# Thread: Inverter Question

1. here is what is happening in a nutshell.....lets start from how i think u have it wired....

inverter at battery....connected to computer somewhere in car...audio output to amplifier in trunk i suppose....

the metal in the car has non 0 impedance(resistance with a non zero phase angle) because nothing is perfect so the metal is going to "impede" the flow of electrons...many people think u have to run a ground wire to the neg post of the battery because thats where u get the best power but for audio this is usually not the case...actually in most installs its the *worst* place to connect the stereo ground....the body of the car is so large that the impedance is low so u can have large current flow no problem even if u ground an amp in the trunk...this is why even tho copper is a better conductor than steel u will actually have a lower impedance in the car's metal than a smallish piece of copper wire of the same length(say from the engine compartment to the trunk)...

u asked about setting the DMM to DC...DMM = digital multimeter, DC = direct current, AC = alternating current...this goes back to people WRONGLY thinking that audio amplifiers produce DC when they are clipping...this is completely incorrect if the amplifier is working properly...it produces a clipped AC signal, NOT DC, even tho it "looks" like a square wave...so what?

well in the car u WILL have a non zero voltage because the metal has a non zero impedance...according to Ohms law voltage = current * resistance...well u have currents flowing all over the chassis of the car...the metal has a non zero impedance so of course your gonna measure a voltage...if u didnt then either u have done something wrong or your meter is faulty...the point is that this 1-5V that was mentioned is not necessarily noise...if u measure it using the DC setting of the DMM i would wager it is NOT noise but just the voltage drop your gonna have..this is easy to show using a piece of wire in your bedroom connected to even a 1.5V AA battery...and we have no noise there(technically everything causes noise but we have to think relative here just like anything else...the amount of noise u have is nowhere near an issue at these levels)....

but here is the kicker....because of this non zero impedance in the chassis of the car, AC currents and high frequency DC(such as u will have with computers and DC/DC power supplies, the car's sparking system, etc) traverse all over the chassis...there will be quiet spots and loud spots(basically spots where u have more currents flowing than at others) and this is directly dependent on the electron flow pathways from the battery to where u mount your equipment...there is no way to know where these quiet spots are unless u test for them...this is something that most car audio installers dont even know which is sad really....

ok so now we get to why the batt is a bad place to connect equipment...first off ground is not ground as any PS designer will tell u, and its because of precisely what goes on above...the impedance of the "ground" makes the ground relative, even on earth where someone mentioned that u have "true" ground...this is also incorrect...when a building is designed for lightening protection etc. such as say a train servicing yard, there is a grid of steel built under the structure where the steel grid intersects usually at about 40ft to 80ft square to help ensure that the ground potential at one side of the building is very close to the potential at the other side of the building so that u dont have these currents flowing through the building...remember Ohms law, if resistance = 0 then current must equal zero as well....the battery terminal is the spot on the car where all the current is returned so it is by definition the noisest spot on the car u can connect a ground wire ....that and the fact that u can measure a voltage difference between the battery terminal and the ground spot of the amplifiers makes a perfect situation for noise to enter your system....

with your system u have a different ground potential for your audio outputs of the computer when compared to the ground of your amplifier(basically u could put a voltemeter on the ground of the audio output of the computer and then put the other end on the ground of the amplifier and u have a non zero voltage) and this is a bad situation for u *especially* because your probably not using a "true sine wave" inverter...you probably using a cheap chopped DC version like the mainstream Vectors...they make the inverters for nearly everybody..radio shack, husky, etc. etc...to help this u can add high voltage capacitors to the output of the inverter to help filtering the noise out of the signal but this is not for the beginner..u can also beef up the DC side of the computer's PS but again this is not for the beginner....

the audio outputs of *aftermarket* headunits in most cars are grounded and the ampifiers input RCA ground is usually lifted off of ground by using a resistor inside the amplfier...this is to avoid what is called "ground loops"(what your dealing with now is a ground loop) but it only works well when the amplfier and radio are at about equal ground potential and both are grounded to quiet spots..

this is why at times people mention that u should just move the ground wire when u have noise...u might hit a quiet spot on the chassis and the noise goes away..this is also why people mention isolators..u isolate the ground wire of the computer to the amplifier so u cant have a ground loop assuming the rest of the system is installed properly...a bandaid but it works in many sitations..it has disadvantages in sound quality tho so no hardcore audiophile worth his salt ever uses one of these...in OEM equipment where u have an OEM amplifier, the cars usually come with a +-0.5V balanced system...people always talk abot needing X output voltage on headunits to have no noise or to have great sound quality, blah blah blah...its mostly based on ignorance...aftermarket manufacturers could easily solve the problem(and some do!!!) by just making balanced audio cards(hint for u guys, using a pro audio card like the EMU by creative will solve your problem if u get a balanced receiver) that we can easily use in the car and mating it with something like a pheonix gold balanced receiver or better yet, make all amplfiers balanced.....this would allow all the "common mode" noise or noise flowing down BOTH wires to be cancelled so that u get just music and no noise...***EVER***....u wanna ensure u never have engine noise again? run balanced....

here is another myth...people talk about running signal wires and power wires down opposite sides of the car....no matter where signal wires are run they will be close to the chassis of the car which means they will be close to all those currents i spoke of earlier and these same currents are the ones that enter our system...i hope it now makes sense that where u run the signal wires inthe car makes little difference if u design and implement your install correctly....

from the above i hope it's evident that it's mostly trial and error for the average person and for those who know what they are testing for it can be a whole ton of work just to get a quiet system....

i hope i didnt bore u guys/gals too much....the above is the tip of the iceberg and i have a headache so i didnt wanna get too indepth....i also hope u have a slightly better understanding of what's going on with noise in the car.....

Kevin

Originally Posted by phatchink
umm what is "DC setting of the DMM...."

I thought the battery would be the most effective ground because well, gound is ground right? so I took the liberity of using 4m of wire and connecting it from the battery to the back of the trunk.
so why would extending the ground wire make it a worse ground?

So to sum it up If i want this noise to be gone I'm just going to use a short ground wire? sounds simple enough, I'm going to try it tomorrow and I'll let you guys know how it goes

o can can you tell me what some of the other easy solutions are just for refrence?

2. WOW!!! I mean WOW tundra, I had no idea what i was getting into That is (to me) already a very indepth detail of why things are happpening and I think i'm beginning to see the light ! Too bad I didn't ahve a chance today to test it out cause I was busy during the day and its dark now.

How do you know all this? Are you somekind of an installer??

O and THANKS for all the info I'm actually going to save what you just said in a file

3. no problem...i hope it helps u have a quiet system....

Originally Posted by phatchink
THANKS for all the info I'm actually going to save what you just said in a file

4. Tundra, Excellent post. Should get it stickyed. I know quite a bit about this issue also, and it frustrates me when local shop fill peoples heads full of crap, basicly.

John

5. ## noise

i dont know that anyone answered your question about what noise is and where it comes from..........noise is distortion in a signal that carries no useful information...............there are two main types of noise additive which can be removed (not always easily) and multiplicative which cannot be removed.......the noise in this case is additive.............the reason people like to run signal wires and power wires on two opposite sides of the car are this: current excites a magnetic field which can in turn excite a current in a nearby wire...signal wires usually have small amounts of current until they reach the amp....therefore the magnetic field of these currents is small and unlikely to excite any current in a nearby wire.......power wires have a greater current and greater field strength making them more likely to excite noise in a nearby wire

im sure there is more to this but im just sharing what i can think of on the spot that might help

6. the part u added about multiplicative and additive noise is WAY above the level of most people reading this forum and assumes an understanding of physics and differential equations....come on now

without getting into hardcore physics the noise is caused by the alternator and *possibly* the inverter....

good info tho for sure

Kevin

Originally Posted by EEKUFX4
i dont know that anyone answered your question about what noise is and where it comes from..........noise is distortion in a signal that carries no useful information...............there are two main types of noise additive which can be removed (not always easily) and multiplicative which cannot be removed.......the noise in this case is additive.............the reason people like to run signal wires and power wires on two opposite sides of the car are this: current excites a magnetic field which can in turn excite a current in a nearby wire...signal wires usually have small amounts of current until they reach the amp....therefore the magnetic field of these currents is small and unlikely to excite any current in a nearby wire.......power wires have a greater current and greater field strength making them more likely to excite noise in a nearby wire

im sure there is more to this but im just sharing what i can think of on the spot that might help

7. ## agreed

i will agree the whining is def from alternator

8. I'm too lazy to read all the posts so disregard if its fixed or if someone already said this.
Radio Shack Ground Loop Isolator

9. Okay... now that we've gone through all that nonsense... here's what *I* think the problem is.

Does the inverter whine when you have the engine running? If it does, you've got a different problem then what I'm thinking of. But if the inverter is okay with the engine running, and you're saying the inverter whines after 15 minutes with the engine OFF, then I think I know your problem:

It is NOT alternator/ground noise.

It looks like it's just time for a new battery.

The whining from the inverter is the "low battery" alert. It comes on after 15 minutes, and stops when you turn off the inverter. The whine is coming from the inverter, not alternator interference or whatever. The whine is supposed to happen; it's a low battery alert, and if you're using a 3 year old stock battery, it may not be holding its charge as well. Get your battery tested, it may be time for a new one.

For preliminary testing, use a voltmeter (multimeter) and check your battery voltage before you turn on your inverter. It should be 12-12.5V. If it's less, it's probably time for a new battery. If it's 12V and you're getting the alert in 15 min, you may have a faulty setup with a drain somewhere. Maybe turn on the inverter and periodically test the voltage to see how fast it's dropping.

10. I agree that the noise he is most likely talking about is "audible" noise from a low voltage alarm in the inverter and not "electrical" noise from the alternator. You said you have the inverter wired up with 4m of wire. What gauge? If it isn't a low enough guage then you will have quite a voltage drop (and fire hazard) by the time it gets to the inverter so your battery may be ok. Ground the inverter to a good point in the truck instead of to the negative battery terminal.

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