Audio Electronics in Days Past

    In the late 60s and early 70s record players had a pitiful power output because the transistors available at the time could not do any better. The good thing was that the record players were portable. I remember my sisters had a monophonic player which probably did not put out more than 3 or 4 watts. Anything more than that required tubes. I have an advertisement from a 1967 Reader's Digest for a top of the line record player "for professionals". It was the same portable case type, stereo and 4 watts per channel! The speakers were contained in each half lid as was quite standard and the only extra professional touch was that it had legs so it could stand on its own rather than sit on a table as was normal with other models. Still, the legs were not very solid and the unit was not very stable so the least touch would send the arm jumping. The price then represented about what $1800 is worth in 2004. A lot of money.

1967 anuncio tocadiscos, advertisement for record player

    After silicon transistors arrived in the 70s it was possible to build much more powerful audio amplifiers. During the late 70s I used to enjoy building all sorts of electronic kits. I was very much into citizen's radio (27 Mhz) and into audio equipment. I had a cottage industry building audio amplifiers for my friends and the friends of my friends. I copied some designs from kits or magazines but I became quite knowledgeable about audio ampliers and designed a few myself.
    In amplifiers using tubes a push-pull output stage required transformers to invert the signal polarity but with the arrival of transistors it was possible to design transformerless push-pull output stages by using complementary PNP-NPN pairs of transistors.
    The basic concept of the complementary pair NPN-PNP amplifier was not difficult to master and I built dozens of them for all sorts of purposes. I modified the design to suit my needs in all sorts of ways but the basic concept is always there.
Basic complementary pair audio amplifier
Essentials of a complementary pair amplifier.
This circuit will work but will produce a lot of distortion



40 W Audio Power Amplifier

    The design I present here was sold commercially as a kit but I designed and built my own "printed" circuit boards which I painted by hand with a felt pen with waterproof ink. I built and sold dozens for fun and for profit. I also used to assemble and sell the loudspeakers, these entirely from kits.
    The power stages worked wonderfully and were rated at 40 watts each which was quite an improvement over the 4 watts we were used to. If I wanted to build an audio amp today, 30 years later, I would build this without hesitation. Maybe I would use different transistors if the originals are not available but the design is still perfect. Probably all those amplifiers I built and sold are no longer in use because the pots used for volume control and the contacts of the switches, all mechanical, have failed but I bet the electronics would still work fine.
    Here is the schematic of a final stage:

40 W Audio Power Amp Diagram, schematic, esquema amplificador de audio 40W.

    The components to the left of the vertical blue line were mounted on a circuit board but the transistors to the right of the line were mounted on heat sinks on the outside of the back of the case and connected with wires. The big output capacitor was mounted directly on the case. The Printed Circuit Board (PCB) had a connector for a cable with the wires that went to the transistors. This sort of modular construction was very useful for troubleshooting because you could easily switch the transistor "modules" with the PCBs and pinpoint the source of any problem. I took pride in wiring everything very neatly so that access and repairs were easily done.
    Parts List:
 R1: 1K, 1W
 R2: 33 ohm
 R3: 150K
 R4: 4K7
 R5: 1K5
 R6: 33K
 R7: 1K
 R8: 2K2
 R9: 2K7
R10: 1K
R11: 10
R12: 220
R13: 1K8
R14: 56
R15: 56
R16: 470 ohm, 1 W
R17: 470 ohm, 1 W
R18: 0.5, 4W
R19: 0.5, 4W
P1: 220 Kohm
P2: 1 Kohm
TR1: BC157
TR2: BC148
TR3: BD139
TR4: BD139
TR5: BD140
TR6: 2N3055
TR7: 2N3055
 C1: 2.5 uF, 64 V
 C2: 2.5 uF, 64 V
 C3: 150 pF
 C4: 10 K
 C5: 160 uF, 40 V
 C6: 1K5
 C7: 32 uF, 64 V
 C8: 27 pF
 C9: 330 pF
C10: 10K
C11: 100 K
C12: 2500 uF, 64 V
Printed Circuit Board
Heat sink for transistors
Screws, offsets, cable, etc.

    When I get another spurt of energy and inspiration I will continue and add a brief explanation about how the circuit works as well as the diagram for the pre-amp, power supply, etc.
    Here is a Schmitt trigger circuit I designed for an application some time ago.


Autor: Alfonso Gonzalez Vespa