Build low power Transistor Amplifier

[BasicHIFI1]

Building a Transistor Amplifier.

One of our members posted the suggestion to 'build a class A amplifier' to start with, as these are reputed to offer good audio amplification characteritics.

As the user of the TEA 2025 based chip amp I am interested in testing a transistor based amplifier to see if it can offer any large improvements over what I have. I am now more trained in listening to music and distortion, so I ready to enjoy the benefits of better sound.

I have with me a transistor radio circuit I completed in 1992 or so, it actually worked, what I lacked was a proper enclosure. I was also rather disappointed that it needed a full 9 Volts to operate whereas commercially produced radios use ony 3 volts.

I located this circuit a few days ago, and it had two transistors marked as "MPS 2907" with the familiar Motorolla sign and the numbers 213 next to that.

http://www.onsemi.com/pub_link/Collateral/MPS2907A-D.PDF

I plan to de-solder the two transistors and rig up a circuit with a 9 Volt battery to test the amplification capabilities of this transistor pair. I am aiming for 2 watts or so output.

Questions:

1. Is this circuit suitable for these transistors?

Common Emitter Amplifier and Transistor Amplifiers

( The output is connected also to the earth, "0v" how is DC current kept out of the output or is it too small to matter ?

(reading this thread as well)
2. Are these transistors a good choice or should I order better ones?

 

[JonSnell Electronic]

1/ Yes if you reverse the polarity. The circuit employs NPN and you chose PNP.

 

[JMFahey]

I plan to de-solder the two transistors and rig up a circuit with a 9 Volt battery to test the amplification capabilities of this transistor pair. I am aiming for 2 watts or so output.

One swallow doesn't make a summer and 2 transistors don not make an amp ... specially 2 puny TO92 transistors.

Questions:

1. Is this circuit suitable for these transistors?

NO.

Common Emitter Amplifier and Transistor Amplifiers

( The output is connected also to the earth, "0v" how is DC current kept out of the output or is it too small to matter ?

That's a very simplified, thus incomplete, classroom example of a single stage transistor amplifier.
As is, can be metered or scoped (fine for classroom use) but can't be used in the real world to directly drive a speaker.

2. Are these transistors a good choice or should I order better ones?

No question of better or worse, you simply don't have an audio power amplifier.

Can you drive home if all you have is a windshield wiper and an exhaust muffler?
Or maybe you need a couple extra parts to consider it a car?
Same here.

 

[BasicHIFI1]

So what do you suggest?

 

[jwilhelm]

There's lots of linestage/headphone amps throughout the forum, plus just about any amplifier input stage can be modifier for this purpose. Standard VAS transistors work fine to drive headphones, but they will need some heatsink.

Ostripper designed a very nice one in the Pitchfork Preamp thread. http://www.diyaudio.com/forums/analog-line-level/272915-pitchfork-pre-amplifier.html#post4291217

 

[BasicHIFI1]

How about this one: DIY Class-A 2SK1058 MOSFET Amplifier Project

I want to drive speakers, 1 watt per side is ok for me.

 

[Mooly]

Those are lateral FET's. Do you realise how inefficient this kind of circuit is, and how much power it uses for such a small output ? In simple terms it gets hot. The sound quality (subjectively) could be excellent though.

 

[BasicHIFI1]

There's lots of linestage/headphone amps throughout the forum, plus just about any amplifier input stage can be modifier for this purpose. Standard VAS transistors work fine to drive headphones, but they will need some heatsink.

So anything that can drive small speakers ? Or large speakers for that matter.

 

[Lazy Cat]

Show us the speakers.

 

[Mrcloc]

Hi. I'm not sure why you want a 1 W transistor amplifier. I would simply use a TDA7052. Much easier.

Here's a 1W schematic. Not a difficult thing to design:

An externally hosted image should be here but it was not working when we last tested it.

 

[BasicHIFI1]

Show us the speakers.

http://www.diyaudio.com/forums/multi-way/287392-new-enclosures-pc-speakers-3.html

 

[BasicHIFI1]

Hi. I'm not sure why you want a 1 W transistor amplifier. I would simply use a TDA7052. Much easier.

Here's a 1W schematic. Not a difficult thing to design:

An externally hosted image should be here but it was not working when we last tested it.

Like this? TDA7052 2 watt audio amplifier

Transistors have better sound? Meanwhile there is this:
DIY IRF610 MOSFET Class-A Headphone Amplifier Project

Transistors are supposed to sound better.

 

[jwilhelm]

So anything that can drive small speakers ? Or large speakers for that matter.

It looks like my post ended up in the wrong thread.

Any transistor can be used to amplify sound, they all have gain, but it's not likely worth the hassle of trying to build a circuit running one one or two. It will sound awful. This will pretty much be the same result for any 1 or 2 watt amplifier. Sure they will operate and make the noise, but they won't actually have enough power to do anything useful. Picture a lawnmower engine in a car.

To get any sort of acceptable sonic qualities, you need some form of feedback comparing the output signal to the input signal, otherwise you will just have louder noise, not true amplification.

If you would like to build a nice lower power amplifier, I would suggest a VHex. It's a nice easy to build design to experiment with, and sound great! http://www.diyaudio.com/forums/soli...-amplifier-simulated-tina-24.html#post4687317

 

[Mrcloc]

Like this? TDA7052 2 watt audio amplifier

Transistors have better sound? Meanwhile there is this:
DIY IRF610 MOSFET Class-A Headphone Amplifier Project

Transistors are supposed to sound better.

1. Yes - the TDA7052 instructable is good.
2. Better speakers have better sound.

If you want an amplifier to sound better, first you need the speakers that can differentiate between a good and a bad amplifier. Speaker distortion levels put even the worst amplifier distortion levels to shame.

That schematic I put there is class AB. Class A isn't worth it, because for 1 meager watt, the effort and expense are definitely not worth it. The schematic I have there can be modified for single-ended or "efficient" class A, but there's no way I'd recommend that. Class AB can provide perfection. The schematic I put there will give really good performance, and will run off a 9V battery, but you may need a few extra pieces of the design to ensure quality and stability. And I doubt it will even need a heatsink.

A simple and fantastic class A amplifier I have built, sounds really fantastic, and comes highly recommended:
3 - 5 Watt Class-A Audio Amplifier - RED - Page80

 

[BasicHIFI1]

A simple and fantastic class A amplifier I have built, sounds really fantastic, and comes highly recommended:
3 - 5 Watt Class-A Audio Amplifier - RED - Page80

That looks manageable, thanks. Nice touch showing the ground rail. Says 24V can it be run off 12? I know when you run a 12 V PC speaker amp off 6 volts it sounds awful, but maybe I can limit the gain of the circuit so this does not happen? Or add a resistor to the volume control.

 

[Bone]

Why not the John Linsley Hood 1969 Class a amp. Nominally 10w on 24v - it will give about 3w on 12v.

Kits are available on eBay here 2pcs 2 Two Channel JLH 1969 A CLASS A Amplifier Board ST2N3055 MKP DIY Kit | eBay and here 2pcs 2 two channel JLH 1969 A CLASS A amplifier kit ST2N3055 MKP | eBay

 

[Ian Finch]

The web (Google actually) is a source of many simple me-too designs tagged "1 Watt amplifier". Mostly they are tube type for guitar effects/headphones/preamp duty. There also many IC, single Hexfet, single Lateral Fet and many basic 1-5 BJT transistor amplifiers that will probably work acceptably well up to 5W but likely have poor fidelity.

If you wan't to compare fidelity at a tiny 1W maximum, it kind of spoils the possibilities when the output is stuck in the crossover region of an AB class amplifier. I think class A is the better option and the only good reason for adopting discrete parts rather than ICs.

Mosfet or BJT, doesn't matter but aim for better linearity than usually possible with a single device, please. To my thinking, designs for the JLH class A 1969 low power kit version or the headphone amp kit version would be a good place to start.

 

[BasicHIFI1]

See my other post of a kind of an amplifier I built way back before the Internet, the amplifier section of a transistor radio:

http://www.diyaudio.com/forums/soli...mplifier-online-simulation-2.html#post4711914

 

[BasicHIFI1]

There's the question of what kind of an enclosure to us. The Lepai- sized enclosure will move al around whenever the on button is pressed, but a bigger case is a waste of space. I plan to use an external power adapter.

Choices are:

Small case, secure in position
Small case, weigh it down with a block of wood
Small case, inside a larger, U-shaped wooden frame open at the front and back

Any other ideas? The componet should match the size of the CD player otherwise it looks rather silly.

 

[Ian Finch]

That all depends on your sense of what audio should look like. DIYs tend to leave aesthetics to individual tastes, skills and resources which vary a lot around the world.

A CD player here is a large 2 Unit rack sized device. Putting a 1 watt amplifier in a matching 2U rack case to suit it would be an hilarious mistake for a device about the size of 2 matchboxes If you said 100W amplifier, you probably would match the appearance of the cases but that is for you to consider and make personal decisions for your time, abilities, cost and availability of parts and finishes.