I think the key here is the aftermarket stereo. Back in the old days when I messed with aftermarket stereo systems, it was quite common to have a "ground loop" problem where you'd hear a whining noise through the speakers and would get louder with engine rpms.

The solution, I seem to remember was to either find a common ground for all the audio equipment and/or get a ground loop filter (or isolator) from a place like Radio Shack.

It sounds like that's what's going on here...

 

I'm not getting any added noise when I rev the engine though, and I'm fairly confident that all of the audio equipment in my car is sharing the same ground, I'll find out though.

 

They used to make a separate noise suppressor that hooks up between the receiver's power cord to the car's power. It's about 15+ years for me, but it used to reduce the alternator noise coming out of the speakers when you accelerate. The power following to your inverter transmits into your receiver via the power connection. Your receiver, in turn, transmit that signal into the speaker making the noise.

Hope I'm not preach to the chorus.

 

Hey that sounds like a low pass filter.

 

Do you really think that this sound caused by the inverter is LIKELY to be above 10KHz??!!!

It's more likely to be a harmonic of 50Hz audible at low audio power.So I think, like the previous post, you're filtering entrely the wrong frequency band with this circuit.

ABC

 

Yes, I do think that part of the noise is high frequency, if it's static-like (crackles and pops). I'm not trying to tune a low-pass filter, just to quickly sort out a problem using basic and cheap components. Total cost of the two capacitors is likely to be less than $5, with the lower capacity cap costing not more than 50c. To get a proper LP designed would require some idea of the reverse input impedance of the inverter. However:

1. By having a 10,000 uF as the main capacitor, you are effectively filtering out any frequency above about 100 Hz, assuming an inverter load resistance of 0.5 Ohm (0.5 Ohm is a reasonable value, given it's exactly 300 W for a nominal 12 V) cutoff frequency is almost exactly 50 Hz.

2. All spikes ("static") have a very high proportion of high frequency signal, and it is good practice to put a small, high quality cap in parallel to high capacity electrolytics to filter out these spikes. This lowers the low pass frequency further, though in practice it does not really affect it, as the total capacity is practically the same (0.1% difference).