Can you record a conversation with a cop without them knowing and use it in your defense?

Answer 1

First, you cannot record conversations over the phone, most states deem it a felony. However, if they are out in public or just 'outside" yes, you can as anyone could hear the conversation, therefore there is no expectation of privacy.

Another View: Use caution in following the advice of the first answer. Many states apply the same criteria to face-to-face recorded conversations as they do to phone conversations. It will depend on your state laws and, in the case of interstate phone conversations, what the state law of the out-of-state party may be.