Radio Modifications

Realistic DX-394

Audio Amplifier Mods

Introduction

The overall audio response of the DX-394 has been described as tinny, hissy, hollow, lacking punch, no bass, or just rubbish. So what do you expect from a small cheap loudspeaker in a plastic box? It will never be hi-fi but you can make significant improvements to the frequency response of the audio amplifier driving the loudspeaker. The mods mainly involve increasing the value of certain capacitors in the audio signal path, see circuit diagram.

Extract from the manufacturers data sheet of the TDA1905 audio amplifier IC (IC4).

Component	Recommended value	Purpose	Smaller than recommended
C147, C148	0.22uF (DX-394 0.022uF)	Input DC decoupling.	Higher low frequency cutoff. Higher noise.
C155	1000uF (DX-394 100uF)	Output DC decoupling.	Higher low frequency cutoff.
Pin 2 HF Decoupling	0.1uF (DX-394 Not Fitted)	Supply voltage bypass.	Danger of oscillations.

In the DX-394 C155 is only increased to 470uF because of its small loudspeaker gives no more bass improvement at 1000uF.

Circuit Diagram

Parts List
Component Type	Value	Quantity
Ceramic Capacitor	0.1uF (104) 50V	2
Ceramic Capacitor	0.22uF (224) 50V	3
Electrolytic Capacitor	470uF 16V	1

The small blue capacitors are resin-dipped high quality multilayer plate ceramic.

Procedure

New Value Capacitors

Additional Components

Replace C155 for a 470uF 16Volt electrolytic capacitor. The new capacitor is physically bigger and you must ensure it does not touch resistors R153 or R171, see photo.

C143 is changed from a 10uF 16 Volt electrolytic capacitor to a 100uF at 16 Volt (use old C155) and a 0.1uF (104) ceramic capacitor is placed across it, see photo.

Solder a 0.22uF (224) ceramic capacitor across C141, C147 and C148.

Solder a 0.1uF (104) ceramic capacitor across C157 to provide HF decoupling to IC4.

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