20 Watt Class A Amplifier - Bode Plot & THD

hey guys what's up eddie oh here with kissandlog.com today i have a class a amplifier it's a 20 watt class amplifier i think this guy is a nice size for class a because they're so inefficient and really 20 watts is a lot of power now it's rated for 20 watts in the 2 to 8 ohms that's pretty nice if it goes all the way down two ohms so what we're going to do is we're going to power these guys up now by the way you can get these on amazon i'll put the link below uh you can get them on ebay too i'll put the links below i think they're 75 bucks or less so it's a nice little board and it's gonna fit on my box nicely it has this big heavy heat sink on this big metal tab and that's one of the reasons i got it because it's one of the few that actually comes with a nice metal heatsink and large t03 transistor packages so i mean you don't always see the big large t03s anymore motorola made uh some really good ones back in the day motorola became on semiconductor and that's the name on these you know whether they're real or not who knows these days but it says made in mexico maybe i don't know anyway we'll see if they work now it also came with these large five watt resistors which is awesome because so often i see the class a amps they just don't put big enough resistors now these resistors are kind of what forces the transistors to share there's four of them there's two pers you know they're two on the positive side two on the negative side so they're in parallel so that's how you can get the power for 20 watts that's that's a nice setup i think it also has a smaller transistor on the heatsink and they mount that up right on the heatsink and right in the middle actually uh so that the temperature affects this transistor the small transistor like it does the big ones that way the bias network can track with temperature so pretty cool and the pot right there to set the bias is right near the bias transistor just you know from what i could see the photographs and the parts that they used from what i could tell it looked like it was worth the money so i wanted to get one and try it out so they come in a pair so and they just came in this box with all this foam and it came just fine and uh anyway yeah just wrapped up in this heat shrunk uh plastic so we'll take this off we're gonna bring the camera over here so you can get a nice close-up uh there's an air coil that you can know there's a couple transistors on heat sinks these are probably the drive transistors for these guys and there's some other transistors on the board to set up you know other bias networks and that and then there's a op-amp for the pre-amplification so it kind of simplifies the circuit and the output stages are only two stages so pretty simple they say there's no feedback i'm going to try to pull down the schematic the schematic i could find had really poor resolution so i'll see if i can get a better resolution schematic and show it to you okay but here for now let's come over to the bench we're going to set up the bias and i might just use these power supplies i have this guy all set up to use this big old capacitor bank with the bridge rectifier this board takes the dc input so you need to uh supply your own bridge rectifier and um and then i have a tour right here to power it too now the thing is is with class a you don't typically need as many capacitors as much capacity because unlike in other amplifiers where they kind of idle at low power and then they go range you know from low power to max power class a sit there and idle at half power so the capacitors are fully loaded and then they go to kind of a no load to a max load and so they're already in the halfway position so if that makes sense the ripple what you're going to get is already going to be there but what we're going to do is we're going to apply some big caps and a rectifier so let's bring right here bench and take a close-up of the boards look at the setup and then i think the first test i want to do is i want to run one watt like i did the class d and i want to do the bode plot and i want to see the what the the body plot looks like i want to see what the phase gain looks like you know i want i want to see where the cutoff frequencies are okay and we'll take a thc measurement along the way and also i want to take efficiency at one watt it'll probably be the worst at one watt but you know but and then uh yeah we'll see if we have time to take it up in power too or that might come next video all right hey thanks guys let's come over bench and get started well and this is the way it came packaged uh and this bubble wrap kind of protect like this the uh heat sinks you can see the heat sink it it extends all the way back here and this is the board right here so you see how thick it is here and it's still fairly thick right through here even pretty nice design i think pretty beefy see the air coil right here that's so you don't have saturation with some kind of ferrite or some magnetic material in there so it's just air you can't saturate it these are the big five watt resistors so we have our input power here and the output's a little difficult to see but it's right down in there let me take this plastic off sorry about that crunchy sound kind of irritating uh there's the led right here there's the pot that we can use to set the bias and that's nice so close close-up of the board so by the way these holes if you can tell it's a larger diameter here and then it drops down into a smaller diameter so you can sink your screw right down in there and probably can't tell that's a smaller diameter but see my lead can't quite go into that the rubber part but on this side it can sink all the way down in there so yeah that that gives a nice you know place to let your screw head sink right down in and then if you can tell there's the mica insulators underneath the transistors and they use some white thermal grease as well you can kind of see it gushing out on a couple of these now you don't really want to see it gushing out everywhere you don't want a lot of that it should just be a real thin film just to fill air voids between the metal and the plastic you don't want a layer of that thermal paste is it's going to be resistive so but it's less resistant than air so the air pockets between the materials all you want to fill in here's a little transistor sitting on the heat sink so that the bias can track the temperature of the heat sinks and you can see the poly caps the yellow and blue caps aluminum electrolyte here's a op-amp here at the end that you get a nice close look at that if you like see the transistors on the heatsinks there that looks like a 2k pot and you can see the transistors right next to it you see the poly cap down here the blue one the blue one over here and these yellow geyser poly caps and there's the output the positive output right down in there so the back side of the board you can see the bolts here holding the t03 cans down you can see the heavy traces underneath the masking there all righty all right i just wanted to show you something i've got my havoc test or the uh uh you know otherwise known as the kite wheats i think ht-118a look at these cool probes i did a review where i showed these probes they're really inexpensive i think they're 10 or 11 bucks silicone big heavy silicone leads nice i put this crazy long gold tip on just to show you and then here's the normal size and there's a little kit that came with it just wanted to remind you guys that there's these probes out there you don't need to spend a lot of money on silicone probes um what i want to do is show you the input see that pin right there this seems so long but that's the input pin right there right the pins are really small well so it goes to the resistor right next to it see that zero ohms okay then it goes through that resistor 2.2 k then on the other side of that resistor we have zero ohms so it goes straight into the op amp so the point is there's no capacitive coupling uh it's a dc inject right into the there is a little looks like there's a little filter here uh with this capacitor i think the second resistor over in this capacitor sets up a little filter on the input for high frequency but otherwise uh low frequency or dc is going to go straight in here so you want to make sure you don't have any dc coming into your amplifier uh the nice thing about that is it allows you to set up where you want your crossover your low frequency to be with your preamp or whatever if you're going to feed the signal directly into here you'd have to go through a capacitor if you're worried about dc coupling okay the other thing i was interested in is the potentiometer right here just wondering where it is right now i would suspect if they did any testing at all they might have set that up for me already and we can still play with it see if it's set optimally but anyway it's right there so we look for three pins it's those three pins there so if i go across here okay it's set up for 294 ohms i go to the middle one okay that's the wiper right there 296 ohms or whatever it is 296 point it's almost 297 ohms i guess if i go between those two it's zero so it looks like the wipers tied to the top to this pin over here okay i'm gonna do something here now i'm going back to this pot uh i notice it jumps right in 295 and then it kind of i don't know if there's a capacitor in there something it looks like something's charging so it settles down right around 298 298.4 0.5 is it okay so it settles in right around 298.6 we're going to call it what i want to do is when i adjust this i want to know if i'm going up in impedance or down so i'm going to take it and make one full turn clockwise just to see which way i go right there okay so clockwise increases the resistance so that's good to know before i start messing around i don't have to do it with power on it all right so we're going to play with a little bit when we get it when we turn it on and when we do we're going to keep track of you know half turns quarter turns whatever and we'll keep track of which way we go and uh how many turns so right now it's turned you know it will just keep being mindful of the angle of that okay okay there's one more thing i wanted to confirm and that i start putting my test wires on the center post is not identified but i know there's a trace on the top side the bottom side there is not but i just want to go to ground because that should be ground and it is okay now you notice i didn't put on continuity i could have got a beep but what if there's 10 or 20 ohms in there i want to make sure it's zero so and with continuity if i keep an eye on it show me that too i guess i thought i'd demonstrate that so this continuity i'm getting here beep but i'm looking up to make sure it's zero ohms and i see a green light up there saying it's short but i'm gonna look anyway that's why i didn't bother to go to continuity just want to point that out okay let's just go over this setup as you can see i have voltage applied i have things running right now sorry about the fan noise uh you can see i have it running just uh here i'm gonna bring it right at 25 volts there we go okay right at 25 minus 25 these leads are plugged into these two stacks of connections and this one right here is a negative voltage coming into this terminal block and this guy here this stack right here is the ground connection that terminal block and then the red alligator underneath is tied to the plus and that one has two leads coming up to this meter reading plus 25 volts and this one's reading the minus 25 so we got plus minus 25 at the input i soldered a wire down to that speaker output and got this i'm using this uh alligator to clamp this uh load to it and then the other side is tied to the ground okay so all the power stuff is right here all the power ground is right here then the ground goes through the board up here to the input and so you can see i'm using scope probes here scope probe on the output scope probe on the input grounds are tied together so my scope probes are safe to be tied like this and my generator i'm using a couple resistors or whatever some parts just to clip onto so i've got the signal coming in from the scope and i'm monitoring it with channel one and channel two is the scope okay all right so let's just go check out the power supply oh by the way this is my 200 watt eight ohm load so that's great for this test and i just propped it up here so the my light wouldn't shine on it too much so oh that works too uh this map works great because it you know has a really high melting point so i can let things get hot on it and it's fine because it's just silicone so it's great so anyway okay there we go now let me show you these uh power supply all right and this is the power supply i'm going to use i you know for a moment i thought i might use these two guys in series uh for the plus minus voltage but uh this guy has enough power it just makes it easier so i use this gw instick and i've got this meter light makes it hard to see but i got that meter reading current this switch goes over here so see they're both the 25 volts says 24.9 25 over there put current 0.33 if i move that current they should yeah they're 0.32 0.33 on this side so yeah that's how i can set it up so that this tracking power supply will make it easy to watch the meters the one benefit of the kiwis is you have to die on both up at the same time so that's the disadvantage but the advantages you have nice voltage current and watts on both outputs so i did a video where i showed these as kind of you know kind of a poor man's uh tracking power supply with lots of current capability all right now let's go to the scope and take a look okay this is the signal running through the scope uh real time everything's powered on right now here let me get over here and show you so channel one is the yellow guy and here i'll get those windows out of the way and it's 83.3 millivolts in right now we're set up for one kilohertz one watt so you can see one kilohertz and the output's about 2.84 which is about one watt and obviously uh you know one kilohertz the phase is gonna be perfect i mean what should be class a amp should be right okay so everything's looking pretty good the next thing we're gonna do see my generator generator one is what i'm using yeah even at one watt i can smell the uh amplifier the heat sinks oh it's starting to get toasty actually wow you know i mean we have 25 volts on each output and each output's putting out 0.3 amps so yeah that's we're putting out some wattage okay and uh you can see we're at 0.03 percent distortion 0.02 something to 0.03 it's kind of bouncing around uh but if we go to measure see we're reading thd and if i go down to harmonics we're going out to 12 hard marks so we're picking up a lot of harmonics and that should be good so that's our thc at one watt okay let me go through the setup real fast okay so we know we're putting in about 83 millivolts okay let's go to our application i'm going to hit the application button on the scope and it comes up with this and i want frequency response now so i come over there select that yes i want to do it okay it's showing me the signals that's coming out right now but let's go to setup inputs channel one yes output channel to yes and that's good because this shows how the you know setup is the device under test generator generator one is what i'm using input with channel one i i got it set up just like that but you don't have to and generator so i want to do well we're 80 millivolts so that looks that looks about right um we're going to start at 20 hertz okay and we're going to stop at 100k hertz i think okay that's 100k that looks good because we don't know how far it's going to go i want to go all the way so okay i think we're ready to run it all right i just shut down the power amplifier and the power supply so it'd be a little less noisy okay so let's uh go to scale scalability plot let's go to phase first and let's put a zero degree out here okay i'm changing the scale right now that's 15 degrees per we can really let me see if i can go yeah so 15 degrees that's best resolution i can get there so uh let's go phase offset and let's get an even number there so we have a zero put that on 30 i think okay so this is zero phase so it was pretty much zero phase and then it went to 15 degrees out at about about 20 30 k so at 20k yeah i don't know it's like 10 degrees out so you know that is pretty darn flush and the gain well here instead of doing that let's just do this let's get cursors on cursor one selected let's leave cursor ones over there cursor two okay so cursor they both start off right now at about 4.2 degrees and about 30.43 dbs and i as i move across you can see the delta it's like under like right there point two that's as big a delta as i've seen so far a point okay 0.7 right there that peak so it's less than a db that's pretty darn flat okay let's go out to 20k right there it's 1db and 20k were about 0.8 db so yeah that's looking pretty darn good guys uh here let's go all the way over here geez it's still okay out 100k it's 1.3 dbs man that thing has some bandwidth wow that's impressive i think i'm just going to leave those 20. wow like that all right so that's your plot pretty nice okay i got it back running again at one watt and what i want to do is you know take it up and just see we're gonna take up the generator and we're gonna just see where the uh thing clips okay oh there it is right there now the top side's clipping the bottom sides just starting to but let's just okay right there just before clip get rid of all that stuff okay that's what it looks like and by the way distortion thc is 0.145 now looks pretty darn good right there's no crossover it's class a okay i took the output up to or the power rails to 28 volts just to see if we get a little bit more power okay goes to like six percent right there and if i go right there it's one more notch it's 0.05 or uh thd right there's 0.145 so that looks looks like uh it's just before the thc gets bad let's go one notch over let's creep it up right there so we go right up to there uh 0.185 you can visually see it and it goes up to about one percent distortion right there right there that's almost one percent so yeah that right there's our max at 28 volts uh on the rails so we're we're almost eight watts that's only eight watts but it's eight watts you know class a and by the way the power supply is up to 0.5 amps so it doesn't change much that's why you don't need a lot of capacitance on the class a because you're already running it you know you're already running it pretty hard all right guys uh i just thought what the heck man let's put a 4 ohm load on it and let's see if we can get the same amplitude okay so i've got on frozen uh display right now i'm going to turn that off so we're just free running i'm going to bring up the power supply okay and power supply is up it's about 25 and a half volts because i think would probably drop some voltage maybe okay here let's turn on the generator output we might drop some voltage on the leads to the board there's our amplitude okay let's start increasing that okay there we go same place so that's good we can go right up to there before we get a uh for our clipping so yeah that's great that means we have good dynamic range because we can hit the same peaks i'm going to freeze that now hold on let me take down a little bit more okay we're about 0.3 we're about 0.35 uh thd right now with four ohms and just freeze that and i'm going to turn off oh okay the power supply is up to about .8 amps right now all right hey so what do you think of this amp uh 20 watts it's rated at 20 watts uh from 2 ohms to 8 ohms so i wonder if that's 20 watts into 2 ohms because in 8 ohms we don't get close to that and i think i'm putting in the right input voltage i'm going to check on that the one thing about this board it did not come and it did not have a lot of specs with it on the you know on amazon either it just looked like a good board and they said 20 watts and uh i'll show you the schematic next time we're gonna do a little more serious testing a little more serious power testing and run this thing through some more pieces because i'm thinking about putting it in this box down here it fit nicely and you know the amount of dissipation you know actually i mean even though we're only putting out close to eight watts and now four ohms it was it was not quite double that i i think i could probably get there i'm not sure but anyway um it's only about 20 well i'd say 30 efficient so class a that's kind of what you expect right so it is going to get hot and you know in this little box i have i think it's pretty nicely sized and if it did put out a full 20 watts it might be too hot and i don't know how much music volume you can listen to right i mean you know one watt when i do one watt test with music it's pretty loud so what do you guys think hey by the way give it a video a thumbs up if you like it that helps the youtube analytics a lot you know yeah it just helps so appreciate that and and well i'm saying i'm appreciating people let me appreciate the patreons thank you guys really do appreciate that thumbs up to you guys and thumbs up to everybody watching the videos and supporting and comments and all that kind of stuff let me know what you guys think about the power level i'm kind of debating between the class d amp which i can get 100 watts easily per side or this one which is probably gonna be closer and eight ohms eight watts uh i think it's going to be a pretty stiff amp i mean i think it you know as far as being able to uh give good solid power out at eight and four ohms and music power is a lot different than you know test bench power when we're doing sine waves you know music's not sine waves continuous it's a lower place and so um anyway we'll do a music test next time and we're going to do some power we're going to listen to some music and see how loud it really is but let me know what you guys think okay because i'm really curious i'm kind of catering on um should i build the this amplifier this class a amp in that box i have here let me show you the box oh there's the box so this guy would fit in it nicely um do i do that or do i put the class d amp in it i have right size of toroid for either one of them i think so what do you guys think let me know hey thanks for watching and next video will be on this amp again we're gonna do a few more tests because i got to decide which one i want to put into that box so i mean from a purist point of view i kind of feel like putting this guy in but i don't know what do you guys think let me know alright thanks for watching and we'll see you next time all right thanks talk to you later all right thanks we'll see you later

Channel: Kiss Analog

Views: 2,751

Keywords: Class A Amplifier, 20Watt Class A Amplifier, 20 Watt Class A Amplifier, Hiraga PS kit, Hiraga, Jean Hiraga, Le Monster, Hiraga Le Monster, Jean Hiraga Power Supply, Hiraga Class A Amplifier, Aleph 5, Aleph Class A amplifier, Aleph 5 Class A Amplifier, Power Supply Kit, Kiss Analog, KissAnalog, Class A kit, Power Supply Design, Le Monster Power supply, Class A Amplifier review, how a class a amplifier works

Id: D_b-i4VSjYc

Length: 33min 37sec (2017 seconds)

Published: Sat Jan 30 2021