OPA548: output Voltage drop (from 11.2 V to 4.4 V) on addition of load(100 W, 8 ohm-Loud speaker )

Part Number:OPA548

Hi,

I am using OPA 548 as a power amplifier to drive a 100 W, 8 ohm speaker in constant voltage mode. Without any load,the gain is as expected.But, as soon as I attach a load to the output pin,the voltage drops drastically. The details are as follow:

Pin1- Vin = +/- 0.5 V 

Gain = 23

Vsupply = 16 V(+ve), Negative terminal grounded.

Vout =11.2 V without load, 4.4 volt with speaker

Current lim = 3A (Tried with maximum 5A as well)

Snubber capacitor- R= 10 ohm, C=0.01 uF (ceramic)

Bypass capacitor-  0.01 uF (ceramic)

Capacitor with power source- 10 uF (tantalum) II 0.01 uF (ceramic)

Heat sink with fan.Surface area-300 sq. cm with two fans(12V)

Any help will be highly appreciated.

Thank you.