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[calx]

Hi guys,

I am new here and I am going to start with a fairly simple (I think) question. I would like to build an amplifier for my A2 level Product design project, maybe a 10 or 20 watt amp. To start with though, I would just like to make a really simple 1/2 watt guitar amp to get some understanding in my head. I have done some basic electronics before and I have had a go at this (see attachment) however I have not had any sound out of it. Some people I have spoken to said the input impedance was too low and that it needs an output stage.

Am I going in the right direction with this? What do I need to do to make it work? Is there a better simple amp that anyone can think of, bearing in mind that, if only for this first one, I would like to avoid chips and use discrete components to get a better understanding.

Thanks in advance peoples for any help.
Callum

[J M Fahey]

That stage is good as a signal preamplifier but does not deliver power, so it can't drive a speaker.
Try this, it's as simple as can get:

You can proto board it first, and later build it on perfboard or veroboard.

[calx]

Thanks man! I will give that one a go
Callum

[calx]

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

[J M Fahey]

Not clear, please draw the schematic you actually built and measured voltages.
Although I would start with the original schematic, as published, and later start modifying.

[calx]

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

[Roly]

I would like to build an amplifier for my A2 level Product design project, maybe a 10 or 20 watt amp.

Being on the other side of the planet I first had to do a bit of research to understand what "A2 level Product design" is, and after wandering around the AQA site (where they never did get around to explaining what "AQA" stands for) I gather that this is a module in the English General Certificate of Education Advanced Levels, what some would call Matriculation, the end of Secondary schooling with possible university entrance next.

---
Summary

This specification is designed to encourage candidates to:

    develop a broad view of design and technology
    develop their capacity to design and make products
    appreciate the complex relations between design, materials, manufacture and marketing.

AS outline

At AS level candidates develop an understanding of a broad range of materials, with emphasis on the life cycle of products, manufacture and final disposal. This specification also considers the broader issues for the designer including the environmental sustainability of products and consumer safety:

    Unit 1: (PROD1) Materials, Components and Application
    Unit 2: (PROD2) Learning Through Designing and Making.

A2 outline

At A2, the specification offers candidates the opportunity to further develop the knowledge and practical skills from AS. Candidates will continue to develop a body of coursework alongside an understanding of the processes and procedures of commercial production and manufacture:

    Unit 3: (PROD3) Design and Manufacture
    Unit 4: (PROD4) Design and Making Practice.
---

As it happens I had a lifetime in the design, manufacture and service of industrial electronics while moonlighting as a musician and music/theater tech building and repairing guitar amps and sundry showbiz-related gear.

If you haven't already an early task is to go out and see what is already available as a "10 or 20 watt" guitar amp.  Call this basic market research, checking out what such a product looks like, what potential customers and users expect.

I started by Googling "10 to 20 watt guitar amplifier" and having a look at the images that brought up.

The first thing to notice is that this got 1.6 million hits including some very heavyweight names such as Fender, Behringer, Ashton and many lesser knowns, at prices ranging from $50 to $1500.  So the first message here is that while this is an interesting design project, and you should end up with a guitar amp out of it, you can forget about getting rich quick by taking the industry by storm - there are many thousands out there ahead of you.

If you are going to build any guitar amplifier some of the things that you must consider include;

- features and facilities
  - inputs and outputs (how many? what levels/connectors? headphones?
  - effects
  - controls
  - speaker size
- the box or case it is going to go in (head and cab or combo?)
- the chassis metalwork to hold the amp electronics and front panel
- the printed circuit board (for ease of manufacture, and rework/repair)
- discreet components or IC's?
- heatsinks and cooling
- power supply, power mains safety (and approvals in different markets CE, C-tick, UL ...)
- ease of assembly
- ease of disassembly and repair
- finish, styling, dress
- costs, "price point"

...and lots of associated details besides.  Many of the above are inter-active meaning that the process of design can have a lot of circularity and backtracking as various factors are traded off against each other during product development.

One particular amp stood out, the Marshall Lead 20, simply because I could find the circuit, and external and internal pix that help illustrate the reality of the design process.

---
Vintage Marshall Lead 20 (Model 5002) 20-Watt 1x10 Guitar Amp in Excellent Condition, made in England, serial number U41410. Features a 10" Celestion G10L-35 speaker. The Marshall Lead 20 is 17" tall, 18 1/2" wide, 9 1/2" deep, and weighs about 28 pounds.
Buy the Marshall Lead 20 Model 5002 Guitar Amp 1x10" Celestion for 199.00!
---

The external and internal views of this amp are attached below.

Here are some different takes on the same idea (not related to each other);


Jade Hot Shot


ESP


Marshall Lead 20 circuit


Some things to think about.

[J M Fahey]

Dear Roly. First of all thanks for your investigation and detailed explanation.
Real Pro 
That said, I think that calx is not going to take the Industry by storm (not an easy task even for heavyweights such as Fender and Marshall), but simply trying to comply with some School Project requirements.
Which is good.
Don't know the time constraints involved.
If calx has, say, 30 days to present a working project and, probably, a working prototype, I guess the design should be kept very basic and to the point.
Something quite standard which can be finished in such a short time, without endless tweaking (which might take *years*, not kidding).
Dear calx, can you please tell us more about your project requirements?
Time?
*Must* it be built into a finished box, "ready to sell", or can it be made out of several boards, a transformer, maybe an L shaped front panel, everything bolted to some wooden board and with everything visible?
I have seen many such projects shown in school "science fairs".

[calx]

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

[Roly]

JM - I posted part of the course requirements that Calx is doing because it's actually a bit daunting.  This is somewhat up-market on a Science Fair project and more like the sort of project that engineering students at uni or senior college have to turn in.

What is being asked of Calx is not just to create a going guitar amp (he already has one of those) but to demonstrate that he understands how to engineer a product for manufacture, something I'm sure you will agree is of a rather different order.

He's not going to put his design into production, but he has to go through the motions and produce a design package that could potentially be then taken to cabinet makers, metalwork shops, and PCB suppliers.  I say "daunting" because I have been through this process many times and it always looks easier than it actually is - as you say "tweeking", or even getting serious bugs out, can take years.

{A company building a prison in my home state of Victoria just went broke, partly because the windows and doors they ordered from China were the wrong size, and as somebody said, having doors that fit seems pretty basic to a prison.}

Given that he has a deadline in about six months it is going to need a heavy application of the KISS principle just to get there.  {Calx; "KISS" stands for "Keep It Simple - Stupid", take the line of least resistance and don't get adventurous, pick the simple and obvious for each step, e.g. no in-built fuzz box.}


Calx; I notice that your course syllabus places some stress on drawing and sketching.  I can't tell you how important I think this is.  You are going to need to produce drawings of your cabinet, chassis, and PCB layout, and before you get to the final stage there may be quite a bit of horse-trading, e.g. you can't get the transformer or heatsink you wanted and what you can get is a different shape with different mounting holes.  So expect to produce several versions of each, and here "version control" becomes vital; all drawings should have a title block showing all the normal guff, but some sort of version number is vital, particularly if you are going to be discussing drawings with people on-line, to make sure that we are literally all on the same page.

I use a date group for this function, so today is 120522 and if there is more than one drawing a day (we hope) a 24-hour time group as well, and since we are all in very different time zones you need to specify GMT/CUT because you are on GMT, I am some 10 hours ahead, and I'm guessing that JM for example is some hours behind.  So 120522-1203GMT should be a serial number that leaves no doubt which is the latest revision.

The honest designer will tell you that there is very little that is new under the Sun, and that most of us (very quietly) operate on the motto "plagiarise and hybridise".  In this particular project you don't have to worry about being sued for copyright, and imitation is not only the sincerest form of flattery, it is also building on the valuable experience of others.

So generally I'd take the Marshall Lead 20 as an "inspiration", since it represents exactly your aim having gone through exactly the process of preparation for manufacture you are supposed to put your design through.

Most commercial small amp designs are crippled at birth by being ... well ... small.  At the 10-20 watt power level I'd suggest that you consider using a 12-inch speaker and allowing for a sealed enclosure of at least 50 litres (that's about root(2) cubic foot); you imitate the simple U-shaped chassis with front, back and floor ('tho you may want to use separate front and back panel sections to allow finish such as anodizing, painting, silk-screen printing of control legends, and so on), and the floor might need tabs at each end to secure the chassis.

You don't want it to look too much like a clone of a Marshall either, so I'd be inclined to make it a bit narrower and taller, but your cab size should be driven by what sizes of timber are locally available that minimise cutting and working.  Here again your will find you produce a lot of sketches and drawings of different arrangements until you find one that satisfies the largest number of conditions.

It will also not be hard to improve on the Marshall heatsinking arrangements and use a rear-mounted heatsink that can actually get rid of some heat.

I normally build my amps out of individual components, like this Marshall, but in your case I think you should give serious consideration to using an integrated chip amp where you can pretty well use the circuit off the data sheet, bolt it to the heatsink, and worry about other things - such as the preamp.

An important point to keep in mind with any amp build is to keep the mains wiring well away from all the other wiring.  This is mainly for safety considerations, partly to stop main-carried electrical noise getting in to your amp circuits, and partly because such segregation is required for type approval in just about every potential market; and consideration of such legal pitfalls should be one of the critical things your instructors should be looking for in your design.


I am willing to provide whatever assistance I can, but ultimately it is down to you to understand your course requirements and fulfill them.  Six months may seem like a long time now but you will be horribly surprised how quickly it will evaporate.  Murphy's Law states that things always take longer and cost more than expected (doubly true for software projects), so getting in early leaves you with some "lead time" up your sleeve when something goes wrong - and it will.  I look forward to you posting your initial design ideas.

[calx]

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

[calx]

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

[tonyharker]

calx see here http://sound.westhost.com/project10.htm . check out the rest of the site as well. A wealth of info!

[calx]

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.

[calx]

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