battery turning on AMP

[trialabc]

Please forgive me if I have posted at the wrong place.

I have tried using a car battery (12V 17Ah) to power on a Spider III amp. I use an inverter to convert 12 to 230Vac and turn. For guitar, I used an electric acoustic for testing.
It works, but the problem is that static noise is heard.

When I lower the volume output of my guitar, the static noise is smaller. When increased, static noise output increases too.
I have also tested using 230Vac from wall, it does not have static noise output. So I think at least it is not introduced by the jet and my guitar. And I am guessing the noise is introduce due to the fan on the inverter.
I have tested the voltage for the inverter output. It does not reach 230, but 190 (AMP is turned on). Not sure if this will affect the performance of the AMP.

Is the something I can do except replacing a better inverter? Or is it inevitably noise will be introduced in this configuration?

[DJPhil]

Almost every cheap inverter out there will have problems like this. Mains voltage is usually a very well formed sine with almost no harmonics (just the primary frequency [50Hz or 60Hz], nothing else), but most inverters output something that looks a lot more like a square wave (even and odd harmonics as well). The higher harmonics don't really cause problems with much, but for audio amps with an unregulated DC supply (most guitar amps) the extra noise can play havoc.

The amp's power transformer will filter most of the higher harmonics because it's optimized for mains frequency, but some of the lower ones may get through. Then what remains goes straight through to the power rails and is picked up and amplified by the preamp stage.

I have tested the voltage for the inverter output. It does not reach 230, but 190 (AMP is turned on). Not sure if this will affect the performance of the AMP.

If you are not using a true RMS multimeter (most sub $100 meters) then it will read incorrectly for non-sine signals. The inverter is probably not producing a square wave, but something partway between square and sine, so it should read lower than nominal on such a meter. Odds are fairly good that the inverter's operating fine, it's just that it's difficult to get clean power with one.

Noise is pretty much inevitable. You might be able to minimize it with some careful filtering, especially if it's just getting into the input stage. It might be possible to use an inline filter as well, though I don't know if there are any designed to filter frequencies in the audio band. Usually they're used for filtering out 100kHz + EMI.
There are inverters out there with sine output, but they tend to be expensive.
Another possible option is buying or designing a DC to DC converter and powering the amp directly from the battery that way. Designing such a power supply for any significant load is very difficult and more than a little dangerous, but you might find something cheap on the surplus market. These tend to use a switching topology as well, just like inverters, but they're designed to output as clean a DC signal as possible. If you found one that was a good voltage match for your power rails it would essentially plug right into them and off you go. Any noise problems would be easy to combat by filtering between the converter and the rails.

Hope that helps some.