Objective Analysis and Perceptual Evaluation of LA-2A Compressors and Vocal Recordings

The LA-2A compressor is a popular optical dynamic range compressor and is much used in popular music productions, particularly as a vocal compressor. There are many claims about sonic differences between different LA-2A versions, but most of these are anecdotal with no empirical evidence to back them up. This paper explores the objective differences between six hardware LA-2A compressors, three vintage Teletronix units, and three Universal Audio reissues. Firstly, it conducts a series of measurements on six compressors, using frequency response sweeps, total harmonic distortion measurements, and tone burst to explore their attack and release characteristics. The results showed subtle differences, particularly around THD and attack and release. Subsequently, an ABX listen test was conducted with 17 trained listeners to explore the audibility of these differences. The test comprised comparing three songs where the vocal tracks had been compressed using the two most dissimilar LA-2As. The results showed a mixture of significant and nonsignificant results, suggesting that discrimination is context-dependent and difficult to discern in typical music productions.