Another modification project. To add a center channel and a tone control to the old PC soundsystem to be used as a simple TV setup in the summer cottage.
Step 1: Analyze the Amplifier Circuit
The original circuit consists of three 30W power amplifiers (TDA2030) for the Right, Left and Subwoofer channels. The Right and Left signal are summed and low pass filtered (12 dB/octave) in the two stages o the op-amp 4558 before it is fed into the power amp. The circuit is simple and with no frills, so it is easy to analyze.
Please read the description in my previous project to learn the easiest way to figure out the schematics.
Step 2: The Original Circuit
The original circuit without the power supply components. The pictures are difficult to view, so download the pdf.
Step 3: The Tone Control
I like to have a tone control in my TV amp, just like in the old times. A simple one-pot circuit is the Big Muff tone control used in tthe Big Muff guitar Stomp Box (also used in my previous project). First I used a 100k pot but the circuit was too susceptible to noise and hum, so I changed to a 10k pot, with much better result. The control gives max + 10 dB treble lift (the white line). The green line shows the bass boost and corresponding treble cut.
The circuit diagram shows that the tone control is inserted directly after the main volume control on the input. It is easy to cut the copper line on the pc board just beside the mid pin of the volume pot. Two wires for each channel carries the input and output signals of the tone control.
Step 4: The Tone Control Board
The tone control circuit is mounted on a small board with the pot placed under the board.
Step 5: The Center Channel
The center channel signal is taken from the output of the summing amplifier 4588. It passes thru the center volume control and into the TDA2040 power amplifier. (Not a 2030 because I had an old 2040 in my drawer). The resistance combination used (8/0,68) gives a gain of 12.
The new stage is mounted on a separate vero board with a separate heat sink not connected to the original metal back plane in order to eliminate the need for electric insulation of the amplifier.
Step 6: The Complete Amplifier
The complete amplifier with the center amp heatsink in the foreground
Step 7: The Final
Some holes had to be drilled for the Center output and the tone and center volume control. I use speakers salvaged from another surround set instead of the tiny original Labtec speakers.