I have to create an audio amplifier circuit in LTspice using OP - amps. it should take audio line level signals, and amplify them to drive a pair of headphones with 100dB/mW sensitivity and 32 ohms impedance. It should also have a bass and treble adjustment filter, this i have managed to create using RC filters to create a band of 20hz-20khz freqency. However im not sure how to calculate what gain is needed to drive the headphones, and the importance of the headphones impedance in these calculations. where should i start?

 

Welcome to AAC!
Is this a school/college assignment?

 

Yes it's a school assignment.

 

We don't do your work for you, we guide you. In order to facilitate that process, you need to demonstrate the ability to communicate what you have learned so far and what decisions you have arrived at. You have mentioned some rudimentary things but appear to not be exactly comfortable with how to interpret them.

ETA: Which definition of "audio line level" are you using? My guess would be 1 V(rms) = 0 dBV, the consumer standard.

 

We don't do your work for you, we guide you. In order to facilitate that process, you need to demonstrate the ability to communicate what you have learned so far and what decisions you have arrived at. You have mentioned some rudimentary things but appear to not be exactly comfortable with how to interpret them.

So far I have gathered that if I supply my headphones with 1mW of power, they will Produce 100Db SPL of sound. But in my circuit to produce that amount of power it would require me to have less voltage than what line level is typically at. This doesn't seem right as I'd assumed that I would have to amplify My voltage by an amount to drive the headphones.

 

We don't do your work for you, we guide you. In order to facilitate that process, you need to demonstrate the ability to communicate what you have learned so far and what decisions you have arrived at. You have mentioned some rudimentary things but appear to not be exactly comfortable with how to interpret them.

ETA: Which definition of "audio line level" are you using? My guess would be 1 V(rms) = 0 dBV, the consumer standard.

Yes thus is what I am using.

 

So far I have gathered that if I supply my headphones with 1mW of power, they will Produce 100Db SPL of sound. But in my circuit to produce that amount of power it would require me to have less voltage than what line level is typically at. This doesn't seem right as I'd assumed that I would have to amplify My voltage by an amount to drive the headphones.

If that is truly the case, then you would be designing an attenuator rather than an amplifier. Consider the output impedance of 32 Ω, does that have an effect the output power level. The reason I ask is that most headphones have a higher impedance than that. I would dig a bit deeper on that, especially since the 100 dB/mW might be referring to electrical power rather than SPL. Usually when you compare two things to express the ratio in dB, they should have the same units.

 

Most headphones today are Bluetooth wireless so nobody makes a headphones amplifier anymore.
Most wired headphones were 32 ohms for each ear. An IC (LM4910) designed for driving 32 ohms headphone produces a max output of 35mW into 32 ohms (1.06V peak/32 ohms= 33mA peak) at a loud level with low distortion that only a few opamps can produce.
Sennheiser HD300 headphones are 18 ohms and produce 118dB with a 1V signal (55mW). The peak current is 55mA.

www.headwize.com was a great site for headphones amplifier designs but it closed 22 years ago.

Look in Google for the Cmoy headphones amplifier circuit and project.
Also look for a Baxandall tone controls circuit.

 

The TS isn't trying to make some top-of-the-line amplifier. It's a course exercise. They are trying to design an amplifier that works well enough to demonstrate that their design works. They need to use circuits and techniques that are commensurate with the course coverage and that show that THEY understand the design process to get to the circuit that THEY designed. Looking up some amplifier circuit online and copying it almost completely defeats the purpose.

 

It seems that teachers do not teach anymore. Students copy truths or errors from the internet today.
The teacher of the TS did not even show him where to start.

The Headwize forum talked about all the details of headphones amplifier circuits but it closed 22 years ago because it became obsolete and people did not need it anymore.

I did not show any of the old circuits and I warned that most opamps cannot provide enough current.

 

It seems that teachers do not teach anymore. Students copy truths or errors from the internet today.
The teacher of the TS did not even show him where to start.

That's almost certainly not the case, but it is certainly easy to walk away with that impression based on the limited view we have here.

Sadly, you can walk through example after example, step-by-step, in class and then give the class a homework problem that is one of the exact problems, verbatim, that you worked in class and then have a large majority of the students have no idea how to start and will proclaim loudly that they were never shown anything like it. For six years I did a small-scale controlled experiment on exactly this. I walked through one of two classic problems commonly used to introduce students to programmer-defined functions in C. I walked through the math behind it (all three lines of algebra), showed them how to write the resulting expression in C, and how to write a function around that expression to take the argument, plug it into the expression, and return the result. The body of the function was a single return statement with a very simple expression in it. After walking through the entire thing in class, including coding it up and demonstrating it, I would tell them that they could expect to see that on an in-class quiz soon. The next class I would ask them to write that function -- and right about 10% would get it correct. I would then go through it from scratch in class and tell them outright that it would be one of the problems on the first exam. The fraction that got it right was usually about 25%. I would repeat the process, telling them that this would be on the second exam. Now we were in the 33% range. I would then tell them that this would be on the final exam and be worth 10% of the total. Before grading the final I would literally flip a coin to place my bet whether the result was above to below 50%, because it was a real crapshoot which it would be -- and that was over a period of twelve semesters. In the case of one of those two functions, I would tell them the one way that seems obvious to do it but that absolutely will not work, and I would show them why, and yet a good fraction of the wrong answers would blindly try to do it that way. On top of it all, in four of those semesters, at least one person complained to the department chair that I was expecting them to know how to do advanced math on exams without any heads up whatsoever.

So what's behind all this?

I have some ideas -- and not just suppositions, but ideas that are rooted in talking to a number of these students, in addition to other instructors. I think a big part -- and increasingly bigger part, too -- is that students are not coming into courses with the foundational knowledge and skills that they should have. The result is that they struggle so mightily with what should be old-hat by then that they aren't in a position to focus on and try to comprehend the new concepts being presented. So those just don't stick and, to the student, it really is just as if they had never been presented at all.

This is one of the several reasons why we insist of seeing a student's best effort instead of just giving solutions, even with detailed explanations. The student has almost certainly seen solutions to similar problems, in the text and in class and often in a slew of online resources as well. But something isn't clicking and so seeing yet one more problem solved for them is unlikely to change that. But by forcing them to meet us at the point they are at, we can better see where there problems actually like and help them struggle through it so that they can move on to the next stumbling block. Unless we help them identify and resolve those issues, we are likely just setting them up for failure.