Recording Problems - Sound level drops

I am using Snow Leopard on my Mac. Whenever I record a vinyl record to Audacity (running a turntable to a USB phono preamp with volume control) the sound level in Audacity drops (in the waveform). I adjust the volume for the recording so it won’t clip, but after I stop the recording, the sound level drops. Is there something to prevent this or is there something wrong with my laptop ?

Any info is appreciated

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