Whoops - Having read your original post in more detail, I see that you need 19v - the adaptor which I've suggested should be fine.

Incidently, you can't use a transformer with DC, inverters work by converting the DC to a rough AC sinewave and then using a step-up transformer.

I'm not sure exactly how the Laptop Adaptors work, but I assume that it uses some sort of solid state bridging technique. Anyway the one I have works fine - it even seems quite happy running my little Canon printer as well.