Sometime last summer, a friend invited me over for a visit. We hadn't seen each other for a while, and both of us like audio. So we had fun playing each other's music and audio gear.
This friend had set up a 2-chanel system in another room, using mainly older gear he already owned, but wasn't using. His speakers were decent quality smaller 2-ways, and he used an old CD player with an older stereo receiver that had no internal DAC. No problem with any of those. But he did go a bit crazy as he bought 2 different external DACs. One was a Topping model, and I don't remember the name of the other. If I remember correctly, he paid at least $100 for one, and over $200 ($250?) for the other. I asked why he didn't use the CD player's internal DAC, and offered the opinion that he had clearly read too many reviews about external DACs.
He had listened to both and already had opinions as to their differing sound qualities. He admitted that he did the comparisons by himself, and asked me if I would help him do a blind comparison. So I humored him. I switched the RCA connectors from one DAC to the other in paired A vs. B comparisons, while he listened. I sat so all he could see was my back – he couldn't see what I did – and I never looked at him. I wrote down what was A & what was B in each comparison, while he wrote down which DAC he thought he heard. He chose a short ~1 minute music passage on one of his CDs.
I asked him to simply write down if he could hear a difference, YES or NO. If you're not sure, the answer is NO. If possible, he could also write down what DAC he thought he heard, but don't spend a lot of time trying to decide which DAC he heard. And don't bother with preference.
We stopped after 10 different comparisons. I got bored listening to that music passage, and so did he. Six of those paired comparisons were A-B or B-A, and four were A-A or B-B. After I handed him my list, he studied it along with his list … and groaned. When I did play those six A-B or B-A comparisons of different DACs, he wasn't good at identifying which DAC he heard. And when I played those four A-A or B-B comparisons (the negative controls), he thought most of them were different.
The clear result, even after only 6 paired comparisons and 4 negative controls, was that he could not tell the difference between them. He said he'd return the more expensive DAC and keep the less expensive one. We never got around to comparing his CD player's internal DAC to either of the external DACs.