Test of the 2 x 30 Watt (now 2 x 10 Watt) audio amplifier on its sound profile update + schematic
Update on the video of yesterday (14 July 2020). Now we are going to listen to the sound of the 2 x 30 Watt (or 2 x 10 Watt) audio (guitar) amplifier. Please read the textbox first (A-E) for all the necessary info. Copyright free music from "the 127ers"; very suitable to test audio amplifiers and they made many relaxing records.
Link to first video is here • Rough test of a discrete 2 x 30 Watt homeb...
Name of first video: “Rough test of a discrete 2 x 30 Watt audio amplifier + schematic”, published 14 July 2020.
IMPORTANT:
A) The sound of this amplifier is pleasant. Much more info in the video. Adaptations that I am going to realize: the 2 potentiometers that set the amplification level of the 2 pre-amplifiers will be be moved to the front, so that the guitar player can add “overdrive” to the sound. In practice: sine waves (an audio sine wave signal from a guitar or whatever audio source) changes in that case into (more or less) square waves. The sensitivity of this amplifier is 100 mV so 0,1 V AC audio input.
B) Very important: the amplifier version with the “3 diodes in a row”, responsible for the voltage drop between the 2 basis of the driver transistors (BD139 NPN & BD 140 PNP), only worked properly on maximum 33 Volt. With higher voltages there is a kind of thermal runaway. That was tested today, 15 July 2020. But, very important, that happened when the amplifier was driven with a 18 KC signal at 1 Volt AC level on its input, both the left and right channel, so a kind of fierce way to overdrive this amplifier.
Anyway: a guitar amplifier must be able to handle this kind of “overdrive”, so that is why I dropped down the supply voltage for the “3 diodes in a row” version of this amplifier to 30 V DC (or 33 V DC) at say 1.5-2 Ampère. That means an output level to the loudspeakers of (say) 2 x 10 Watt.
C) to prevent a too high output current to the loudspeakers (=protecting the end transistors for strange “loads” or a shortcut) I mounted at the end of each channel (after the end-electrolytic capacitors) a resistor of 1,5 Ohm at 10 Watt (2 resistors of 3,3 Ohm/5 Watt in parallel).
D) I added “cooling plates” (fins) to the supply transformer (230 V/50 Hertz to say 24 Volt at 2 Ampère). Reason: that transformer had a very low quality. That low quality means that such a transformer in fact has a too high “idle current”. So when the audio amplifier takes a tiny (quiescent) current the supply transformer gets warm or even hot due to that, even when nothing happens (=no audio signal in)....
E) I mounted this low quality transformer into this amplifier to give this handicapped transformer a (good) use, but it is much better to buy a good quality transformer for 24 Volt at (say) 1,5 or 2 Ampère to supply the circuit. A good quality transformer gets (only) a little bit warm at idle current, when the amplifier is “at rest” (=receives no audio input signal). And the steel core only gets warm when the amplifier works at its maximum, so with the maximum current that the amplifier(s) take.
F) Don't forget the 500 pF capacitor at the input. Important for stability, acts as a high cut filter above 18 KHz, prevents oscillations.
By the way: the showed transformer in the video burnt out this evening, I have ordered a good quality 24 V-2 Amp. new one via "allekabels", website in the Netherlands.
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