# Designing amplified speaker for legacy car navigation system

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 at adafruit. 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?
                              R
[MAX9744 SHDN] ----+----\/\/\/\/\--- [MAX9744 GRND]
|
+----- [NAV MUTE]


• To be able to help you, I need the specs of the Nav output and the input of the Max9744 and the BC23A, and at least partial schematics. – Guill Dec 4 '15 at 0:35
• The MAX9744 specs are here: adafruit.com/datasheets/MAX9744.pdf - I doubt the NAV system is documented anywhere. I think the best I could do is get the voltage when the "stereo mute" is active? – rrauenza Dec 4 '15 at 21:33
• If you are trying to feed one amp from a low level signal, what you want is a preamp - probably an op-amp circuit, not a power amp. – Chris Stratton Jan 9 '16 at 20:56
• @ChrisStratton, so rather than the route I took, I could have kept the BC23A and built a pre-amp to boost the BC23A? – rrauenza Jan 10 '16 at 1:22

I used the mute functionality from the navigation system's 10-12 volt input to shutdown the amp, adding a diode to protect the source of the mute signal.

Note: The SHDN and GRND pins are on AdaFruit's packaging, not the MAX9744 chip itself.

simulate this circuit – Schematic created using CircuitLab

The values were chosen based on AdaFruit's packaging of the MAX9744 and their schematic. This is what I wrote on their forum:

I've learned a lot about voltage dividers -- I was bench testing resistor values and found that a value of 10K didn't activate SHDN (I really want SHDN, not MUTE, since 99% of the time, this device can be off.) I then found the diagram for the ADAFRUIT MAX9744 board. Turns out adding another resistor to GRND is essentially a voltage divider with a 10K connected to 3.3v.

The MAX9744 specs say that the voltage has to go under 0.3*Vdd for LOW (the internal 3.3 volt Vdd) and over 0.7*Vdd for HIGH. So, I determined (using the voltage divider formula and 0.3*3.3v=1.1v) that I needed to connect SHDN to GRND with something less than 4.2K. But I also didn't want too much current from the MUTE signal, which would also be connected to this resistor, so I picked something on the high end. Then I picked a value for the other resistor that would raise the voltage greater than 0.7*Vdd assuming a voltage around 11 volts (which is what I measured from MUTE, and the diode drops another volt.)

I put it all in an aluminum box (also grounding the box.)

I ended up with a ton of audio noise from the car. I recorded the audio on my iPhone (don't have oscilloscope), put it in Audacity, and noticed what was probably the spark plug noise along with the alternator noise.

I isolated the cause of the noise to the car power supply using an external 12v wall adapter power supply on an extension cord.

I added a 12v voltage regulator (NTE1914) per their recommendation with a 100uf electrolytic cap on either side -- didn't make much of a difference.

I then bought a prepackaged power filter for cars, IBNF10.

The one I received had a 1000uf 16v electrolytic cap, a 1uf 50v electrolytic cap, and a ferrite toroid with about 10-11 wraps:

Noise is only barely audible now. I am curious, should anyone read this, why the 1uf cap wasn't a ceramic one. I'd heard that you usually pair a bulk electrolytic cap along with a small ceramic cap in order to soak up the high frequency AC noise.

• There's an easy to use schematic editor - button on the editor toolbar - to put the final polish on your answer. I can't make out the ASCII circuit schematic. – Transistor Jan 9 '16 at 21:01
• Oh, that's awesome! – rrauenza Jan 10 '16 at 1:17