Audio Preamplifier with Digital Volume

Audio Preamplifier with Digital Volume

Audio_pre_with_Digital_volume_1.gif (21739 bytes)

�� . �� . �� (Fig.1) �� . � �� . �� S1 (UP), �� S2 (DOWN), �� , �� , �� IC1A-B ,�� R-S (IC1C-D ), �� IC3. � �� IC2B �� . �� S4 �� RESET � �� . �� , �� S3 �� MUTE, �� . �� 8V. �� , �� A,B,C,D, �� IC2-3 �� Fig.2, �� E,F,G, �� (Fig.3).

The circuit is a preamplifier with digital regulation intensity of sound. He is separated in three departments. In first schematic (Fig.1), is found the circuit of control of electronic potesometer. The control become from two pressing switches. The S1 (UP), that put up the intensity of sound and the S2 (DOWN), on the contrary, that lower the intensity, checking them two oscillator follow a bistable multivibrator R-S (ICIC-D), with that determined the direction of counter IC3.. The gate IC2B ensure deactivate the counter in the limits of region of regulation. With the S4 we have the possibility selecting the level of sound, in which it will come back after RESET or when we give supply in the circuit. This operation is very useful when we select null level of sound, then with switch S3 we have the possibility of going to situation MUTE, to rapid nihilism of sound. It will be supposed you are careful the supply of circuit, which is symmetrical � 8V. The logic conditions, from outputs A,B,C,D drived on correspondingly inputs IC2-3 in Fig.2, the points E,F,G linked on correspondingly outputs from power supply (Fig.3).

Part List

R1-4-7=100Kohms	C1-3-4-5-8-9-10=100nF	IC3=4029
R2-5=10Kohms	C6-7=10uF 25V	S1-2-3=Push button switch [n.o]
R3-6-8-9-10-11=1Mohms	IC1-2=4093	S4=DIP switch

Audio_pre_with_Digital_volume_2.gif (24928 bytes)

�� (Fig.2), �� 16 �� . �� IC2-3 ,�� . �� . �� IC1A-B �� IC4A-B ,�� . � �� IC4A-B �� , �� R38-40 . �� , �� , �� .

In second schematic (Fig.2), become the choice of resistors of divider, what is connected in the 16 entries of one analog multiplexer. Here the multiplexers they are two IC2-3, one for each channel. In the circuit they exist also two dual analog opamp.. One, in the input IC1A-B and one, in output IC4A-B, that make also a light gain. The gain of IC4A-B can change depending on our own needs, changing the value of R38-40. If we add in the input a rotary switch, so that we make choice of various sources of input, then we will have a complete qualitative preamplifier, exempted from the problems that have classic mechanic potesometer.

Part List

R1-2=47Kohms (All Resistors 1/4W)	R37-39=100ohms	C8-10=100pF mylar or ceramic
R3.......R34=Tolerance 1-2%-See Table1	R38-40=2.2Kohms (See Text)	IC1=TL072
R35-42=100Kohms	C1-2=10uF 63V MKT	IC2-3=4067 (MUX)
R36-41=1Kohms	C3-4-5-6-7-9=100nF 100V MKT	IC4=NE5532

Table 1
Resistors		Step
Channel L	Channel R	~2.5dB	~3dB
R3	R19	4K7	3K8
R4	R20	3K3	2K7
R5	R21	2K7	1K8
R6	R22	1K8	1K2
R7	R23	1K5	1K
R8	R24	1K2	680R
R9	R25	820R	470R
R10	R26	680R	330R
R11	R27	470R	220R
R12	R28	330R	150R
R13	R29	270R	120R
R14	R30	180R	82R
R15	R31	150R	56R
R16	R32	120R	39R
R17	R33	82R	27R
R18	R34	220R	68R
Total		18K5	12K7

Power Supply

Audio_pre_with_Digital_volume_3.gif (9985 bytes)

�� (Fig.3), �� , �� .

In third schematic (Fig.3), he exist classic power supply, with separated the supply�s for the analog and digital department.

Part List For Power Supply

T1= 2X15Vac-30VA	C3-4-6-7-9-10-12-13=100nF 100V	IC2=7808T
D1....4=1N4002	C5-8-11-14=47uF 25V	IC3=7915T
C1-2=4700uF 40V	IC1=7815T	IC4=7908T

Sam Electronic Circuits 3/02

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