I simulated your amplifier circuit. The input has a transistor that pulls up the voltage very well. But it has only a resistor to pull the voltage down. As the voltage gets lower then the current in the resistor becomes too low to work properly. Then the bottom of the output waveform clips.
You need to replace the pull-down resistor with a constant current sink or "a boostrapped" pair of resistors.
Your amplifier doesn't have a voltage gain, it has a small loss.
Presumably you are assuming he posted a 'full' circuit?, I assumed he was only posting small parts of circuits, which weren't intended to work (because they wouldn't).
Hi Nigel,
My sim had the wrong transistors and wrong resistor values but it showed that the pull-down resistor can't properly drive the PNP transistor. I added bootstrapping and changed the bias voltage a little and even though there is no negative feedback, the output now looks pretty good.
His woofer is a tiny 5.25 inch one that might blow up if it receives more than 18W (18 Watts), even though it might be marked "100w" (100 whats).
The diodes in the circuit bias the output transistors in class-AB so their average current is low. If the diodes are replaced with a resistor then the output transistors would operate in class-A and would get very hot due to the high current all the time. If the diodes are shorted then the output transistors operate in class-B with crossover distortion.
I wonder if a lousy old LM324 opamp would show its crossover distortion in a simulation? Even with plenty of negative feedback its very low bandwidth would have a terrible time trying to eliminate its crossover distortion.
Yes, no problem with that - but it all depends how much power you want from your amplifier - I suggested with a well designed transistor amp you may be able to get 10W (20W bridged) from your 20V supply - which you still haven't explained?. With a MOSFET amp you won't get anywhere near that.
Anyway, I simulated the bootstraped curcuit with a PNP one-transistor input stage, but it did not help with gain.
That's because it's nothing like an audio power amplifier - for a start (as Audioguru also said) the driver needs to be PNP - but you still have a LOT of circuitry missing and incorrect.
The woofer is an 8ohm 5.25"" with a rating of 70W rms, and form my listening tests, is has as much bass as a standard 8 inch woofer.
I actually do not really need more than 15 watts, with my current designs I can get about 0.5w of peak power and thats almost enough for listening moderatly loud in the same room. The reasons for more are for listening in other rooms and always having a little more just in case.
Your PNP input transistor is backwards. It should be an NPN. Its biasing is bad because when it warms it draws more current and will saturate. When it cools it draws less current and will be cutoff. You will need to adjust the trimmer pot for each transistor because they are all a little different.
The PNP output transistor is upside down.
Without any negative feedback the distortion will be horrible, about 40%.
It is too bad you saved your schematic as a fuzzy JPEG file type. I made it more compact.
Here is the amp with Q2 as a PNP with DC negative feedback so its biasing stays correct. It is pretty sensitive since now, it is nearly at full output with an input of only 46mV RMS. Its distortion is horrible, about 40%.
Here is the amp with Q2 as a PNP with DC negative feedback so its biasing stays correct. It is pretty sensitive since now, it is nearly at full output with an input of only 46mV RMS. Its distortion is horrible, about 40%.
One more transistor (NPN) and it's finally going to be an amplifier! - although personally I'd add at least another one, replacing the horrible two diodes with a Vbe multiplier!
If you insist on playing with simulators, why not get an existing amplifier and simulate that? - from what you've posted so far you don't have the faintest clue how to design an amp?