Designing amplified speaker for legacy car navigation system

04-25-2017, 01:47 AM
We have a 2003 Honda Odyssey with built-in nav and stereo. The two were integrated -- the nav audio went through the stereo, there was also a mute signal to signal the stereo to play the nav audio on the front speakers.

I replaced the stereo a few months ago which severed any navigation audio. We bought a BC23A 15 watt powered speaker as a workaround. After wiring it to the nav system, the audio was too quiet. (I turned the powered speaker up, and turned the nav system all the way up.) Hooking the BC23A directly to an ipod gave me audio much louder than I needed, so I know the speaker works.

My assumption is the line level from the nav system is too low for what the BC23A is expecting.

I began thinking I should make/buy a small amp to raise the line level before it gets to the BC23A. Then I found the MAX9744. The specs say:
The outputs are Bridge-Tied-Load which means you can only connect speakers up directly. Don't connect the outputs to another amplifier!
So I can't put it in series with the BC23A. Then I realized that this amp is probably sufficient to replace the BC23A.

The specs for the MAX9744 say this about the input level:
This audio amplifier takes in stereo audio, either using a 3.5mm stereo jack or terminal blocks. Line in audio is a-OK. The audio inputs are not differential! The ground connection is connected directly to the power ground, this chip simply does not handle differential inputs. However, the inputs do have blocking capacitors, so if your audio levels have DC bias, its OK to connect them up directly without extra audio blocking caps.
I believe this is sufficient to put in and connect directly to a speaker, maybe recycling the BC23A as a non-powered speaker.

A friend suggested I amplify as near the source as possible -- so I am thinking of running the amplified output from behind the stereo to the remote speaker, rather than running line level to a modded BC23A.

My questions:

Does this sound like a reasonable plan?
How important is it to place the amp near the source? It would be convenient to just gut the BC23A and put it in there! (Maybe I should just do that initially since this isn't hi-fi music we're talking about.)
I would like to use the "stereo mute" signal from the NAV system to turn the amp on and off -- to reduce hiss when not operating. The amp has a SHDN pin that shuts down the amp when grounded. I believe the nav system has a mute signal (high) when speaking. What's the simplest / reliable way to switch this? Is it as simple as this? What value should R be?
[MAX9744 SHDN] ----+----\/\/\/\/\--- [MAX9744 GRND]
+----- [NAV MUTE]

04-25-2017, 08:35 AM
You can always try it out .. and I would agree with your friend, if the signal is that low coming out of the nav, that line/wire should be as short/close as possible to avoid noise issues.

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