The NFS-320 and the FCPS-24 (assuming there are any in that system) do not put out FWR.
<URL url="Notifier & Fire-Lite: Filtered or Full Wave Rectified? text=“viewtopic.php?f=60&t=6616”>Notifier & Fire-Lite: Filtered or Full Wave Rectified?
Electromechanical horns do sound raspy on FWR, however.
When you compared them to 9838s, are you talking about when a 9838 sounds smooth and nice or when it sounds raspy (if powered by FWR) ?
Another possibility is that the NAC is overloaded, and unable to provide enough power for the signals on the circuit. In that case they should have installed a NAC booster panel such as an FCPS-24S8 in the NAC to keep the power source strong.