Clarifications on Ruby amp build

[Jtommi]

Hi,
I'm new here and pretty new to DIY stuff also.
I live in Belgium (Europe) and am currently in a boring callcenter job.

To begin in DIY guitar electronics, I built a Blue clipper from Tonepad (which works fine) and repaired my Marshall MS2 (IC and a diode had to be replaced).
Now I want to get in something more fun and thought about making the Ruby amp (http://www.runoffgroove.com/ruby.html) which you probably all know.

Since I want to make it a 2-Channel amp, I have to understand where the distortion happens. If I understand the schematic correctly, the distortion is done by the 386, is that correct?

[J M Fahey]

Yes, so no "2 channel" for you.
Keep it simple and, worst case, add a clean booster (or a plain old simple distortion, such as an MXR Distortion + ) before it.

[Jtommi]

So there are still two options:
- keep the 386 clean and switch between a clean signal and an overdrive (as you suggested) and put the whole in a box
- switch between the gain knob (overdrive) and a normal resistor (clean)

What is he buffer used for? Is it just to brng the guitar signal up and thus could be called a preamp?

[Roly]

Welcome @Jtommi.

Since the LM386 is a very low powered amp, about half a watt, you don't get much output level before it starts to overload and distort; in other words it lacks power headroom.  For "clean" you may be better off considering a design that uses a chip that is a bit more powerful.

In this design the FET buffer is used because the input resistance of the LM386 is (only) 50k ohms.  This is fine for most uses but passive electric guitars sound quite dull when they try to drive a load this low, so the FET stage is used simply to present the guitar pickup with a much higher resistance, 1.5 megohms in this case, meaning less loading, where the pickup's natural resonance tends to "sing".

For more detail you will find a discussion of pickup loading and its effect on the natural resonance of the pickup here.

[Jtommi]

Thanks for that complete answer.

For "clean" you may be better off considering a design that uses a chip that is a bit more powerful.

That would mean that I have to go higher than 9V on the supply?

[Roly]

That would depend partly on the impedance of the speaker you use, the higher the speaker impedance the more voltage is required to develop a given power, but you could go to say 12 volts on batteries, or any voltage you like using a mains power supply, which is no big deal.

If you don't feel comfortable doing the mains wiring there are lots of ready-built power supplies for a range of voltages and currents, plug packs and supplies for printers and the like that are being chucked out every day that you could scrounge and press into service.

Amps based on small chips like the LM386 are fun to tinker around with and learn on, but are not terribly useful, whereas an amp that can run say 5 or 10 watts has more real application, say if you are gigging or jamming with a drummer.

[Jtommi]

I wanted to avoid 12+ Volts because I don't understand the whole power amp thing with transformers and also because these transformers are hard to find or very expensive.
But if I can use normal power supplies, that changes everything. I might even have a 12V power supply laying around.

Do you have a suggestion for an easy and loud 12V amp after I've completed my standard Ruby?

[J M Fahey]

Search for "12V car amplifier schematic", you can easily get 4 to 10W with a humble TDA2003 and almost 20W with a TDA2005.

Start with this one:
http://www.generalguitargadgets.com/projects/13-amps/60-tda2003-practice-amp



Just checked, their PCB layout is *horrible*, there definitely are better ones.
Even on stripboard.

[Jtommi]

That looks pretty easy. Maybe I'll skip the Ruby entirely.

Does it require a heatsink. (I'd think that everything that comes in a TO220 package should have a heatsink attached)
And what voltage rating do I need on the 100uF and 1000uF capacitors?

[Roly]

That looks pretty easy. Maybe I'll skip the Ruby entirely.

Yeah; I guess the point here is that chip amps are of pretty similar complexity (i.e. not much) until you start to get well up in power rating, and as @JM shows above you can get up to a more useful 8 watts, or more, with a very similar simple circuit and a different chip amp.

There are lots of electronic mail order houses such as Jaycar, Maplin, RS Components, etc., etc., and it's worth getting their paper catalogues as well as exploring their web sites.  Many advertise in magazines such as Elektor, Practical Electronics, etc.

Lots of electronic stuff gets junked these days, stereos, computers and so on that can be a rich source of sundry components and heatsinks that can be had for a bit of scrounging, "dumpster diving", and at junk shops and garage sales.

Caps rated for the supply voltage or greater will do.

[J M Fahey]

Yes it does, but the simplest one, such as


Since you have 12V tops there, 16V rated caps are perfect.
Of unavailable, next higher: 25V.

Gain in that amplifier is R2/R3  + 1 , with 10X gain minimum recommended.

A good "starting" value  is 20X gain or 2200/100 ohms .
gain should be enough to provide some crunch with volume on 10, confirming you reached full power, *or* you can builf a simple distortion pedal , MXR Distortion+ is a classic, and use it as a preamp.
It can be set to "clean", of course, unlike others.
http://www.diystompboxes.com/smfforum/index.php?topic=49764.0

[Roly]

Unfortunately the SGS-Thomson datasheet contains an error when it comes to providing the PCB layouts.  Fig 17 says it is for the single TDA2003 circuit, but in fact it's a duplicate of the two-device bridge circuit shown below that.

So I went digging for some alternatives, and put these up for comment;

(generalguitargadgets.com refuse permission for hotlinking their (questionable) layout, so  xP them.)







Some heatsink ideas;











The last is an example of overkill, looks like a heatsink from a computer CPU which can be recovered free from a junked computer (or even perhaps from your local computer repair shop), but there are two rules of heatsinking;

- you always underestimate what will be needed,
- more is better.

HTH 

[Jtommi]

Wow, I never got so much details on a forum, thanks a lot to all.
As mail order I always use Futurlec because they're very cheap and only 4-6$ international shipping. It takes some time to receive the order and some parts aren't exactly as ordered but I can live with that (especially after comparing the price of a TL072 at Jaycar and Futurlec : 1.95$ vs 0.25$).
Unfortunately garage sales are rare here in Belgium and junk shops are overpriced.
But fleamarkets (don't know if it's the correct word) are pretty common here. (it's like many garage sales in one open place)

Yesterday I learned a lot about transformers and rectifier circuits so I'll probably build my own PSU, but how can I know how much VA the transformer has to provide?

[Roly]

Yeah, fleamarkets, swap-meets, all sorts of names for turning one man's "junk" into anothers' "trove".  I'm lucky in that I live in a country area where the regulations are not strictly enforced and I can do a lot of "shopping" at the local rubbish transfer station, pick over piles of unwanted TV's, computers and other electronics, often still in working order, that people don't want any more.  One friend, a fellow tinkerer, once remarked that I had a "very tastefully chosen junkheap" to pick over for parts.

VA or "Volt-Amps" is another way of saying "watts" and as implied it is the product of the voltage and the current multiplied, P=E*I.

As a rule of thumb an amplifier is about 50% efficient, so the power supply should be rated for at least double the output power (plus a bit of safety); so an amp expected to deliver 10 watts to the speaker should have a power supply capable of at least 20 watts (VA).

If you were a manufacturer you would want to cut things fine because overbuilding eats into profits, but for the home builder overbuilding has some other considerations.

Using a larger than needed transformer might be a good idea because projects tend to grow and you may want to add a preamp and some indicator LED's later; you may want to later rebuild the amp into something a bit more powerful and it would be handy if you didn't have to rebuild the power supply; larger transformers tend to provide "stiffer" supplies that don't sag in voltage as much under load; and of course a larger transformer will tend to run cooler (and last longer) if it is lightly loaded.  The extra cost of the next size up may be recovered in the longer term.  This is also why transformers with multiple voltage taps might be more useful later than one with just the voltage you require right now.

[Jtommi]

Using a larger than needed transformer might be a good idea because projects tend to grow and you may want to add a preamp and some indicator LED's later

That's exactly what I was thinking about...
In a local shop (on its webpage) I found a 36VA 2x12V (10,90€) and a 48 2x12V (12,90€), they seem perfect and aren't to expensive. They'd allow to go to 24V and are cheaper than their counterparts in 1x12V ( 24VA 1x12V for 12,90€ or 42 1x12V for 16,90€).