Home Audio on a Budget: Subwoofer Lullaby

Well… you want a home theater, you have some pieces, but missing something. I’m in the same boat. I already had my Sony STR-DE495 receiver and a decent set of L/R speakers, but the setup was… underwhelming. It needed weight. It needed that low-end rumble that makes you feel the movie and juicy maid music.

yes, subwoofer.

My hunt began with a budget. A very, very small budget (I am still learning about audio setups like this, so I don’t want to spend much). I was looking for an active subwoofer in the 25-50 EUR range. Welp, it might be hard.

What I found was a wasteland of two things:

1. Passive subwoofers, which are just speakers in a box that need an amplifier I don’t have. (In retrospect, maybe that was the easier path. Oh well.)
2. Rather underwhelming active subs with 15-25W of power older than I am, and pricy as heck.

I need to think about something different…

The World of Wireless

After weeks of scrolling, I had a thought. Soundbar subwoofers. They’re usually wireless, which means they get their signal wirelessly, but they have their own power cord and their own internal amplifier. They have to be active units.

This led me to a Philips HTS5120 subwoofer unit. The reviews for the original soundbar system were good, with no one really complaining about the bass. The spec sheet boasted 45W of power, which was double or triple what I was finding.

I found one. I haggled, and finally got the price down to 25 EUR!

It arrived, I plugged it in, and… nothing. Of course. The wireless part meant it was waiting for a signal from a soundbar that I did not have and would not be buying. Time to open it up.

Show me what you got, bad boy

Inside, the power supply/amplifier board was connected to the RF receiver board (the wireless part) by a single plug. This plug, a 12-pin connector, was the key.

I got lucky. Some kind soul on a HiFiGuides forum had already tried something similar. They only labeled four pins (Left In, Right In, STB, and Ground), and their pin locations were different, but it was enough. It was a starting point. It proved it could be done.

I spent a few minutes with a multimeter and a piece of paper, tracing the wires from the 7-pin header on the amp board (CON2) back to that 12-pin plug. I confirmed the audio inputs and ground, and then labeled the rest. This is what I ended up with.

And here’s the board itself

Easy.

First attempt

I grabbed an old RCA cable, chopped it in half, and got to work.

• RCA Signal -> AL and AR (twisted together, it’s a mono).
• RCA Ground -> GND.

What about the other pins? MUTE and MUTEN (which I figured was Standby)? Well, they’re probably active-high, so they need to be grounded to turn off. I wired MUTE and MUTEN to GND.
I plugged it in. My Sony STR-DE495 receiver was ready. I played a track.
Nothing. Silence. Not even a hum.

The Pain

This is where this project went from a quick 1-hour DIY sub to endless trial and error. For nearly eight hours, I was stuck.

I re-soldered everything. I checked my RCA cable. I tried grounding only MUTE. I tried grounding only MUTEN. I tried grounding neither. I stared at the amplifier board until my heart was nothing but eternal darkness.

I was just poking around with a multimeter, feeling defeated.

I knew the amp had power. I just couldn’t get it to wake up. The problem had to be that standby pin, but only after all those hours I’ve actually used my head and found a service manual with all circuits used in this subwoofer.

After looking at it for some time, and asking AI to help me understand it (I am terrible at electronics and reading stuff like this), I’ve come to a conclusion. Let’s wire 5V to STB, and see what happens. I didn’t want to try that, because earlier I’ve checked what the original RF board was doing, and there was 3.3V on the STB, so it must have needed to be ground…

I moved one wire.
I plugged it in. Turned on my receiver.
…and the buzzing just about exploded my ears.

I’ve shorted something somewhere, but well, it works!!!!

That was the secret sauce. It (MUTEN/STB pin) needed 5V to come out of standby mode.

The final, working wiring on that 12-pin plug:

• RCA Signal -> AL & AR
• RCA Ground -> GND, GND (there are two grounds)
• Jumper Wire -> MUTE to GND (This is needed, ground it)
• Jumper Wire -> MUTEN (Standby) to 5V (This is the magic)

Finally

I cleaned up the wiring. Then came the drilling. I don’t have power tools. So I sat there for 20 minutes, drilling a hole for the RCA jack by hand with a metal drill bit. In the process, I cut myself and realized I’d picked a drill bit that was too small anyway. But I tried. I eventually muscled the jack in.

It’s now a fully functional, 45W active subwoofer that I got for 25 EUR.
It plays… really well. Maybe too well. It’s incredibly boomy, and I have the subwoofer gain on my Sony receiver set to its absolute minimum.

My next step is to wire a potentiometer in-line with the RCA input to act as a proper gain control. But for now, I’m just happy to finally have bass.

Was it worth the 8 hours of suffering? Uhh, I hope so? It’s too boomy for me, but when the volume is lowered on the receiver, it actually sounds pretty damn good.

I’ve also added a rocker switch that interrupts the 5V-to-Standby jumper.
Now I can turn the amp “off” without yanking the plug from the wall.

More or less finished product, I still have some issues with RCA wiring causing the subwoofer to buzz and not get signal from time to time, but I’ll fix it after adding the knob.

Finally, that’s the end.

P.S. there is still bad connection/short somewhere and depending on how I move the plug it works or blows my ears with buzzing. I’ll fix it… someday.