You seem to be very knowledgeable about this subject. I have some Google Nest Audio speakers that sound fantastic, but have the same problems as bluetooth speakers, and lack a 3.5mm input jack to convert into normal speakers. Do you have any recommendations on how to do this, for someone with minimal audio knowledge and some basic EE?
The Google Nest Audio speakers are kind of a special case. They only sound good because they use a sealed, extremely rigid cast aluminum sealed enclosure with a high excursion driver. The performance of these speakers with a regular crossover and amp will be poor, due to the low efficiency of the enclosure/small driver.
To get around this, Google put in the TI TAS5825M smart audio amp. By measuring the speaker parameters through V/I measurement and a model, it drives the speaker in a closed loop way with far more power than it would actually be able to handle nornally to compensate for the resistance from the enclosure air pressure, and throttles to maintain the coil at a safe temperature. The chip also does DSP to compress the audio signal, cutting the peaks off the bass as needed when the volume is turned up so volume is maintained at the cost of bass.
One way to explore could be to just feed I2S audio from an I2S ADC i.e. PCM1808 to the digital input of the amplifier. The processing is internal to the amp so theoretically you won't lose the tuning. However this may turn out to be a relatively annoying reverse engineering project with fine magnet wire involved.
Note: I2S is different from I2C - the amp will likely have both. You will likely need to keep the original system around to program the amp over I2C (or capture the transaction and replay it) - otherwise you will likely get no audio.
The "raw" audio performance of this device (just an amplifier connected directly to the internal speaker and dsp on the computer) is impressive, kicking out bass down to 40Hz. It will, however, not last long like that. Reports online are that these blow speakers easily even when used with the default amplifier.
I would recommend that if 3.5mm input is desired, to replace them altogether with the IK Multimedia iLoud Micro Monitors. These have sound quality just as good as the Google at similar size, with the same DSP tricks, but have regular inputs and no smart features.
Thanks for the comprehensive answer, I'll look into the I2S audio solution.
You're right that iLoud Micros sound similar, they're 3x the price (The Nest Audios were sold at $50/each on sale). Definitely worth it, I just like tinkering with things.
