Failed Attempt - Symfonisk (Sonos Play 1) to 3 Ohm Subwoofer

This was intended to be a project to expand on some other projects and teardowns i'd seen online to utilise an Ikea Symfonisk / Sonos Play 1 as a wireless driver for a subwoofer. Other projects had utilized the Symfonisk to create wireless speakers out of more high-end equipment.

I have a very limited knowledge of electronics, but enough to be dangerous and enough logical thinking (given the time) to try anything out.

The subwoofer projects I had seen inspired me. I have an old Bose Acoustimass 5 passive subwoofer sitting around. These subs are a stereo sub box, with the woofers sitting at 6 ohms (i measure 5.7 ohms) each.

My goal was to use the Symfonisk to drive the two relatively small woofers, in parallel (therefore 3 ohm), utilizing the technology utilized in the Symfonisk - particularly, the Texas Instruments TPA3166 amplifier.

The following are my investigations, theory, and the rapid end!

I wanted to understand how the Symfonisk was generally set up. So with the help of the TI TPA3116 data sheet and a multimeter, I started prodding around.

First step: The Symfonisk speakers.

The Symfonisk "bass" woofer is 4 ohms. The tweeter is 7 ohms.

From the TI datasheet, I noticed that the chip has a maximum power output control utilizing the voltage on the "PLIMIT" pin.

I speculated that the sonos app adjusted this voltage up and down, depending on the in-app setting.

It turns out that this is not the case. The Symfonisk PLIMIT is tied directly to the GVDD, therefore always setting the output of the TPA3116 to maximum.

In the photos, you can see there is no change in the voltage at the PLIMIT pin regardless of the volume limit set in the app. The photos show 100% and 22% app setting with no change to the PLIMIT.

I wanted to understand what the Symfonisk set the TPA3116 gain at.

There is open circuit from GVDD to the GAIN/SLV pins of the amp.

I think this means a gain of 20dB and input impedance of 60k ohm.

From what I can see on the tiny circuit board, the input capacitors are 2.2uF.

Combining this with the gain and input impedance from the previous step, I calculate the high pass filter is set to 1.2Hz.

Reading the text from the datasheet, I read this as meaning there's a flat frequency response down to 12Hz (10x the pass filter setting).

From some other videos (maybe even an Instructible), I understood the Symfonisk processor basically receives the digital audio signal, processes it by combining stereo into a mono signal and then splitting that mono signal by frequency - high-frequency sound to the right channel of the TPA3116 input and the lower frequencies to the left channel of the TPA3116 input.

The TPA then amplifies these channels, sending the right channel to the tweeter and the left to the woofer.

Reading the TPA3116 datasheet, I noticed the chip has a "Mono" mode where the left inputs are tied together, to ground. This puts the amp into mono mode, effectively doubling the single-channel output, but also lowering the amp's minimum speaker impedance - down to 1.6 ohms.

The attached extracts from the datasheet indicate this system.

One complicating factor was that the Symfonisk sends it's processed high-frequency sound signal to the right channel and the low frequency to the left. Ideally, for my subwoofer project, it would have been the other way around with the low being sent to the right channel and the high being sent to the left. Meaning I'd only have to de-couple the Symfonisk processor's high frequency from the amp.

My idea was to basically cut the signals off between the sound processor and the amplifier chip, short the left channel of the TPA3116, and send the low-frequency sound to the right channel.

This would have meant reconfiguring the outputs and the LC filter configuration.

Basically, this is where it all became too hard. Trying to undertake this microsurgery on the board and potentially modifying the output LC filter to suit the mono configuration was going to be tough. As I said, I'm not an expert at this, so I had no idea if my theory was correct and if it was even going to work.

I'll add more to this instructible later if I become inspired, but I hope my failure might inspire or otherwise help someone else in their quest!