We reduce can when it is used with another verb or in any other unstressed position in the sentence. It sounds like "ken" or "kn." For example, we say "I ken do that, I kn see it." We don't reduce can't, so it will have a clear vowel /æ/.
When can and can't are stressed (eg, "yes, I can" or "no, I can't."), can is usually longer in duration, while can't is more abrupt. "Yes, I caaaan." "No, I can't."
Here are some examples of can and can't in stressed and unstressed positions in sentences.