Discrete audio amplifier project PT11.2 output bias and thermal stability

[@laxr5rs]

Happy Birthday, Snickers!

[@joeyscott4299]

Thank you for everything you share John ! You've helped me out a lot with understanding how audio amps function .I've been a Sub for 8 or 9 months . I never thought I would get into electronics but about a year ago I started playing around with LEDs and small scale solar/battery wiring and now I am hooked . I've always been in to amplifiers , home and PA set ups , but never worked on them . After a while of research and hands on with circuits I kinda came full circle back to the audio I've always loved . Now , at 46yo , I've started doing my own repairs and have built a couple small amps . My thanks go to Mr. Carlson , Big Clive and You most of all because you break things down in a way that is easy for the beginner to understand .

[@Larr.y]

I've always had a vague interest in electronics, but although I wouldn't have the first clue as to what you're talking about here, what keeps me coming back primarily are the cameo appearances from Snickers, and also the fact that your voice has a quasi-ASMR effect which becomes somewhat hypnotic after a while. So I find your videos very enjoyable in spite of their subject matter being completely over my head ... that's serendipity I guess!

[@creepingmee]

I quite literally laughed at the "baked" snickers thing. Glad to see you haven't lost your sense of humor John. Keep up the good work!

[@rogeronslow1498]

Hi John The problem with connecting the Vbe multiplier transistor to the heatsink is that there is a very long thermal time constant of the heatsink. This delay causes the bias overshoot you witness. A solution is to use a smaller Vbe transistor like a TO-92 or similar on short flyleads and connect it thermally directly to one of the power device tabs. This dramatically improves the response time of the bias correction with very little overshoot. It's not as mechanically elegant as a transistor screwed to a heatsink but the thermal performance is much improved and well worth it.

[@HillsWorkbench]

That's why I'd use a diode to sense the heat, metal cased or the BE junction of a transistor rather than the bias transistor itself, which if heated changes gain, adding to the desired BE junction tracking.

[@flurng]

Happy Birthday, Snickers! Thanks for yet another STELLAR performance - the channel wouldn't be the same without you! (Oh, and please TRY to contain your enthusiasm!)

[@1959Berre]

Happy birthday Snickers! It is good to see you love that cat. My cat passed away 3 years ago at the age of nearly 16; still miss him dearly. You're right about those thermal pads, they stink; way too thick. They behave like heat insulators while cold. I always use those flimsy mica pads with thermal compound on both sides. To avoid temp over compensation consider including a temp feedback circuit based on two or three diodes in series, monitoring the heat sink.

[@mikedavis1127]

one of my favorite things about some makers channels is their workbench creatures. seeing their furry buddies always seems to add some connective content to the overall thing. im sure theres an actual word for it but anyway its just something ive noticed - i guess its something many can universally relate with beyond the video topic you have Mr snickers ave and big clive have had their pets on vid from time to time to be honest i probably would have clicked the video regardless of the topic just to see snickers ;) always a joy to see him and thanks for all the great informative vids have made several 7205 and 2822 circuits that greatly benefited from your insight. thanks and happy birthday to snickers!

[@Markoul11]

I would prefer to get rid the resistors and the potensiometer and the transistor at the bias circuit and put instead a 3.6 V zenner diode. This would give about 1.8V bias on each Darlington transistor, enough to operate the amp at class AB without crossover and small d.c current. Also you can thermocouple the zener with the output transistors. A 3VZ has a negative temperature coefficient about -2mv/C.

[@peteb2]

Cool cat Snickers.... still miss the olde guy who hung out at my bench the last 19 yrs!

[@gatekeeper65]

So sorry about your cat. My 17 year old cat passed away in July, and my 14 year old on new years eve. They were my kids, and and i'll honour their memory by going to the shelter and saving another one. I hope you do too. Take care.

[@peteb2]

gatekeeper65 yes... what is it about us electronics types & our tendency for feline pets? Maybe it’s that they look after themselves unlike a dog that needs walking & so more valuable time we’d need to spend away from the bench! Ever since I was a young kid there was a pet in the form of a cat and the pattern followed me into adulthood. My marriages might not have worked but there was always some single stray cat that adopted me and kept watch over my testgear & workbench.... at my age though having formed a really close bond over 19yrs of MorrisTheCat’s existence like none I’d ever had before with a mere pet I don’t believe I can face it again for now... I agree though when & if I feel it’s time to honour him as you call it, it’ll be a trip to adopt another. I guess all has something to do with my age & my recognising my own mortality.

[@JohnAudioTech]

I grew up with cats and dogs around. Each one had its own personality and some were more endearing than others. I think my friend said it best about human and pet relationships. With pets, there is no contest of egos as there can be with a human companion. Pets depend on us to give them care and love and they show affection in return. Sometimes I stop and think of the pets I had lost to time. Many times it was too soon. Cats work best for me because I can give them plenty of food, water and a clean litter box and go off on a short vacation for 3 or four days or just out for the day. I hope Snickers stays with me for many more years. After him, I may be done with pets as I want to pull up stakes and do a lot of traveling without worrying over a pet.

[@awesomekiertechtips3838]

Maybe you should use low gain transistor in the bias. and increase the biasing resistor and trimmer/pot a bit.

[@btouw8558]

Don;t understand, I find this stuff very usefull tp know, I watch until it is finished.

[@demoncloud6147]

Snickers thinking : " Why my Human is 'meowing' (talking to) himself ? "

[@yngvenystrand167]

The BD139/BD140 pair should also be thermally connected to the heatsink. Because now they are not heating up to the same temp as the output and Vbe multiplier transistors. This causes the drop in the quiescent current with increasing temp as their Vbe voltages don't decrease (-2 mv/degC) equally with the heatsinked transistors. The Vbe multiplier compensates for four Vbe drops and should be at the same temp as the four transistors in the output darlington pairs.

[@killcar5nbike2]

Indeed, the Vbe multiplier on an EF stage has 2.4v across it. The drivers should share the main heatsink for EF stages.

[@JohnAudioTech]

Good point. During a hard excursion, the drivers would have warmed as well and would track better. Heating the heatsink externally with the drivers separated would lead to overcompensation.

[@killcar5nbike2]

Worth noting if the Vbe of the compensation transistor drops by say 10mv, the collector voltage will drop by 40mv as the Vbe multiplier runs 4x Vbe in this configuration. This is why the drivers need to be mounted to the same sink as the outputs. If you're interested I have a vid on my channel about adjusting the compensation output of a Vbe multiplier.

[@KrisisVal]

The title should be “snickers birthday!”

[@joinedupjon]

Happy birthday Snickers! It's hard to believe that 2006 was actually quite a while ago now. Agree about the video length - 15 or 20 mins is IMO about optimum

[@filpatogames7583]

Hi, Bro, I am making this circuit of you, one thing I did not understand that why you did not use diode biasing for output stage? Why you did biasing with a NPN transistor? Plz take time to give me detailed answer!

[@electroniquepassion]

👍😁

[@TheTrueVoiceOfReason]

Happy Birthday Snickers! 2006, huh? Wow, that means I've been watching you and John since you were just a young kitty and John went by another channel name. And John, yes, your style of video lends itself well to the 20 - 30 minute maximum length. (30 is stretching it, though) Thing is, I've always liked your videos because they are an interest of my own hobby. You, Uncle Doug, Shango066, The Guitologist, D-Lab, and a number of others, some who have moved onto the great bench in the sky, have been my KZbin compatriots. And I do what I can to share your name across other channels. Heck, I even mentioned you when Paul from PS Audio was fumbling through trying to explain how transistors work in amplifiers. (He's a fun one to watch when he tries to explain a lot of the esoteric unexplainable audio phoolery) Hope you and Snickers enjoy the day and also hope all is well with everything else.

[@JohnAudioTech]

Thanks for hanging around all these years. My original channel started in 2008. I'm trying to find my first video so I can post it. Its got to be on one of my backup drives somewhere.

[@TheTrueVoiceOfReason]

@@JohnAudioTech '08? Well, it has been quite a while back. 2 years old is still a young kitty. Hope you can find your old channel's stuff as you had a lot of good vids there. And seriously, it is good to have you as part of the YT family / community. My best to you, Snickers, and your family (you are definitely a good son!)

[@davekazoroski6548]

Yes videos should be about 15 to a maximum of 30 minutes. Most folks don't have time to view long ones. Plus by setting yourself a time limit, you are forced to make sure that you are covering the most important points.

[@SuburbanDon]

John, in a power amplifier of 10 watts max, would a solid ground plane be safe to use instead of a star ground system ?

[@JohnAudioTech]

You should be okay with that.

[@Reynicolerio93]

do you have any issue by using a smps as power supply for these kind of amplifiers?

[@wallybaldemor8523]

I saw a thermistor in pioneer amplifiers

[@milenedejong1400]

question john. are those power transistors to220 which you will be using in the final project? and are those the 2sc5200 and the 2sa1943 . i was thinking you would use the TO-3P .. maybe you just used these to demonstrate. i am a little confused (maybe just my expectations) .. great video as always btw..

[@JohnAudioTech]

I will be using the TO264 versions. these TO-220 are the FJP5200/1943. Contains the same die as the larger transistors (de-rated power dissipation because of the smaller TO-220 package). I use them because they are cheaper and plug right into socket boards and perfboards for prototyping.

[@milenedejong1400]

tnx for the reply john.. we are all stocking parts to built the amp soon :) .. btw , did you ever see this document about the improved vbe doubler ? www.hagtech.com/pdf/vbe.pdf

[@bijouxbijouxbijoux]

Happy birthday Snickers!!! 🎁 🎈 🍰

[@KrisisVal]

Snickers is the same age as me (almost)!

[@gatekeeper65]

Scratch Snickers' cheek for me. He's a handsome fella.

[@twotone3070]

Snickers the mobile Catalanche.

[@Dazzwidd]

The JAT CAT

[@charlesklein7232]

how did you get your cat? feed them canned cat food, not "dry." they like dry, but it has no values, its not really healthy, it just tastes good. it basically cereal.

[@MagruderSpoots]

Also second. You should have an all Snickers channel.

[@peteytwofinger]

pwetty nice fwend name of snickers ...

[@DiscoverRajivVlogs]

Snicker looks cool.

[@grhinson]

Cats seem to be ubiquitous among electronic videos

[@victimovtalent6036]

Less talk do less cat!

[@brianksiazek767]

Happy Birthday Snickers!!!!! (Meh)

[@MagruderSpoots]

First.

[@ItsDextrin]

lol

[@JohnAudioTech]

Good job Mark. An achievement to be proud of.

[@MagruderSpoots]

@@JohnAudioTech LOL! I couldn't resist.

[@terencekaye9948]

NO sound

[@TMS5100]

obligatory cat tax.

[@PeteVanDemark]

Happy Birthday, Snickers!