–6.0

59

34.0

117

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 8

Ω LOAD

Power dissipated by a load is a function of the voltage swing

across the load and the load’s impedance. As load imped-

ance decreases, load dissipation becomes increasingly de-

pendent on the interconnect (PCB trace and wire) resistance

between the amplifier output pins and the load’s connec-

tions. Residual trace resistance causes a voltage drop,

which results in power dissipated in the trace and not in the

load as desired. For example, 0.1

Ω trace resistance reduces

the output power dissipated by an 8

Ω load from 158.3mW to

156.4mW. The problem of decreased load dissipation is

exacerbated as load impedance decreases. Therefore, to

maintain the highest load dissipation and widest output volt-

age swing, PCB traces that connect the output pins to a load

must be as wide as possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply’s output voltage

decreases with increasing load current. Reduced supply

voltage causes decreased headroom, output signal clipping,

20166895

FIGURE 6. External 3D Effect Capacitors

20166894

FIGURE 7. External RC Network with Optional R

3DL

and R

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