Engine noise, no luck with bypass filter

[silentblackhat]

I built a preamp based on the LM386. I have rca inputs into my Alpine headunit in my car. without saying all the stuff i did to do this, i spliced the output of the speaker to the posative of the RCA. I put the ground of the speaker output to the negative of the RCA. so it would play on both channels, i hooekd the positive of the single output of the preamp to both posatives of the rcas.(this runs a computer microphone). Currently i have the amp in my dash running off of the cars battery power.

the problem that i am having is that when the key position is "on"(not running engine) i get a constant pitch noise which changes when the fuel pump charges and things like that. when i start the car i get the same noise but with the alternator noise also. BUT with the car on the Acc. position, everything is clean, no noise at all.

I tried to put a .1uf cap between the posative and ground of the amp to bypass the noise to the ground but that didnt help any.

Can anyone help out? i checked the ground pointand there isnt any voltage drop or anything to create a closed loop. Thanks

[azira]

Honestly, the LM386 is a piece of junk, you should use a Burr Brown OPA 134 or TL072 or ... well look at this link D.Self opamp reviews . I would investigate the power supply to your microphone equipment for engine noise getting into the system. Otherwise the power to your head unit is dirty.

I'm not exactly clear on how you wired your headunit RCAs though. Can you try explaining that again with more details or a diagram?
--
Danny

[Cal Weldon]

I have moved this to Car Audio

[ghudnub]

The noise definitely indicates there is a ground loop.

You have to isolate the output of the amp from the microphone input of the computer. There are off-the-shelf ground loop isolators that you can plug in series with your RCA's.

I'm not really clear on your current setup though...

[Perry Babin]

The engine noise is more likely to be due to a ground loop than a poor quality op-amp.

I don't understand why you're using the speaker wires to connect to RCA cables. Maybe you could explain this a little better. For most head units, the speaker negative is floating at ~1/2 of B+ and the RCA shield is grounded so it's not a good idea to connect them together.

If you're using an op-amp as a mic preamp and you're grounding the input of the mic power supply to chassis ground, there's a good chance it's causing the ground loop. To help eliminate it, you could try grounding the mic power supply to the case of the head unit.

If that doesn't work, you may want to try running the mic preamp from batteries. This would allow you to verify that the preamp and its power supply are indeed working properly. Running it off of batteries would eliminate the need to ground it to the vehicle which would eliminate the possibility of ground loops. You can find battery holders that will hold 8 AA batteries which will give you ~12 volts.

If the preamp works well from the batteries, using an isolation transformer (a ground loop isolator) will break the ground loop.

[sagarverma]

hi silent

SAME PROB HERE(100%)

i did one simple think.got a computer smps(old and dead).took out its i/p inductor a nd used it in series with the power supply.i.e one end of inductor on battery and other to the i/p of the system(amp,preamp common power supply).

believe me,i tried everything from caps,10nf,100nf etc etc,ground loops and the list is long.but after all pains this simple method is what bailed my amp out of this quaigmaire of noise.

[silentblackhat]

Before I did this, i used a 9volt battery and it worked fine. no noise at all.

the way that i have it hooked up is the power supply for the cd player split in 2. 1 goin to the headunit and 1 goin to my preamp. i have the amp grounded by my shifter(i have a 1999 honda civic).I basically took 2 rcas, cut the ends off of one side of the wire so its the RCA head and then just the bare wire on the other side.
i have the output of the pre-amp connected to the positive of the rca input and the 'negative' wire of the rca not connected, i had the negative of the rca hooked to ground b4 and it did this same thing . It works well when the car is off but only has noise when its turned on.

does that make it more clear? Am i connecting everything right? im still trying to figure out whats wrong with it. thanks for the help so far.