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curves Amplificateur 1998.

But.

After previous different tries, I decide to limit myself to 2 ways of output at the farthest, and I forget my previous tries of home cinema. Besides with classical CD, this does not bring thing big.

Purpose:
- I need the best report of quality / price.
- I limit myself therefore to a modest power, like 2*30W on 8 Ω as usual.
- I need the minimum of noise, that it is thermal noise, and especially 50Hz of the sector!
- I need a selector of entrance. A manual commutation is enough for me.
- I need a grader of serious and aigue. I shall only have to have an increase of level following curves physiological of the ear. An manual adjustment is enough. And even better, I accept a manual commutator instead of a potentiometer to have a better coupling of both ways.
- I need an headphone output.

Introduction.

Here is therefore the ampli in this wholeness.
To the left there is supply, where a part are besides concealed.
In the bottom center it is the preamplifier (volume, balance, bass and aigue adjustment).
In the top center it is the ampli with transistor in TO3 package.
On the right, it is the headphone amplifier.

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I am now going to make zoom on certain parties of assemblage

Infrared remote control, decoder RC5.

On this photograph, we see the infrared decoder, comming from Elektor, which allows me to have a remote control for volume of the amplifier.
It is drived by an infrared receiver which does not appear on this photograph, and it pilots a motorized potentiomètre ALPS which appears on the top-right photograph of the card of the preamplifier.
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This is the schematics:
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And the infrared reception cell is on the following schema:
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... it is an autonomous cell put outside the box.

The driver for the motorized potentiomètre ALPS is the following:
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Preamplifier.

Here is a zoom on the preamplifier on the following photograph.

Therefore on the left we see the regulator. It uses traditional 7815 and 7915  (on the small radiator with fins), and next two huge capacitors of filtration of 1000μF which serve for slicking voltage completely provide 7815 and 7915  and erase all HF imperfections of these components.

Against the metallic front face, we see two big blue commutators for the bass and aigues. This circumspection allows me to provide coupling answers curves between both ways thanks to capacitor and 1%/2% resistor. Result is better than the traditional potentioeter which have up to 20 % of drift between both way. It should provide better sound picture.

On the right, there is the ALPS motorized potentiometer for volume, drived by the present circuit on the previous photograph.

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Remain some operational amplifiers (the blacks animals in 8 legs). This was in the origin (1996) of OP227, that I changed in OPA627 in about 2000, then I replace them again with AD825 double sold by Sélectronic (and also by other entreprise under a similar form):

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A zoom on the preamplifier.

We can see that every operational amplifier is encircled with two capacitors every time, which make 10μF or 100μF. In fact every operational supply of amplifiers are decoupled of main supply by a small RC filter (R=10Ω).

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The preamplifier schematics are:
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A zoom on the inout selector.
I made it simply by using one standard commutators.
We see behind a part of the preamplifier.
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The Supplies.

On the following photograph, appears this time the full supply.
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It is based on a 300VA transformer, what I added some additional winding to sub-drawn several different voltage.
- Main winding provides 2*18Vac (alternative) for the power supply of the amplifier, which is +-25Vdc (continuous). It is accomplished with about 2mm² wire.

Secondary windings are accomplished in enamel wire, like 0.4 or 0.6mm ², and intended for a low power.
- There is then a winding type 2*20Vac for the preamplifier supply, which becomes ±28Vdc, then ±15Vdc after regulation.
- It must have something like 2*8Vac for the headphone amplifier supply who will appear on a photograph farther. This becomes ±6.5Vdc after regulation.
- There must be another winding. Probably it is of 2*28Vac, which becomes ±38Vdc and who is regulated in about ±30Vdc and who also serves the power amplifier for the voltage part. This second supply serves for supporting a fixed voltage on the electronics of the power ampli, and so minimized some forms of distorsion.

This is the power supply ±25Vdc

As every supply, positive and negative part are separated and filtered separately, then connected together close to the place of use.
Photo

 
Here is a photograph of this supply.
I raise it to show where is put inductance for the CLC power supply ±25Vdc : simply underneath the supply card!
By putting it at 20cm of electronics, I don't meet side effect due to potential radiance. It is a total silences, what certain trader amplifier does not have.
It apparait in the way the diodes of bridge rectifier, but I am not frankly persuaded of their usefulness, seen the battery of filtration which waits behind.
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Second supply of output amplifier, like regulated ±30Vdc:

*** NO SCHEMATICS ***

But I have a picture:
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This is the supply of preamplifier, regulated ±15Vdc.

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Supply of headphone amplifier, regulated ±6.5Vdc.

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Every continuous supply is accomplished thanks to PI filters, either CRC for low power, or CLC for high power.

CRC means a serial resistance put between two capas connected to the ground. One of this capacitor is supply by traditional bridge rectifier, and the other capa provides desired and filtered supply. CLC is the same principle, but I use a inductance instead a resistance. A resistance is more practical when currents are weak, because they acquire so a broad elimination of all parasites on the supply. But for high currents, this resistance should be too weak (<1Ω), and this would have no more effect. I prefer then the inductance that are more effect in these conditions, without introducing too high serial resistance.

The capacitor value is in general well brought up, for instance 2*60000μF for the power supply, a total of 2*15000μF for the preamplifier, and about 2*16000μF for the headphone amplifier. This circumspection that I takes every time allow to acquire silent and robust supply, able of providing high current without weakening, and get closer to battery behaviours.

Headphone Amplifier.

Now, we have the headphone amplifier on the next photograph.
It is a pure class A amplifier (easier to make in low power). The output transistors are put on a metallic carpenter's square, together with regulator LM317 and LM337.

There is a good battery of capa as in every circuit or supply to have low serial resistance in the supply. This also avoid to have impule current everywhere in the amplifier.

Electronic circuit is very simple itself, it is about a simple-stage with feedback build as follower.
This requires few components, and used with a pure circuit classify A, they acquire stability and transparence without too much expenses.

The visible inductance on this photograph serves in galvanic isolation in high-frequency of supply in comparison with the rest of circuit.
It is just a small additional circumspection, history to slow down possible curls of mass in high frequency via ways not always foreseeable.
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Schematics of headphone amplifier:
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And the protection linked to headphone is the following:
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Power Amplifier.

And definitely here is the power amplifier!

Four big capacitor of 10000μF in the middle for the power supply. This allows to have in high frequency a supply extremely near thepower  transistors. Assemblage is more or less symmetrical, it is always the same supply for both ways, and possible diaphonie trouble are short-circuited by paying attention to ground, by using for instance a reference point central and unique on this trial circuit.

Both vertical circuit are in fact protection. Supplies are cut in case of DC voltage on output or in case of too strong audio power at 10kHz (restricted in 10W max because of the used tweeters). The operational amplifier must be used to adjusted the output voltage as close as possible to 0V.

On electronics side, I use a system of two serial differentials. There is not symmetry in this schematics, but there is however a kind of symmetry in the electronic behaviour.

The output is polarized in order to have 0.5A DC current by channel thanks to the grey potentiometer.

All this things provides 0.005 Ω output impedance, to which is needed to add the same value for the external cablage.

Photo


The schematics is:
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A zoom on the protection circuit, describe shortly with the previous picture.
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Then it is the schematics of protection circuit of headphone amplifier:
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Soldering, Réalization.

Let say a small photograph to show how components are linked together.
Ordinarily I use a hole island prototyping board, but here I had on the hand a full line prototyping board.
Components are welded over, and linked up between them with enamel wire of 0.2mm ².
An exception for the supply power, where I use of the standard cable of 2.5mm ² that I welded just like that above.
Some tores appear here and there. I use this to produce a galvanic isolation in high frequency, history to avoid possible curls of mass between supply via the transformer in high frequency.
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A zoom, on another board supply.
I often use the prototypiing board as "double-face", " double components ". I mean components on both faces, and cablage on both faces.
We see in black chemical capacitors, in yellow poly-switch fuse, and some plastic and ceramic capacitor to ameliorate the frquency behaviour of chemical capacitors.
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Result.

First listenning (1996).

- There is no noise on output. Even with the ear on medium speaker or tweeter, we hear nothing.
- On variety CD, everything goes out to marvels. The bass are clean, medium-aigues is flawless, in short no defect of poor quality equipment is present.
- The total cost is close to 6000 french francs (about 1000$), and I do not regret the investment.
- Brasses pass flawless.
- No form of aggressivity at first glance. Listening always remains soft and nice.

Second listenning (2004).

When I created the class A amplifier, it appeared that the amplifier 1998 really met a difficulty:
- The sound message remains blurred on classical music, for instance on symphony, when it crosses one pushed orchestral (I do not know the official name) very lively. Instruments become, during this short period of time, more difficult to separate between them.

New configuration.

Till the beginning of 2004, this amplifier is use in full audio range.
It will change change with the arriving of class A amplifier.
Go on "configuration" at this address : [Cliquer ici ]




realization of a classifies AB amplifier - personal realization - infrared remote control RC5 - preamplifier - headphone amplifier- headphone output - filter in pi


Created  : 01 June 2004.
Last update : 01 august 2007.

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