Single mosfet power amplifier - Page 1
Forum section: electronics / beginners

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 10:56:32 am
content: Hello there,since I'm working with the preamps I've been quite intrigued with a simple project of a single chip power amp. I can't find enough information on the net and the forum, so I decided to open this thread.The configuration I posted is the one which works better for me, I tried many, but I can't made them work properly. This one is quite noisy, but the sound is loud enough.

author: xavier60
author_rank: Super Contributor
timestamp: December 12, 2017, 11:41:07 am
content: The bias divider needs to connect directly to the Gate. Connect R4 to the Drain rather than to the rail. The negative feedback will make biasing the MOSFET less critical. Do you understand the disadvantages of that configuration? Passing DC through the speaker's voice coil.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 11:44:34 am
content: Yes, it's a class A amplifier. Passing DC through the speaker's coil just causes heating and will reduce the potential loudness of the speaker, before it starts clipping, as the coil will already be pulled/pushed away from the normal position.

author: danadak
author_rank: Super Contributor
timestamp: December 12, 2017, 11:49:07 am
content: Class A design -http://www.electronics-tutorials.ws/amplifier/mosfet-amplifier.htmlhttp://sound.whsites.net/class-a.htmhttp://www.audioholics.com/audio-amplifier/amplifier-classesRegards, Dana.

author: xavier60
author_rank: Super Contributor
timestamp: December 12, 2017, 11:53:27 am
content: Also, 24v is too high for that sort of class A amplifier. The speaker will not like more than a few volts across it. So the rest of the 24v will be across the MOSFET, causing a lot of unnecessary dissipation. 5v would be more suitable.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 12:50:19 pm
content: Quote from: xavier60 on December 12, 2017, 11:53:27 amOr add a resistor between the MOSFET's drain and source and connect the speaker to it, via an AC coupling capacitor. It should also improve the damping factor and reduce the distortion slightly.

author: retrolefty
author_rank: Super Contributor
timestamp: December 12, 2017, 01:33:56 pm
content: Well it might have wished to be a class A stage but without any DC bias applied to the gate this thing is just a noise maker.

author: Seekonk
author_rank: Super Contributor
timestamp: December 12, 2017, 02:39:00 pm
content: Single transistor output designs were common in late 60's era car radios. However, they used a transformer to block the DC.

author: hayatepilot
author_rank: Frequent Contributor
timestamp: December 12, 2017, 03:17:35 pm
content: Such a circuit can be used as a headphone amplifier but it needs a big capacitor in series with the speaker to block DC.http://electronics-diy.com/class-a-mosfet-headphone-amplifier.phpGreetings

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2017, 09:40:52 am
content: Quote from: hayatepilot on December 12, 2017, 03:17:35 pmThat's a source follower, so it has no gain. The current sink will make the impedance much higher, on the negative cycles, than the positive, causing distortion.

author: hayatepilot
author_rank: Frequent Contributor
timestamp: December 14, 2017, 10:40:20 am
content: Those simple circuits will always have relatively high distortion because they lack any form of feedback.And you can't let any form of DC get to the speaker because you will fry the speaker.Thats why you need a capacitor on the output or positive and negative voltage rails.

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2017, 11:25:06 am
content: Quote from: hayatepilot on December 14, 2017, 10:40:20 amI agree that's not good, but it won't necessarily fry the speaker. If the current is low enough, to keep the I2R losses low enough, the speaker will remain within its power rating, it will be fine. Suppose the DC is 125mA and the speaker is 8R, 0.25W, the power dissipation, due to the DC will be 0.125W, so it won't fry it. However, the maximum RMS AC power into the speaker will only be 62.5mW, which is tiny.

author: hayatepilot
author_rank: Frequent Contributor
timestamp: December 14, 2017, 03:46:57 pm
content: Yes, that is true.We need to know what type of speaker he wants to use and what audio quality is expected.

author: Ale
author_rank: Contributor
timestamp: December 14, 2017, 04:00:44 pm
content: Well, it's quite easy to say. My actual goal is to make a simple power amplifier to test my other preamp circuits. Of course I have many others power amplifier at home, but I love the homemadeAnyway, I have:2 x woofer 8" 60W 4ohm1 x woofer x 8" 8ohm 50W2 x full range 6" (I will measure them tonight if needed) 40W 8ohmto choose from.I don't have the precise specs, I bought them for RMS Watts, but I suspect they can't reach it.

author: Audioguru
author_rank: Super Contributor
timestamp: December 14, 2017, 05:36:47 pm
content: It is silly to heat your home with a class-A amplifier that has lots of distortion.Why not make or buy a normal class-AB or modern class-D amplifier that has much higher output power, very low distortion and a very good damping factor.

author: Ale
author_rank: Contributor
timestamp: December 14, 2017, 05:53:03 pm
content: I'd like to build some by myself and class A seems the easier to do and with few components, for now I don't have p-channel mosfets, so I excluded push pull... can you suggest me something please?

author: Audioguru
author_rank: Super Contributor
timestamp: December 14, 2017, 06:12:36 pm
content: Mosfets need a gate voltage that is much higher than their source pin output so a very high supply voltage is needed for a complementary pair to be used as outputs in a class-AB amplifier. Also they have a very wide range of gate voltage that turns them on. Therefore bipolar power transistors are used in most class-AB amplifier circuits and ICs. But they aren't simple if they have good quality.Here is an audio site with many good articles and projects:http://sound.whsites.net/index2.html

author: hayatepilot
author_rank: Frequent Contributor
timestamp: December 14, 2017, 06:29:48 pm
content: If you just want to have a simple amplifier and you don't care about the design challenge then I would suggest you use some variant of a single chip amplifier.For example theTDA7297Or the LM3886 which needs a split power supply.You can get those for a couple bucks and the sound quality can be very good.

author: Ale
author_rank: Contributor
timestamp: December 15, 2017, 08:41:13 am
content: I built a TDA2050, well... I think 15 years ago. Right now I'd like to use mosfets or BJTs, for me it's interesting to study them. I've got some 5n60c, IRF740 for mosfet and BD711, 2n3773 for bjt at the moment, can I build something decent out of them? Or should I buy something else?thanks

author: hayatepilot
author_rank: Frequent Contributor
timestamp: December 15, 2017, 10:14:21 am
content: This is probably one of the simplest audio amplifiers you can build:http://diyaudioprojects.com/Solid/ZCA/ZCA.htmYou will have to test how well the circuit works with your mosfets.There are apparently some audio purists who think that less transistors equals a better sound...Greetings

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2017, 12:45:04 pm
content: Quote from: Ale on December 14, 2017, 04:00:44 pmYou won't be able to get anywhere near that amount of power with a single transistor, class A amplifier. The transistor dissipates the same amount of power, when there's no sound as it does when it's operating at full power output and the maximum efficiency of a class A amplifier is only 25%, unless you use a massive transformer and even then it's only 50%. To get 60W out, you need a power supply and transistor capable of dissipating 240W.It's possible to make a small, single transistor amplifier, capable of a half a watt or so, but it's much easier and cheaper to use an audio amplifier IC, such as the LM386.http://www.ti.com/lit/ds/symlink/lm386.pdf