The general answer is no, and you must be very cautious about this because you can actually be criminally prosecuted for doing so. There are exceptions laid out in Penal Code 633.5, but you should contact a lawyer before you even consider doing it. In some circumstances, police can authorize you to record the perpetrator without his consent. Such information can be very valuable.
CA Penal Code Section 633.5
Nothing in Section 631, 632, 632.5, 632.6, or 632.7 prohibits one party to a confidential communication from recording the communication for the purpose of obtaining evidence reasonably believed to relate to the commission by another party to the communication of the crime of extortion, kidnapping, bribery, any felony involving violence against the person, or a violation of Section 653m. Nothing in Section 631, 632, 632.5, 632.6, or 632.7 renders any evidence so obtained inadmissible in a prosecution for extortion, kidnapping, bribery, any felony involving violence against the person, a violation of Section 653m, or any crime in connection therewith.
Learn more about domestic violence and violent crimes.