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limiter.jpg (13140 bytes)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.

Following circuit is the simplest limiter possible. It nicely shows how the limiter works. Good circuit for unassuming use.

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

cpresorg.gif (5275 bytes)
Output voltage vs. input voltage (with C8 placed)

Schematic diagram

limiter2.gif (6019 bytes)

Parts list

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

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)

T1 - BF245C
T2 - BC556B, BC307
IC1 - LM386

Don't place the C8 if you don't want to use whole dynamic range. Then you will get much better S/N ratio.
You may adjust release time by C7 and R4 values.

PCB layout

lterdps.gif (6367 bytes)

lterdps2.gif (7485 bytes)