How many know that I have been a good builder of audio amplifiers? I’m not talking about the easier way, something like assembling your computers. But the hard way. Getting printed circuit boards, buying each and every component seperately, placing them on the pcbs, soldering them, and getting them to work. Actually, I have been doing this since I was 15 yrs of age. Is any one surprised? Well, kids these days are building robots at the age of 10.

Anyways, let me talk about the amplifiers. When I was 15, I could only play with audio cassettes, since CD Players were a bit expensive. So, I broke an old tape recorder that we had, a very old National Panasonic tape recorder and took out the cassette mechanism or the cassette deck. That had already served for about 15 years, as the tape recorder was bought in my family the year that I was born. And after it died out, the mechanism served me for 3 more years. I dont know where I threw it after that.

I replaced the mono playback head with a stereo AX head, and removed the recording head from it. Changed the pinch roller, and bought a new motor and a set of belts and fly wheel, and the mechanism was ready.

Next thing I needed was a pre amplifier. A Pre-Amplifier is an amplifier that boosts the very weak signals that comes out from the coild of the head, which are induced by the magnetic variations on the tape, to such an extent that it can be fed to the amplifier.

The design of a Pre Amplifier can be very tricky. If you use one mismatching component, all that you will hear is noise, and not the actual music thats recorded in the cassette. And if you dont bias the transistors properly in the pre amplifier, you will end up recording noise in the cassette that you are playing.

I always prefer transistors to those stupid ICs. ICs consume a lot of energy, release a lot of heat, and are stupid. You cant do a single alteration in the circuit thats inside the IC. Its not open source

A Japanese transistor, with number C 138 is my favourite first stage pre amplifier transistor. a Its an NPN transistor, and the boost that it gives to the signals that come in microvolts from the head, are simply awesome. This is usually a single transistor stage, which just boosts the signal and gives a 180 degree phase-shift as well to the signal. We usually try to make it a Class C amplifier to improve the effeciency.

The second stage is a Class AB amplifier, with 2 transistors, and both of them biased thru a preset, thru which the amplitude of the signal can be controlled. But this is a stupid idea that most disagree with. If the values of the components are chosen incorrectly, we would end up creating a low pass or a high pass filter with it.

The 2nd stage amplifies the signal to a better leve. I usually prefer amplifying the signals thru 4 stages of amplification before it gets into the power amplifier, while most do it right from the first stage of the pre amplification process, and thereby get a very poor sound reproduction.

After the pre amplifier stage, the signal is passed on to an equalizer.

People of these days, electronics hobbyists if any, can ignore the above preamplifier section, since thats history now. These days you get CD ROM drives for about Rs 800, or $20, with a built in pre amplifier.So, all you need to do it is to connect the ourput of the drive to an equalizer or the poweramplifier as desired.

The equalizer is a circuit that gives the signal a +10 DB boost or a – 10 DB buck, and this can be done for each and every band of the signal. The most common equalizers come with 5 or 10 bands. If you see winamp, you will find a 10 band equalizer in it.

The equalizers are usually made with Op Amps. IC 741. Any engineer cant have passed out from engineering without hearing about this IC. On a 5 Band equalizer, there are 6 OP Amps, or Operational Amplifiers. The first OP Amp boosts the entire range of Singnal to +10 DB, and this signal is fed thru potentiometers to the other Op Amps. which would be tuned to a particular band of frequency.

The level set by the potnetiometer will be again amplified by the corresponding OpAmp, and we get to tune the sound wave t the perfect style we like. Pop, Jazz, Rock, anything.

The Output of the equalizer is then fed to a noise reducing circuit, which will reduce the noise caused by the 50Hz applications around the amplifier. If you think any other freuqency is noise, you can designer a noise reducing filter for that too.

Then comes the power amplifier. I build power amplifiers only with transistors. And that too, only with British Transistors. I dont trust any other transistors. British Transistors start with the series BC and BD, where BC is the series of low power transistors, and BD High Power. There is a german series called SL, but unfortunately, though I admire German effeciency, I dont like this series.

For a power amplifier, I would use a BC547 to boost the equalizer’s output of 5 Volts to 15 volts. This 15 volts is boosted to 45 volts, and at the last stage, the 45 volts is amplified to 150 volts.

The output of the amplifier should usually be with an impedence of 8 ohms, so as to get optimum deliver of sound and compatiblity with speakers. This particular amplifier could deliver an RMS of about 1500 watts, if, suitable speakers were connected. But I was confined to 2 x 8 inch woofers, and 2 tweeters. And still I had fun.

I still have a big bag full of circuit diagrams here, and I am planning to scan them and put them online for display. So wait for it.