Stereo encoder with limiter

New design is available at

This solid stereo encoder is more than a "stereo bug". It can be used for quality stereo broadcasting.
The limiter ensures right modulation level for very wide input audio level range. Output amplifier allows to connect the stereo encoder directly to MPX or mono input of a transmitter.
To assemble and set-up this stereo encoder some experiences are needed.

Technical parameters

Supply voltage: 11-15 V stab.
Supply current: 40 mA
Output MPX voltage level: adjustable 0-8 V p-p

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Schematic diagram

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Part list

R1, R11 - pot. 10 k
R2, R12, R25, R30, R32 - 47k
R3, R7, R13, R17, R34 - 10 k
R4 - 180 k
R5, R15 - 47
R6, R16 - 240 k
R8, R18 - 82
R9, R19 - 330
R10, R20 - 2,2 k
R14 - trimmer 500 k
R21 - 1.5 k
R22 - 820
R23 - 10 M
R24, R29 - trimmer 10 k
R26 - trimmer 50 k
R27 - 2.7 k
R28 - 150 k
R31 - 470
R33 - trimmer or pot. 50 k
R35 - 10

C1, C3, C11, C13 - 1 u
C2, C12, C29 - 220 p
C30, C35 - 1 n
C4, C14 - 1 u / 25 V tantal.
C5, C15, C25, C27, C28, C31, C33, C34, C43 - 10 u
C6, C16 - 220 n
C41 - 33 n
C8, C18, C23, C24, C39 - 100 n
C9, C19 - 18 n
C10, C20 - 3.3 n
C21, C26 - 10 n
C22, C42 - 100 u
C32, C44 - 1.8 n
C36 - 10 p
C37 - 47 p
C38 - 22 u tantal.
C40 - 220 u

T1, T3 - BF245C
T2, T4 - BC556B
IC1, IC2, IC4 - LM386
IC3 - BA1404
D1 - 1N4007
D2 - Zener diode 2.7 V / 0.5 W
X1 - crystal 38 kHz - buy here
socket 2x3

For increasing the output level connect 10 uF capacitor between Gain pins on the IC4.

C5 and C15 are not required if the audio source is strong enough (usually is).


  1. Limiter adjust
    Place halfs of the 2x3 socket instead of T1 and T3. Set R14 to 180 k. It's reccomended to connect headphones to output of the limiter (Q1-Q4) for this time.
    Find two BF245C (T1, T3) transistors from approx. 10 pieces with similar properties. This means that if you fetch different audio levels to input of the limiter, the balance must not vary. Try it in wide input level range.
    Then eliminate small differences by R14.
  2. Stereo encoder adjust
    Set R26 exactly to half, R24 and R29 to zero. Connect the device to a transmitter. Adjust right pilot level by R33.
    Set R1 and R11 to max. Fetch some audio signal to input. Adjust right audio level by R24 and R29.

Now is the device set-up. You can adjust MPX level by R33 at any time.


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PR = wire bridge

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