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Topic: audio line level isolator? (Read 912 times) previous topic - next topic

winner10920

I have a project where basically its just a msgeq7 taking an audio signal and giiving that to a 328 for led display purposes, however to make it more uiversal id like to isolate the audo signal to the msgqeq7, so what would the easiest way to accomplish this? First thought is a ac analog optoisolator but it wouldn't work at line level voltages right? Then I was thinking a smll isolation transformer which is basically my current plan, but are there better ways? And if the transformer is the best way any suggestions on that?

Docedison

What impedance is the Line Level Signal? 600 ohms? Radio Shack makes a fine 600 - 600 ohm isolation transformer and many line outputs are 600 ohms. I got some LM1036's. There are unique in that you can vary the output level with a DC pot... So I am using 10K Digipots with my Arduino. THere's many ways to solve your issue but you need to post a little more information before an educated response is possible. A transmission gate or CD4066 would work very nicely providing you use coupling capacitors (10uF) and some load resistors (10K) to provide a discharge path for the coupling capacitors... (Eliminates "Pop's" or large dc transients from charged capacitors). Very simple to do and the CD4066 has 4 gates... 2 extra's for??? Low current and about 125 ohms on resistance. They are perfect for low level < 7.5VPP audio applIcations. I've used them to switch crystals in and out of circuit on several 40 MHz transmitters (8.00+ MHz crystals). I attached a CD4066 data sheet as well.

Bob
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winner10920

It would definetly be limited to the 20hz-20khz range and the source impedance would not be clearly defined, that's the goal, something that'll work with a cheasy mp3 player, nd a ipod, and a legit stereo system without the worry of connecting grounds and accidentally frying something (which I did : (, very expensive mistake, I blew my 700w amp in my car)
which also perhaps makes me think, would it be easier to isolate the power supply of my electronics to kinda like isolate it from the other side?

winner10920

How would the cd4066 effectively isolate the audio signal? Id love to try it I've actually got one or two laying around

DVDdoug

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How would the cd4066 effectively isolate the audio signal?
I'm not following that either.. it's not an isolator...  I think he's using it to eliminate switching noise with the digital pot...

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that'll work with a cheasy mp3 player, nd a ipod, and a legit stereo system without the worry of connecting grounds and accidentally frying something (which I did : (, very expensive mistake, I blew my 700w amp in my car)
That shouldn't be a problem if you use the line-level input to your 700W amp.    Line-level signals (and loud headphone outputs) are in the ballpark of 1 Volt.  700W into 4 ohms is more than 50 Volts, and of course the voltage constantly varies with the music and the  volume setting.    For that kind of range  (~1 - 50V), you'd need some sort of switch or pot for high-low sensitivity and some sort of protection for your low-voltage electronics.    You are going to have the same high-voltage issue with the transformer... There is the potential for overloading it, and if it burns-out it could short and fry the amp.

I would guess your 700W amp has a "bridged" or differential (push-pull) output where both speaker connections are "hot" and there is no speaker ground.  If you ground the amp's ' - ' output, you'll fry it! 

The MSGEQ or 328 should generally work grounded to the car chassis, with no ground to the amp's output.   The only issue here is frying the MSGEQ or 328 with 700 Watts...



You can build a differential amplifier with an op-amp.   It would be "safe" for both the power amp and the differential amplifier, whether you ground one of the differential inputs or not, as long as you DON'T ground the power amps differential output).  But, you'd still need voltage dividers and possiblly some protection diodes to protect the op-amp from high voltages.

winner10920

That is exactly what happened, definetly not the smartest of my silly ideas, but I thought it would be easier to just put a voltage divider on the speaker output and hook the ground of the msgeq7 to the other side of the speaker, but as I quickly found out that only makes alot of smoke, I can't believe how dumb that was, but it gave me the idea to isolate the input (tho I guess properly used I wouldn't have the problem) its more in case I make another dumb mistake, also I think in case the pwm frequency of the leds switching causes a ground loop type problem

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