Thread: Inverter Question

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?