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Old 26th July 2004, 06:58 AM   #1
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Default Any good, cheap ideas?

I know this is probably pointless ,but here goes. I have a senior design project in the coming school year. Was going to try for a tube amp but they don't teach you about tubes these days. Any who, i am thinking sold state amp and wanna know what would be challanging but doable....(I am taking 16 hours). I have background as an Audio guy in studio and live and three years of EET behind me. But don't have enough in my head to come up with ideas on my own. i am not looking for schematics here, just wanting ideas on something that will be worthwhile, do-able and somewhat challanging. I am sure somebody can give me some starting points that will get me cracking open some high dollar text books i have laying around here. thanx guys.

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Old 26th July 2004, 11:34 AM   #2
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Your question is difficult to answer from the information you give us. It would be helpful if you define cheap with a dollar figure and some idea of what would interest YOU.
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Old 26th July 2004, 04:29 PM   #3
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Hi sauuuuuce;

Not pointless at all. I think this is a great post.

First tube thoughts If you really want to do tubes there is a lot of information on the net that would make it pretty straight forward to do that. The cost is going to be fairly high but you will probably be the only one doing it which is kind of fun. The cost is higher because you have to use some expensive "iron" (transformers and/or chokes) or use lots of tubes for an output transformerless design. Also Each tube costs many times more than each transistor. However a decent tube amp can be easily made from very few tubes. Another way to keep the costs down is to consider a pre amp or other line level project where you won't need an output transformer.

Since you are into studio you might consider a tube microphone pre-amp. Many people feel that feeding a solid state mixing board with a tube pre-amp improves the results considerably. A well designed tube pre-amp will tend to compress rather than clip hard and as you know live mic'ing usually involves some pretty substatial transients. You could even make a small hybrid mixing board (say four to six channels) using tube inputs and BJT for mixing and EQ stages. It would be an energetic project but very rewarding. If you start with the mic pre-amp part and don't finish the rest you still have a useful project.

If you want to try tubes go over to the tube forum. They have a sticky thread of tutorial information. Also check out Steve Bench's Tube Site He has a nice section on load lines that is very helpful as well as several interesting and unique designs. His stuff is often written at a more challenging level but it is another useful resource. Also look at Svetlana Tube Zone. They make new tubes and have a lot of data sheets on their products as well as some very interesting and helpful technical papers.

Some Solid State Thoughts I am working on a SS amp design. At the moment I am using a simple CE input stage with voltage divider bias driving a complementary pair output loaded with a costant current source rather than a load resistor. This is single ended class A and currently has no interstage feedback. Once I get it built I can see whether modifications including feedback need to be added.

I am using the CCS load to minimize distortion and increase output before clipping. In the simulations that I ran I got about 10% more output voltage using CCS v.s. load resistor. I am not confident that the distortion results of my simulations were really valid so I am not sure how it will affect that goal.

I may also try adding CCS loading to the voltage amplification stage as well but I am trying to make the initial design as simple as possible. It would not be expensive to add them just more design work.

Low or zero global negative feedback single ended designs are a growing area and are a very challenging area of study. These types of design have room for lots of creativity for keeping distortion under control in each stage and using natural interstage interactions in your favor. Your goal is to keep 3rd order distortion significantly below 2nd order and try to virtually eliminate all higher order distortion product before adding any global or inter-stage feedback.

You could try making a design which allows you to switch between zero GNF and moderate GNF while keeping the gain the same for the purpose of comparing objective measurements with subjective listening results. I would suggest that your objective measurements include not only traditional distortion figures (not THD but dB down figures for each order of distortion) and comparative scope traces as discussed Here .

I guess the main thing is to find something more that building the same old differential input push pull output with GNF design that has been the standard for so long. Try to find new, or at least less common, ways of improving the sound and performance of an audio system.

The area of psycho-acoustics is very interesting as we try to find a way to truely represent what we hear by subjective measurements and might be an interesting area for you to study.

Well I hope some of these ideas will help spur your creativity.
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Old 26th July 2004, 09:25 PM   #4
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Default thanks and here

shaffer: thanx for the ideas....this definitley gives me some stuff to start looking into.

kWski: I am thinking well unnder 1 grand. I'll have some loan reimbursement coming to me and that's about all i can dedicate to the project. As far as the amp....I am not concerned with PA level power for this so maybe like 10-100W. What i plan on spending most of my time on is minimizing distortion and things as such. So; low power, low distortion stuff.
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Old 27th July 2004, 03:39 AM   #5
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Excellent, wow your cheap is alot higher than my cheap. Actually $1000 should be more than enough cash for your project. I am a solid state single ended true class A junkie. As we all know these amplifiers dissapate lots of power all of the time. How about a single ended class A solid state amplifier that idles at a lower bias when there is no signal as a design project?
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Old 27th July 2004, 11:45 PM   #6
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Default Cheap???

A grand? My cheap is more like $50. It is possible to build a good pwr amp for less than $50, but you have to rip some parts out of old junk. (my specialty) If you can find pwr transformer, filter caps, and a good heat sink, everything else costs reletively very little. well for solid state.
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Old 28th July 2004, 02:49 AM   #7
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Ah the wonder of student loans.....A grant and a merit fee remission paid the tuition so the a bank is supplying the cash for the design project and the government is paying the interest on it........Of course that is not the case with the 20 grand or so I racked up in unsubsidized loans before I became an independent student(apparently you are not independent until the school year you turn 25, maybe i should have waited a few more years!).........

Thanx for the ideas. This will get me looking around....anything else you can suggest just post away.
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Old 28th July 2004, 07:02 AM   #8
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I suggest an smps powererd class ab amp of 200w or so in an enclosure of 5x2x2. This is what I'm on anyway.
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Old 28th July 2004, 04:54 PM   #9
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Here's a perspective from someone who still very much remembers his senior design project. Having had some industry experience, I helped my group choose a project I knew we'd complete in the time given but our advisor didn't think we had enough "challenge" in our project and requird us to add a bunch of features. Ended up our project (and many other peoples) did not get fully completed successfully.

I would suggest that you choose a design that is simple in principle but fully work it out on paper like a real world design would have to be done. When you build it compare to your calculations. A design similar to a Zen + bride of zen would be straight forward but still have enough components and tradeoffs to parameterize that it won't be boring, or I would argue, too easy either. After you have that squared away and you choose a particular bias current, calculate the heat. Do the electrical equivelent modeling for your heatsink and transistors and determine how hot you expect the heatsink to become and if it's adequate. Then build, test and determine if you were successful in designing the amp. 10-watts class-A 2stage design fully analyzed, for a senior project is a healthy task.
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Old 2nd August 2004, 05:38 AM   #10
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Default Idea Number 1

Here is my first go 'round with this project. Kinda decided to go through some text books and came up with this so far. Anyone see any earthshaking problems here? Fire Away.
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