Current state:
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Wish I had the Topping A90 in the stack, but my old THX AAA 789 amp is not terrible.
Doing a bit of a bake-off between the D90SE and my Gustard X16. I have levels matched to within 0.01 dB or so.
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(Ignore the shape of the plots. That's just the native response of my Focusrite Scarlett 2i2).
There's really not much between these two...as expected. The Topping struggles with some MQA albums, like Katy Perry's
Smile (don't judge me). The Gustard had minor clicks during soft passages on DSD until I updated the MCU firmware (fixed on new X16 DACs). None of the DACs I've tried under $1k are perfect, but these two definitely sound amazing.
Both are XLR via short cables into the Topping Pre90/Ext90, so the load should be identical. I'm not a big fan of A/B tests, but I thought this would be fun.
I can share plots of the different filters (amplitude and impulse) tomorrow if anyone is interested.
Edit: transport is the iFi ZEN Stream, but I now suspect that the skipping I heard on Ms. Perry's album is wonkyness in the iFi when the USB and S/PDIF outputs are grouped into the same zone in Roon. I'll switch to a pair of RPi4s running VitOS to see if the problem goes away.
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