No, the Sound Amplification Input extension is not supported when you are using System 7.5.3. If you are using System 7.5.3 on a Power Macintosh 7200 computer, you should use a microphone which supplies the proper line level signal, such as the Apple PlainTalk Microphone.
The following Tech Info Library article provides additional information about the specifications of PlainTalk microphones:
Article 15884: "
PlainTalk Microphone: Specifications."