System 1 is pretty similar to MoeSizzlac's simple helmet speaker system which can be found here, with a couple of small changes to improve volume and clarity. I had initially planned on using an electret microphone, but wasn't happy with the volume level I was getting. Electret microphones are quite sensitive, so you can't turn your amplifiers up very far before the system starts feeding back. I hopped on Ebay and for about 10 bucks found a microphone designed for handheld radios. This microphone is dynamic which tend to be less sensitive and more directional than electret mics. I also ordered some cheap preamp boards, since the mic didn't have a built in preamp like the electret mic. From the preamp I wired up two little amplifiers, the same ones Moe used I think, and hooked up a speaker to each one. I mounted the grills of the ODST helmet, and crammed the amplifiers and preamp into the right cheek. The microphone attaches to the helmet right in front of my mouth with some velcro.
So, this design is pretty loud, but I'm a sound guy and I figured it couldn't hurt to try getting it a little louder. I designed a notch filter centered at the frequency of the feedback I was getting (about 1500HZ), and added it into the signal chain. It does eliminate feedback at that frequency, but is a little too drastic in the way it effects the overall sound. I think I may try designing a little equalizer to put in the left cheek.
If I have time today I'll do a video comparison with and without. Like I mentioned in my post, I think the notch filter is a tad aggressive. You probably only need to attenuate 3-6 db at the frequency of the feedback to help. Id like to implement a dual band eq with adjustable gain and frequency for each eq point. This would make it far easier to dial in an eq curve that takes out just enough to cut out feedback, but not to where it significantly effects overal volume. If you're curious, the filter circuit I used is called a twin-t.