Cal. Commercial Code § 3308

(a) In an action with respect to an instrument, the authenticity of, and authority to make, each signature on the instrument is admitted unless specifically denied in the pleadings. If the validity of a signature is denied in the pleadings, the burden of establishing validity is on the person claiming validity, but the signature is presumed to be authentic and authorized unless the action is to enforce the liability of the purported signer and the signer is dead or incompetent at the time of trial of the issue of validity of the signature. If an action to enforce the instrument is brought against a person as the undisclosed principal of a person who signed the instrument as a party to the instrument, the plaintiff has the burden of establishing that the defendant is liable on the instrument as a represented person under subdivision (a) of Section 3402. (b) If the validity of signatures is admitted or proved and there is compliance with subdivision (a), a plaintiff producing the instrument is entitled to payment if the plaintiff proves entitlement to enforce the instrument under Section 3301, unless the defendant proves a defense or claim in recoupment. If a defense or claim in recoupment is proved, the right to payment of the plaintiff is subject to the defense or claim, except to the extent the plaintiff proves that the plaintiff has rights of a holder in due course which are not subject to the defense or claim.