I will be sure to bear this stuff in mind     

OK, I will try out the power stage and see what happens.

It does seem that a lot of the larger companies are as much if not more interested in cost and ease of manufacture than longevity. I guess if things lasted forever then they would never sell any new stock, totally wrong but you can kinda see why it happens.

Yes that is roughly the course I am studying - I have done some form of DT for 6 years but the A level course goes more into the manufacturing processes and material theory etc than just simply "build a CD rack". I intend to study \mechanical/electronic engineering at university, so an electronics based project would seem to make sense.

The metal surface of the bench is just a 2mm sheet steel. It is completely removable and is there as you suggested to turn a regular work bench into a welding table. I would remove the top before any electronics, it is just 3/4 inch ply underneath. Out of interest, would a metal bench be likely to damage components?

Cheers, Callum

I'm on it!

I figured when I checked it over that using the chassis as a heatsink wasn't the greatest idea and it is, I believe, the power amp section that has gone.

I will clean it up and continue with the research and then when my last exam is over with (at the end of the week) I will start with some more serious design and maybe ordering some parts.

GIYF - I like it,  I get your point

Once again, thanks for your help and I will keep you informed this week as to any decisions and stuff:)
Callum

Ok, like I say, I believe the amp is the Carlsbro is the colt lead 45, here are some pics of it.

Does anyone know of where I can get a decent schematic of a power amp with the chip?

Thanks again, all help is greatly appreciated:)
Callum

Tried LTSpice, thats a nice little program

I will put some pics up here tomorrow, some of the components have been robbed already, but mostly intact. Don't know if It works but I believe there is a little reverb unit in there too.

I will do some research over the weekend into parts and cost and stuff which will influence which schematics I go for (as well as complexity :L), and I will start putting some stuff down on paper as to the structural and aesthetic design. As JM says, the metalwork facilities at school, and at home (in the process of making a 125cc Kart with a mate) should make the chassis not too much of a problem.

I have a fairly firm Idea in my head of what I want it to look like - Similar to Marshalls in terms of size and stuff but I think I fancy a white vinyl/Tolex covering as this should help the amp stand out from the crowd a bit! (even if it doesn't sound great) Get some credit in the course for branding/packaging so I think my brand will be "White Noise" - you heard it here first when it is a multinational audio company! haha, well gotta have ambition

Had a look on ESP at some of the power supplies there and I reckon for the 20 Watt, an 18 - 0 - 18 transformer with the supply design provided should give the approx. 25V required.

Thanks again, I will keep you posted.
Callum

Thanks, I downloaded LTSpice and it looks pretty good, will have a play around to learn how it works but doesn't look too complex.

The Carlsbro is the colt 45Watt lead (I think) have't found model number but from what I can gather this is it.

So that preamp is a good bet then Can they both be run off the same power transformer? As one is 15v and the other 20v.

Thanks again guys
Callum

Ok, thanks,
So you reckon OpAmps are the way to go for the preamp?
And that looks like it will be helpful, I will download it and try it out!
Thanks again

just to add something to that last comment, I have seen this: http://sound.westhost.com/project27b.htm

The 100Watt amp here is probably more than I want but the Preamp schematic is what I was interested in. This uses op amps, which I am not set against I just fancied using discrete components as I said before. Is this a good example of a preamp? Can a similar thing be done as well using discrete components, or would that be over-complicating the process?

Thanks, Callum

I will check that site out. Can that circuit just use a guitar input or does it need a line level input from a preamp? That's the bit i'm struggling with - something that I can plug my guitar in one end and have it power a decent speaker at the other. All I seem to be finding are power amps that require a line level input - obviously more than a guitar can give. I'm sure I must be missing the blindingly obvious... 

By the way, I just built that circuit in the first reply to this thread and had no luck, I used 2n3904's and a2n3906 instead of the ones in the schematic, as that is what I have, and I subsituted the 1k8 resistor for a 2k2 as again, that was just what I had. Other than that, It was the same so do they matter? I will come back to it tomorrow withn a fresh head, and make sure I haven't protoed it wrong.

Thanks again.

Also, what is this an schematic of (http://wiringschematic.net/wp-content/uploads/2012/04/p10_fig22.gif) ?
What are the input/output ratings and stuff?
Thanks

Thanks again, you sound pretty experienced in this field, what is it you do?

I have an appreciation for the pace at which the time seems to run out from completing GCSE and AS levels in this subject, and I fully agree that the KISS principal, which I already try to use in a lot of projects, definitely applies here - some sound advice I think. Can you recommend any cheap/free software for the design (and maybe testing) of schematics?

I have had some good fortune though. I was at a guitar lesson last night and told my tutor about the project, as he used to be an electrical engineer, and he had an old Carlsbro amp in his attic. The amp itself does not work but there are still a lot of components that can be used on my project like the power transformer etc, which I am still  trying to find the rating of. The best part is the 12 inch Celestion speaker that was with it, so he has probably saved me £30 - £40 in parts there because this was a freebie!

I will do some research and let you know what comes up. In the meantime, do you know of a source of preamp schematics for inspiration/reproduction as I have a book with some fairly simple power amp schematics, however the preamp is proving harder to find.

Thanks again,
Callum

Yea, thanks Roly for doing all the research, you now probably know more about the course than I do! I had been interested in basing it loosely on a Marshall as my current amp is a Marshall 100watt, so that info on Marshall practice amps is really helpful.

In terms of the project, we will have approx 6 months worth of lessons and the work I can put in in personal study periods and my own time, so the deadline is not too tight. It does need to look like a finished product so it will need to be in a cabinate - I am going for a combo design - but I am not too worried about the cabinate construction as I have made something similar in the past and have some experience in this kind of thing. The circuitry is the bit that I have to learn about before construction as this is the area I am least experienced in.

There is no brief as we have "free rein" to make what we want, as long as it is within the capabilities of the workshop.
Thanks again guys,
Callum

I think I have solved this one, I was looking at it the wrong way

Right I have been trying some stuff out just on the basic circuit whist I am waiting for some more bits to arrive. I have a question now to do with the biasing potential divider circuit. I was trying to bias it with roughly 3V (that's about enough right?) and I used a 22k and a 47k to start with (will alter them for impedence later) wich gave me just less than 3V when I calculated it from a 9V supply, and worked out about right in practice. However, When i added the transistor in, the drop across the lower went to the 0.7Veb and the upper resistor went to the remaining 8V ish. Now I know the transistor set the 0.7V itself, but why does this then alter the divider voltages? I know this is a simple question, but I can't work this one out -_-
Any suggestions would be great,
Cheers, Callum

Thanks man! I will give that one a go
Callum