the transformer you are using so far is to use only for halogen lamps, not for LED bulbs, the vast majority on the market LED bulbs for low voltage (12V DC) need to use a special stabilized ballast for LED technology, this is and rectified ( DC). There are very few ampoule led easy connect on the market, which can be operated with AC voltage
(labeled 12V AC / DC). This in turn should not be operated on electronic ballasts for halogen technology, because these electronic ballasts provide a high frequency AC voltage, which would greatly reduce the life of the LED lights.

closing words on this topic:

If you want to switch to LED technology, then correct or not, i.e. always operate LED bulbs at special ballasts for LED technology, otherwise the life of the LED's is shortened and the cost is gone.