What was the first power cable that you noticed a difference in the sound?

Nordost Blue Heaven on my Sony surround receiver. I was able to try it for free. I came back a week later and gave the store the money. It transformed my system which I used for music and video. I still own those cables. Blue Heaven is 16 gauge so I would recommend the Red Dawn for it now. I use a Puritan Classic Plus on my Marantz surround processor and a Straightwire Pro Thunder on my NAD M 25- 7 channel amp. I have a separate 2 ch setup that has nice power cables. Probably the first cable you should upgrade even before you start auditioning speaker cables. 

The first power cord that I ever made I thought made a bigger difference than an Acrotec 6 nines power cord that I thought didn't make much difference.  Based on the pseudoscience (voodoo) of the time, It's braided of 18 gauge stranded wires. I don't remember how many but it was fairly thick.  The entire thing was wrapped with strips of bike inner tubes to keep it tight, with the entire thing wrapped with that thick magnetic tape, the kind used in shops, and work areas to hold tools in place with heat shrink on top of that and a blue braided cover and decent plugs.  All truly done with nothing special materials.  The idea of the magnetic tape at the time was to scatter or lessen outside electrical interference but I guess the braiding process takes care of that and I might have reintroduced noise with the magnetic tape.  

   Honestly, I never paid much attention to stuff (power cords) like that at the time but years later I loaned it to a friend to get his take on them. A month later I went to pick them up and I was their a few hours listening to his system with the power cord attached to his amplifier.  As I was leaving he had the standard cord reattached and was playing a live Tony Williams quintet recording  I just heard and his drum kit just collapsed to the left speaker.  We put the homemade cable back in and within a few minutes back to listening, the soundstage changed most notably Tony's drum kit was spread from the left speaker to the center-left of the stage.  Neither of us could figure out why that would happen but it was repeatable.  This was really one of the first times I noticed such a big change with cables and a power cord at that.  Since that time I paid much more attention to powering my system.

There is one simple answer to this:

If you heard any difference between a standard 12AWG power cable and a $500 Audio-Woo-Woo cable, it was confirmation bias or just delusion. 

It's such fun talking about this stuff with studio engineers. You know, the guys working in multimillion dollar studios, who record and master the music you buy. They laugh endlessly about the absurdity of audiophilia nonsense. But the exotic power cables gets the most laughs. You'll hear phases like "more money than sense" or "crazy deluded muppets".

Virtually everything the audiophilia community is obsessed with defies the laws of physics. Or is of such minuscule consequence that merely moving the speakers a few inches will negate any claimed effect.

If you want to spend some money, get an electrician to run you a nice clean 20amp power supply in Romex 10AWG solid from your fuse box to a set of six hospital grade power outlets. You can get the lot from Home Depot. Top it off with a decent breaker like an Eaton 20amp ACFI/GFCI and the. forget about the whole thing.

The first power cord that made a significant improvement for me is the Shunyata Venom V10 NR.  To my ears, it sounds best on my DAC.  A few years later I purchased a Shunyata Delta  V2 NR. Wow, that was an even greater improvement in clarity and  realism. I also use Shunyata Venom Digital power cords on my iMac and DDC.  Happy listening!

"If you heard any difference between a standard 12AWG power cable and a $500 Audio-Woo-Woo cable, it was confirmation bias or just delusion."

- Well, that clears it up, never heard that point of view before 🙄