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[J M Fahey]

It's an excellent preamp.
First proto board it, then design a PCB (or use perfboard/veroboard) and build it.
I guess you have access to some metal fabrication shops at school, make a "C" or "U" shaped aluminum chassis. at least 18Ga (1mm) or over.
Easy to work with and no worrying about rust.
Couple that preamp with some TDA2030/50 or LM1875 amp and you are done.
Download and learn to use Bancika's DIY Layout Creator, an impressive tool.
http://www.softpedia.com/get/Others/Home-Education/DIY-Layout-Creator.shtml
I prefer the Windows version, because a lot of people used it and you can learn by looking at their designs.
There's an advanced Java based version which works under *any* Operating System, but it's not as popular and does not open the earlier examples.

[calx]

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

[Roly]

I am a retired electronics tech and spent a large slice of my life designing, building and servicing industrial and bio-med electronics.  I've also moonlighted as a muso (keys and guitar) and music/theater tech/soundie/lighting operator since I was a teenager.

Free circuit modeling/drafting software?  Sure, Google LTSpice. There is also a strong Yahoo users group.  A word of caution about circuit modeling - the ultimate model is a hardware prototype; it automatically includes all the stray resistances and coupling capacitances that cause trouble.

I have no experience of it, but I've been looking at the LM3886 chip amp which seems to already have a lot of community support for guitar amp applications, builds to look at, people's problems to learn from, printed circuit layouts to be informed by.  It's quite a bit more powerful than your intended 20 watts, but when it comes to guitar amps it's wise to have some reserve up your sleeve (particularly heatsinking).

The guitar teacher is a double hit because the old amp provides you with a testbed and some bits, and your guitar teacher being experience in electronics and locally accessible to actually look at your problems (and you will have problems) is very valuable indeed.

What is the model number/name of the Carlsbro amp?

The circuit I attached is from Rod Elliot's excellent ESP site and you will find a related guitar preamp there that is well worth a close look;

http://sound.westhost.com/project27.htm

Like the one I posted from Rod's project 27, project 10 requires a preamp.  There is a good reason why guitar amps of the 20 watt class aren't Class-A, and that this one is for the audiophiles.

It is fairly normal that the master volume control is at the point of transition between the preamp and the main or power amp, and that the signal voltage level at this point is typically around 1 volt, or "line level".

This means (generally) that pre and main amps can be fairly easily mix-and-matched.  Take a look back at one of the circuits I linked to above and you will see a simple preamp that uses a couple of FET's and very simple top-cut "tonestack".  If you go for a dig on the net you will be spoiled for choice, but Rod's Project 27 preamp would be hard to beat.

I see JM has mentioned some other chip main amps which you should investigate.  As part of your project you should note the different IC's and circuits that you investigate, and why you decided for/against them.

There is no reason why you couldn't go with a transistor preamp, but these days even a fairly "ordinary" op-amp such as the TL07x is pretty damn good and easy to use, and you can go up-market to pin compatible things such as the LM833 which is still almost free to buy and close to perfect, or there are other "ultra" specialist audio op-amps which are not quite free and so perfect they make your eyes water (but are a bit of overkill amplifying guitar which has a typical output level in the hundreds of millivolts, not microvolts, coming via a thrash-metal stomp box  ).

[calx]

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

[Roly]

A Google search turns up three different Carlbro Colt 45's; L/ead, B/ass, and K/eyboard.  I'll bet there are only minor differences.

Since this amp is rated for quite a bit more power then you intend there are likely to be various useful bits for prototyping, not least the power supply (which may actually be a bit too high in voltage for your needs).  A pic or two of the chassis insides would be helpful.

As for your preamp power supply, it is normal to derive that from the main amp supply via a regulator of some sort, often a simple zener diode.

You should be developing a list of preamp and main amp options to explore, and gathering datasheets of suitable chip amps and IC's used in your options (e.g. TL07x series op-amps), the TDA2030/50 and LM1875 chip amps suggested by JM, get catalogs from Farnell, RS Components &c, and have a look at what is available in the way of power transformers, bigish electrolytic caps, and hardware such as jack sockets, and dress stuff like knobs and case corners.

You should be developing your first conceptual drawings of what it might generally look like externally and internally, and how the various parts might go together.

I still sketch a lot using pencil and paper, but I also have a favorite drafting programme, an old thing called EasyTrax which is professional abandonware and free, originally intended as a PCB layout programme which it still does well, and also useful for more general drawing.  But everyone has their own taste and some people think highly of Eagle (for example).  I don't, but whatever floats your boat and gets you results.  If you are going to get a PCB done you need to identify your local suppliers and find out what format they like artwork to be submitted in, and be influenced by that.  Ditto for any metalwork, panel designs, and so on.

[calx]

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

[phatt]

Hello Calx,,
I assume you refer to the 20 Watt Class A Amp on ESP site?

It has no safety overload protection, it wastes a lot of power, it runs hot which *Generally* means it will die early. Good luck,, you will need it.

If You want it to work without hassle DON"T go near Class A.
Leave all that silly stuff for the HiFi freaks to debate.
When it comes to Guitar Amps Class of operation is meaningless BS (pardon the swear words)
Good Guitar sound is all about tone shaping not class of power stage (the difference will never be heard by the masses)

IMO less than 30 watts is a waste of land fill.
(Unless of course you only wish to hold bedroom rock concerts)

If the supply transformer in the Carlsbro delivers 35-0-35 VDC then just purchase an LM3886 power chip and that will leave time to focus on the preamp.
I have one running from 36-0-36 supply and although it will never impress hifi geeks it does a darn fine job for guitar power stage.
The preamp BY FAR will be where the time is spent. As most of the tone tricks happen there. Get it wrong and no amount of fancy class A drivle will solve it.

Yeah,, repeating Roly's Q,, What model Carlsbro?
It maybe worth saving.
Phil.

[spud]

+1 on what Phatt said -

I know your were leaning toward discrete components but really, go with LM3886 (or something similar) and say you "rationalized" it as being the most efficient, cheapest, least complex way to build the power amp - get some project points there.   

Like he said, focus on the pre-amp to get that right and pop in the LM3886 (with appropriate power supply) and away you go!

Jim

[calx]

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

[Roly]

I think my brand will be "White Noise"

I like it.   

Wot Phil said about Class-A. +1

Except for anything below 30 watts only being suitable for land fill, I agree with Phil and Spud.

Personally I'm not all that taken with all-in-one chip amps and I couldn't see myself going that way for a guitar amp - HOWEVER - what Spud says is right.  Looking at it from a manufacturers point of view chip amps are a very attractive option when you consider how much messing about they save on a production line, and that's what we have to keep in mind here with this project.

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

GIYF

I'm going to look in a minute, but it is normal with most IC's that the data sheet contains a representative circuit, and many manufacturers simply follow this; so do what I am about to do, go to;

http://www.alldatasheet.com/

...and download the .PDF data sheets for the LM3886, the TDA2030/50 and LM1875 chips that JM suggested, and have a good meditate on them. {you have to click to view, wait for it to come up locally in your PDF viewer, then name and save to your datasheets folder}.

Put that speaker in a plastic bag before it grows a fur overcoat from drill swarf.  Once a tiny bit of swarf finds its way inside to the pole gap you can just about chuck it as a hopeless cause; otherwise it might be useful.

Google the transformer manufacturer name and type number - you might get lucky and find its spec.  The voltage rating of the two big blue electros will give you a clue to the secondary voltage.

You will notice that Carlsbro have used the chassis as a heatsink for the output pair.  This is a clue as to why this amp is junk.  Using the chassis as a heatsink, particularly a steel chassis as this appears to be, is one "saving" too far, and a short path to amp history.

Real heatsinks are made from ali and have fins mounted vertically where air can freely circulate past them, and that doesn't mean boxed up inside the cab.  Guitar amps tend to get used on stage under hot lights and they need to be designed conservatively for heat flow.  Whatever the nominal output wattage, you are going to have to get rid of at least as much as waste heat, and on-stage ambient of 40 degrees C are not uncommon.

Notice that the more significant controls have larger knobs.

Yes, that is a spring reverb tank.  By all means save it for a project, but I council against trying to include reverb (or any Fx) in your design - you have enough on your plate already.

The Carlsbro sure is a filthy mess, but it looks complete to me and this is the sort of thing I refurbish and bring back to life, so I wouldn't wreck it.  It may be useful to provide power generally, and a preamp for testing your mains amp, a main amp for testing your pre - maybe.  The thing is, clean it up and have a look at how a manufacturer has created an amp similar to your intent.

[calx]

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

[phatt]

Re *First pic*.
I'd being injecting a signal into that connector on the left a fair chance the psu and power stage may even still work.
Fire it up with the light build limiter and see how it comes up.
I'd say the preamp section is cactus, I keep that stuff for spare parts to build prototypes.
Roly has years more experience than most of us so in His hands I have no doubt it would be fixed.

Yes a lot of gear has been built only using the thin case for heat sinking.
I found and old BSR Fender Bass rig years back that had melted the plastic back panel into and unreadable mess halfway slid off the metal back panel.

At that time I was still learning and had not seen much of the designs and how they where made but even with my limited understanding blind freddy could see that a bigger heat sink was needed.
Just blew me away that a Name like Fender could sink that low.

I was able to save it by sandwiching a 4mm plate of alloy along the back to help.
No more issues.

*****Word of caution;
I hope you don't have plans on building electronic gear on (What looks like) a Steel welding bench?
Phil.

[J M Fahey]

Just curious:
is *this* what you are getting into?
http://en.wikipedia.org/wiki/Design_and_technology

[calx]

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

[phatt]

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

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

Damage to components will be the least of your problem when you are DEAD.
Steel is highly conductive one small mistake which you will likely never see coming and you are history.
I once read a comment Re electricity;
"Don't mess with anything you can't outrun"

You may need to catch up on safety issues before playing with this stuff.

Yes swarf and grinding grit and sensitive electronics don't generally mix well.
Again both are conductive and one flick of swarf in the wrong place and it all goes bang when you power it up.

I work on a wooden bench standing on a 20mm thick off cut of rubber conveyer belt.
All metal jewelry and stuff comes off before the power goes on.
Never work on electrical stuff without shoes on your feet.
Phil.