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Why it is important to use a limiter?

Audio signals as music or speech have big dynamic ranges. There are silent and loud sections. These audio signals aren't too good for a transmitter, which requires audio signal with constant level on the input. Limiter is a device, which weakens loud signals and intensifies silent signals. On its output there is signal with constant level.

Technical specifications

Supply voltage: 10-14 V stab.
Supply currents: 20 mA
Audio input: impedance 20 kOhm, optional preemphasis
Output voltage: 0,4 V rms
Dynamic range: >50 dB

Output voltage vs. input voltage

Schematic diagram

Parts list

Resistors:
R1 - 68 k
R2 - 22 k
R3 - 180 k
R4 - 10 M
R5 - 10 k
R6 - 47 R
R7 - 240 k
R8 - 1,5 k
R9 - 820 R

Capacitors:
C1 - 100 p (ceramic)
C2 - 1 n (plastic)
C3, C6, C12 - 100 n (ceramic)
C4, C11, C13 - 100 u (electrolytic)
C5 - 0,33 u (tantalum)
C7, C8 - 10 u (electrolytic)
C9 - 220 n (plastic)
C10 - 47 n (ceramic)
C14 - 470 n (plastic)

Misc.:
T1 - BF245C
T2 - BC556B, BC307
IC1 - LM386

PCB layout