Need sugggestions for making class B audio amplifier

[ROCKET SCIENTIST]

Hello guyz,
How are you all? Some days ago I've bought a mini 2 inch subwooofer speaker which has the rating of 12 ohms and 8 watts printed on it.
I wanna make a nice sound system for home use. I'm new to design a class B amplifier. Though I've done some study on class A, B and AB, I'm still confused about where should I start. :roll:

Please, suggest me some design techniques. What should be the power supply? Does it have to be dual rail or single rail?
I don't wanna use batteries. Rather I can use a 12-0-12 Transformer which I already have in my hand. Should I use +-12V or the total 24V? I've bought a pair of TIP41C and TIP42C transistors for the push-pull section. Also, I have some commonly used transistors like 2N2222A, BC547 and one TL082 OP-AMP too. :lol:

Waiting for suggestions from experts. Thanks in advance. :-D

Rocket scientist.

 

[crutschow]

I find it difficult to seriously consider a 2 inch speaker as a subwoofer (some tweeters are that large). Don't expect a lot of bass. ;-)

You might consider this amp. You could use your transformer to generate +/- 12V for power.

 

[FvM]

You might consider this amp.

It seems to me that due to design faults the circuit can't provide more than a few 100 mW undistorted output power.

 

[crutschow]

It seems to me that due to design faults the circuit can't provide more than a few 100 mW undistorted output power.

It would help if you described these "design faults".

 

[betwixt]

Without dong any math, the op-amp will not be able to provide enough current to properly drive the transistor bias requirements and also the volume control, while 'sort of' overcoming the cross-over distortion, only does so at certain volume levels.

Brian.

 

[ROCKET SCIENTIST]

Oops !!! I've made a typing mistake. Actually my mini subwoofer is 3 inches of diameter. Still, as you told, I shouldn't expect a lot of bass, but obviously it would be somewhat greater than that of a 2 inch cone. It's price is 1 USD here in local store. Thought I could get started with it.

Thanks for the circuit.

- - - Updated - - -

It seems to me that due to design faults the circuit can't provide more than a few 100 mW undistorted output power.

Can you suggest a better one for me?

- - - Updated - - -

Without dong any math, the op-amp will not be able to provide enough current to properly drive the transistor bias requirements and also the volume control, while 'sort of' overcoming the cross-over distortion, only does so at certain volume levels.

Brian.

I've done some little math. To get 8Watt rms max. with sine wave across 12 ohms, from the formula, P=[(Irms)*I(rms)]*R
we get, I(rms)=816.5 mili apms. So, I(peak) = I(rms)/0.707= 1.155A and I(p-p)=2.31Amps.
Which means we need at least (2.31*12)=27.7 or 28 volts DC as a single power supply. For dual rail it will be +-14 volts DC.
Now the question is transformers are usually rated 12V-0-12V rms.
Do I have to use some kind of voltage booster to get the desired supply voltage?

Correct me if I make any mistake in the calculation.
Thanks again.

Rock-Sci.

 

[BradtheRad]

This simulation depicts roughly what you can expect from a class AB amplifier:



It will make everything easier if you use a bipolar supply.

Adjust the potentiometers for optimal waveform, particularly an absence of crossover distortion. (As see in the XY scope trace at right.)

 

[E-design]

With the 10k fixed resistor, there are some clipping on the negative cycle and oscillations near the crossover that show up in my simulation. With that reduced to 2.2k and the 0. 22uF cap made smaller (22nF); I get 4.5W rms audio at less than 0.1% THD with a 1kHz test input. It seems to work well from 20Hz (THD 0.3%) to about 10kHz. At 10kHz, distortion (THD 0.4%) starts rising up to 2.5% at 20kHz.

This is the result with a pure resistor load. Performance with a real speaker load can be very different with other unexpected surprises.

 

[BradtheRad]

Which means we need at least (2.31*12)=27.7 or 28 volts DC as a single power supply. For dual rail it will be +-14 volts DC.
Now the question is transformers are usually rated 12V-0-12V rms.
Do I have to use some kind of voltage booster to get the desired supply voltage?

A 12 VAC transformer produces sinewaves which peak at 12 * 1.414, or 17V.
After you add a rectifier(s) and smoothing capacitor you can expect 15 or 16 VDC, with little or no load.

As you add a load, that high volt level will drop... depending on the load, and your transformer's power rating.

So you may get as much as 14 V at normal volume levels, then see it drop a volt or two when a strong bass note comes through. It will drop below 12V if you demand more beyond the transformer's power rating.

It is common to increase the value of the smoothing capacitor, to sustain volt level through moments of high power demand.

 

[FvM]

This is the result with a pure resistor load. Performance with a real speaker load can be very different with other unexpected surprises.

The good thing is that the speaker impedance can't fall below the coil DC resistance which makes most of the nominal speaker impedance. As long as you don't use LC crossovers, there shouldn't be big problems.

 

[ROCKET SCIENTIST]

Which software did you use to make the simulation? I used multisim and the result was much worse than yours.

- - - Updated - - -

With the 10k fixed resistor, there are some clipping on the negative cycle and oscillations near the crossover that show up in my simulation. With that reduced to 2.2k and the 0. 22uF cap made smaller (22nF); I get 4.5W rms audio at less than 0.1% THD with a 1kHz test input. It seems to work well from 20Hz (THD 0.3%) to about 10kHz. At 10kHz, distortion (THD 0.4%) starts rising up to 2.5% at 20kHz.

This is the result with a pure resistor load. Performance with a real speaker load can be very different with other unexpected surprises.

what kind of difference?? As far as I know inductive load takes more current from the supply than the resistive ones. Can this somehow drop the supply voltage?

 

[E-design]

Which software did you use to make the simulation? I used multisim and the result was much worse than yours.

Can you post your Multisim file?

By using some cheap darlington's in a different drive setup(Class_B_mod), you can get more voltage swing and more power(6.5W).

 

[betwixt]

You should note that in the original schematic there is no R4 component. When set at one end of its track, there is effectively no feedback from the output point back to the op-amp input so it can't compensate for the dead band in the output transistor biasing. I do not have the simulator here but try dropping R4 to a realisitic output impedance of the first op-amp stage, say 20 Ohms and see what happens at different volume control settings.


Brian.