Car Subwoofer Amplifier Review – ElectroSchematics.com

Taking apart a circuit board or module and reconstructing its complete schematic is a valuable skill. We can see how someone designed circuitry to gain ideas that can be applied to our work. And taking an in-depth look at an existing design provides great lessons on how to do (and not to do) things!

This review/teardown post shows an arbitrarily picked cheap subwoofer amplifier module that was part of an old 2.1 channel car audio system.

Quick Overview

The PCB, as is, a compact one (65mm x 45mm) with a slightly lengthy heatsink for the power amplifier chip. The single-sided PCB has neatly arranged thru-hole solder pads for stereo audio input, volume control potentiometer, subwoofer and the DC power supply connections. The module runs on the common 12VDC (14.4V maximum) automotive power supply.

Quick Inspection

I went ahead and traced the PCB artwork. To create the schematic, first we must derive how components are wired together. This takes a lot of flipping back and forth, but a well illuminated PCB inspection setup can help reduce the difficulty behind that irksome task.

The circuit has two key segments – an active subwoofer filter and the subwoofer power amplifier. On quick inspection, the entire build is a very close replica of an Indonesian design.

After some cleaning up, here’s the first part of the schematic.

Now it is obvious what the TL072 IC was doing! It seems to be a classic example of using an op amp as an active LPF (low pass filter) – the essential frontend of a subwoofer amplifier. TL072 Datasheet https://www.ti.com/lit/ds/symlink/tl074h.pdf

We can see that the operating voltage of the subwoofer filter was close to 9V as there’s a zener diode based shunt regulator onboard to render 9V from the 12VDC input. Further, since this circuit is designed not to operate with a split (+/-) power supply, we can also see some design trickeries. Let’s dig into this later!

So, what about the subwoofer power amplifier circuitry? This is the schematic of the subwoofer power amplifier. This is wired around the obsolete Sanyo IC – LA4440. As you’re aware, the LA4440 is a 6W 2-Channel (Bridge = 19W typical) Power Amplifier IC with built-in audio muting function. LA4440 Datasheet https://www.mouser.in/datasheet/2/308/LA4440-1192769.pdf

It turns out, this is a pretty little BTL (bridge tied load) mini subwoofer power amplifier circuit which’s quite capable of producing a sensible bass kick on the car subwoofer. In this bridge amplifier configuration, first and second channels (ch1 and ch2) of the LA4440 IC operate as noninverting amplifier and inverting amplifier respectively. The output of the noninverting amplifier (Pin 12) is applied as input to the inverting amplifier (Pin 7) through a potential divider.

As mentioned earlier, it is just another replica of Indonesian work. This is the original!

While examining the resemblances, I found the circuit under test had some notable differences especially in terms of the electrolytic capacitors used by the module maker. Intentional or not – who knows? If you feel the same way, take note of the value variations of the electrolytic capacitors!

I could go on and reconstruct the module, but that’s not very interesting at this point. Also, I wouldn’t want to rebuild it as is. I’d at least follow the original subwoofer filter design idea, and probably other ideas for the subwoofer power amplifier. I’ve already collected a bunch of TDA series power amplifier and NE series op amp ICs anyway.

Different Chinese subwoofer modules are available online nowadays. I want to try those out to get a better feel for what they do. I bought a couple of TP3116D2 and TAS5630 subwoofer amplifier boards from Amazon. Onward to those monsters, later!

TP3116D2 Datasheet https://www.ti.com/lit/ds/slos708g/slos708g.pdf

TAS5630 Datasheet https://www.ti.com/lit/ds/symlink/tas5630.pdf

Quick Test Report

Since the module calls for a “line level” (https://en.wikipedia.org/wiki/Line_level) input, I tested the module by feeding a music bit with ample low bass content (https://www.youtube. com/watch?v=vaI3W2Tcmag) available through the “Line Out” socket of my Bluetooth speaker.

And then, I listened it through a 4Ω/40W open subwoofer wired directly to the output of the module. The power source used was a 12V/7Ah SLA battery.

Sadly, the results were not as good as expected. I know, subjectively the subwoofer doesn’t really have any sound. And, a steep subwoofer filter means that there’s very little sound from the subwoofer except sub-bass. Anyway, things here are a little weird!

What’s Next?

Now it is the time to look at the entire circuitry a second time to set everything up. An in-depth tweaking and testing the module will take some time and effort, and it does not have to be very practical or effective when playing on a cheap module.

I went through a step-by-step test procedure and confirmed that the real culprit was the power amplifier circuitry (the filter circuit itself presented some problems, but that is trivial). In the end, I interchanged two electrolytic capacitors (C1-C4) to fix the problem, which was a successful attempt. Below is the updated schematic:

The bottom line

To get enough juice to your subwoofer, your electronics need to be able to handle the job. Since there are several factors working against you, I need you to go to the blog of a veteran audiophile. This is vital if you’re just dipping your toes into the world of high-end audio. The right setup of a subwoofer can create magic. If you get it wrong it will destroy the ability to create music!

Conclusion

We examined a small car subwoofer amplifier module. While not easily seen, the core design has a few minor flaws. The components selection is a little odd. The subwoofer filter circuit is moderately good, but we need to inspect it closely again. The real problem is the subwoofer power amplifier. Overall, it’s not too bad!

My next post will continue with a new experiment following the same design idea, but with simple and different approaches. Stay tuned!

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