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MAX9744 and Car Nav System
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MAX9744 and Car Nav System

by rrauenza on Sat Dec 05, 2015 3:16 pm

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 was 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 (12v supposedly) from the NAV system to turn the amp on and off -- to reduce any noise/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?


Code: Select all | TOGGLE FULL SIZE

  [MAX9744 SHDN] ----+----\/\/\R/\/\--- [MAX9744 GRND]
                     |
                     +----- [NAV MUTE, +12v]


(I've also posted this here, http://electronics.stackexchange.com/qu ... ion-system before I ordered the MAX9744, but I'm not getting much response yet after a week.)

Thank you!

rrauenza
 
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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Sun Dec 06, 2015 11:13 am

Sounds reasonable. I wouldn't worry too much about the distance between the source and the amp. If the wires are twisted, that should reduce any interference.

If the mute signal is active high, then you need an inverter to change it to the active-low mute signal the amp wants.
See this post for instructions to build an inverter
viewtopic.php?f=8&t=67170&p=341373&hilit=transistor+inverter#p340601

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Re: MAX9744 and Car Nav System

by rrauenza on Sun Dec 06, 2015 2:22 pm

I don't think I need an inverter -- here's the truth table:

Code: Select all | TOGGLE FULL SIZE
STEREO       MAX9744
MUTE         MUTE or SHDN
------       -------------------
HIGH         HIGH (enable MAX9744)
LOW          LOW (disable MAX9744)


SHDN - digital Shutdown pin. Connect to ground to turn off the entire chip and put it into low power mode
MUTE - digital Mute pin. Connect to ground to turn off only the audio output stages, its faster than shutdown and keeps the digital i2c audio levels


i.e., I want the MAX9744 normally muted (SHDN or MUTE to ground) until the Nav systems signals STEREO MUTE, which indicates directions are coming. This signal should take the improvised Nav speaker powered by the MAX9744 out of "standby."

So I think what I need is just some protection to isolate the two systems ... ?

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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Sun Dec 06, 2015 3:53 pm

You can use an optoisolator, like this one:
http://www.digikey.com/product-detail/e ... -ND/720234
You can put a 12V signal with a resistor in one side, and get a 5V signal out the other side

Or, you can use a resistor divider. Put a 4.7K and a 2.7K resistor in series to ground on your mute line.
you'll get a ~5V signal from the point between the two resistors.

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Re: MAX9744 and Car Nav System

by rrauenza on Sun Dec 06, 2015 7:04 pm

I'm not following. The MAX9744 says to short out SHDN (or MUTE) to GND to disable the amp. I need to interrupt that short when NAVMUTE is high.

Are you describing this?

Code: Select all | TOGGLE FULL SIZE
[NAVMUTE +12v]----/\/4.7k/\/---+---/\/2.7k\/\---[MAX GND]
                               |
                               +------[MAX SHDN or MUTE]


and

Code: Select all | TOGGLE FULL SIZE
[NAVMUTE 12v]---\/\/\280ohm\/\/\--[OPTO ANODE   1]
[NAVMUTE GND or MAX GND?]---------[OPTO CATHODE 2]

[MAX MUTE or SHDN]-[OPTO Vo  4]
[MAX GND]----------[OPTO GND 5]
[MAX 12v supply]---[OPTO Vcc 6]

* 280ohm = 14v / 50ma

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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Mon Dec 07, 2015 11:37 am

You want to use MUTE, not SHDN.

Actually, I just looked at the schematic again, and you want to drive MUTE at 3.3V, not 5V. So that means using a 7K resistor and a 2.7K resistor. Sorry.

If the MUTE signal from the car is active low, then the resistor divider will work. When the car's mute is low, you'll get low on the MUTE pin, and the amp will mute.
When the car's mute is high (i.e., not muted) you'll get 3.3V on the MUTE pin, and the amp will enable.

If you use the optoisolator, your schematic looks right, except you want to use a bigger resistor on the anode - use something like a 3K. You don't want 50A going into the isolators LED.

Also, you want to power the isolator at 3.3V, not 12V

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Re: MAX9744 and Car Nav System

by rrauenza on Sat Dec 19, 2015 12:10 pm

I've had to put the MUTE aspect on hold -- after I installed it (without the mute functionality), it was really really noisy. Picks up every electrical noise in the car and amplifies it.

So... I've bought a small metal box and will install the box behind the nav/stereo and will run a powered/amp'ed speaker line to the remote speaker. Hopefully that will solve that problem, otherwise I've exhausted my electrical engineering knowledge :).

My understanding is that I should also ground the board to the metal box for RF isolation -- since the power is 12v and chassis, I think that means I can connect the box to the chassis (nearby metal) or I can split the negative lead inside the box and also bond/ground it to the box?

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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Sun Dec 20, 2015 10:53 am

Split the negative lead and ground it to the box.

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Re: MAX9744 and Car Nav System

by rrauenza on Sun Dec 27, 2015 1:06 am

I've found some time to work on this!

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.)

In the end I think I used the following (looking at my notes, not the board), also adding a diode to keep current from going back into the mute signal.

Code: Select all | TOGGLE FULL SIZE
MUTE --- >| ---  7700ohm --- SHDN ---- 3300ohm --- GRND


I put it in an aluminum enclosure with the GRND pin connected to the box.

It works!

What I then found was still a lot of noise when it played with the car running. Buzzing. I found a crutchfield article about noise and it suggested attacking the ground first.

I completely reworked my power supply path - I was using a line tapped previously from the drivers side, run all the way to the passenger side, and then back to the stereo. The ground was connected at the passenger side to the wall. Apparently that's a pretty long ground. And it's a long 12v.

I changed the power to be tapped off the stereo's power, right next to the device. I also tapped into the ground in the stereo wiring harness.

The noise level went down A LOT.

But I'm still getting a lot of noise -- more of the higher pitched whining.

I've twisted the leads in and they are pretty short. I also twisted the speaker wires.

I'm at a loss now what else might improve the situation except that I've seen people apply ferrite toroids. I don't know if my noise is from the power leads or the line level leads from the nav system. I did install the optional capacitor onto the board.

Suggestions?

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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Sun Dec 27, 2015 10:11 am

Noise is a tough problem. As you say, it's typically a grounding issue. I'll see if anybody else here has any ideas.

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Re: MAX9744 and Car Nav System

by rrauenza on Sun Dec 27, 2015 12:14 pm

... And apparently cars are the worst for noise!

I may also try the diyaudio forums - they have other threads on noise. I'm tempted to buy a hobby oscilloscope so I could at least see if it is from the power, mute, or signal/audio. I suspect it is power ... And that radios just have really good noise suppression on their power lines.

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Re: MAX9744 and Car Nav System

by adafruit2 on Sun Dec 27, 2015 5:54 pm

its almost certainly coming in on the power lines. Just for analysis you can try powering the MAX9744 from 12V battery pack (8 x AA's for example), that could help isolate it a bit

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Re: MAX9744 and Car Nav System

by rrauenza on Mon Dec 28, 2015 1:13 am

Data!

I don't have an oscilloscope, but I have an iPhone to record. Apple says, "the maximum audio bit-rate for an audio recording on the iPhone is 64Kbps with a mono channel." Not sure what that means for a sample rate.

Here's the recordings. I loaded the voice memo m4a's into Audacity via ffmpeg and then saved them as 16 bit signed WAV's.

Car Off: https://www.dropbox.com/s/m3bpjg6v1o1mm ... f.wav?dl=0
Car 1000: RPM: https://www.dropbox.com/s/nt1j7rkdnyp6g ... M.wav?dl=0
Car 2000: RPM: https://www.dropbox.com/s/sp7wlr7e5uc8o ... M.wav?dl=0

I did a frequency/spectrum analysis in Audacity, but ... I dunno, it doesn't seem helpful.

If I look at the audio and count the ticks, Off has no ticks -- 1000 RPM has ticks 10 ticks/0.206s, or 43hz and 2000 RPM has 10 ticks per 0.1s, or 100hz. Not surprisingly it correlates to the RPM. So, the alternator?

I know the MAX9744 has a capacitor on the power input, but it must not be big enough for this application?

The car's max rpm is 8k, 2k of that red. So the range of noise is theoretically ~40hz to 400hz.

And yeah, maybe I'll run my wall-wart PS and verify it is the car PS -- but I'm pretty sure it is, especially since it got better when I improved the positive length and ground quality.

Underneath it also has some higher frequencies -- I haven't been successful in isolating those in audacity. The spectrum seems pretty wide

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Re: MAX9744 and Car Nav System

by adafruit_support_rick on Mon Dec 28, 2015 9:14 am

100hz is too fast to be a bad plug. If the drive ratio of the alternator is 3:1, which seems reasonable, then that would be the right frequency.
Did you put the ferrites on the power wires or on the audio wires?

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Re: MAX9744 and Car Nav System

by rrauenza on Mon Dec 28, 2015 12:30 pm

I haven't tried any suppression yet -- still researching. Apparently what I need is a LC filter (or CLC?)

I could just order this: http://www.amazon.com/uxcell-Aplifier-S ... B00BGAKIUK

Or head to a stereo shop and see what they have.

Or build my own. I haven't found a design yet that I know is ideal for a car application -- my understanding is there should be an inductor and two caps. Some designs recommend a foil capacitor with a small value and a electrolytic capacitor at a high value to capture high and low frequencies.

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Please be positive and constructive with your questions and comments.