I replaced R4 with two Diodes as previously suggested (I got rid of the cross distortion), although I'm not sure how this works.

In real circuit? Or simulation only?

 

The crossover distortion is not what you are hearing. It would barely be noticeable.

Apologies to any golden-eared audiophiles out there, but I am certain it is not what he's describing.

 

In real circuit? Or simulation only?

The crossover distortion is not what you are hearing. It would barely be noticeable.

Apologies to any golden-eared audiophiles out there, but I am certain it is not what he's describing.

I recreated the circuit according to the following diagram (I changed the bias of stage 1 because I didn't have a 20kΩ) and thus I changed RE1 to get the desired current such that Vc1 = Vcc/2.

Edited:
I had forgotten to connect the smartphone ground to my circuit ground, never mind about the striked message below:

Without the input connected, I hear a noise that sounds like a sin, and the frequency gradually increases forever. I used a cap in parallel with the power supply as well, but nothing changed. Also, the two power transistors Q4 and Q5 are getting very hot and I can smell them (they still haven't been destroyed), but I'm pretty sure I built the circuit correctly. VB4 is about 6V and Vb5 about 5V, they have a 0.7V difference from LTSpice.OP but I'm using the BD135 & BD136 instead in the real application.

 

Without the input connected, I hear a noise that sounds like a sin, and the frequency gradually increases forever. I used a cap in parallel with the power supply as well, but nothing changed.

Never mind about that, I forgot to connect the ground of the smartphone output to the common ground of my circuit.

Now I'm getting that Vibrating sound, that has somewhat of a mid-frequency.

 

This is what the vibration sounds like:
The air-like noise is not from the output speaker but from the recording.
https://soundcloud.com/babaliaris%2Fweird-vibration-sound-3-stage-amp

 

Maybe you built the circuit on a solderless breadboard that has too much capacitance between the rows of contacts and the wires all over the place. Then the circuit oscillates and maybe the connections are intermittent. The unshielded wiring causes electrical hum pickup.
Your diodes have no part number so they might be the wrong type instead of silicon.
The transistors might have their 3 pins mixed up.

 

Maybe you built the circuit on a solderless breadboard that has too much capacitance between the rows of contacts and the wires all over the place. Then the circuit oscillates and maybe the connections are intermittent. The unshielded wiring causes electrical hum pickup.
Your diodes have no part number so they might be the wrong type instead of silicon.
The transistors might have their 3 pins mixed up.

Diodes are the 1N4001. I'm not sure for the rest.

 

This is what the vibration sounds like:
The air-like noise is not from the output speaker but from the recording.
https://soundcloud.com/babaliaris%2Fweird-vibration-sound-3-stage-amp

 

Wait, it actually sounds pretty great. This vibrating sound does not get amplified. In matter of fact, you can only hear it by putting your ear really close to the speaker.

I'm uploading a video right now, I will post it shortly.

 

This is the result:
Notice that at some point the gain drops briefly. My 100$ old guitar amplifier does that all the time as well.

The air sound at the end is from my computer's fans

 

Hello,

Instead of the two diodes, a potmeter with a transistor can be used as a Vbe-multiplier or rubber diode:
https://en.m.wikipedia.org/wiki/Rubber_diode

Bertus

 

Notice that at some point the gain drops briefly.

It wasn't a gain drop, a notification came to my phone and when that happens the phone automatically lowers the output to zero.



Finally, I managed to make it after 2 years and completely understand what I'm doing! @Jony130 might remember me, he answered almost in every post that I made over the last 2 years.

Many thanks for your patience, Behzad Razavi for his Electronics 1 & Electronics 2 YouTube series, to my professor who gave me some advice when I said to him I'm ready to give up, and to Adel Sedra for his excellent book.

I've got the basics, now I need experience and probably I must read The Art Of Electronics.


And with some LEDs

 

This noise (vibrating sound) is normal, because you have a ground loop and lack of supply rail filter, so the noise from the mains voltage is normal.
As for the burning smell. It's probably Q3 because you have decreased RE3 resistor to 47R. So you increased the current thus, the power dissipation in Q3 also increased.
And the push-pull stage.
I vote to bring back R4 to 220R instead of two diodes and RE3 to 100R.
Also the C1 and C2 polarity. The plus side of capacitors (longer leads) should point towards the push-pull stage.

 

I'm leaving for Military Service in one month from now

So you join the army? Or maybe you have mandatory military service in Greece? I also spent a year in the army back in 2004. Best of luck in your Military Service.

 

Also the C1 and C2 polarity. The plus side of capacitors (longer leads) should point towards the push-pull stage.

Oh, I did not think of that, it makes sense since they go directly to the DC base of the two power amps.

So you join the army? Or maybe you have mandatory military service in Greece? I also spent a year in the army back in 2004. Best of luck in your Military Service.

It's an obligation here in Greece, you can not avoid it. Every male under the age of 18 (or older in my case if you are a University student). It's 8 months for me because I chose to serve in Cyprus (we are allies) where they are in need of more soldiers so they offer some benefits like shorter time and a small salary as well (normally we don't get paid).

Anyways, thanks!

 

My breadboard melted


Probably it's because of the paralleled load (Speaker plus 4 LEDs with their series resistors) right?

I changed the load in LTspice to 1Ω and the current reached 1.5A (9Watts heat dissipation from the power amps). The breadboard says it can handle 2 Amps.

Edited:
I've also thought of making some out-of-phase outputs for the LEDs, so they do not blink at the same time (a more awesome effect). But I can't think how to do it.

It's really hard to keep them out of phase with high pass filters, and when I'm adding the LEDs they get paralleled with the filter resistances, lowering them, thus messing with their frequency response and cutting out frequencies.

Without LEDs:


With LEDs:

 

Don’t drive LEDs and loudspeakers from the output of the same amplifier. Look up colour organ circuits.

 

My breadboard melted


Probably it's because of the paralleled load (Speaker plus 4 LEDs with their series resistors) right?

Yes, the average power dissipation in a single transistor of the output stage will be around P = (0.5Vcc^2)/RL = 0.45W.
Your transistors will have a terminal resistance between the junction and the ambient equal to RThja = 100 °C/W (without heatsink).
This means that the output transistor junction will have almost 50°C (0.42W * 100 °C/W) higher temperature than the ambient temperature.
This is why your breadboard will try to meltdown, and this calculation does not take into account the load caused by the diodes.