Low Noise Technology

LONO™ technology offers significant improvements in the acoustic performance of toroidal transformersPicture10


  • Quiet with DC superimposed on the input
  • Quiet with AC line over-voltage
  • Specified noise performance
  • In-rush current reduced by 40%


LNTIt also represents the culmination of years of research at PLITRON into the causes and nature of acoustic noise in magnetic elements and the methods for improvement. An important side benefit of this technology is that inrush currents are also greatly reduced. Over the past 25 years, toroidal transformers have replaced conventional transformers in many applications because, among many other benefits, they are quieter under normal conditions. But, because of the their highly efficient nature, standard toroidal transformers become noisy under adverse line conditions, such as overvoltage and DC offset, (and tend to draw high inrush currents during start-up). With increasingly deteriorating power lines over the years, these adverse conditions are becoming more prevalent. PLITRON LONO™ toroidal transformers suit a variety of applications, but the features of these products are probably best utilized in the demanding Hi-End Audio industry, where growling transformers can represent a huge bear of a problem.


The acoustic performance of transformers is calculated and specified during the design stage. In custom applications, customer engineers work closely with the Plitron design team to establish acceptable performance limits and define specifications.

Noise measurement system

Under four adverse line conditions, spectrum analysis of the noise performance is verified using Plitron’s standardized Noise Testing Chamber. Results are graphed as Sound Pressure Levels (SPL) in decibels (dB) vs. frequency (20 Hz to 10Khz) and assigned a Noise Criterion (NC) curve number. The NC specification is internationally recognized and describes noise levels in dB SPL. For reference, a good recording studio’s noise level is typically NC 15 to ensure a virtually silent noise floor, while in offices NC 30 is usual. LONO™ transformers measured in our Noise Test Chamber under adverse mains conditions produce less than NC 19 at 1 metre distance. At 2 metres they produce less than NC 13. Every doubling of distance results in 6 dB reduction. With nominal line conditions, LONO™ transformers produce NC 10 or less at 1 metre. For more detailed information about this new technology and our measurement system, request Plitron’s White Paper on Transformer Noise. Reduced In-Rush Current PLITRON LONO™ transformers provide lower inrush currents, typically 40% less than standard toroidal transformers. This feature avoids nuisance fuse interruption, can reduce or eliminate the requirement for external devises used for inrush, and allow lower current rated power switches.

Related Information

Also see Measuring Acoustic Noise Emitted by Power Transformers presented at the 109th Convention of the Audio Engineering Society. This preprint was prepared by Menno van der Veen and the research staff at PLITRON. Check out the product line: Low Noise Power Transformers for Solid State Amps.


Plitron Manufacturing Inc, an ISO 9001:2015 company, was established in 1983. Plitron specializes in research, design and more - Read more ...