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Help -- car stereo is acting up with hand made amplifier

  1. Jul 10, 2014 #1
    I am adding 2 extra rear speakers in my jeep, I have some rca cables comming from the rear speaker pre-amp outputs from the back of my pioneer stereo.
    My plan which works beutifully is a dc-dc boost module bringing the cars 12v up to 20 for the main amplifier.

    The main amp is a 2x25 watt class d module I got from online which actually works suprisingly well,
    but this is what is going on...

    I have the modules laid out in the back of my car with the 12v line and ground line I am going to use.
    You need to keep in mind that both the module and the stereo are using single supplies, and so the rca ground is just connected to the entire cars ground, and sence the module is a single supply it too has the imput ground just connected to the entire cars ground through the modules negative powersupply pin.

    So when everything is connected and all grounds are grounded, I turned on the module and I herd a big pop out of I think all the speakers in the car (but I couldnt tell) and there was no audio,
    only when I took power off of the module did I hear audio even through the rear speaker I had hooked up, but it only lasted as long as the decoupling capacitors were charged.

    I messed around with it and found that even just having the modules input ground connected to the rca cables ground was enough to make the pop even though I shouldnt have to ground the input anyway because it is already grounded through the negative power because it is a single supply.

    if I do not connect the amplifier modules ground to anything and I connect the audio signal from the rca cables to the audio input on the module then I hear a tiny bit of music but ALOT of weird distortion.

    It sounds like one of these (the stereo or the amp module) are a dual supply, but I did a test connection and both the stereos audio ground and the amp modules audio ground are just the same darn thing!
    Meaning the car batteries negative.


  2. jcsd
  3. Jul 10, 2014 #2

    jim hardy

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    A question well stated is half answered, but your 'description' is very confusing.

    have you tried a car audio forum ?
  4. Jul 10, 2014 #3
    I know its confusing and It was very difficult to describe but car audio forums do not know how what they use works, just how to use it... and how to brag.

    Is there anything I can clarify to help you understand?

    Pretty much I have a perfect setup that should be working but the answer to the problem lies within how it is acting, (in terms of poping speakers and stuff
  5. Jul 10, 2014 #4


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    You are missing something. Troubleshoot this from a different approach. Feed the amp module from something isolated. Figure out one thing at a time.
  6. Jul 10, 2014 #5


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    yep, great idea

    make up an audio patch lead to feed audio from an mp3 player etc to go into the power amp

    you also need to be confirming the voltage supplied to the power amp

    is the car stereo output still working normally without the power amp plugged in ?

  7. Jul 10, 2014 #6
    I have already already made sure of the voltage supply and have tested the amp using an ipod.
    I have used the preamp outputs before with somthing else and they worked fine so I am assuming they still do but I will go ahead and check.

    another thing is that I found out when I do not have the modules left right and ground inputs connected to anything then the speaker pops real loud upon powering on.
    but It pops the exact same way as when I have the rca cable conected to the inputs and I have all the speakers in my car including the one hooked up to this amp module.

    So it is almost acting as if it is not grounded... but then the speakers hooked up to my stereo pop too!

    really weird huh?
  8. Jul 10, 2014 #7
    the first thing you should do is isolate the problem, power the stereo directly from the car battery, then the inverter from the car battery(test the output voltage) then the amp running from the inverter from the car battery.

    I suspect the class-d amp's output is connected to the car ground somehow(which is a big no no).
  9. Jul 10, 2014 #8
    you can also use a DMM to see if you have a ground loop, check the inverter ground and your rca grounds (you want the car grounds and to be close to 0 ohms), not lifted.
  10. Jul 10, 2014 #9
    Good thinking, I did try the amp only off of the car battery and it is doing the same thing as it is when hooked to the 20v inverter, (I did check 20v btw)
    Google tda7492 and you can find the general circuit for it.

    Because it is a class d it is bridged so there should be no connecting to ground.

    Also I noticed that even when the power was off on the module and I touched the input ground pin to the ground on the rca cable I BArely herd a tiny scratch from my regular car speakers. But I suspect this is just weird sensitivety from the preamp... idk
  11. Jul 10, 2014 #10
    What is a DMM and ground loop?
    I am sure I know what they are but considering I am completely alone in this hobby of mine I dont have all the vocabulary down pat.

    ... OH!
    "Diode Multi Meter"?
    No, digital multimeter!
    Ya I have been useing the diode tester on my digital multimeter to test connection, explain the ground loop.
  12. Jul 11, 2014 #11
    did you try hooking the amp up to a speaker when it was running directly off the battery using your ipod as the source?
  13. Jul 11, 2014 #12
    Yes, and it worked fine.

    About a week ago I was making a guitar amp which used a split supply crossover network which I made by hand and a similar single supply class d module.
    this is when I learned that unless connected to seperate power supplies you can not mix the two types smtogether. And that truely seems like that is what is happening here.

    OH! Is that a ground loop?
  14. Jul 11, 2014 #13
    a ground loop is when one connection to ground has some resistance going to it's ground connection, the (other) properly grounded connections will see that ground as lifted (meaning it has some voltage potential) and it will put that lifted signal at ground.

    hope I explained that right, some other pf'ers can chime in here as I'm no EE, I barely qualify as a tech, all my knowledge is self taught.
  15. Jul 11, 2014 #14
    I understand I think,
    car is in the garage, alarm is armed, and people are asleep... so I will try that tomorrow morning.
    but what I still dont get is how my car speakers are poping.. I know that the stereo amplifier inputs are in parrelel with the rca cords, in otherwords there is only one preamp, but how the heck would that happen? Ground is only the reference point so I dont see how connecting ground to ground would effect the audio like that.
  16. Jul 11, 2014 #15
    imagine you had resistance between the ground points, all sorts of nasty things could happen.
  17. Jul 11, 2014 #16
    Because its changing the reference!
    One amp is useing 0v as reference while another is useing a lifted ground as reference.....that is some trippy stuff right de'er.
    And that very well could be the problen In some stupid way so I will make sure to check that tomorrow morning.

    I love going deep into thought about resistence and electrical referencing theory and then remembering that electrons actually move from negative to posotive...haha.

    ok besides the ground loop I was thinking there coukd be the posobility of somthing going wrong with the signal the preamp is putting out and tge signal that the module is meant to recieve, like maybe the stereo rca output is tabbed before the preamps output capacitor or somthing, (because the preamp has a single supply it requires an output capacitor to block dc).
    I happen to have a pre amp circuit on my breadboard right now so I am thinking I may simulate what is going on tomorrow and see if that goes anywhere.
  18. Jul 11, 2014 #17


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    Yes, dodgy earths are responsible for more automotive electrics problems than any other one cause. Modern cars are a nightmare to re-wire (which is what you are effectively doing). Is the Engine Management System fighting you in some way because it thinks there's a fault? Can you power the car radio and your amp directly from the car battery? The radio security system would need to be sorted out, too or you'd need to initiate it every switch on.
  19. Jul 11, 2014 #18
    Not sure if this will be of help but....
    Years ago i was setting up a home theatre system. Got it all connected and turned on the amp and hear a loud POP and the amp cut out.
    Turns out that when I wired one of the speakers the 2 leads of the wire were touching creating a short
  20. Jul 11, 2014 #19

    jim hardy

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    some of these things have a remote control line for the amplifier so it won't come on unless the radio is on

    i think the one i saw about twenty years ago used the radio's antenna control wire to tell the boomer amp it's okay to be on now

    doubtless there's a computer menu nowadays..

    i cannot imagine modern stuff not including fade 0n/fade Off

    but if you took your 12 volts from the radio feed, it might experience an inrush that'd explain a pop

    and you can't return amplifier power through the signal wires, as you already found

    you say this is a home made rig? Can you post a schematic?
  21. Jul 11, 2014 #20

    The way its wired is of course the stereo is just connected to all the leads comming from back of the stereo and the amp is running in parralel with my subwoofer power cable, so I have 8 guage wire running strait from the battery and another 8guafe wire grounding underneath a seat.
    then a 22awg wire comming from my subwoofer amp posotive 12v terminal to my amp module and another 22awg wire running strait from my subwoofer grounding point to my amp module

    I am not reading any resistance between the rca ground and the dc-dc converter ground
    Last edited: Jul 11, 2014
  22. Jul 11, 2014 #21
    Don't trust hand held DMM's too much. They don't measure small resistances very well. If there were only 0.5 Ohms between grounds then I know meter wouldn't pick it up and it's supposed be a top-of-the-line Klein model.

    There are other ways to measure small resistances with accuracy. You can use a wheatstone bridge for instance.

    For troubleshooting purposes you can just run a new wire from ground point to ground point and see if that fixes the problem.
  23. Jul 11, 2014 #22
    I rigged it myself byt tgis is none of my own circuitry this time, for the schematic of the amplifier you can just google tda7492 and tgere is a general circuit.

    but I just brought the amplifier inside and tried this whole thing on my bread board sence I happen to have a single supply preamp circuit laid out and it is doing the exact same thing as it does in my car. So it sounds like it is somthing with the module although the module works just fine it just is not accepting a signal useing the same - as ground.
  24. Jul 11, 2014 #23

    jim hardy

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    sorry, but i just can't decipher that description.

    1. what voltage do you measure from the battery's negative terminal to your amplifier's negative terminal ? You'll need long wires for your meter to measure that.

    2. TDA7492 datasheet page 22 fig 28 ,
    not that darn google middleman one
    shows mute and standby control lines, differential inputs with jumper labelled "single ended" , differential outputs, clock sync input, etc etc etc . You haven't mentioned whether you have used these terminals in accordance with the ST instructions.

    you'll have to stop flailing before anyone can help.
  25. Jul 11, 2014 #24

    this is the module.

    but like I said I have tested everything and simulated the circuit.
    I also read 0v between like I should between negatives.

    But guys I just found a 2x50 watt car amplifier that is smaller and less expensive than this project was goingto be given that it worked.

    Thanks for all the help and teaching me a few things. This is one that I will continue to think about.

    ttyl fellow electronic bro's
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