...amplitude of the peaks is smaller than that of the valleys by a few db.
Here's what a waveform looks like after post-processing.
Today I tried recording without the phono-preamp and normalized the output to see if it's a problem with the way the tonearm was grounded...At first glance it looks OK. But looking at it more closely it's hard to say if it's any better as certain parts still appear to have the same problem but it looks obfuscated by noise or whatever else might have gone on in between.
There was something interesting I noticed about the preamp. When I disconnect the right channel from the input but keep the left one in I hear noise. The noise goes away when I touch the right channel of the rca cable (which is unplugged). Doing this the other way, keeping the right one in and pulling the left one out results in no noise. The noise is on both left and right output.
The left channel is louder than the right but that's not a major problem for me now