Talking about developing a 15 Watt Class B audio Audio Amp pt 4: definitive schematic and demo

Please read the description first before you comment. Part 4 (definitive schematic) of the developed class B audio amplifier, approximately 15 Watt audio output.

The definitive schematic shows some (minor) changes:

1. A somewhat higher quiescent current, realized by 2 x a 1 K parallel resistor, paralleled to the original value of 680 Ohm (result: better sine wave forms, lower crossover distortion)

2. A 10 N (=0,01 uF) capacitor from the collector of the BD 139 (Driver) transistor to ground, minus. This wiped out all the parasitic oscillations immediately

3. The experimental setup with the 470 K trimmer and the 1 K resistor in series between the bases of the end-transistors (BD 743 C, NPN and BD 744 C, PNP) to (+) and (-) was replaced by a fixed value resistor of 470 K. The quiescent current could be regulated in an easy way by what was told in (1)

4. A fuse was added. The circuit was tested on overdrive on its input (say 1 V AC at 4 KC) with 3 automotive car-lamps each 21 Watt in parallel during 1 hour. Power supply current run up to (say) 2.5-3 Ampère. Heatsinks did not get extremely warm

5. The BD 139 needs a small cooling plate/heatsink, say 2 x 2 cm aluminum, thickness 1 mm or so

6. I tested the 2M-1M resistor combination at the first driver transistor base, the BC 557 b (PNP) by doing minor changes to these resistor values, the effects were nihil, regarding the pureness of the sine wave (cross over). Solution on (1) worked best

7. The circuit was tested on its durability: heatsinks never got hotter than 45 degrees Celsius with extreme loads at the LS output. No need for special measures to prevent it from thermal runaway. Say gluing the diodes on the heatsinks of the end-transistors, etc.

8. Did many tests with the 3 diodes (all 1N5408) in series. Three diodes worked best. Please note: in general “every” Si Diode can be used for this purpose, though the “breakthrough” (barrier) voltages of Si diodes can differ in the 1/10 (one-tenth) of a Volt range, such tiny differences will have a direct (!!) effect on the quiescent current and (thus) how warm/hot the end transistors get when there is no audio signal at the input of the amplifier

9. A primitive “output shortcut protection” can be realized with a standard 12 Volt automotive filament lamp, type H4. The 2 filaments are paralleled (video). You don’t need that for normal (not rugged) use, when you use it as a (say) 15 Watt audio amplifier for household use with a fuse of (say) 2 Ampère in the 35-40 Volt power supply lead. That “car lamp protection” has a slight effect on the frequency response on high audio output levels. When you bridge the lamp with a switch you can get the best out of it, output protection on or off.

There are 3 preceding videos of this 15 Watt Audio amplifier where you can see/study the development process, with all the electronic measures/adaptations to get such a 1974 circuit “working”.

Pt 1 https://youtu.be/tJfAxQ2LCfU
Pt 2 https://youtu.be/tFqf92nzICc
Pt 3 https://youtu.be/NsCFazb3t48\

All these video’s show the development process and how you can you develop such an amplifier yourself.

An oscilloscope + a sine wave generator from 20 Hertz-20 KC with a variable audio output between 0 Volt and minimum 2 Volt (better: 2-10 V AC audio out) is absolutely (!) necessary, otherwise serious developments and tests are not possible.

Interested in audio? Buy my book “Schematics 2, Audio Amplifiers and Loudspeaker Boxes”, via the website of LULU. Author Ko Tilman.

Do you want direct success with other audio amplifiers?

Go to:
A) Nice sounding audio amplifier with the 2N3055 (NPN) and the 2955 (PNP). Can be made for mono or stereo, always works good is here: https://youtu.be/cq2fd6zI-mk
B) Audio amplifier with the TDA 2003 CHIP for say 12 -15 Volt (say mono 5 Watt). Two of these Chip amplifiers mean: stereo. Usable in automotive/car audio. Circuit is here: https://youtu.be/KSJndhH95M4
C) Audio amplifier with a few transistors, e.g. the BD 139 NPN and the BD 140 PNP for say max 18 Volt (mono say 1-2 Watt) is here: https://youtu.be/jU6VI2WpGqk
D) Audio amplifier with the TDA2615, it is the best and cheapest stereo Hi-FI Chip audio stereo amplifier that you can get now, 2022, the sound is extremely pure: https://youtu.be/1id_cNdDLvs
E) Update & VLOG about that 3 stage audio amp made with the TDA 2615: https://youtu.be/BVnNIydDTKc

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 Use the “looking glass” there, searching via keywords, say “audio” or “amplifier” in this case. My books about electronics & analog radio technology are available on the website of "LULU”, search for author “Ko Tilman” there. https://www.lulu.com/search?adult_aud...

I keep all my YT videos constant actual, the original video’s with the most recent information are always on YT. Avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Upload 20 September 2022.
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