comprises an audio power amplifier, a 12V dual power supply, and an audio level indicator. The power amplifier used is the popular Ic-TA7230, which delivers up to 7-watt (RMS) power per channel into a 4-ohm load. This ic has in-built short circuit protection and over-temperature cut-off. A suitable heat sink must be con- nected to the ic to prevent thermal run- away. potmeters Vr8 and VR9 are volume controls for left and right channels respec- tively, while Vr7 is the balance control.
A dual power supply is used for the circuit. The +12V section uses a π filter with capacitors in the parallel arms and an inductor in the series arm. However, for the –12V supply, the inductor of the series arm (as used for +12V supply) is replaced by a resistor. Filters are provided to reduce the ripple factor and thereby reduce hum (noise). please refer inductor details in parts list.
The audio level indicator is built around Ic KA2281. lEds d14 through d23 are connected at its outputs to show the audio level of each channel in five steps. The input to this audio level in- dicator is derived from the output of the power amplifier. The gain of this level indicator can be varied by changing the
values of resistors R49 and R50.
assembly
The complete circuit, with the exception of the audio level indicator, can be assembled on a single pcb. A separate pcb is used for the audio level indicator and for mount- ing lEds (d2 through d8). Single-sided, actual-size pcb for the complete circuit is given in Fig. 7. The component layout for the pcb is shown in Fig. 8.
Use sockets for all ics except ic1 and Ic9. shielded wires must be used for con- nections to stereo head and all potentio- meters. The pcb must be mounted away from power transformer and dc motor.
Inductor l2 should also be placed away from the power transformer. if inductor l2 is difficult to procure or fabri- cate, it may be substituted with a 5-ohm, 5W wire-wound resistor.
A suitable cassette drive mechanism and cabinet may be used to assemble the stereo cassette player. readymade cabinets and cassette mechanisms are available in the market.
Adjust potmeters Vr1 and VR2 for minimum distortion at higher volume level. Use separate aerials for FM trans- mitter and FM receiver. ❏
Readers queries:
Q1. i would like the author to clarify my
following doubts:
In the circuit, radio input and FM transmitter are meant for mono signals. What are the changes required to convert it into a stereo transmitter for cordless stereo headphones? What is the range of the FM transmitter and how can we extend it?
The transmitting function is present only when ‘tape’ mode is selected. For transmission function in other modes, what are the modifications required?
The author has further mentioned that in normal mode, mid-range frequency components get attenuated due to capaci- tor c41 (0.047µF). can I add a condenser mic preamplifier with a mixer circuit in this mode to obtain karaoke function?
m.p. murugesan Juticorin
Q2. You have not given the con-
you give me a procedure for checking the circuit step by step, as my project does not seem to work?
Y. srinivas Eluru
Q3. i want to use only the preampli-
fier part along with volume, treble, and bass control functions of the stereo cas- sette player circuit as i already have a STK459 Ic-based power amplifier. please tell me where from should i bisect the circuit.
• FM receiver has only the mono output as per the circuit connections. if i provide similar signal (audio) to the cir- cuit, will the sound be heard from both the speakers? If not, what should I do to get the output from both the speakers?
• If I give output from a stereo Walk- man headphone to AUX-IN terminal of the circuit, will the amplification be done? Also, will the tone mode support be avail- able for AUX- IN?
• What is the function of bilateral lEds in the circuit?
chetan devi Through e-mail
The author, Rejo G. Parekkattu, replies:
A1. Usually, the FM/AM radio receiver
kits are provided with a mono audio out- put. However, there are stereo decoders based on chips like TBA1330/KA2261. If one has such a circuit, connect the left and right outputs of the circuit to pin 4 of Ic3
50 metres. To increase the range of the transmitter, a power amplifier stage can be added to it. please refer ‘long range FM Transmitter’ circuit by pradeep G, published in dec. ’99 issue of EFY (or Electronics projects, Vol. 19). only the power amplifier stage of this transmitter, built around transistor T2 (2N3866), is required to be added.
For this purpose, wind coil l2 of the dec. ’99 circuit (or Electronics projects, Vol. 19) over coil l1 of my circuit. The transistor T2 in Section I can be replaced with another transistor having higher current rating. However, such long-range transmission is illegal.
For transmission in all modes, a number of changes will be required in Section-I, which I think are quite un- necessary. One can directly tune the FM/AM receiver to any radio station and therefore an extra radio transmission from the cassette player is not required. The only problem is that auxiliary sig- nals cannot be transmitted by using this circuit.
if transmission in all modes is re- quired, connect the circuit given in Fig. 1 here to the outputs of Section I, and con- nect its output to the base of transistor T3. The circuit employs two voltage followers to prevent loading of inputs.
disconnect pins 7 and 10 of Ic4 (HEF 4017) from the circuit. Then connect pin 7 to pin 15 of Ic4 via diode d1, instead of pin 10. Also remove diode d25 from the circuit. connect the base of transistor T2
to +12V via resis- tor R13 (1.2-kilo- ohm), instead of connecting it to pin 7 of Ic4.
You can mix the output of a con- denser mic pream- plifier with the out- put of section ii. in the normal mode, your sound can replace the vocals of the record to some extent, giving some sort of karaoke function.
For stereo FM transmission, some special circuit will be required. it needs some type of encoding before transmis- sion. realising this function with discrete components is not easy. ics for this special application might be existing.
A2. lEds d2-d5 are coloured bilateral
lEds, which have only two terminals. It is analogous to two lEds connected back to back, as shown in Fig. 2. If such lEds are not available, d2-d5 may be replaced with ordinary lEds. in this case biasing resistors R15, R17, R19, and R21 are not required. The connections will be similar to those of lEds d6-d8.
Ic µA741 cannot be used in place of lF353. If lF353 is not available, any other 8-pin dual op-amp like HA17458, Tl072, NE5532, etc may be used.
For testing the circuit, check whether the outputs of the decade counters are changing in order, on pressing switches S1 and S2. If they are not function-
Fig. 2: Depiction of bilat- eral LEDs
ing, check the related components and replace the Ics, if necessary. The other components in the circuit are less likely to get damaged.
if all connections are correct and all components are right, the circuit is sure to work, as the prototype made by me is working perfectly.
A3. To use STK459 as power ampli-
fier, you can avoid the power amplifier circuit based on TA7230 and directly connect the output of section 2 (Fig. 5 in the article) to the inputs of STK459 power amplifier through volume and balance controls.
The mono output of the FM receiver is mixed equally in both the channels by the circuit, so you can hear the sound from both the speakers.
Amplification will surely be performed on the AUX.IN inputs with tone mode support.
bilateral lEds are used to display the selected function (TApE, AUX, RAdIo, and TRANSMIT).
If you use STK459 as power amplifier, use a separate power supply (+18V, 3amp or above) for it. (The power supply circuit in the construction project will not support STK459.)
The pcb design plays an important role in reducing distortions such as hum. lines carrying high current, such as lines to the speaker terminals, should be kept separated from the sensitive parts of the circuit, such as components in the pream- plifier section.