i got the roger armstrong first up and it was terrible! i got it as it was CHOICE recommended and it had bad interference etc. my BIL and SIL also got it and found the same thing... we all got rid of it within 1 month...
I then bought an angelcare for DS1 - and we love it. but i dont think you want the sensor pads etc?
for a normal sound monitor we just bought the oricon digital monitor and it is wonderful, no interference or even buzzing etc. it has amobile parents unit too which is great if you are going into diff rooms/outside etc when bubs is asleep. i love it. you can also see room temp on the parents unit and i think it plays music etc if you want to (havent used this) and has a night light (havent used this)