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Simple class A amplifier how it works

billybob

Active Member
I have built a class A mono amplifier hopefully for a sub. With four C5200 transistors in parallel here is a picture of my setup and schematic...
The output seems a little distorted at high volume, and it really doesn’t get very loud. How can I fix this? I am still trying to learn how audio amps work and the inductor and 4 ohm resistor is throwing me off. Right now I am not impressed with this amp.
 

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crutschow

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Right now I am not impressed with this amp.
Not surprising.

A Class A amp is very inefficient and is not really suitable for the high power a sub requires
For high power you need a class AB or D amp.

Also you show DC going through the speaker, and that's a no-no.
 

billybob

Active Member
Not surprising.

A Class A amp is very inefficient and is not really suitable for the high power a sub requires
For high power you need a class AB or D amp.

Also you show DC going through the speaker, and that's a no-no.
I don’t like it either, the cone is always pushed out. I knew class A was bad heat efficiency wise, but I thought that sound quality was fairly decent and better than a class D. Learning the hard way most YouTube videos are fake. Is there an equivalent class AB circuit I could look into for a mono sub? Maybe using the NpNs as a darlington setup or something?
 
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unclejed613

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Most Helpful Member
you need to use a 16 ohm resistor as a secondary load, and couple to the speaker through a large (20,000uf or larger) capacitor to get the DC off the voice coil. unless you have a speaker that has ferrofluid cooling, the voice coil will quickly overheat and burn out unless you isolate the DC from the voice coil. you should also have some kind of bias adjustment for the transistor bases, so you can set the DC level at the output capacitor to 1/2 the rail voltage (6V in the case of a 12V rail). also, the output power here is dependent on the rail voltage and getting the output to swing as close to "rail-to-rail" as possible. the biggest problem you are going to have with the transistors working against a resistor is a huge amount of even harmonic distortion. a better collector load to balance against would be a constant current source also running in class A. the amount of standing current will be determined by the required output power. i'm not a fan of class A amps, but if you want more power and less distortion, and to not fry your speakers, you will need to get a bit more complexity here... you probably should also have a gain and buffer stage before the output transistors.
 

audioguru

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Your speaker has no impedance. Most car speakers are 4 ohms and the amplifier is powered with the 13.8V of a charging car battery. A class-AB push-pull audio amplifier will produce a maximum output voltage of 11V peak-to-peak which is 11V/2.828= 3.89V RMS. Then the maximum undistorted power in the speaker is (3.89V squared)/4 ohms= 3.8W which is fairly low power. The power in an 8 ohm speaker will be only 2W.

A bridged amplifier has two amplifiers for each speaker, one amplifier for each of the the two speaker wires to almost double the output peak-to-peak voltage producing a maximum undistorted output power of 14W into 4 ohms. Use a bridged amplifier IC, some are stereo.
 

Nigel Goodwin

Super Moderator
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I have built a class A mono amplifier hopefully for a sub. With four C5200 transistors in parallel here is a picture of my setup and schematic...
The output seems a little distorted at high volume, and it really doesn’t get very loud. How can I fix this? I am still trying to learn how audio amps work and the inductor and 4 ohm resistor is throwing me off. Right now I am not impressed with this amp.
Well it's not really an 'amp' at all, and is completely useless.

There are countless audio amplifier designs out there, and NONE of them look anything like that - for good reason.

As AG said - basically with a car battery supply and a 4 ohm speaker you can only get 4W single ended, or 16W bridged - in the case of a class A amplifier (a proper one, not the random collection of bits above) you're much more likely to get considerably less than that, plus loads of heat.
 

tomizett

Active Member
What Nigel says is true, but given that the OP is a beginner, it provides an opportunity to point out some really obvious problems and develop the design into something that looks more more like a conventional amplifier. Potentially a good learning exercise.
 

audioguru

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Most Helpful Member
Using random parts and trial-and-error is the worst way to learn electronics.
To copy the design of an audio amplifier then you simply look in Google for thousands of them, only a few of them do not work.
To design your own audio amplifier then you need to learn the details of electronic amplifier circuits.
And Never Ever put DC in a speaker.
 

billybob

Active Member
Using random parts and trial-and-error is the worst way to learn electronics.
To copy the design of an audio amplifier then you simply look in Google for thousands of them, only a few of them do not work.
To design your own audio amplifier then you need to learn the details of electronic amplifier circuits.
And Never Ever put DC in a speaker.
Can anyone recommend a book on audio amp circuitry and the concept of how it works. I feel kinda stupid jumping into this design head first so quickly and probably should do a lot more research on the “ins and outs“ of audio amplifier designs. Books help me learn faster than a short article online, and most of the time subtly increase in complexity, so any recommendations to get me started would be great.
 

crutschow

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I thought that sound quality was fairly decent and better than a class D
For a sub, the small distortion of a well-designed Class D amp is more than sufficient.
It takes significant distortion to be audible in a sub.
 

audioguru

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I modified your circuit to make it produce as much output power as is possible with only some distortion as a class-A circuit with a 12V supply and driving a 4 ohm speaker.
Its input resistance is very low. Its distortion is fairly high.
Its gain is very low.
Its output power is only 0.69W.
Its efficiency is only 5.3% and it heats with over 12.9W.

Books? I used them 53 years ago.
 

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billybob

Active Member
Here is a fairly good amplifier:
You’re awesome, I’m gonna take trip to the library to actually understand how these amps work.
 

audioguru

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First ya godda learn about transistors, diodes, resistors, capacitors and speakers.
 

MacIntoshCZ

Active Member
You’re awesome, I’m gonna take trip to the library to actually understand how these amps work.
art of the electronics - TAOE, pdf can be find via google
 

billybob

Active Member
First ya godda learn about transistors, diodes, resistors, capacitors and speakers.
I know a good amount of electronic components and how they work, it’s just their effect on audio amplifiers and how sound is controlled by them.
I guess wave forms also trip me up, I need to get an oscilloscope.
 

audioguru

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My class-AB audio amplifier uses two complimentary (NPN and PNP) emitter-follower (no voltage gain) output power transistors Q2 and Q3. The NPN pushes the output voltage up and the PNP pulls it down (called push-pull). They each have low-value emitter resistor R3 and R6 to help match their characteristics.

The two diodes between the bases of the output transistors are mounted on or near the heatsink of the output transistors and match the base-emitter voltages of the transistors allowing a small class-A current to flow to prevent crossover distortion and match heat changes.

The capacitor C2 from the output to the R7 and R8 resistors that bias the bases of the output transistors produces "bootstrapping" so that the positive-going output swing is as high as is possible and increases the open-loop gain.

The input transistor Q4 produces a fairly high open-loop voltage gain but the AC and DC negative feedback in resistor R1 from the output to its base reduces the voltage gain and reduces any distortion. The ratio of R1 and R2 (about 12) roughly produces the voltage gain of the amplifier but is reduced to 8.9 by the input resistance of the input transistor.

Most audio amplifiers have a differential pair of input transistors (one for the input and the other for the negative feedback).

The sound is not controlled so that it is high fidelity with a flat audio frequency response with very low distortion. A tone controls circuit can be added to control bass boost for a tiny speaker or cut highs for a cheap speaker that shrieks.

I simulated my amplifier design but I did not build and use an oscilloscope it to see if it oscillates. Most audio amplifiers and all opamps use a small value capacitor to cut high frequencies when phase shift causes oscillation. This "frequency compensation" capacitor would be mounted at the base and collector of Q4.
 

schmitt trigger

Well-Known Member
I am assuming that you want to learn by designing and building your own, rather than simply copying an application note.

The absolutely best book on audio amplifiers is by British engineer and author Douglas Self: “Audio Power Amplifier Design Handbook”.
Expensive and better suited to more experienced people.

A better book for a novice would be “How to Design and Build Audio Amplifiers” by Mannie Horowitz
 

unclejed613

Well-Known Member
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