Micro Details in Push-Pull Output Transformers

Having stumbled upon some reports on the loss of micro details due to a collapse of permeability in output transformers, and being of an inquiring nature, Hans Polak decided to have a detailed look. This supposed phenomenon would be detected with push-pull (PP) tube amplifiers with output transformers at low levels, near our auditory threshold. For this investigation he focused on the output transformer (OPT) in isolation.

Power amplifiers present a unique set of testing challenges that are usually not encountered with digital or low-level analog circuits, as they must supply significant current as well as voltage to their loads. This comprehensive checklist on what is needed for testing power amplifiers covers the fundamental measurements, with specifics on how to best accomplish the tests using Audio Precision's APx series audio analyzers and APx500 measurement software.

This article is another great contribution by Claus Futtrup, this time with direct support from Jeff Candy, the two creators of Speakerbench, the free to use web-based application for low-frequency analysis and design of loudspeakers. Another follow-up article for the Bass Reflex Alignments article series that was published in audioXpress and Voice Coil, this time, the authors discuss the important Quasi-Alignment Families where the fundamental idea is trying to approach a roll-off similar to the Butterworth B4 alignment (a maximally flat response) but compatible with real systems for which the driver Qt will not allow a true B4 alignment.

After discussing computation of discrete alignments such as B4, BL4, LR4, IB4, and CD4, this new article in the "Computation of Bass Reflex Alignments" article series explores the real-world situation where it becomes impossible to have a system design that exactly matches a certain driver Qt, normally described as an alignment error (or misalignment). The technique presented allows designers to modify tuning towards a desired target response function.

The fifth article by Claus Futtrup in his series about loudspeaker bass reflex alignments explores the concept of transitional alignments, which is helpful in bridging between filter types. The example details a transitional alignment between Butterworth B4 and Linkwitz-Riley LR4. This article series started in May 2024 with “Computation of Bass Reflex Alignments," the series expanded with classical alignments such as the B4, the LR4, the IB4, and most recently, the CD4 vented alignment, in the August 2024 issue.

In the fourth article from Claus Futtrup in his series about the invention, concept, and historical perspective of loudspeaker bass reflex alignments, he explores the Critically Damped Fourth-Order (CD4) bass reflex alignment. This alignment remains one of the least known and hasn't been discussed in literature related to loudspeaker design, even though it allows for interesting output properties. This article was originally published in Voice Coil, August 2024.

Before computers were broadly available for loudspeaker bass reflex box simulations, engineers relied on "Alignments" for their designs. Such box alignments are typically named after equivalent filter functions that the high-pass response of the box was aligned after — for example, Butterworth (B4) and Bessel (BL4) — to name a few. This article, the third in this series, explores the rarely talked about Inter-Order Butterworth (IB4) alignment, mentioned originally by Richard Small and Neville Thiele.

Before computers were broadly available for loudspeaker bass reflex box simulations, engineers relied on "Alignments" for their designs - typically named after equivalent filter functions that the high-pass response of the box was aligned after — for example, Butterworth (B4) and Bessel (BL4) — to name a few. In this article, Claus Futtrup now focuses on the Linkwitz-Riley (LR4) Bass Reflex Alignment.

The first in a series of great value for speaker designers, this article provides an updated approach for the mathematical presentation of the various alignments, applicable to all computerized development tools. This is effectively an explanation of the code available in the Alignment Chart of the Speakerbench web application that Class Futtrup developed in collaboration with Jeff Candy.

The use of standard filter alignments represents a foundational concept in the loudspeaker industry, with an impact nearly as significant as the invention of the electro-dynamic loudspeaker principle itself. As one of the creators of Speakerbench, a web-based speaker design tool, in collaboration with Jeff Candy, Claus Futtrup is in a unique position to discuss the evolution of enclosure designs based on this electrical filter theory, including the concept of an alignment chart.