Low Distortion audio power amplifier

[Adgj533]

I do have a +22, -22 (max being 30v/5A i think)bench DC power supply, and the school has one too. so ill  use that
Also, Im trying to put this on PCB using eagle software, any tips?

If you use two power supplies for the positive voltage and the negative voltage then adjust their voltages to be the same.
I have never used Eagle software.

I want to buy the transistors and other parts tmrw, what should i look for when buying it?
Also I saw Daves how to NOT blow up ur scope video, I was kind of confused by it. Is it safer if you have a separate ground for your scope? and connect circuit grounds and PSU grounds together?

[Zero999]

Don't connect the amplifier's 0V rail to mains earth and it should be fine.

[Adgj533]

Hey im trying to put this on a breadboard and giving it input from my phone using a 3.5mm jack to 2 wires (trying to play a song and hopefully it will be amplified by speakers). I found an old 60W 6 ohm speaker at home. But wouldnt that just be DC voltage since its coming from a phone with a battery or do I need function generator hooked up too?
Also I need a higher voltage to go in, how can i get that?

[Audioguru]

That audio output from your phone is not DC, it is audio. Audio is AC.
If the output level from your phone is not high enough for this amplifier to produce its full output power then you need to change the resistor in the amplifier that sets its sensitivity, or add a preamp to the input of the amplifier.
You can feed the amplifier with a function generator if you want.
A solderless breadboard cannot pass the high currents this amplifier uses. An 18V peak into 8 ohms produces a current of 2250mA. The contacts on a solderless breadboard might be able to pass 100mA.

[Adgj533]

2 questions:
1) can i reduce the input voltage to like 50-100mv so that the output wont be high and still be manageable to do in breadboard
2) how do I find the correct parts to build this thing, i was searching digikey and there are a lot of transistors with diff wattage etc, idk which one to choose
audioguru ur a life saver man, thanks for all your help these past few days
really appreciate it

[Audioguru]

2 questions:
1) can i reduce the input voltage to like 50-100mv so that the output wont be high and still be manageable to do in breadboard

The power supply voltage(s) of an amplifier and the speaker impedance determine the maximum output current and power from an audio amplifier. All amplifiers have a volume control to adjust the input level.

2) how do I find the correct parts to build this thing, i was searching digikey and there are a lot of transistors with diff wattage etc, idk which one to choose[/quote]
You simply calculate the maximum current and heating in each transistor. Since I am posting now I cannot see the schematic to see if capacitor and resistor values were already chosen.
Are you supposed to design the amplifier of just put it together?

[Kleinstein]

One could change the gain to a higher value to make it work with an 100 mV input level. However, this will reduce the feedback and would incease distortion. The inverting type of circuit looks good on paper, but it need the stage before the amplifier to deliver some current and this stage will add to the distortion, especially at higher gain.  So the proper way would be to add a stage before this amplifier to bring the 100 mV level to some 1-3 V level first.

The output stages with 3 transistors in a NPN/PNP/NPN  (and complementary) combination with local feedback can be tricky. It can depend on the speed of the 3 transistors to have a suitable combination.  So it is not only wattage to look at, but also speed. Finding suitable sets of transistors is where you might need the simulation for.  Depending on the transistors used it might take some extra pF range caps to stabilize this. This is likely the really tricky part with this type of amplifier. The nasty part is that also parasitic properties of the layout may come into play here. The shown circuit is one of those that can give pretty good performance if done right, but could as well end up as a MHz range power oscillator if a different brand of caps / transistors are used.

[Audioguru]

The schematic on the first page of this thread shows that all parts are already chosen. Only the two Oriental transistors are not supplied at Digikey but a quick look at their datasheets shows that they are huge high voltage transistors. Huge high voltage transistors are not needed since the amplifier's collector and emitter resistors for them shows that their current is very low so that any little 44V or more Western transistors will work. A few of resistors must have their current and amount of heating (therefore their size) calculated.

[Adgj533]

yeah i was given the schematic by professor, so I just have to put it together, but he didnt mention what type of transistors we should buy etc,Im gonna use 2N3904 and 3906. Hopefully they work
 thanks for your suggestions

[Adgj533]

hey man, i put it on a breadboard, and the output looks like this, similar to a capacitor charging, any idea why?
Also the gain is only 6, supposed to be around 18 according to my calculations

[Audioguru]

You cannot measure the gain when the output has such severe distortion.
If the wiring on your breadboard is as messy as the wiring on your schematic then it will not work.
The contacts on a solderless breadboard cannot pass the high currents of this amplifier anyway.

[Adgj533]

Hey AudioGuru, Thanks for your input but I was able to build it in a breadboard by limiting the current on the power supply, and there were 2 other kids in the class that did it using the full current limit, one them were using a bigger resistors and a heat sink.
Btw, now I have to use PCB to implement it, and I was wondering if I can put a pot after 100k resistor (pls look at original clean schematic) can I control the volume?
Also one of the groups did use a PCB and they only used a small speaker and the song played fine. No distortion etc
He used a knob to control volume but idk where I would place that. any ideas?

[Audioguru]

I was wondering if I can put a pot after 100k resistor (pls look at original clean schematic) can I control the volume?

The ratio of the 100k negative feedback resistor and the 6.8k input resistor sets the gain of the amplifier to be too high with noise, distortion and poor high frequency response or low gain with oscillation. A volume control for an audio amplifier goes at its input as a signal voltage divider without affecting the gain. But since this amplifier has an inverting input then it has a low input resistance. I do not know if your teacher will allow you to change its input to be non-inverting with a high input resistance which is more suitable for an input volume control.