Building a simple power amplifier - Page 1
Forum section: electronics / beginners

author: jackbob
author_rank: Regular Contributor
timestamp: October 05, 2011, 03:06:31 am
content: Hello everybody this is my first post on the eevblog and I thought where better to go to ask people who understand what I'm taking about. I have been doin electronics for years but have failed at making a simple power amplifier too many times. So here's what's up... I have pairs of 2sc5200 and 2sa1943 bjt's. I want to build a simple amplifier with them. I would be running them at around-30 0 +30 volt rails. What type of preamp transistors would work best? I tried using 2n222 and 2n2907 and didn't work so well. Don't know if I'm building it wrong or using the wrong transistors. I just want to build the basic of the basic not with a bunch of components to make it sound absoulutely perfect just the bare minimum. Any help would be greatly appreciated and if you have any schematics you could make or recommend I would love to see them.

author: amspire
author_rank: Super Contributor
timestamp: October 05, 2011, 05:17:31 am
content: Designing good power amplifiers is not at all easy, particularly if it is all discrete components.There is biasing of the preamp and driver transistors, maximizing linearity of the amplifier before feedback is applied, biasing the output stage to minimize crossover distortion and have it remain correctly biased as the transistors heat up, Stability after feed back is applied into capacitive and inductive loads, avoiding enough output offset to put significant DC current into the speakers. And then there are the enhancements live output short circuit protection. Not sure it can be done without experience and a lot of calculation.Anyway, this might be a place to start:http://english.cxem.net/amplifier/amplifier4.phpAnd this link is interesting because it has a lot of comments on the design of a popular ETI-466 power amplifier, including links to the articles and schematics.http://home.alphalink.com.au/~cambie/ETI466Web.htmRichard.

author: jackbob
author_rank: Regular Contributor
timestamp: October 05, 2011, 06:07:58 am
content: Thanks for your help I will look into the links you posted. There is a lot more to it than I thought. I have been doing electronics for a while but mainly dealing with power mosfets for use in smps and inverters. Bjt's seem a little more complicated with the biasing and such. In mosfets you just need a gate and pull down resistor. I thought about using an op amp to drive the bases of the output bjt's instead of having more transistors to drive them. Do you think this is possible. Op amps seem simpler than a preamp stage. Thanks,Jacob

author: Balaur
author_rank: Unknown
timestamp: October 05, 2011, 06:26:16 am
content: I had a lot of fun in my youth with a homemade Quad 405 amplifier.The main information page is here:http://quad405.com/Keith Snook has very lots of information on this amplifier:http://www.keith-snook.info/QUAD-405-mods.htmlA popular implementation is the Mod-3 version that you can find on some page in this document:http://www.keith-snook.info/Schematics/QUAD%20405%20schematic%20evolution%20.pdf(You can drop the triac protection circuitry and use +/- 30 V rails)

author: david77
author_rank: Frequent Contributor
timestamp: October 05, 2011, 11:29:44 am
content: When it comes to anything to do with audio circuits I found Rod Elliot's site very helpfullhttp://sound.westhost.com/articles.htm. Have a look through his articles and look at his projects. I've built several of his amps and I have based other designs on stuff learned from him.

author: jackbob
author_rank: Regular Contributor
timestamp: October 05, 2011, 08:42:38 pm
content: Thank you everybody for your help the liks you posted will keep me busy for a while.

author: bearman
author_rank: Regular Contributor
timestamp: October 13, 2011, 09:08:06 pm
content: Not sure of your application but quick and simple audio amps can be made with ICs like the TDA2030. This is one of the 5 pin chip family of audion amps that look like a TO-220 package.There is a slew of these out there to choose from. I have found some TDA2030 for 50 cents USD. Can't beat that. According to datasheet they can be as clean as .5% THD.You will need significant heat sinking capabilty with a dual polarity 30 volt supply. These also have built in short circuit and overheating protection.Might be an option.B