A Problem#
I wanted to add an amplifier to improve the audio quality in my Honda Accord Euro. The problem with my Honda’s radio, a very common one for factory head units, is that it has no line level outputs. Most reasonable people would just by so called line out converters (LOCs) and be done with it, accepting the minor quality loss in favour of better amplification. Unfortunately for me, I’m not like most people, I just had to make my own solution to the problem.
Working on a Solution#
Having opened car radios in the past, I was confident in removing, disassembling, tinkering, reassembling and re-installing the car radio.
I found the relevant datasheet for the main processor - a specialised unit for audio applications, with inbuilt ‘DSP’ and audio input and output.
I designed a preamp circuit around a NJM2068 op amp and a separate power supply and control board - which served to both power the preamp and provide a +12V trigger for the power amplifier.
Prototyping#
I created prototype boards using the toner transfer method, installed the circuit into the radio and success - kind of. The preamp worked, although a little quiet, but it had a glaring problem - engine noise. As soon as the engine was started, an incessant whine and whir could be heard over the speakers.
A second, more carefully routed and constructed prototype significantly reduced the noise problem, but it was not gone entirely and with different resistor values to change the op amp gain, the audio signal was no longer too quiet. I built a new power board, based around a miniature isolated dual rail DC-DC converter - again an improvement, this time, it was usable, but only with the use of ground loop isolators between the headunit’s new outputs and the amplifier. While it is usable, it isn’t what I set out to achieve when I started the project.
Wrap Up#
Unfortunately, this project remains unfinished; time and budget having been spent. I’ll likely pick it up again at some point in the future.