Audio Engineering

Preamplifi ers and Input Signals 215

conditions, but will also occur in low-pass fi lter systems if too high an attenuation rate is
employed.

The ear is very sensitive to slew-rate induced distortion, which is perceived as a
“ tizziness ” in the reproduced sound. Transient overshoot is normally noted as a somewhat
overbright quality. The avoidance of both these problems demands care in the circuit
design, particularly when a constant current source is used, as shown in Figure 7.48.

In this circuit, the constant current source, CC 1 , will impose an absolute limit on the
possible rate of change of potential across the capacitance,C 1 (which could well be
simply the circuit stray capacitance), when the output voltage is caused to move in a
positive-going direction. This problem is compounded if an additional current limit
mechanism, CC 2 , is included in the circuitry to protect the amplifi er transistor ( Q 1 ) from
output current overload.









In Current limit

Out

NFB

0 V

0V

Figure 7.46 : Typical amplifi er layout causing slew-rate saturation.

Input

Output

Figure 7.47 : Transient “ ringing. ”