Simpler Power Amplifier

There is a way to emulate a triode, tetrode or pentode vacuum tube here on ec that works also in real life?

Yeah with transistors. The only difference is the way you bias them. Well the V-I curves are different as well, transistors have a sharper cutoff regions than a tube does, but for all intends of purposes, that can be neglected. Since tubes transform voltage to voltage the closest analogy are MOSFETs. You just bias them as one would bias a transistor, just use higher voltages as the supply rails. It's very crude, but it got me designed some pretty good circuits, like a Wien Bridge Oscillator I created a few weeks ago

P.S The way tubes enter cut off is what makes them sound so good. You see with transistors as soon as you overshoot your output and overload your amp, the output gets cutoff in a very sharp way, the wave gets completely flattened, which produces a few very odd number harmonics, which sound very distasteful to the ear. Well in vacuum tubes the cutoff isn't as steep as with transistors. They tend to round up the wave, thus producing mostly even order harmonics, which, up to a given point are very favorable to the ear. They sorta enrich the tune. I personally use a very weak negative feedback with my tubes, because I like to have some second harmonic in my sound.

The output is in 40wrms

No, it's 20W. The Power of AC is P=U^2/R or P=I^2.R and you have to measure the RMS voltage and current.

I'm sorry, I was wrong to consider the RMS calculation. Rated power; P=12.8^2/4=40 W

RMS; 12.8x0.707= 9Vrms. So P=9^2/4=20Wrms

Yes, that's correct, although the Pmax rating is also important. In every amplifier it's given in the manual, because the RMS value is only for sinewave. The music power is rated as the maximum power e.g. 40W in this case.

Instead of using a sinwave they use a rectangle to demonstrate max power. And "musical power" is measured at 10℅ THD cuz it wont be close to double the pure sin power. Thats just a marketing trick. No car-dealer like the unit kW they prefer horse power, cuz the value is higher and customers likes higher numbers and compare car 1 in HP with car 2 in kW! At the end they get cheated, like buggzy. So just divide by 2 "max power" or "peak power" or """Musical Power""" and you get a value to compare cars. No need to invent a marketing number. Have fun

Actually I haven't gotten cheated. I never compare RMS with Music Power. I always look at the RMS value, because that's within the boundaries I'll use the amp. The max power is at 10% THD as you stated, and I never crank the amp at such a level. But to be honest most amps have a very good THD characteristic up to a given level, and then it suddenly spikes to 10%. For instance you may have your amp working at 0.5% THD at 50W and it'll increase to 10% at 60W, and the extra 10W you won't even feel, but the distortion you'll surely hear

But you said Pmax is an IMPORTANT parameter to look up in every amp manual. LOL, a typical buggzy.

Not exactly what I said. I said that the Pmax is also an Important parameter BECAUSE it's always given in the manual. It shows the absolute maximum power you can draw, before the distortion takes the apprehend.

Also not correct. If the amp provides a high bandwidth and the test signal is at low corner e.g. 100Hz a rectangular signal can reach alsmost Pmax without any distortion

Except when the distortion is at the point of cutoff. Then the frequency is irrelevant.

Cutoff does not mean distortion. Its for a good amp just less magnitude. Anyway, harmonics >1000 e.g. for this amp seen from 100Hz as 1. harmonic won't have big impact to THD cuz its already only 1/1000 -60dB of base harmonic. And this is only a test scenario, cuz no highend audio source provide frequencies above 20kHz.

Cutoff does not mean distortion? How come? Cutoff is the most prominent source of distortion an amp can have.

You mean cutoff=clipping?

I mean "cutoff" on sense of frequency and not clipping

By large number of ' Bootstraps' you mean 2?

I build bass guitar amps. Interestingly, people notice if the amp and speaker rig won't respond above 20K!

THD is not very useful for musicians. They want to know what kind of distortion will happen at low mid and fully cranked up volume. And in my opinion, no, you cannot remotely emulate a tube sound with transistors. Not anything like it!