Simple class A amp improved with a constant current source

[@fredfabris7187]

I’ve got a couple things. First I don’t think the video is too long I prefer a longer video than a short one. Thank you for re-explaining the constant current source because it’s helpful. I love your videos and the detailed conversation about how circuits work is hard to find online. Much appreciated

[@markweikle5645]

"that's not going to keep greenpeace from kicking down your door." I launched a little of my eggroll on that. Lol!

[@northox]

Never get bored of John's video.

[@RexxSchneider]

The problem with biasing a MOSFET for class A is the relatively large range of Vt. its threshold voltage. The datasheet for IRFZ44 indicates a value between 2 and 4 volts. Coupled with a high forward transconductance of around 2.7 A/V at 1A, you can see why small variations of the bias voltage on the gate can produce large current swings at the drain. In the original circuit the 100K pot was fed from a 22K resistor allowing the gate voltage to vary from 0V to 7V. The resulting change in drain current past the threshold voltage was ameliorated by the 0.47R source resistor and 8R drain resistor, which represents a dc gain of about 8R/(0.37R + 0.47R) = 9.5 (the 0.37R is the reciprocal of the estimated forward transconductance). That's not too bad for sensitivity. When you replace the 8R drain resistor by a constant current source, you're providing a high dc impedance to the drain and dramatically increasing the dc gain, so that the bias point becomes very "twitchy". In fact the mismatched temperature coefficients of two diodes against one PNP emitter junction will make that bias point very temperature dependant. However, the ac gain does double because originally the load is 8R in parallel with the 8R speaker, and now it is a high resistance in parallel with an 8R speaker. So to stabilise the dc, you sensibly introduced a load of negative feedback from the drain to the gate. That has the effect of reducing the circuit gain, effectively becoming the ratio of the 10K feedback resistor to the value of the trim pot that's setting the gate bias voltage. Assuming the drain voltage is round about half the supply (4.5V), that suggests the trim pot will have a value between 8K (if Vt=2V) and 80K (if Vt=4V). The gain will therefore be between 1.25 and 0.125. Great for dc stability, but not so good for use as an amplifier. You could, of course, increase that 10K resistor and replace the trim pot by a trim pot in series with a resistor R, then bypass the trim pot with a capacitor that has a smaller reactance than the resistor R. That would allow you to set the dc gain small, but have a larger ac gain set by the ratio of the feedback resistor to the unbypassed resistor R. The ac gain is then independent of the setting of the trim pot.

[@theschnilser7962]

Reminds me of the time when I had a very simple Class A amp with just a decoupling cap, two resistors, a TIP3055 and a 12V light bulb as a current source. It was not even soldered, but on a breadboard like this one. That thing was not very loud either, but enough to enjoy some music while doing some electronic stuff or gaming.

[@pilobond]

I wonder if you can try using LM317 in current source mode as a constant current and do a comparison video. Thanks for wonderful videos.

[@leiferickson3183]

Hey! No peek at the distortion spectra with the current source? Inquisitive minds want to know.... I love your videos by the way.

[@researchandbuild1751]

Its still class A , distortion will ne pretty much the same as his last video but just more output power

[@ferrumignis]

@@researchandbuild1751 Constant current load means the MOSFET operates with (almost) constant transconductance, and John also changed the biasing circuit to add more negative feedback so I would expect to see some improvement, especially as maximum output is approached.

[@ScottGrammer]

17:32 Congratulations! You have just invented the push-pull amplifier! Well, almost. It is a push-pull amplifier, but the positive side is not driven. But that's actually a good thing.

[@darthbubba866]

Welcome to the world of Class-A audio amps; warm sound, warm listening room! 😄

[@johnnytoobad7785]

When used in low power audio circuits a "CCS" on each supply rail can really reduce THD. (even in simulation) Not sure why but it works ! I have a schematic for a headphone amp that uses a green LED for the voltage ref. Different color LED's provide different voltage drops.

[@gkdresden]

11.6% COP ist a very good value for a constant current class A design. It corresponds to about 20% plus for a usual class A power amplifier design with an idle bias current of half of the current at its nominal maximum power. The limit of class A design is a COP of 25%. BTW: I like your mixed type BJT / FET circuit design. :)

[@agl33]

Great video John, I recently started watching your DIY amp videos and they're very inspiring, amazing work! Thanks & cheers!

[@luisbulek65]

Thanks Johh, good video. This music apparently is north east brazil?

[@theschnilser7962]

Maybe it matches the style, I dont know. I think its some generic stuff from the YT audio library.

[@PhG1961]

The bench looks soooooo familiar.... ;-)

[@hannescamitz8575]

Can't see the issue, he has at least 3 or 4 dm2 free space too fiddle around in. 😉

[@PhG1961]

@@hannescamitz8575 Indeed, it's the same over here... however, all the free space gets easily filled up. Sometimes temporalliy, but more often temporalliy with a more indefenately time span. My wife calls it all rubish and junk. I justify it as high tech equipment with sometimes a vintage look...

[@hannescamitz8575]

@@PhG1961 I got spiders in the basement so she won't even go down there. 😉 And my oldest kid calls it my secret laboratory 😎

[@PhG1961]

@@hannescamitz8575 Indeed ! Laboratory is the right description. Depending on who is visiting, I call it my man cave or hobby shack. However, I prefer 'high tech laboratory'.... Since my equipment is getting older, it's now high tech with a vintage look... ;-)

[@debmalya6144]

A small thing - what if we use a buck or boost or buck/boost converter modified as a current source - it may increase efficiency and still be a class a amp at the same time

[@johnsonlam]

Next video: Did audio grade components have dramatic effect in circuit?

[@gkdresden]

Lol, what means "audio grade"? Devices which measures terrible? :D

[@nsfeliz7825]

what exactly is "audio grade" ? is such a thing mentioned in the transistor data sheets? never seen one . who decides what is "audio grade"?

[@johnsonlam]

@@nsfeliz7825 Good question, here I refer to those "claim itself audio grade", then test it in the same circuit and find out how "high" the audio grade is.

[@gkdresden]

@@nsfeliz7825 I would define "audio grade" as low noise, high speed, highly linear devices.

[@jeffreysainio2572]

Looking at the result "out of the corner of my eye" I saw a class B amp, for all the components required.

[@chrisredfield5278]

Thank you so much sir you are the best.

[@briangoldberg4439]

Double the output power at the same current draw sounds like a win to me.

[@user-su5sq5ib3i]

Would like to try this!

[@CoquiAudio]

nice video bro, when you get a chance try the Hiraga class A

[@amitghosh6966]

Please do a design on making a 10 watts and 20 watts @ 4 ohms amplifier using transistors

[@HillsWorkbench]

He already did that twice with two amps with more power. For less power just reduce the rail voltages. kzbin.info/www/bejne/Y3XOYmeQlsxsbtE

[@Canadian_Eh_I]

Great video! Do you do custom amplifier upgrades?

[@roberthorseman7432]

That's tidy up to my bench.

[@TRONMAGNUM2099]

Excellent!

[@FSXgta]

Is it true that Class A amps need a long time to warm up? If so why is that

[@19janiboy96]

The amount of current that a Transistor conducts is temperature dependent, as the Transistor heats up the amount of current wil vary until its reached its thermal equilibrium.

[@ferrumignis]

It's true that some of them do! This is really about bias stability vs temperature, if an amplifier has poor bias stability then it won't be operating at the correct bias point until its operating at the temperature it was calibrated at. A well designed amplifier shouldn't suffer from this problem.

[@horacewonghy]

Depends on transistor/mosfet Just look the data sheet

[@ferrumignis]

@@horacewonghy The datasheet won't help in this case; it tells you the maximum operating temperature and may show how some parameters change over temperature, but its the job of the amplifier designer to take this into account in their circuit design which. A good amplifier will maintain the correct operating point of the transistors as any parameters change over temperature.

[@horacewonghy]

@@ferrumignis I remember that some mosfet data sheets has temp relation graphs. like irf

[@hooligun90]

Great video!

[@neccron9956]

What about using a LM317 and one resister as a CC source?

[@JohnAudioTech]

its drop out voltage is a bit much for this circuit.

[@kuadidzv8726]

very nice explanation sir😍😍😍

[@hans429]

Wow! I ditnt know you have aLawnmower tractor!!

[@NocturneVid66]

John, I really enjoy your videos. You’re doing great work improving these inexpensive circuits. If you want to move up to the next level, check out the technical articles posted by audio legend Nelson Pass on his First Watt website. He has done a number of fun designs using current sourced Class A Mosfet amplifiers.

[@raviranganathan6616]

Superb

[@listerine-pr5lt]

I am sure that Green peace is goin to kick my door but the question remains how I get help from Red Cross to save my life?

[@laxr5rs]

Cool!

[@KuntalGhosh]

Hey man can u design a class ab amplifier using 2sc5200 and 2sa1953 (i think i got the part no. Wrong) , it should have around 250w rms on 4ohn load! I was looking at lots of different circuits but i was unable to find proper diagrams

[@jakk2ahao]

John i need it too please. 200w @8 ohm would be nice.

[@KuntalGhosh]

@@jakk2ahao yup. U would need a pretty high supply voltage

[@researchandbuild1751]

With that kind of circuit the power supply will be just as important as the amplifier itself, thats a lot of power you will need at least 350 watts power supply! Maybe a converted old pc supply with a step up dc to dc comverter on it, but would require a beefy boost conveter You need 42 volts rms so that means 84 volts peak to peak! That is starting to be a dangerous voltage so layout , etc will be important, and as i said the power supply will also be a demanding project for such an amplifier

[@KuntalGhosh]

@@researchandbuild1751 use a fucking transformer!! Why bother with smps & boost converter shit. Use a center tap 45-0-45 transformer. & U will have +-45v available.

[@KuntalGhosh]

@@researchandbuild1751 check this amp. I paid 30$ for this 40w amp. Its made by an old Indian company called ahuja. The circuit they have used is well capable of doing 150w photos.app.goo.gl/bmjpfgXykzNRY9kK6

[@CircularMirror7]

If the bench isn't messy then there is no work is getting done. I am also need to clean my bench.😂

[@ford1546]

class-A is such an impractical amplifier if you think about the heat and power it needs and how much heatsink you need. you can get very good class-ab amplifiers and if you buy hypex ncore then you have a very good class-d

[@elpechos]

Class A is good for learning and experiments though

[@linandy1]

Do class A really sound better? All those audiophiles say it does.

[@william62922]

👍👍👍👍👍

[@KlaweKlapki]

class A is hot but its worth it

[@northox]

Why?

[@ferrumignis]

@@northox A good class A amplifier has very low odd harmonic distortion which provides a more pleasing sound. However the distortion on a modern, well designed class AB or even Class D amplifier is so low that the advantage is negligible.

[@horacewonghy]

Easy,Perfect but hot

[@MassimoTava]

Im sending you a gift box full of random goodies, not that you dont have enough to cover already.

[@JohnAudioTech]

Much appreciated. Please let me know when shipped so I can check the PO box. Thanks!

[@MassimoTava]

@@JohnAudioTech you should have it Saturday.

[@andymouse]

Green peace ! hahahahhahhah :)