A significant key for the success of arrhythmia diagnosis using ECG is detecting the atrial electrical activity (AEA). Despite extensive research, there is a diagnostic problem in detecting AEA in some arrhythmias, especially when the AEA-wave is hidden in other waves. Our proposed method utilizes the well-known linear combiner usually used for noise reduction, and adapted it for AEA detection. The physician/user marks one prominent AEA segment. Then, a synthetic signal is created that contains an isoelectric line in addition to a Gaussian in the delineated segment. The 6 precordial leads, lead I, and lead II, serve as reference signals, so by finding the appropriate weight coefficients, their linear combination is forced to converge to a signal that is similar to the AEA signal. At the final stage, the resulting signal is band-pass filtered and the peaks higher than a certain threshold are determined to be AEA-waves. Sensitivity of 94.0% and precision of 90.2% were achieved in detecting AEA from the standard 12-lead ECG for various arrhythmia types.