Stereo preamplifier 2.1 , tone control + subwoofer, useful preamplifier circuit to be used in active boxes, combined with power amplifiers. It has a bass filter for the subwoofer channel and a stereo channel with bass/mid and treble tone control. With independent volume adjustment for the satellite speakers and subwoofer.
About the 2.1 Preamplifier
P1 sets the overall volume, C6 couples the audio signal to the IC1A amplifier, configured as inverter and 1.5 gain works with buffer. IC2A forms the tone control with bass and treble option. P2 adjusts the output volume for the satellite speakers. R17 and R18 connect the stereo signal to the amplifier IC3A, configured as non-inverting and gain 11, if necessary you can change the gain. IC3B was a high pass filter with a cutoff frequency of 49Hz, IC4A a low pass filter with a cutoff frequency of 157Hz and IC4B forms a low pass filter with an adjustable cutoff frequency via P6 from 58Hz to 157Hz, P5 adjusts the volume of the bass channel.
The cut-off frequency can be adjusted according to your needs, for this use circuit calculators with operational amplifiers.
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Also get to know our 2.1 amplifiers
- TDA2030 2.1 amplifier board subwoofer circuit diagram
- TDA7294 2.1 TDA7293 2.1 power amplifier circuit
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Here I used JRC’s original NJM5532 operational amplifier, you can use any dual operational amplifier.
The power supply uses rectifier bridge in DB-1 package and can be DB102, DB103, DB104, DB105, DB106 or DB107. For filtering we used C23, C24, C12, C13, C27, C28, C21, C22, C25, and C26. We used voltage regulators LM7812 and LM7912, optionally you can use 3 mm leds to indicate the power status. The transformer should be a double type from 12 + 12 to 15 + 15 VAC. Avoid using the same transformer that feeds the power amplifier.
This preamplifier can be used with several of our amplifiers posted here. You can reuse burned-out home-theater speakers, with the option of using a subwoofer and two or more satellites.
Schematic of the 2.1 preamplifiers with tone control and bass filter
Printed circuit board of the preamplifier with tone control and subwoofer output
Bill of material to assemble 2.1 stereo preamplifier, tone control + subwoofer
Last Update: 10/21/2022 10:07 AM
| Parts | Value | Description | Quantity |
| 1/4W resistors | |||
| R1, R4, R5, R9, R10, R13, R14, R29, R30 | 1K | Brown, Black, Red, Gold | 9 |
| R2, R11, R17, R18, R26 | 68k | Blue, Gray, Orange, Gold | 5 |
| R3, R12, R19, R21 | 100k | Brown, Black, Yellow, Gold | 4 |
| R6, R7, R8, R15, R16, R20, R27, R31, R32 | 10k | Brown, Black, Orange, Gold | 9 |
| R22 | 150k | Brown, Black, Yellow, Gold | 1 |
| R23, R24, R25, R28 | 6.8k | Blue, Gray, Red, Gold | 4 |
| Capacitors | |||
| C1, C2, C14 | 47pF (47) | Ceramic Capacitor | 3 |
| C3, C7 | 10p (10) | Ceramic Capacitor | 2 |
| C4, C8 | 2.2n (222) | Film capacitor | 2 |
| C5, C9, C15, C16, C17 | 100n (104) | Film capacitor | 11 |
| C6, C10 | 1u (105) | Film capacitor | 2 |
| C11 | 10u/25V | Electrolytic Capacitor | 1 |
| C12, C13 | 2200u/25V | Electrolytic Capacitor | 2 |
| C18, C19 | 220n (224) | Film capacitor | 2 |
| C20 | 10n (103) | Film capacitor | 1 |
| C21, C22 | 220u/25V | Electrolytic Capacitor | 2 |
| C23, C24, C25, C26, C27, C28 | 100nF (104) | Ceramic Capacitor | |
| Semiconductors | |||
| IC1, IC2, IC3, IC4 | NE5532N or equivalent | Operational amplifier dual | 4 |
| IC5 | LM7812 | 12V positive voltage regulator | 1 |
| IC6 | LM7912 | Negative voltage regulator -12V | 1 |
| B1 | DB107 | Bridge rectifier | 1 |
| LED1, LED2 | Led 3 mm | Green or red led | 2 |
| Miscellaneous | |||
| P1, P2, P3, P4 | 50K | Double potentiometer | 5 |
| P5, P6 | 50K | Simple potentiometer | 1 |
| CN1 | POWER | KRE 3-Way Connector Terminal 5.08 mm | 1 |
| CN2 | IN | JST-XH-03 2.54 mm 3-pin connector or equivalent | 1 |
| CN3 | OUT | JST-XH-03 2.54 mm 3-pin connector or equivalent | 1 |
| CN4 | SUBOUT | JST-XH-03 2.54 mm 2-pin connector or equivalent | 1 |
| Solder, Wires, PCB, Box, transformer, etc. | |||
Download files for this assembly, includes Gerber, PNG, PDF, and postscript PCB files.
- NE5532 IC Datasheet in PDF
- PDF Datasheet of the DB107 bridge rectifier
- PDF Datasheet of the LM7812 voltage regulator
- PDF Datasheet of the LM7912 voltage regulator
Buy equivalent board in AliExpress
