TRACON Controller reported having communication problems due to a headset issue resulting in the controller issuing a wrong altitude to an aircraft that had them below the MVA. The controller was able to issue a new altitude and the aircraft arrived safely.

Narrative

I was having frequency issues told the supervisor. He changed out the bottom part of my headset. Tried 3 frequency checks with two aircraft nothing was heard. Unplugged and replugged the headset did a frequency check both aircraft heard me. Gave Aircraft X a base turn and decent to 7000. Noticed they were descending below 7000. Questioned it they said I gave them 5000 I told them to level off and issued a low altitude alert but I gave them the wrong MVA altitude. The computer alert went off after or while I was issuing the low altitude alert. Then I pointed out the air field. And gave them a visual approach via the RNAV Y course. Aircraft landed with no other issues. After being relieved I noticed that when I touched the cord on my lower headset everything cut out till I hit the cord again. Better equipment.