An endless variety of stereo amplifiers arc available in the market. But the prices of good quality amplifiers, equipped with graphic equalisers etc, are not within the reach of all audio enthusiasts. The cheaper varieties are generally based on discrete transistors, and suffer from drawbacks such as uncontrolled hum, ineffective tone controls, poor bass response, thermal runaway etc. The use of interstage transformers in such circuits reduces their versatility.
This article describes the functional and constructional details of a compact and complete solid-state stereo amplifier module, which is capable of providing an output of over 8 watts into each channel. The module contains an inbuilt 6-band graphic equaliser, which can be used to control and blend frequencies and tones over a wide range, thus ensuring perfect sound reproduction.
The module can be used as an AF output stage of a record player, cassette player or just as an independent stereo amplifier itself. The system offers a high degree of flexibility as it can operate in environmental temperatures between 0s and 70°C, and operate off voltages in the 5-20V range. The system is internally protected against input and output short-circuiting. The module can automatically protect itself on being accidentally connected to a high voltage supply line (up to 40V). The noise and distortion level is extremely low and no turn-on transients occur.
The main feature of the module is its low cost (which is less than that of a commercially available graphic equaliser kit itself!). The complete circuit costs just around Rs 200.
The block diagram of the complete amplifier module is given in Fig. 1. The circuit of the complete module is given in Fig. 2. Each channel consists of an independent 6-band graphic equaliser, power amplifier and a common power supply.
The graphic equaliser is built around a dual op-amp LM1458. The gain of the op-amp at various frequencies is determined by the setting of the corresponding pot. The audio frequency spectrum is covered in six different bands of tone control. The frequency control pots and the corresponding frequencies controlled by them are listed in Table I. VR13 and VR14 serve as volume controls for the left and right channels respectively.
As the circuit provides excellent control over the frequencies, use of the-conventional graphic equaliser which requires at least a dozen op-amps was deemed unnecessary.
The power amplifier section is built around two low-cost, monolithic power amplifier ICs LM2002A. The ICs are current limited and thermally protected. The input is fed to the non-inverting input pin 1 through the l0 mF coupling capacitor. The output is coupled to the speaker through the 2200pF capacitor. The 1-ohm resistor and the 0.1 mF capacitor, serially connected across the output terminal (pin4) and ground, help in noise reduction. At a supply voltage of 20 volts, each amplifier IC provides an output exceeding 9 watts into a load of 4 ohms, and about 1 watt at a supply voltage of 5 volts. Ihe output power at different voltages may be calculated from the manufacturers graph given in Fig. 4.
It is preferable to operate the module at its upper voltage limits to obtain the maximum output power. The circuit can operate off voltages between 5 and 20 volts. The power supply circuit for the amplifier module is given in Fig 3, The circu it is capable of providing an output of 18 volts with an accuracy of about 25 per cent. XI, Dl, D2 and C29 drop the mains voltage to about 24 volts, which is then stabilised at 18 volts by IC4. The LED indicates the presence of output voltage.
The complete circuit may be assembled on a single PCB, pattern of which is given in Fig. 5, and the components should be mounted as shown in Fig. 6, Pin connections of all ICs and transistors are given in Fig. 7.
Tip temperature of the iron should not exceed 275°C under any condition. Shielded cable should be used for all input and output connections and colour coded ribbon cable should be used for connecting the pots. Proper heatsinks should be selected with the help of the manufacturers data shown in the graphs in Figs 8(a) and 8(b). The common heatsink should be bolted to the PC’B at an angle of 90*.
If the module is used as an independent amplifier, it may be enclosed in a metallic cabinet. The cabinet itself may be used as a heatsink in this case.
After construction, the module should be tested for proper operation. The module should be switched on and an audio signal fed in, with the volume controls at ‘maximum’ position. Let it remain on for about 10 minutes and then check whether the tab temperature remains below 40°C. If not, check for improper contact within the heatsink, and apply some heatsink compound if necessary.
Now, feed in an audio signal from a tape recorder and cheek the graphic equaliser for proper boost/cut in the corresponding frequencies. If proper response is not obtained, check for open wires. If hum persists, check for leaky capacitors,or defective rectifier diodes, in case the mains supply circuit is used.
1. The circuit may be used with the less commonly used 2-ohm speakers in order to obtain higher output volume.
2. The LED dancing arrays, the circuit of which is given in Fig 9, may be added to the main module. The LEDs flicker with si brightness level which is dependent upon the output power.
3. The quality of the speakers must not be compromised as the quality of Ihe output mainly depends upon the speakers. The prototype used 15cm, 10-watt ‘Philips’ speakers.
4. Active or passive crossover systems may be used at the outputs; this would facilitate the use of different speakers for different frequency ranges.
5. The module may be used as a stereo cassette/record player, after feeding Ihe input through a suitable preamplifier.
I have assembled the circuit of ‘8W+8W Hi-Fi system with 6+6 Band Graphic Equaliser» published in EFY June ’92 issue and it is giving a lot of problems as follows:
On increasing the volume a lot of noise is heard. Only at very low volume the circuit gives better results.
Secondly, in the circuit the common line is connected to VR13, VR14, LSI and LS2. When I switch on the set a crackling sound is heard continuously and ICs get heated. Whereas on disconnecting the common line from LSI and LS2, I am getting slightly better results at low volumes. Also, Graphic Equaliser part is not working very satisfactorily.
The author, Mr Amrit Bir Tiwana, replies:
The main problems are most probably due to long earth wire loops, high input signal voltage and probable use of non-shielded wires at input terminals. Since the design of the circuit was tested by me. EFY lab, and so many EFY readers, try to check the above facts and you will get good results.