asterisk users - Mar 2011 - 1.8 and no alsa input

Is there a way to configure asterisk 1.8 and ALSA so I dont read 
anything "in" for the input port.

I tried this in asound.conf
pcm.nullpcm {
    type null
{

then in the alsa.conf file
input_device=plug:nullpcm

This did not seem to work as I still get feedback.

Is there a way to do this?
Thanks,

Jerry