CLASS A AMPLIFIER - Page 1
Forum section: electronics / projects

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: July 02, 2017, 11:46:43 pm
content: Has anyone for fun ever built a class A amplifier based on the complimentary pair of mj2955 and 2n3055 power transistors, or better yet come up with a design utilizing multiple output stages in parallel? I can never find an appropriate schematic on Google images and would appreciate any advice (yes I'm a beginner)

author: ruairi
author_rank: Frequent Contributor
timestamp: July 03, 2017, 12:14:11 am
content: I'm sure there are some audio folks here (I am) but the real amp builders hang out is DIYaudio.com

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 03, 2017, 01:26:28 am
content: Why do you want class A? It's just a total waste of power.Using multiple output stages of class A would be just a big room heater that occasionally plays music.Use class AB and there would be a lot more designs out there.incidently I have 2 mono class a amps edit: oops not sure what transistorsI never use them though as I am worried about forgetting to turn them off.

author: DimitriP
author_rank: Super Contributor
timestamp: July 03, 2017, 01:48:09 am
content: Yes , you are in the wrong placeHead over toDIYAudio.com, and on your way there, stop here :http://www.firstwatt.com/pdf/art_amp_camp_1.pdf] http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

author: cat87
author_rank: Regular Contributor
timestamp: July 03, 2017, 08:35:41 am
content: If you want to build a very nice class A amp, I'd recommend reading Douglas Self's book "Power Amplifier design Handbook" (6th edition preferably).He has some really interesting designs in that book. It covers everything from designing the amp, thermal management, to part selection.There's also a design for a trimodal amp. It's a Class A/AB/A amp, where you can go from class A to class B with the flip of a switch.

author: German_EE
author_rank: Super Contributor
timestamp: July 03, 2017, 09:23:35 am
content: Don't listen to the naysayers, a Class A amplifier will work just fine providing you specify your heatsinks and power supply accordingly. Yes, you will heat up the room, but so does your PC, and that does useful work without anyone complaining of the thermal issues.Build it, have fun, your ears will thank you.

author: Delta
author_rank: Super Contributor
timestamp: July 03, 2017, 09:58:02 am
content: I thought you didn't need a complementary pair for Class A....?

author: cat87
author_rank: Regular Contributor
timestamp: July 03, 2017, 11:56:25 am
content: QuoteIt depends on what kind of class A amp you're building. To get the most efficiency out of a class A, you can do a push-pull configuration. And for that, you would need a complementary pair. But if efficiency is not high on your list, then you can also go with both final BJTs being NPN. Though you'll need a much bigger heatsink.

author: cncjerry
author_rank: Unknown
timestamp: July 03, 2017, 04:08:15 pm
content: Nelson Pass designed a class A amp in the 70s that a lot of us built. It required a hell of a power supply. I used a power supply out of an old IBM computer called a "mid pack". I still have it in the closet after using it for years. Sounded great, used a ton of power, generated a lot of heat but nowhere near as much as my Biamped Krell setup.The equivalent amp to the Pass design would be the Krell KSA 50s which you can find used for not a ton of money, probably as much as what you would have in any 30-50 watt home brew amp.

author: Gyro
author_rank: Super Contributor
timestamp: July 03, 2017, 06:22:29 pm
content: Has everyone forgotten John Linsley Hood so soon?http://sound-au.com/tcaas/index.htm

author: c.coyle
author_rank: Newbie
timestamp: July 03, 2017, 07:33:29 pm
content: Google "JLH amplifier". Then do the same search on Ebay. Lots of tweaks and upgrades since the original 1969 design. Great, great first audio amplifier build.

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: July 03, 2017, 08:52:01 pm
content: This thread is going be very old (possibly a few weeks or even a month or more depending on shipping from China) by the time I get to put anything together how ever in the book I read in class told me a push pull stage in a class A will waste 75% of its power by heat. I'm playing in a very unusual speaker so I'm not sure that 30-50watts rms across the load is enough. I read about the krell ksa's they're certainly nice amplifiers and worth what you would pay for them, but I wonder why they stuck resistors on the output stage in a voltage divider wouldn't this be inefficient. causing the overall amplitude to lowered by an amount determined by their resistance?There is do much info I'm drooling all over it lol

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: July 03, 2017, 08:57:55 pm
content: And those little jhls are Cool I've seen those before on eBay and almost bought one but then bought my Astro mix amp pro. I game a little and it has active Feed back so I don't end yelling at every one lol how ever this will be something I intend conduct analysis on ( buy one for strictly music) and look at it on an o-scope for fun

author: Gyro
author_rank: Super Contributor
timestamp: July 03, 2017, 09:15:21 pm
content: If you buy them, just remember that those right angle Aluminium things are just heatsink brackets. They need bolting to some very substantial heatsinks!

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: July 03, 2017, 09:47:27 pm
content: I've seen YouTube videos of those huge things ( comparable to the krell design) isn't the Jhl design basically a complete amp that the out put can connect to the input of another jhl in series and create higher amplification? It appears to be so and if this is how it was constructed then it is truly beautiful and are IBM midpacks still available

author: cat87
author_rank: Regular Contributor
timestamp: July 04, 2017, 05:45:12 am
content: QuoteAre you talking about those resistors on the Emitters of the BJTs ?

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: July 05, 2017, 02:39:31 am
content: Yes on the out put

author: cat87
author_rank: Regular Contributor
timestamp: July 05, 2017, 05:44:36 am
content: Those are actually there to stabilize the BJT's characteristics (thermal and large signal behavior) Because a BJT's internal emitter resistance is subject to change due to temperature and current through the transistor, it is needed to have those resistors in place. That way, you actually have them in series with the emitter's internal resistance, thus making up for any change in the re value (i.e the re will change, but the external Re will be fixed)You'll notice that they regularly have values between 0.1 Ohms and 0.33 Ohms. That's what's needed in most cases, to swamp the actual value of the built in re.

author: ale500
author_rank: Frequent Contributor
timestamp: July 05, 2017, 05:52:42 am
content: I don't remember the MJ2955 but the 2N3055 had 15 kHz as transition frequency...at least the original Motorola version.... if my mind doesn't play tricks.I thought class A amplifiers where mostly done with tubes (no flames please!), I am curious, I thought class AB was a bit more popular on the transistor side...

author: BrianHG
author_rank: Super Contributor
timestamp: July 05, 2017, 06:13:31 am
content: For a class A amp, I would favor a mosfet source follower with either a resistive pull-down for the least possible performance, or, a second n-channel mosfet configured as a constant current source pulldown in a balanced configuration (the simplicity of the n-channel mosfet having no audio signal fed to the gate, basically it's being used as a programmable resistive load, the design is still considered a single ended class A amp), give you a better linear range and drive for your speaker.Now, you can also design a class A amp with approximate 50% efficiency, but, this calls for a center-tap power audio transformer like the ones in vacuum tube amps, fed by 2 n-channel mosfets with the gates having a mirror audio image. (Center-tap at VCC or GND depending on design, with each side biased and driven by mosfets with a mirror audio signal at the gates...Bipolar here in emitter follower configuration may have a tighter, flatter response.) Though not as flat response as direct drive, when it comes to class A, you're obviously looking for that ascetic warm sound. Note since you will be feeding the transformer with 50% of your supply power in DC, it will get nice and warm sound or not.These 2 design topologies are easily searchable at diyaudio.com with schematics.