The Gainclone amplifier is well know amplifier among the audio fans. It’s based on the National Semiconductor’s LM1875 chip. This article shows an alternative design using the TDA2050 by ST.
The original implementation of the chip can be found inside the datasheet:
The development of the new topology started as a group project in which many people participated. The original forum thread can be found here(in Bulgarian):
The amplifier includes a delay circuit that keeps the speakers disconnected for few seconds while the amplifier settles to its nominal working condition. This will prevent the amplifier from introducing the unpleasant pops in the speaker during power up.
Here is the complete schematic for this project:
As you can see the NFB loop was modified.
Here is the original schematic suggested bu ST:
Comparing the two schematics we can see the differences. The modified circuit has the following advantages:
1. Eliminating the electrolyte capacitor in the NFB loop and using a non electrolytic one. This results in a lower overall distortion and sound improvement.
2. Using a low value resistor in the NFB loop (R2) we introduce a much lower noise in the signal path.
3. Using two identical capacitors at the input and NFB loop will result in cancellation of the any kind of distortions introduced by the capacitors due to the differential structure on the amplifier.
4. No DC gain. This results in no DC offset at the output.
To prevent the circuit form unwanted roll off at low frequencies, the NFB was further modified to prevent DC gain, but allow low frequencies to be amplified with enough gain.
The speaker delay circuit was designed by a fellow forum mate Bis and was adapted to suit the needs for this project. It is a simple but efficient. It uses a standard 555 timer. The time delay is set with the value of C2 capacitor. The relay used in the schematic is a one pole 10A power relay. I recommend Omron G2R series relays here. But you can use any other brand you like. The schematic shows a low cost version with relay by Goodsky:
This is how the PCB looks like:
Few words about the grounding. You will notice that the signal ground and the power ground are separated. Each ground should be connected with its own ground wire directly to the center of the power capacitors. (pin1 and pin 3 of X2 terminal). The 12V supply should be coming from a separate low power transformer.
The original PCB artwork is available for download HERE and as usual the design is free for personal usage. The commercial usage is of course not allowed.
THANK YOU FOR READING THIS ARTICLE