Archive for April, 2010

Telephone Amplifier

April 7th, 2010 by Thomson | No Comments | Filed in Coreelectronics

A telephone amplifier enables more than one person to follow a telephone conversation. The unit described here, in common with all normal units of this type, requires no direct connection to the telephone. Instead, the special pick-up coil has a built-in rubber suction cap that enables it to be easily attached to the telephone base. This produces a very weak signal from the magnetic field radiated by an inductive component inside the telephone, but satisfactory results can be obtained if it is fed to low noise, high gain amplifier. It would of course be possible to use a much simpler circuit if a direct connection to the telephone were to be made, but this would make installation more difficult and it is ILLEGAL to make a direct connection to a Post Office telephone anyway.

Telephone amplifier Telephone Amplifier

The preamplifier stage of the unit is based on IC1 which is a low noise op amp having a FET input stage. This is used in the conventional inverting audio amplifier mode and the negative feedback network, R1, 4, sets the voltage gain at about 40dB. (100 times C11 reduces the gain slightly at high frequencies in order to obtain an improved signal to noise ratio.

C4 couples the output from the preamplifier to volume control, RV1, and from here the signal is coupled to the power amplifier by C5. The output stage uses the TBA820M, a class B amplifier which will give an output power of a few hundred milliwatts rms. The closed loop voltage gain of the device is determined by the value of R6, about 25dB. (180 times) with the specified value. This gives the required very high overall gain in conjunction with the preamplifier’s gain. C7, R7 and C8 are needed in order to maintain stability.

The quiescent current consumption of the unit is only about 5mA, but this rises to as much as 50mA or so at high volume levels. The best position for the pick-up coil on the telephone base (not the handset) can be located with a little experimentation.

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VMOS 10 Watt Amplifier

April 6th, 2010 by Thomson | No Comments | Filed in Coreelectronics

At first sight this circuit may seem to be a straightforward Class B design having an emitter follower, complementary output pair and Darlington Pair common emitter drive stage. However, the output devices are, in fact, complementary VMOS transistors used in the source follower mode (the FET equivalent of the emitter follower).

VMOS 10 watt amplifier free projects VMOS 10 Watt Amplifier

R1 and R2 are used to bias the unit to give the optimum quiescent output potential and they provide overall negative feedback, which improves the quality of reproduction. D1 and C4 are boot strapping components, enabling the gate drive voltage to Q3 to go above the positive supply potential, giving improved efficiency to the circuit. R3 is the main collector load for Q2 and PR1 is used to give a standing bias on the output transistors that gives a quiescent current consumption of about 25mA. The thermal compensation circuitry normally used is totally unnecessary in this circuit, since VMOS devices do not suffer from thermal runaway. In fact the quiescent bias current will drop slightly as the output devices heat up, but not sufficiently to give rise to significant crossover distortion.

C2 and C5 provide DC blocking at the input and output respectively, while C1 is a supply decoupling component. C3 gives a degree of high frequency attenuation and aids the stability of the circuit.

Although the current in the driver stage, only about 1mA, may seem to be totally inadequate, it is in fact more than sufficient since the VMOS devices have extremely high input impedances and consume no significant, input current. This is one of their main advantages over bipolar devices. One disadvantage in this particular application is lower efficiency due to the higher threshold voltages and on resistance of VMOS transistors in comparison to bipolar devices. However, the circuit will give an output of 10W rms using a supply voltrage of about 33V or so (with a current drain of up to about 600mA). An input of about 500mV rms is needed for maximum output.

Note: The output devices do not have internal zener protection diodes and the appropriate handling precautions should be taken.

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Cassette Preamplifier

April 3rd, 2010 by Thomson | No Comments | Filed in Coreelectronics

Used in conjunction with one of the cassette mechanisms currently available on the surplus market (or a mechanism removed from an old recorder or player) this preamplifier circuit makes an inexpensive but useful cassette player for use with a hi-fi system.

The output signal level from a cassette tape head is typically about 500 micro Volt or so at middle audio frequencies for a mono head and about half this level for a stereo type. The preamplifier must, therefore, provide a considerable amount of voltage gain in order to match this to a hi-fi amplifier, since these require a signal level about 1,000 times higher. It is also necessary for the preamplifier to provide equalization, because the output from a tape head rises at a rate of 6dB per octave. However at higher audio frequencies, tape heads are not very efficient and require a much less rolloff.

free electronics projects cassete preamplifier Cassette Preamplifier

Q1 and Q2 are used in a conventional two stage, direct coupled, common emitter amplifier and the frequency-selective negative feed-back through C3 and R4 provides the appropriate equalization. These also set the midband voltage gain of the input stage at about 46dB. With such a low input level it is obviously necessary to use low noise transistors (Such as the BC109C) in order to obtain good results. Running Q1 at a low collector Current, about 200uA, also helps.

Q3 is used as a low gain common emitter stage, which provides the additional amplification. R9 introduces negative feedback, which controls the voltage gain of Q3 and the specified value gives a gain of about 14dB. For a stereo unit R9 should be reduced to 390R in order to give increased gain, to compensate for the lower output of a stereo tape head.

When playing a Dolby B encoded cassette SVV1 can be closed; this gives a small degree of treble cut which provides a reasonably flat overall response.

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