pHAT DAC to car stereo without AMP?


#1

So, background: I’m thinking about getting a Pi Zero with a pHAT DAC to solve a minor problem with my car stereo - while connected via bluetooth (to my Android phone) it won’t display the current media playing or let me use the steering wheel controls. There’s a USB drive and the stereo system will read songs from there, but it would be convenient if it worked for podcasts on my phone as well. But the stereo is old enough to support the Apple Accessory Protocol, so my (hopeful) solution is to wire a Pi Zero W to connect to the Android phone as a bluetooth speaker, to the stereo as an iPod (as a USB gadget + 3.5mm out), and bridge the two with both audio and data.

My actual question is if the pHAT DAC will work without an amp. My understanding is that headphones won’t work with the pHAT DAC because it won’t amplify the signal, but powered speakers will work without a separate amp. If that’s the case, my car speakers should accept the unamplified signal in the AUX port and output audio just fine. Unless the stereo system is expecting an amplified signal since it’s supposed to take a headphone line-out? I don’t actually know, so I’m hoping somebody that knows more about electronics and sound system can at least give me a definite “yes it will work” or definite “no it will not work” or a general “it’s impossible to tell without knowing more about your stereo system” (in which case I’ll stay on the safe side and get a mini amp or another pHAT with an amp).

Secondarily, I’ve done some soldering (rewiring a “greeting card circuit” to activate when a box opens), but this would be my first project of this kind. As I understand it, assuming it works as a USB gadget and I can program it to fake an iPod, I will need the following hardware:

  • Raspberry Pi Zero W
  • pHAT DAC
  • USB Male breakout and wires to combine the power and data into a USB Male plug
  • 3.5mm male-to-male cable
  • A male header for the Pi
  • A female header for the pHAT
  • Some kind of case to protect all this and hold it together

Is there anything obvious I’m missing? Is this the kind of project that would warrant a heatsink, for example?

Thanks in advance.


#2

Ideally a line-out is the right way to go, since your car has an amplifier stage within the stereo, and presumably also adjustable volume control.

In this case the line-out on pHAT DAC would be no different to connecting a phone and turning the volume on the phone down to a sensible level- which you probably do without realising anyway.

Your stereo should be able to compensate just by turning the volume up, and by using just one amplification stage you avoid introducing additional noise and potential clipping.

This should handle the USB connector side of things tidily: https://shop.pimoroni.com/collections/raspberry-pi/products/zero-stem-usb-otg-connector

Although you might want to use a USB extension cable on top of that, if the USB port in your car is somewhere exposed.

What you suggest sounds… possible… although I’ve only looked briefly into using USB gadget features. I don’t know you’d go about faking an iPod, or passing through transport controls to the bluetooth device.


#3

Thanks, that’s extremely helpful! I was just planning to follow the instructions from NODE (I linked to the wrong page, there’s another with instructions for a USB dongle by simply soldering on a USB breakout and wires), but that Stem simplifies things a lot.

I realize the iPod spoofing part is the dodgiest step in my plan, but I’m much more comfortable messing around with software than hardware and so far I’ve found instructions for setting the Pi into gadget mode as a serial device, and that the Apple Accessory Protocol is a serial protocol. The rest I’ll have to figure out once I’ve got hardware to experiment with.