My stereo amplifier has some heat issues. - Page 1
Forum section: electronics / beginners

author: WyverntekGameRepairs
author_rank: Regular Contributor
timestamp: December 19, 2019, 04:25:15 pm
content: I have created and prefboarded a class-A amplifier that uses a Darlington pair consisting of a MJE3055T connected to a D1453 Horizontal Deflection Output transistor. I have two of these pairs, one for Left audio signal and one for Right audio signal, thus creating a stereo amplifier. I have chosen to use a 19K dual potentiometer to control the gain (or volume) via the bias on the 3055Ts. (It has 2 potentiometers in one package basically, and they remain isolated from each other. This allows the use of a single potentiometer dial without accidentally joining the signals from both sides).The power supply I am using is 12V (it reads about 13-14 volts though, as these inaccurate linear power supplies tend to do) at around 1.5A. I'm dissipating a lot of the energy through a huge resistor capable of handling ~200 Watts of power. It has a value of 20 Ohms, and is placed at the positive voltage input of the amplifier circuit. I can understand this sucker getting quite hot despite dissipating around only 8.45 Watts (assuming the power supply is delivering 13 Volts). Now, here is where it gets tricky. I also have a 1KW resistor with a value of 2.2 Ohms. This beast is bigger than my forearm, and is most definitely overkill for the application I'm currently using it in.Since we get the power dissipation of a resistor by calculating the current across it and then using that to calculate the power deing dissipated across it, let's do some calculations here.Assuming our power supply is delivering 13 Volts steady, let's calculate the current across the resistor. I = V / R, so 13 / 2.2, which gives us 5.9091A. Now we use this to calculate the power dissipated by the resistor. P = I * V, so 5.9091 * 13, which gives us75.8182 Watts.Holy mother of God. It draws so much current that the power supply drops down to 6V to compensate for the current draw (because the transformer is used to delivering about 1.5A, not 5.9-6A). That's pretty intense.With this huge resistor in place, it has a few benefits and drawbacks. The benefits are that the lower supply voltage allows less distortion, as well as posing less of a risk to the signal capacitors that provide isolation between the signal inputs and the power circuitry. The drawbacks are that the resistor is sucking up a lot of current and wasting quite a bit of energy in the form of heat. If you leave the resistor on long enough, you can actually feel it getting about 7 or 8 degrees warmer than ambient. It also is bulky, and very hard to put in a discrete place (obviously, as this resistor is supposed to be big, mounted on a panel, and not-so-discrete in the first place). However, the biggest drawback that is very obvious here is quite naughty: Heat. Not in the resistor itself, but in the transistors. The transistors have less resistance preventing them from entering thermal runaway or getting dubiously hot. This causes the transistors to get up to around 10 degrees above ambient. I have had to mount them on some pretty good sized heatsinks to help dissipate that heat.Using the smaller but higher resistance resistor (which dissipates ~8.5 Watts) allows higher voltage and thus more distortion, but cooler transistor operation (the transistors only get around 2 or 3 degrees above ambient when using that resistor). However, using the 1KW resistor with lower resistance allows less distortion due to more current draw reducing voltage for compensation... But also causes more heat to be dissipated by the transistors.What would you do in this situation? I'm curious to see if there is a sort of "sweet spot" I can achieve by putting resistors in series or parallel to fine-tune the balance between heat dissipation and distortion.NOTICE: This project is meant to be incorrect. Things are supposed to go wrong. I'm practicing troubleshooting methods, as well as my soldering, by building this small little project. This is for amusement purposes only, and experimentation. This is not meant to be a real project. Do not post comments saying I built it the wrong way, I already know it is not the right way to build an amplifier. As I said, this isintentionally designedto mess up.

author: madires
author_rank: Super Contributor
timestamp: December 19, 2019, 05:08:51 pm
content: A circuit diagram would be helpful.

author: mikerj
author_rank: Super Contributor
timestamp: December 19, 2019, 05:47:54 pm
content: Class A amplifiers are horribly inefficient, and using a resistor as a collector load is the worst thing you can do in terms of efficiency. Switching to an active load would give some improvement, but if you want to stick to a linear amplifier and efficiency is a concern then consider a Class A/B design.

author: Gyro
author_rank: Super Contributor
timestamp: December 19, 2019, 06:57:17 pm
content: You need to check the SOA of a horizontal deflection transistor very carefully before attempting to use it in linear mode. Also, if you're overloading the transformer that badly then it is going to die, imminently!See here:http://sound-au.com/tcaas/index-1.htmto see how to do it properly - you're clearly not at the moment.

author: WyverntekGameRepairs
author_rank: Regular Contributor
timestamp: December 19, 2019, 11:10:44 pm
content: Quote from: Gyro on December 19, 2019, 06:57:17 pmNot to worry, I'm replacing that adapter with a transformer that was meant for audio amplifier applications. It's a very interesting one, using a toroidal core. Anyway, it has high-current outputs that are capable of satisfying the power-hungry resistor.Also, there's no such thing as "doing it right" when you are messing around experimentally and trying unconventional methods out. You can't learn until you screw something up!Quote from: mikerj on December 19, 2019, 05:47:54 pmI agree, class A are actually terrible when it comes to efficiency. But think about it, this issupposedto be inefficient. It's a very experimental thing. I'm not planning on releasing it to the public, if that is what you are saying. God no, I'd never release this monstrosity as a professional or even hobbyist-grade product. This is just for my amusement.However, I will definitely try an AB amplifier. I'm curious to see the results.

author: madires
author_rank: Super Contributor
timestamp: December 20, 2019, 11:25:52 am
content: Your circuit has a lot of problems:- no input stage for driving the Darlington with sufficient current- no biasing- no feedback- no output cap to keep DC away from the speaker

author: Gyro
author_rank: Super Contributor
timestamp: December 20, 2019, 12:02:56 pm
content: Just seen the added schematic... and I was worried about the niceties of SOA?EDIT:Quote from: WyverntekGameRepairs on December 19, 2019, 11:10:44 pmOf course there is! There are fundamentals, like not implementing something that is going to toast your speakers with high current DC.Go to the site I linked and do some serious reading.

author: WyverntekGameRepairs
author_rank: Regular Contributor
timestamp: December 20, 2019, 02:48:56 pm
content: Quote from: madires on December 20, 2019, 11:25:52 amYes, and so do I.Quote from: madires on December 20, 2019, 11:25:52 amNo input stage? Where do you see that?Quote from: madires on December 20, 2019, 11:25:52 amTechnically, the gain controlisbiasing the signal transistor positive. See connections of potentiometer.Quote from: madires on December 20, 2019, 11:25:52 amWhy do I need feedback? This is asimple experimental fun little thing for occasional use, not on-all-the-time-use. Keyword here issimple.Quote from: madires on December 20, 2019, 11:25:52 amAdded. Thanks for that one. Nearly forgot about that.Also, don't crush my dreams, that's my job silly. If I fuck up, I'll learn from it myself. I did not ask to be taught how to build an amplifier. I presented a problem and asked what you would do in the situation, assuming you know this is a silly little project for amusement purpouses only.Quote from: Gyro on December 20, 2019, 12:02:56 pmGoing on a rant here, but...Calm down, Nikola Tesla the Great and Perfect. You clearly missed the part where I said this was for amusement purpouses only. Also, I just added an output cap to filter out DC. You can't expect me to do everything your way, you know. The entire point of this project is to screw something up and learn from it. Hot transistors is one of those things. I was asking what you would do in the situation, assuming that you understood the fact that this is a silly little project where safety is not too extreme of a concern because this is not going by regular standards, so I could getactually helpful suggestions. Telling me I'm "doing it wrong and I need to read up on my fundamentals" is not a suggestion relevant to the question at hand. If I wanted to read up on the fundamentals, I would have asked for a link. Don't crush my dreams, that's my own job to do. If I blow something up or burn thingts out, so be it. Hell, as long as I actually learn from it, it is not for nothing.

author: madires
author_rank: Super Contributor
timestamp: December 20, 2019, 04:49:50 pm
content: Quote from: WyverntekGameRepairs on December 20, 2019, 02:48:56 pmDo you see any input stage?Quote from: WyverntekGameRepairs on December 20, 2019, 02:48:56 pmThe pots are fed by the amp's output and act as feedback. That doesn't provide a stable biasing.Quote from: WyverntekGameRepairs on December 20, 2019, 02:48:56 pmActually you have feedback via the pots. I should have written "no proper feedback". You want feedback to stabilize gain, for more bandwidth and for lower output impedance.

author: WyverntekGameRepairs
author_rank: Regular Contributor
timestamp: December 20, 2019, 04:59:29 pm
content: Quote from: madires on December 20, 2019, 04:49:50 pmDespite this post being focoused on troubleshooting methods and solutions to small specific problems, I do actually find this helpful for the REAL amplifier I am building. It will be a class AB amplifier using real audio transistors. I actually didn't see any use for the more "enhanced" feedback (the kind that is "correct") until now, thanks to your comment. I was debating whether or not to put in enhanced feedback, because typically more joints and wires means more interference and points of failure. I think that if I put in the feedback you have indicated here, it would actually improve the sound (at the cost of having more wires, of course).So in that case, thanks!

author: Gyro
author_rank: Super Contributor
timestamp: December 20, 2019, 06:16:48 pm
content: Good luck with your endeavour.