Rough test of a discrete 2 x 30 Watt homebrew audio amplifier on thermal runaway + schematic
In this video: how to test a homebrew power audio amplifier. It is a very coarse test (of a 2 x 30W stereo amplifier). Important corrections about (aligning) the amp, watch (C and D and E) in this textbox, update 15 July 2020. Next video here • Test of the 2 x 30 Watt (now 2 x 10 Watt)...
Warning (!): don’t do this test with your (expensive) home audio set, it can be defective (unrepairable) within a short time.
Important (forgotten to tell): supply the amplifier under test always via a fuse of (say) 4 or 5 Ampère (slow type). The “load” in this case is two 21 Watt/12 Volt incandescent lamps in series.
Corrections:
A) I often say that 30 Watt or 60 Watt is dissipated “inside the (2N3055) transistors”. But of course also a good part of the energy is dissipated in the automotive lamps.
B) On 14.56 I say that I “put out the voltage”, but I mean: I disconnect the signal generator.
C) I tested the "diode version" of this amplifier today (15 July 2020) and it proved that even 45 Volt was too high (!) for the diode version of this amplifier (smoking hot transistors). A good DC voltage is maximum 33 V. So a transformer of 24 Volt at 1-2 Ampère with a bridge rectifier (25 Amp type) works properly (24 V x 1.4 = 33.6 V).
D) the amplifier must be aligned with a voltmeter, setting the potentiometer of 500 K to a value where you measure half of the supply voltage between "X" and ground (-) minus, so not (!) between X and Y (somewhere I say that in the video).
E) don't forget the 500 pF capacitor at the input (important for stability/oscillation prevention)
The book “Schematics 2, audio amplifiers and loudspeaker boxes” is available on the website of Lulu. The schematic in the video is on page 20 of this book. The showed amplifier is a (say 30 or 40 Watt) audio/guitar amplifier.
You can align the minimum/absence of crossover distortion by biasing the amplifier propery, setting the “working point” with the 500 K potentiometer. On point "x" there must be half of the supply voltage opposite to ground (aligned with the 500 K potmeter).
When all is OK, solder the 500 K potentiometer out, measure it and replace it with a fixed value resistor of the measured resistance. Aligning depends on the supply voltage, a good safe maxiumum is 33 Volt for the "diode version" (don't go higher) and 45 Volt for the "transistor-zener" version (in my book).
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My books about electronics & analog radio technology are available via the website from “LULU”, search for author “Ko Tilman” there.
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Regarding all my video’s: I constantly keep them actual, so the original video’s with the most recent information are always on YouTube. That is the source, and search there. When my video’s are reproduced or re-edited on other websites/channels you can not (!) be sure about the original content (=really working electronics with real properties for a purpose) and important adaptations to the circuits.
Be aware of that, I saw on the internet my circuits reproduced in a poor or not proper way.
I also found that people probably republish my circuits under phantasy names and/or with phantasy properties, attributing electronic properties to them that they were never made for, so phantasy properties. Sometimes they want to find gold with them. I take distance from all these fake claims; I cannot help that it happens, sorry. Upload 14 July 2020, update 15 July 2020.