if the total emitter current exceeds 5 mA. Thus, maximum

output voltage can be increased and much lower distortion

levels can be achieved using load impedances of at least

25 k

Ω.

INPUT IMPEDANCE

The input impedance of pins 4, 5, 24 and 25 is defined by in-

ternal bias resistors and is typically 50 k

Ω.

The SELECT IN pins have an input impedance that varies

with the BASE and TREBLE control settings. The input im-

pedance is 100 k

Ω at DC and 19 kΩ at 1 kHz when the con-

trols are set at 0 dB. Minimum input impedance of 30.4 k

Ω at

DC and 16 k

Ω at 1 kHz occurs when maximum boost is se-

lected. At 10 kHz the minimum input impedance, with the

tone controls flat, is 6.8 k

Ω and, with the tone controls at

maximum boost, is 2.5 k

Ω.

EXTERNAL SIGNAL PROCESSING

The SELECT OUT pins (6 and 23) enable greater system

design flexibility by providing a means to implement an ex-

ternal processing loop. This loop can be used for noise re-

duction circuits such as DNR (LM1894) or multi-band

graphic equalizers (LMC835). If both are used, it is important

to ensure that the noise reduction circuitry precede the

equalization circuits. Failure to do so results in improper op-

eration of the noise reduction circuits. The system shown in

Figure 3 utilizes the external loop to include DNR and a

multi-band equalizer.

TONE CONTROL RESPONSE

Bass and treble tone controls are included in the LMC1982.

The tone controls used just two external capacitors for each

stereo channel. Each has a corner frequency determined by

the value of C2 and C3 (see

Figure 4) and internal resistors

in the feedback loop of the internal tone amplifier. The

maximum-boost or cut is determined by the data sent to the

LMC1982 (see

Table 1).

The typical tone control response shown in Typical Perfor-

mance Curves were generated with C2 = C3 = 0.0082 µF

and show the response for each step. When modifying the

tone control response it is important to note that the ratio of

C3 and C2 sets the mid-frequency gain. Symmetrical tone

response is achieved when C2 = C3. However, with

C2 = 2(C3) and the tone controls set to “flat”, the frequency

response will be flat at 20 Hz and 20 kHz, and +6 dB at

1 kHz.

The frequency where a tone control begins to deviate from a

flat response is referred to as the turn-over frequency. With

C = C2 = C3, the LMC1982’s treble turn-over frequency is

nominally

DS011028-6

FIGURE 2. Input and Mode Select Circuitry

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