Electronics-Lab.com Community

# Help needed urgently: Audio Amplifier Project.

## Recommended Posts

Hi there, greetings from Denmark..

I have a school project where I have been given a specific assignment, to construct and analyse a PCB circuit that works as a Class AB Audio Amplifier, a project which is also an exams project. From this we're supposed to learn something about cross-over distorstion, input & out impedance, complementary pairs & bootstrapping, though we haven't really been given any theory about it, which is really bad.
To make it easy for those of us who have this assignment have been given a kit (K48) from kitsrus.com to construct & analyse. Not the brightest idea that we were given this assignment just before the holidays began, which means that I only had been able to construct the circuit and do some of the measurements/probing at school. So I have had to do the measurements through simulation because I don't have access to any of the auxiliary devices, DC amplifier and measurement tools needed to do this. I have used Multisim Power Pro 10.0 to do the simulations, most of the measurements are very close to my own measurements I have done with the board, the problem is that all current measurements are very wrong according to my calculations. The worst part is that Im getting very close to the deadline.

Below I've added both the multisim circuit diagram and the circuit diagram which I've used to construct the PCB Circuit, which also has some of my calculations on it.

- Multisim diagram
[img width=680 height=543] - K48 diagram

My theory is that the DC current is derived from the voltage and viceversa in multisim, so that Im not really able to simulate
that Im using one pair of 9V batteries in parallel. But this only a theory to why the current measurements is off. Because when I probe the current from the DC output i get around 13,2mA.
Another theory is that my measurements + calculations are wrong.

/Incarnate

##### Share on other sites

9V batteries in parallel make only 9V but with twice as much current available.
Your circuit uses two 9V batteries in series to make 18V.
The peak current into the 8 ohm speaker is 1A but little 9V batteries cannot supply anywhere near 1A.

The currents in your circuit add up to 13.2mA as shown with the battery current without a signal. The idle current in the output transistors is too low at only 1.8mA to reduce crossover distortion. Turn the 200 ohm pot to increase the idle current to 10mA to 20mA for very low crossover distortion.

The writing on your drawings is fuzzy and hard to see because you saved them as a fuzzy JPG file type and posted them over at Photo Bucket instead of saving them as a very clear GIF or PNG file type and posting them here.

##### Share on other sites

9V batteries in parallel make only 9V but with twice as much current available.
Your circuit uses two 9V batteries in series to make 18V.
The peak current into the 8 ohm speaker is 1A but little 9V batteries cannot supply anywhere near 1A.

The currents in your circuit add up to 13.2mA as shown with the battery current without a signal. The idle current in the output transistors is too low at only 1.8mA to reduce crossover distortion. Turn the 200 ohm pot to increase the idle current to 10mA to 20mA for very low crossover distortion.

The writing on your drawings is fuzzy and hard to see because you saved them as a fuzzy JPG file type and posted them over at Photo Bucket instead of saving them as a very clear GIF or PNG file type and posting them here.

You're absolutely right, they're in series, my fault.
The pictures are a bit fuzzy, but if you zoom in you should be able to see it more clearly, but Im not sure how much of it you can understand whats written on the K48 Diagram since Im since most of it is written with danish terminology.

Im a bit unclear what you mean by idle current? I have reduced the cross-over distortion on the PCB Circuit via an offset adjustment. Im not sure how I can get it to 20mA idle current by lowering / increasing the 200 ohm pot, but if increase it to around 83% resistance 200 ohms I get around 19,8mA.

Im not entirely sure on how I find the impedance for the input and for the output, I mean which components do I have to use in my calculations in order to find in and out impedance?

I've tried reading something about bootstrapping, but I can't seem to quite understand it fully, could you explain it simple how it works?

Thanks.. =D
##### Share on other sites

The idle current is the current drawn when no sound is produced.

For low distortion the circuit needs to be operating in class A-B which occurs when both Q5 and Q6 are conducting slightly when there's no signal. Adjusting RV2 alters the base voltage of Q5 and Q6 and thus the idle current.

The voltage on the speaker goes below 0V so R4 is used to help pull down the base voltage of Q4 and Q6; this action is known as bootstrapping.

The output impedance will be a little below 1.2R because there is little negative feedback.

What is the impedance of the speaker? with C3 at only 100

##### Share on other sites

The idle current is the current drawn when no sound is produced.

For low distortion the circuit needs to be operating in class A-B which occurs when both Q5 and Q6 are conducting slightly when there's no signal. Adjusting RV2 alters the base voltage of Q5 and Q6 and thus the idle current.

The voltage on the speaker goes below 0V so R4 is used to help pull down the base voltage of Q4 and Q6; this action is known as bootstrapping.

The output impedance will be a little below 1.2R because there is little negative feedback.

What is the impedance of the speaker? with C3 at only 100
##### Share on other sites

I think you should look in Google for Class-A, Class-B  and Class-AB amplifiers since your teacher doesn't teach you anything.
Also look for "RC calculation for frequency response".
Also look for "bootstrapping".

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.