The Measurement of Audio Transformer Colouration

This thesis presents and discusses research undertaken into the detection and measurement of colouration produced by audio transformers. It is common for transformers to be subjectively described by audio professionals as ‘warm’, ‘fat’, ‘smooth’, etc. however there is little evidence to show if there is an audible difference and if there is a correlation between these levels of perceptual attributes and the performance of the device. Therefore, the research question was defined as: is there an audible difference with a transformer?

A review of the objective and subjective elements of the study was conducted. First the history, application and operation of the audio transformer with an aim to understand the objective measures and performance of the device with focus on the nonlinear distortion response. This also includes the design and testing of a suitable test circuit used in the measurement method. The subjective review concerns perception of distortion and testing methodologies for investigating audible differences. The conclusion of the review is that few pieces of research exist showing the relationship between device, distortion and perception.

The testing of each transformer involved the use of a specially designed test circuit using a variety of measures including THD+N and frequency domain analysis, to provide the most information about the operation of the device. Using the same setup, the device responses to a variety of samples were recorded and implemented in a double-blind triple-stimulus with hidden reference test using trained listeners in accordance with the ITU-R BS.1116-3 recommendation. The test results were then analysed for a random distribution using a 1-tailed binomial test. The results of the analysis show a high likelihood that the bass samples were audibly different shown by the significant p-values of all 3 samples at less than 0.001. A slight correlation seems to exist with THD+N, 3rd harmonic distortion and level however with no other obvious trends, it was concluded that the distortion and therefore the audibility is programme dependent.

It was concluded that transformers are likely to produce a level of distortion deemed audible although the effect is considered to be programme dependent.