Class A MOSFET amplifier using one transistor - with schematic

[@1959Berre]

All the best wishes to you and Snickers. And may your health be perfect in these troublesome times.

[@Gersberms]

Thank you for the very easy to understand circuit, I'll have to spend a bit more time trying to really understand how the negative feedback works. I like that you're highlighting the fact that mosfets use voltage to control a current, it's easy to see why that is so useful in this circuit.

[@ao6640]

I breadboarded one myself a few months ago using a K3878 (and similar ones I had lying around) using a zener to bias the gate. I thought it was pretty cool, untill I discovered chip amps and learned how to design a proper class AB output stage thanks to your videos.

[@ivanlukic2360]

Hi John! Greetings from Serbia. I've seen many of your youtube videos and I like them very much. It's nice that you test budget priced gadgets and explain simple circuits. For most users that's all that is needed for enjoyment of music. Spending too much money on the amps and speakers makes no sense, especially for home listening. A simple LM1875 project can bring a lot of fun and enjoyment for very little cash, while, sometimes, even hundreds of thousands of dollars can bring only frustration and headache.

[@rolandmetivier4437]

I like these types of amplifiers, lots of niceness coloring the signal

[@iblesbosuok]

Glad to watch you again, sir.

[@MichaelBeeny]

Great for people that like the valve sound, efficiency and heat.

[@EngineeringEssentials]

By valve , you must be talking about soft clipping and 2nd order harmonics

[@woodowoodo1405]

even with the ordinary royalty free music, the sound was comparatively good to my ears. At a stretch, may be worthwhile developing an alternative JAT SE amp concept covering all bases and tastes ;)

[@KissAnalog]

Thanks John - always look forward to your videos!

[@copernicofelinis]

As a suggestion, on the schematics you could add the bias voltages and the bias currents. It will make is easier to understand the circuit. Voltages in green and currents in red will match the power supply s display.

[@sixtyinsix]

From me: Nice video John, thx!👍 From my gf: Happy Birthday Snickers! 🎉🎊😺🎂🍧

[@laxr5rs]

Cool! Thanks! [Sounds great. Good description]

[@bobdavelisafrank_4850]

I love these little single ended circuits! Not the highest performance, but simple and practical for low power loads -- I use a small scale one of about 20mW to power my headphones. The distortion cleans up a lot if you can put the gate-source 'junction' through a feedback loop of another amplifier, which was a popular old design!

[@andymouse]

I always love theses ..keep meaning to play myself...must buy new scope ...cheers.

[@ivanlukic2360]

I also have one suggestion for future video. Since many of your projects are about small IC based power amplifiers, it would be nice to explain how to add headphones output in parallel with speakers output. The gain and power is usually too high for headphones, so some voltage divider is needed. The video could be about how to calculate values and power of proper resistors.

[@AmazonasBiotop]

Thanks John! 15 year that is great. :)

[@EngineeringEssentials]

👍😍 true

[@springwoodcottage4248]

Super interesting and educational. Thanks for sharing!

[@mikedenallo6997]

THE man is Back!

[@SinaAtalay]

John thank you so much!

[@researchandbuild1751]

I built something like this a long time ago. Gets really warm lol!

[@EngineeringEssentials]

Ita an electronic heater 😁

[@dfn03]

Me too, I was running it without a heatsink. Surprisingly it didn't die instantly.

[@listerine-pr5lt]

Snickers doesn't look to be interested in Audio Amp anymore.

[@ao6640]

He's waiting for quantum amplification technology to arrive...

[@researchandbuild1751]

By the way, RF mosfets are lateral, and would probably work well for audio

[@johnsonlam]

Thank you John, for such a simple circuit. Is it possible to step by step improving the circuit to make it better? This is also a kind of fun to see something improve by changing little by little. Also want to bust a myth, I don't believe change a component will have dramatically improve in quality, good sound should be: always improve the circuit first, then take care of the component later.

[@EngineeringEssentials]

I agree 👍

[@tenlittleindians]

The guy that invented Trainwreck amps did not share your opinion. A great story here: kzbin.info/www/bejne/j3iXnqd6rsied8U

[@johnsonlam]

@@tenlittleindians When I watched the video, I just feel Fisher is "fine tune" his amp with different component and different layout, without coming to circuit design first. I didn't get the message of "component quality over circuit design".

[@EngineeringEssentials]

That's a great video John, 🙏. Gain of this amplifier depends on the gate biasing resistors, can you do a video by changing value of gate bias resistors? Just want to know one more thing, is it possible to clean up output just by adding a constant current source?Or a negative feedback is a must? Thank you🙏

[@jeffmassey4860]

In addition to a Constant Current Source,how about trying an incandescent light bulb?

[@jaybird57]

Havent seen thoes radio shack horn tweeters in years.... cool.

[@jp040759]

Great video all around.

[@Spentelectrons]

Id be curious to see the difference in output on an 8ohm resistor versus an 8 ohm speaker. For example I'm simulating my load as 8ohm but I'm physically operating it against two speakers in series that total a resistance of 6.4ohm to a standard volt/ohm meter. Is their a more accurate way of simulating a speaker? Maybe just a different ohm value? A resistor/inductor/capacitor in parallel? Does the difference matter?

[@jeffm2787]

Wondering why you don't use the FFT to figure out the clipping points, perhaps have both the FFT and Waveform on the screen at the same time. The FFT would show distortions that you would be hard pressed to see on the sinewave.

[@JohnAudioTech]

Yes, that is the right way to do it but I'm just lazy.

[@adaminsanoff]

Very educational and helpful video!

[@robtitheridge9708]

very interestin. i wounder if you could swop the big resistor for an out put transformer?

[@killcar5nbike2]

Yes, I've took the idea to extremes using a universal tube output transformer and a high voltage supply. The output becomes very linear as the current modulation through the MOSFET is in the ma range. However I ended up with having to use IRF840 to survive the voltage present across it. At clipping onset my design puts 500v across the output. I did a video on it, but it's more a badly phone recorded video of designing it, rather than the schematic and testing of a working amp. Most output transformers, especially single ended are going to be high turns ratio ones designed for valve impedance matching.

[@JohnAudioTech]

I did this with a similar circuit using a bipolar junction transistor. I used an inductor as it is low resistance to the DC bias currents and passes them through. The varying signal voltage is high impedance to the coil and that passes through the speaker that is connected across the coil.

[@jamesgilmore2389]

Excellent video. Thank you.

[@chronobot2001]

Nice 1950's swinger music.

[@tenlittleindians]

So your using your preamplifier as a volume control? Adjusting your preamplifier up and down does what? Increase and decrease the voltage and the amperage? Does an increase in voltage or amperage have more of an effect than the other on this circuits input or are they slaves together and acting linear? I'm not too sure I have explained my questions without adding more confusion.

[@JohnAudioTech]

I use the preamp as it has an easy to use dial control to adjust the source signal level. The music player volume control adjusts in large steps that is hard to fine tune. Also, the music player I use output is below standard line level so the preamp boosts it a bit to a usable level.

[@Aris-wz4kq]

i have build one of your previous amplifiers (the Amazingly simple class A amplifier published by you in December 19) and i m very happy with the result. Do you think this one is better ? in quality and gain .Because if you think so i will be happy to build it.Thank you for your educational and intresting videos

[@jamescox4823]

hi, was your scope on the drain or speaker for the 4x gain?

[@roboticdart6138]

Hey John, I always learn a lot from your videos and I have been wondering where the pilot signal comes from and what its purpose is, does it come from the preamp or from the scope?

[@JohnAudioTech]

I made the test signal in a program called Audacity and added the 4.5KHz with 1% amplitude of the 1KHz tone. Its purpose is just to allow me to visually compare the harmonics (distortion) created in the amplifier on the scope.

[@roboticdart6138]

Thank you sir 👍

[@boldvoltage639]

Seems like it could be a good headphone driver with maybe an opamp CC, would higher impedance be a problem?

[@klaushollinetz7494]

Please do another version with an equally simple CCS instead of the resistor - it could be another sort of a smaller Zen amp?

[@ncuzdvarga4918]

Good day john, what serial of mosfet? Thank's.

[@Lithium09]

Please make a video on ground loop isolation. After connecting Bluetooth module my amplifier is making noises. Is there any way of getting rid of it besides using a separate power supply? There are dc transformers (isolators) in market but they are very expensive. They alone cost more than my assembly (Bluetooth module and amplifier) please help.

[@dooda1193]

keen

[@trickyrick8621]

Would love to here some your thoughts on the High Quality HIFI Digital CLASSD Amplifier Module MERUS MA12070 2X80W

[@robertalabla]

Is the 4.7uF input cap a non-polar/bipolar or a regular electrolytic?

[@davidgriffin79]

Interesting. So, keeping it Class A, how would you remove the harmonic distortion?

[@lenoircportfolio]

Hi John ! May I ask you what is the output level of your preamp ?

[@danielauen7790]

What do you know about the TDA8512J chip?

[@aboveaveragejoe6291]

Is it 1v p-p out of the preamp?

[@PaiSiraseth]

nice cat

[@com456dj]

Nice Video

[@samppazzz]

This fet is not very linear for different voltages vs current...

[@RexxSchneider]

As with most FETs, the transfer characteristic when used as an amplifier produces a drain current approximately proportional to the square of the difference between the gate voltage and the threshold voltage. This produces second harmonic distortion, but the ear tolerates that far better than odd order harmonic distortion. For comparison, a bipolar transistor has a transfer characteristic that produces a collector current approximately proportional to the exponent of the base-emitter voltage, which is far less linear. In all cases, we apply negative feedback in amplifier circuits that reduces the gain but improves the linearity. It also makes the performance much less dependant on the characteristics of the individual transistor and allows circuits to be reproducible.

[@user-yh7zc9ke4s]

@@RexxSchneider your comment is pure gold for me

[@t1d100]

Fun

[@basterma]

Snickers

[@thekillerdog5228]

Anyone want to make me happy, if u can tell me why when i built this it worked but as soon as i build it on a breadboard it doesnt, just heats up so u think hmm easy bad breadboard right! New evga broadboards and same issue, so i rebuilt it by hand and omg! Still dont work, so now im like hmm maybe a blown part NOPE! 7MONTHS later and im still doing the same b.s i have over 100x transistors now and other parts and still still still cant get it too work! What! But i can build class ab etc no problem! Like im ether being haunted by a electronic ghost or all these videos are capper then bitcoin.

[@KlaweKlapki]

something for me

[@joebloe1152]

Meow.