Handbook for Sound Engineers

Consoles 899

tells the input attenuator to drop the input signal suffi-

ciently. The whole circuit operates in check—the bigger

the input signal, the bigger the potential overload, the

bigger the control signal, the more the attenuation.

Below the tripwire—the threshold—the whole circuit

behaves the same as an ordinary straight amplifier. Fig.

25-76D is about as simple as a decently performing

limiter can get and was first noted in the original

mid-sixties Philips cassette recorder.

The LM386 is a small power op-amp commonly
used to drive headphones or small loudspeakers but
works well just as an ordinary amplifier, in this case at
some 30 dB gain. It is used here for its power output
stage, which is hefty enough it can ignore the diode
rectifier and side chain loading effects. This diode
conducts when the positive-going output signal exceeds
about 700 mV and charges a reservoir capacitor, Cr.
This is buffered by an emitter follower (TRf) feeding the
gain controlling transistor TRg. When the voltage on the
base of TRf is sufficient to force conduction through the
two base-emitter junctions, TRg turns on, causing an
increasingly low-impedance path to ground at the input
to the amplifier. It forms a potentiometer, with the
source resistor Rs attenuating the input signal to the
level at which the rectifier and two transistors are just
conducting. In this circuit that amounts to a posi-
tive-going output signal of about 2 V (the added voltage
drops of the rectifier and the two transistor base-emitter
junctions). Simplicity has its drawbacks and in this
instance they are noise and distortion. Although the
distortion is in a different league from a diode clipper,
transistors are not ideal VCAs and are somewhat
nonlinear in this application. If, however, the signal

Figure 25-75. Dynamics Input/Output Plot

+20

+10

0

10

20

30

Output level–dB reference

40



40 

30 

20 

10 0 +10

Input level–dB reference

Gain 15 dB

Expansion

1:5

+20

Linear

1:1

Compression

2:1

Limiting

> 20:1

Figure 25-76. Simple limiters.

Clipped

output

A. Diode clipping.

Germanium

Gentle

clipping

B. Soft diode clipping.

D. Simple feedback limiter.

Voltage controllable

gain/attenuation

element

Amplifier

dc control

voltage

Rectifier

C. Block diagram of a feedback limiter.

Rectifier

E. An FET-based limiter.

Release Tantalum

FET bias &

threshold

FET

Sidechain