The Brewers looked to have had all three points in the bag at Sixfields after establishing a convincing 3-0 lead in the first half through Andy Corbett, John McGrath and a Chris Dunn own goal.
But the visitors had to withstand heavy Northampton pressure to cling on for a valuable away victory as John Johnson and Andy Holt pulled goals back and Billy McKay headed against a post as the Cobblers came close to salvaging a point.
Sampson said: "We deserved to get something from the game with our second-half performance and we had enough chances to do that.
"But our first-half performance was very poor and I take full responsibility for that. We didn't put them under pressure and gave away some poor goals but the second half was completely different and all credit to the players, they were unlucky not to get a point.
"We needed to go more direct and show more urgency in the second half. We did that and put more balls into the box but it wasn't to be."
Source: PA
Source: PA