how to make this circuit produce an output of 50watts on a 8ohm speaker connected on the output?

crutschow

Joined Mar 14, 2008
27,171
Sorry that's not at all practical.
You show a Class A amp for low-level signals driving a high-impedance load.
The changes you need to make are to use an entirely different circuit design.

For 50W into 8 ohms, you need an audio power amplifier with a push-pull, Class AB or Class D output stage.
The easiest is to buy an IC audio power amp rated for a least 50W.
 

Alec_t

Joined Sep 17, 2013
12,060
You will also need a power supply to provide 50W plus some margin. 50W sinewave into 8Ω implies a drive signal of 20V rms, which is nearly 60V peak-to-peak.
 
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Ian0

Joined Aug 7, 2020
2,175
See
https://www.passdiy.com/project/amplifiers/zen-variations-1
for a similar design
There are some people who seem to like to design power amps that way; but @Alec_t is perfectly correct about the power supply; and amplifiers of that design are at best 25% efficient, so you'll need a 400W power supply for a stereo pair and to dissipate 300W of that as heat.
It's not completely impractical, but I wouldn't advise it. The quality of the output measured in total harmonic distortion is "disappointing". Even a valve amp performs better (by most normally-used measures of performance) and is more efficient. Those who believe in "Subjectivism" may disagree.

PS. That 0.05uF capacitor is much too small, you won't get any output below 3kHz.
 

Audioguru again

Joined Oct 21, 2019
3,172
A simple calculation shows that a 0.05uF capacitor feeding a 1k resistor cuts frequencies below 3200Hz and here the 10k resistor and transistor make the cutoff frequency higher as I show on my graph.

The 8 ohm speaker is shorting the transistor because a coupling capacitor is missing between them.
With a coupling capacitor the maximum output power is only 0.000077W (77uW) which is almost nothing.

For 714 thousand times more power you need a Power Amplifier Circuit.
 

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sparky 1

Joined Nov 3, 2018
540
How does a big old amplifier sound on that speaker.

There are many inexpensive solid state amps depending on application. A fender 10G has TL072's to allow basic tone controls,
the the final stage can be a module like LM1875
 
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Ian0

Joined Aug 7, 2020
2,175
I suppose the simplest way to make it drive 50W into 8 ohms is to increase the supply voltage and add a unity-gain buffer to give it some output current.
amplifier.png

The performance isn't great, but it demonstrates the principle. Loads of 2nd harmonic distortion. The subjectivists would love it.
 

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Audioguru again

Joined Oct 21, 2019
3,172
The LM3876 amplifier IC in post #9 is obsolete and not stocked at Digikey.
The amplifier in post #14 with horrible 1% distortion will be fine for playing acid rock noises.
 

Ian0

Joined Aug 7, 2020
2,175
The amplifier in post #14 with horrible 1% distortion will be fine for playing acid rock noises.
I never said it was great, but it's the only post so far that fulfils the TS's remit of "how to make this circuit produce an output of 50watts on a 8ohm speaker".
People pay good money for that amount of distortion! A 300B single-ended triode design (with or without pure-silver-wound transformer) will be no better and might set you back £10000.
(Do people use their 300B singled-ended triodes for acid rock? Perhaps not, they're generally no more than 25W).

Whilst we're on the subject of rubbish circuits, the second circuit in post #9 is also a bit dubious. No base-emitter resistors on the output transistors? Shoot-through problem? An just how old is that circuit - when did we stop using 50 ohm and 500 ohm resistors and adopt the E6/E12/E24 series?
 
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