I want to make a big party boom box, with a 100watt subwoofer and four speakers of 25 watts each. But we need a better amplifier and preamplifier board which can meet the requirements. I searched a lot but some of the amplifier systems are very expensive or some of them even do not support 5 channel surround sound output.
That’s why I ought to design my own preamplifier and amplifier board. So, we need a maximum output of 200watts but before that a better surround sound preamplifier board is needed. This article is sponsored by JLCPCB and if you sign-up to JLCPCB you will get a new user coupon of $54 to place the prototypes order.
Why we need preamplifier:
Audio output from Aux or Bluetooth doesn’t fulfil the voltage rating demand required for an amplifier to run. Also, these systems only support two channel mentioned as left channel and right channel. But we have to raise the input level before giving it amplifier. In this stage we can use operational amplifier buffer stage with adjustable gain this will not only isolate the input from output but also provide a desired signal value.
Basic Idea of working and components selection:
Because we are dealing with frequencies between 0-14khz (suitable audio range). Which can be further divided into 3 categories- bass, mid and treble. For lower range frequency (for bass) we need a high slew rate and low noise ic that’s why for this we can use JRC4558D. It will fulfil the requirements of 100watts subwoofer. For speakers we can use lm324 four operational amplifier ic which can be used as a buffer in 2x gain multiplier for the all four speakers.
1. JRC4558D dual operational amplifier
2. LM324 Quad channel op-amp
3. 100K, 47K preset (for bass and volume control)
4. Resistors: 1k, 3.3k, 4.7k, 6.8k, 8.2k, 10k, 22k, 27k, 47k, 100k, 150k
5. Polyester film Capacitors: 100nf and 1nf
6. Electrolytic capacitors: 1uf, 4.7uf, 47uf, 1000uf
7. In4007 diodes
8. Bluetooth chip
9. AMS1117 5v (voltage regulator)
Circuit is perfect for a 2.1 amplifier system, I only added the subwoofer bass option, but in future I will try to add some more features like frequency control, subwoofer volume control, treble control and mid adjustments. Here are using a small 5volt voltage regulator to power the Bluetooth board.
Always used polyester film capacitors instead of ceramic capacitors for good frequency response and low noise. In last, I made a suitable circuit for proper filtration and rectification of linear power supply.
I designed the schematics in EasyEDA and manufactured it from JLCPCB in just $2. JLCPCB is providing the PCB prototypes in the lowest price and the quality is always amazing. You can see those glossy finish on the PCBs. You can check the latest news updated on the webpage of JLC and they are making the projects more practical with dual side real PCBA assembly service.
If you want to use the same designs as mine then Download them from here. And if you sign-up to JLCPCB using this link you will get $54 new user coupon as a welcome reward which can be used to order the PCBs, PCBA, stencil and 3D printed parts accordingly.
Power supply used:
Because both of the operational amplifier works on dual voltage levels, either we have to make a voltage divider circuit or we can use center tapped transformer method to get the desirable voltages. My friend designed a rectified and filtered power supply with load protections, you can see that from here.
Our PCB has Onboard power supply, 4 channel speaker audio outputs and 1 subwoofer output, 5volt supply for the small Bluetooth system, Subwoofer and volume control presets, Desirable gain output for the speakers.
Working and observations:
High frequency has a little more gain over bass, so the lm324 can be modified to give lower gain by changing this resistor here. If you want to know more about OP-amp workings as buffer then read this article. The bass gives a very...Read more »