Symmetrical Audio Class A preamplifier [3]

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� ������������ ��� �������� ��� �����, ����� ���� ������ ����������� ������������, ��� ��� ������ �� ��� ����� ��� ��������� �� ���� � [Class A]. ��� ���� ������� ����������� ����������, ���� ������������ ������������ ������, ���� �� ���������� �� ����� ���� ��� ������. ��� ������ ������� ����� � ����� ������������ ���������� [Q1,2] ��� ���������� ���� ��� ���� ������� �� ���������� �� ������� ��� ���� �������������� �� ����� ''������''. ������� �� ��������������� ��� ���� ���������� ���� �� BC550C ��� BC560C, ����� �� ������ ���� ���� �� ''����������'' ����� �������� �� ����� ��������������. �� �� TR1 �������� ��� ��������� �� ���������� ��� ���� �������� [Offset] ���� ����� ��� �������. ������ ���� �� ���������� ��� �� �������������� �� ��� ����������� 82� ��� ���� ��� R6-7. ��� ������ ��� ������ �� ������ ����� ��� �� ������ D1 ��� D2 ������ �� ������������ ���� ����� ��� ���������� Q3 ��� Q4 ����������, ���� �� ����� ����� ������� �����������. �� ���� ������ �� ����� ��� �� �� ���������� Q8-Q10 ��� Q9-Q11 ����������. � ������� ��� �������� ������� ������� �� �� TR2, ��� 20mA ��� ����������� �� ����� ����� 200mV, ��� ���� ��� ���������� R25. ��� ������� � �������� ��� ������� ���� ������� ���������� �� ����� ������� ������ �� ��������������� ��� ������� C12 � �� ��� ������������. �� ������� ��� ��������� �� ������� �� ������� ������ ��������� ��� �����. ���� ������ 1 �������� �������� ������ ��� ���� ������, ��� �� ��������� ���� �������. 

The preamplifier that appears in the figure is a completely symmetrical preamplifier, from the input as the exit and it works in Class A. It does not have somebody innovation, simply it uses good solutions, in order the result is what should. Such proposal is the use transistors matched [Q1, 2], that finds in the same chip with result the thermic and other characteristics are ''perfect''. It can they are replaced by simple transistors as the BC550C and BC560C it's enough you enter in the fatigue to matched you pairs with similar characteristics. With the TR1 we can if it needs we regulate the Offset voltage in the exit of unit. It can however be suppressed and it's replaced with two resistances 82 ohm in the place of R6 -7. A point that it should I accent it's that diodes D1 and D2 it should they are placed very near in transistors Q3 and Q4 respectively, so that they have common thermic behavior. The himself should become also with transistors Q8 - Q10 and Q9 - Q11 respectively. The regulation of out current becomes with the TR2, in 20mA that corresponds in 200mV fall across resistance R25. If we want the response of unit in the low frequencies she becomes ''flat'' it should jumper the capacitor C12 or him we abolish. The currents that run through the circuit in various points appear in the figure. In table 1 exist various voltages in utmost materials that they will help in the regulation. 

 

Table 1-Measuring points [across]
R3 1.9V D2 1.9V
R4 1.9V R19 1.35V
R10 1.9V R24 1.3V
R11 1.9V C7 1.6V
R8 0.2V R20 0.8V
R9 0.2V R23 0.8V
R13 1.3V R25 0.2V
D1 1.9V R26 0.2V

Parts List
R1-17= 1 Kohm 1/4W 1% metal film R25-26= 10 ohm 1/2W 1% metal film C12= 2.2uF 63V MKT 5%  [See text]
R2= 47 Kohm 1/4W 1% metal film R27= 100 ohm 1/4W 1% metal film Q1= SSM2210 or BC550C [See text]
R3-4-10-11-14= 820 ohm 1/4W 1% metal film TR1*= 100 ohm Top adjust cermet trim pot. Q2= SSM2220 or BC560C [See text]
R5-12= 220 ohm 1/4W 1% metal film TR2= 10 Kohm Top adjust cermet trim pot. Q3-6-9= BC560C
R6-7= 39 ohm 1/4W 1% metal film C1= 1nF 100V MKT 5% Q4-7-8= BC550C
R8-9= 82 ohm 1/4W 1% metal film C2-3= 1nF 63V MKT 5% Q5= BF256A or BF245A
R13-18-19-24= 270 ohm 1/4W 1% metal film C4= 22pF 100V ceramic Q10= BC337
R15= 4.7 Kohm 1/4W 1% metal film C5-6= 100uF 16V Q11= BC327
R16= 5.6 Kohm 1/4W 1% metal film C7= 1uF 63V MKT 5% D1-2= LED 5mm [RED]
R20-23= 10 Kohm 1/4W 1% metal film C8-11= 47uF 25V J1= 10 pinhead with 2.54mm step
R21-22= 390 ohm 1/4W 1% metal film C9-10= 47nF 63V MKT 5%

 

Sam Electronic Circuits 03/04