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.
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.
I am now going to make zoom on certain parties of assemblage
Infrared remote control, decoder RC5.
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. Photo
And the infrared reception cell is on the following schema: Photo ... it is an autonomous cell put outside the box.
The driver for the motorized potentiomètre ALPS is the following: Photo
Here is a zoom on the preamplifier on the following photograph.
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.
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
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
A zoom on the inout selector. I made it simply by using one standard commutators. We see behind a part of the preamplifier. Photo
On the following photograph, appears this time the full supply. Photo
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
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. Photo
Second supply of output amplifier, like regulated ±30Vdc:
Every continuous supply is accomplished thanks to PI filters, either CRC for low power, or CLC for high power.
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
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
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.
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
And the protection linked to headphone is the following: Photo
And definitely here is the power amplifier!
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
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.
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
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.
A zoom on the protection circuit, describe shortly
with the previous picture. Photo
Then it is the schematics of protection circuit of headphone amplifier: Photo
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. Photo
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. Photo
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.
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.