Why does the denoising autoencoder always returns the same output?

I am trying to implement a denoising autoencoder (DAE) to remove noise from 1024-point FFT spectra. I am using two types of spectra: (1) that contain a distinctive high amplitude spectral peak and (2) that contain only noise peaks. If I understood correctly, I can train the DAE using the corruputed spectra (spectra+noise) and afterwards…

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