Circuit" the circuit has output level flat over the audio frequency range of 20
Hz to 20 KHz. What changes are required in the circuit to get flat output level
from 100 Hz to 25 KHz.
I believe the high-end frequency limitation in this circuit would be the rms-dc
converter used for a detector and the low end would be the 10uF capacitor in
the output. To set the low limit to 100Mz then the new capacitor value would be
1/100*6.28*RL where RL is the load resistance.
To extend the response to 25kHz one could perform a measurement and see what
the response is at 25kHz, if the response isn’t satisfactory, try an rms-dc
with more BW like the AD636 or AD536A.