EE student needs help to graduate!!

[alizawi]

I'm bulding a preamp-powe amp system for my senior project, and I have a few questions:

I'm using the LM3867 for the power amp design, basically following the steps on page 18 of the data sheet to give 40 watts output, but I'm having an issue of finding the right power supply at such high current. I would like to avoid building one, so I have been searching for chips or tryin to find AC-DC supplys, but its hard to find anything that gives +35V, -35V....any suggestions?

another issue with this chip is that im not using a regulart PCB, i'm hand wiring the whole thing on a board (using sockets, etc...), so I have to drill holes so the LM3876 can fit then I will have to solder it....is this method going to cause me any problems?

and lastly i'm using LM1458 for my premap design, is this chip reliable, or would you recommend the LM386

thanx

[anatech]

Oh my!

You may need to build your supply unless you are made of money. It looks like you want the easy way out. No good will come of this.

You need a 50VCT transformer to get your +-35VDC supply - HINT!

The 1458 is not a good choice for a dual audio op amp. But it's your project. At least it's tame. You need a +- 15 V supply for this (or there abouts).

Sockets are not a great idea for output chips expected to deliver high current peaks. Take my word for this.

Cruise the chip amp forums. You will see your project in there somewhere. All kinds of hints.

If you are an EE student, you will be expected to design these or similar systems. I would fully expect you to be able to research this on your own. So hit the "search" button at the top right of the page.

-Chris

[DcibeL]

If you are a senoir engineering student, I would think that designing a linear power supply would be of no trouble at all. Could be done in an afternoon to pick all the parts. In my course (electronics technologist) we are not encouraged to pick audio amplifiers as our thesis as it is much too simple to take a design off the internet and build it. We are encouraged to design projects ourselves (usually incorporating a microcontroller), and as far as audio amplifiers go, why design something when you don't have to (ie already many solid and proven designs).

If you really wanted to just "buy" a power supply, the best solution would probably be to use two 24V switching supplies, one connected backwards to provide -24V. I haven't looked at prices, but I would think that perhaps a linear supply would still be cheaper, even if you have to design it yourself.

There is no problem installing the LM3867 on a perfboard, drilling holes where nessicary. If the amp board are mounted in a chassis, the LM3867 can be screwed to the heatsink, and the perfboard mounted to the bottom of the chassis, and neither of the two actually have to touch. I would use heat shrink where ever possible to reduce the possibility of shorting pins together.

Good luck.

[Narcisse91]

Sounds to me like you're trying to use an op-amp for the preamp, an op-amp for the power amp, and purchase a supply to power the whole thing. You're not really designing, you're implementing.

Maybe your professor is ok with that...


If I were you, I would look into finding a unique design goal. Gotta do the supply. Design a nice quality switching supply, and there is your project. Then just use it to power the amp.

But making an op-amp function is not a challenge. At least, it shouldn't be, to a senior year EE student.

[EC8010]

I'm afraid I have to echo the previous sentiments. As a lecturer, we had a thing called "the Honours challenge" that we looked for in undergraduate projects. Simply assembling a power op-amp to the manufacturer's application note and adding an op-amp to drive it does not count as design and certainly doesn't meet the Honours challenge. You need to do more to justify a degree...

[ctviggen]

I don't remember being able to design anything when I was in school. We learned a bunch of math. Ask me what the inverse fourier transform of something was, and I could tell you. Onthe other hand, ask me to design an FIR with certain criteria or a first order filter with a -3dB point of X, and I'd be clueless. I didn't know that resistors came with wattage ratings, for instance. There was certainly no way that I could create an amplifier (although given a schematic of one, I could analyze what it does).

[alizawi]

Thank you all for replying... a few comments/questions about what was said

anatech:
why is the LM1458 not a good choice? Getting 15 V shouldn't be a problem here, i may just use the MAX666.

DcibeL:
An audio amp project was the very first suggestion made by my professor!! the bulk of my design is actually in the preamp which includes all the tone control. Nevertheless, implemanation can be just as challenging as design, since you have to deal with issues that wouldn't appear on paper. I'm not sure I understood what you mentioned about mounting in a chasis

Narcisse91:
My prof is ok with that...got any suggestions to help?

EC8010:
WE need more people like you to maintain the integrity of engineers

ctviggen:
I feel exactly the same way...Everthing I am doing is self taught/researched. The 75% of students in my sneior project class using microcontrollers have no idea how to program them so they will probably find something on the internet and modify it

[HFGuy]

Hello, Iam currently a graduate student (CMOS analog designer).

I will give the same advice to you that i give my undergrad students in the labs i TA ( is stick to 4th year classes, i dont want to be a babysitter). Dont think of your thesis as a requirement to pass, think of it as an oppertunity to show your skills. When going to a job interview your thesis is something you can show, proof you know what your doing. Very few companies will care about your GPA, they want proof you can think.

Personally i am not a big fan of uP based thesis mostly because i dont think you get alot out it. Every EE student should have atleast 1 uP classes (who doesnt love the 68HC11). You might want to remember to include more design, implementation is good to be able to show a final project but for the most part thats a tech's job.

In case your interested, for my thesis i designed a 2.4GHz frequency synthesizor in 0.18um CMOS. My good buddy designed a RF navigated robot (won top thesis in canada). A great thesis will seperate you from the hundreds (or thousands) of other graduating egineers. Anyways you really shouldnt be using opamps for ampification if this is your thesis. Design a discrete amp.

[alizawi]

Thanks HFGuy, what you say makes perfect sense.....however, i believe the project should somewhat be an application of what I learned in my courses, although this is requiring more research than I had expected....uP projects as u said are worthless, and don't apply much of what i studied......and my engineering knowldege and strengths for the most part lie in digital control systems and Comm. systems, which cant really be implemented into a thesis, so whats left but good old opamps and transistors. I just like to be realistic.

[Narcisse91]

You'll find in your first couple of years of designing, what you learned in school barely gets you started. Based on a couple comments that have been made, I think I left school with more design experience than some people here (but probably less math and physics), and every time I work on something fresh, I always come across things I don't know.

So this is your chance to not only apply what you learned in class, but to prove to the world that you can take what you learned and use that information to design something you've never seen before.

My suggestion - if you don't want to do the whole circuit from scratch, pick a portion. Power supply, preamp, tone controls, or power amp, and design it from scratch.

Power supply - linear supplies are not tricky, but you can add a ton of features to them. Inrush current limiting, softstart, AC line filtering, line regulation, all that fun stuff. Switching supplies are a lot more complicated, and tough to build properly. That's no reason not to try, though.

Preamp - research class A or class A/B amps. Get a good understanding of how they work. Understand every aspect, from transistor configuration and biasing to gain control. Then build one with a gain of 1 to drive your power amp. Build it from scratch with discrete components. Your own design, with your own component selection.

Tone control - you mentioned it would part of the preamp, you could always break it out. There are a lot of ways to do this. Most simply, use a tone control chip. Avoid that method. design a circuit to break out the frequency you want control of, design a circuit to adjust each one's gain seperately, and design a circuit to recombine them.

Power amp - again, research class A or class A/B amps. Figure out all the stuff I mentioned in "preamp". Design and build from scratch.

My suggestion would be to pick at least one of the sections above and design and build it from scratch. You'll have to look at other people's work in the learning process, but try not to include anyone's circuit. If you do, make sure you understand what every component is doing and why.

I'm sure everyone who has posted would be willing to help when you have any questions, we're just trying to make sure you get the most out of this project.