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[Versione Italiana]

Milk-Mint GainClone.


The audio amplifier described in this page uses the LM3875TF chip by National, the driving force behind one of the most famous "Do It Yourself" projects of the last years. It's interesting an interesting device capable of good sound and the advantage to employ a very small number small number of external components,which makes it easy to be constructed. In short... a classic GainClone!
This time dressed in a fancy way, at least for this kind of device, but with some original and unusual technical solutions.

For more specific information on GainClone, a web research is all you have to do. You'll find out discussions, schemes, variations, tweaking and much more. For a quick revision of "what is it", you may want to refer to the specific page on Richard Murdy's site, one of the first to discuss this particular project.
Many others have written on it, in an even broader and more detailed way. For instance, Nick Whetstone, in his famous site "Decibel Dungeon", has an entire section called "GainClone Pages", which deals with all of its aspects. In a minimal or "inverted" version, with or without Pre, with regulated power supplies or not.
Not to be missed.

Compared to the standard configuration, I added a few small variations. In this case, the power supply tension is of +/- 35 Volt, higher of some ten volt for branch than the one used by Richard. This allows the chip to supply more power. A little more than 50 Watt per channel on 8 ohm, for less than the 0,1% of distortion.
(For details about the tension/power relations, look through the datasheet) The electrolytic capacitors used as filter are Panasonic FC of 2200µF/50V (two per channel) and the input capacitors is made of Polypropylene of 2,2µF.
The potentiometer, indicated as R1, can be set in the range of values going from 20 to 50 Kohm. Richard uses a 12 positions Stepped attenuator of of 35k, I use an Alps Blue of 20k log. This is not a problem.
The main difference is in the way the potentiometer attenuates the signal.
At the same sound level it may occur that the 20k, set at 12 o' clock, gives a sound of an intensity similar to the one of 35K, set at "1" o' clock. Some people prefer a high range of control in the part in which the sound is low (high attenuation), others the opposite.

As a general rule, it's always preferable to have an attenuator of less value than the input impedance. In the case of the scheme reported above, the impedance value is more or less the one of the R2 resistance. That's to say 22Kohm. The ideal thing would be to use attenuators of 20K or less.
The transformer employed is a Toroidal encapsulated Talema, of 300VA-2x25Volt, visible in the group photo. The straightening up diodes consist of two Fagor bridges of 25A-500V. The group is completed by a six poles/double sector switches, by Palazzo, (employed as an input switch) a toggle switch for the "ON/OFF" commutation,another double sector used as "MUTE", a small IEC panel mounth with integrated fuse and Input/Output connectors.
Finally, scattered on the floormounting, you'll find all the elements that make up the device.

Here on the side, you can see the already polished detail of the self-supporting central body, seen from opposite angles.
They consist in some layers of wood overlapped and pasted to each other. Multi-layered in two depths: 6 and 10 cm thick. It's not the quickest way, it needs precision and clear ideas on how to place all elements, but it allows you to build up even complex models without too many difficulties. Once the layers have all been glued, you may start to rough out and polish. A couple of coats of filling paste, will partially cover the wood texture, which, though it may look thin and well polished, if not covered, may end to be noticeable even under the paint. For faultless models, I suggest to give first a coat of "base"
(a pretty thick paint, usually white or grey, which dries up quickly), rub it and then give the final colour.

In this case it's a delicious milk-mint (emerald green), set against the dark-silver grey used for panels, for tootsies and the shielded covering of the transformer, obtained with a close pipe of pvc, its inner side coated with an aluminium adhesive tape.
The same tape, with infinite patience, has also been used to completely shield the body. You can see it in that ugly and blurred picture of the inside (sorry for that!).

I'm not crazy for the box style, but I'm aware that it might be difficult to imagine a different covering.

As a matter of fact, if even the main producers actually provide the same old hat since many years (talking of aesthetics...), I'm not surprised at the fact that many DIY fans can't think of anything different.
As for me, I usually start placing the main components. This grants me to decide the approximate size
of the object, which are the best points to place the tootsies (if possible, always three) or which are the best indicated
paths for the signal and power supply cables (the aim is to get them to be as short as possible and rationally separated
from each other). Only then, I start and imagine the external shape. Once the model is designed, it's like assembling it is easier to assemble.

The three tootsies are placed as a triangle with the heavy toroidal in the centre. This solution makes the amp stable like granite. The "on" switch is placed on the bottom panel, a green led, not far, lightens the floor under the amplifier. The aluminium element in the upper side is meant to dissipate both channels and measures more or less 150x100x5 mm.
I tried the amplifier at a cracking volume for at least a couple of hours, without the heatsink ever overheating. It's hot, but it hardly gets to temperatures higher than 40-45°C. Maybe during the summer, with 30 degree in the shadow, it will prove good to make toasts (I hope not so!:-)

I will keep myself informed, now it is no longer mine.
It is a gift, I hope it may bring a lot of satisfaction!...:-)


Some short remarks.
The GainClone is a very good amplifier. Valuable components, a compact layout and a correct managing of the masses, together with a good power supplying, are the key to getting high performances. The simplicity of the circuit allows to lodge it in a small-sized cabinet.
If you have a little time, patience and a certain manual skill, you can move further, designing a new one, or inserting its parts in a pre-existing case, fit for the purpose. (But always keeping in mind and considering all the issues connected to safety and electricity use).

For instance, never thought of the rectangular backed clay vases? Yes!, those used for plants! You can find them easily in a variety of shapes and colours. Backed clay, as ceramic, it's a very compact material, has a good insulating power and is partially a thermoconductor.
If you also find, who sells China vases for Bonsai.. I'll say no more!;-)

- Marco Saccani, january 2006 - www.moxied.com


To carry through the most part of the projects is necessary to work with devices connected to the electricity network, which can be lethal if unskilfully managed.
For that reason, we do not consider ourselves liable for any damage to people or things, due to the partial or total reproduction of the matters presented in these pages.

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