Introduction

This site contains several articles on audio and general electronic circuit design. The first article describes an algorithm for the design of vented-box loudspeakers that uses a technique that's well suited for software implementation. The second deals with using the freeware programs LTspice and FreePCB together to design printed circuit boards. The third discusses the design of a popular kind of phono preamp circuit using active RIAA equalization. The fourth discusses some improvements to the publicly-available SPICE models for the ON Semiconductor MJL3281A and MJL1302A power transistors. Brief descriptions follow below.

1. Vented-Box Loudspeaker Design Including Conventional and Assisted Alignments of Arbitrary Order and Box Quality Factor

This article covers three topics in vented-box design. The first is a new algorithm for design with a given driver that allows any box quality factor to be specified. This eliminates the lookup procedure implied by the alignment charts and tables of the classic literature on the subject, replacing it with an algorithm that solves a single nonlinear equation in one unknown using conventional root-finding techniques. Using this procedure, one is no longer restricted to the discrete set of box quality factors imposed by the classic techniques.

The second is the extension of these techniques to assisted alignments of arbitrary order n. Thiele's notion of alignment class is extended to higher-order systems that require more than one class number to fully describe how the system transfer function is partitioned between the loudspeaker and external filter.

Finally, high-pass filtering techniques for reducing cone excursion below the box tuning frequency are explored. Some performance comparisons are made between assisted alignments and systems designed in the normal way with a high-pass filter added after the fact.

2. Flanking Sub Processor

This article was inspired by some comments by Wayne Parham at the Audio Round Table forum. He proposed the idea of stereo subs (called "flanking subs") placed close enough to the main speakers that they could be crossed over to the mains in the frequency range of 100-200 Hz, reducing overall system frequency response variation in that range. My own subwoofer installation requires that the subs be close to the mains because of limited available space. I wondered if it might be possible to simultaneously reap the benefits of stereo flanking subs and mono distributed subs by designing a circuit that provides a mono output at the lowest frequencies and transitions relatively quickly to stereo in the frequency range over which the flanking subs do the most good. I came up with a design that accomplishes this function, and the article describes the rationale for its design.

3. Using LTspice as a Schematic Capture Tool for FreePCB

The LTspice - FreePCB article shows how to use these two freeware programs together to design printed circuit boards. Using netlist export in LTspice, a netlist is created and imported into FreePCB. The default behavior of LTspice when assigning pin numbers to devices in the exported netlist often results in incorrect pin numbering. This in turn would result in an incorrect circuit board design if nothing were done to correct the problem. This article introduces the concept of "pin-swapping subcircuits" for SPICE. These pin-swapping subcircuits can be used together with user-created LTspice symbol files to fix the LTspice pin numbering problem. Once this is done, LTspice and FreePCB become a powerful and free set of tools for creating PCB designs from a working simulation.

4. Phono Preamp Design With Active Equalization

This article covers the design of a simple and popular active RIAA phono equalization circuit. The equations developed can also be used for the design of an inverse RIAA network used for lab testing of phono preamps. Emphasis is placed on a technique for allowing the designer to explicitly specify the time constant associated with what's normally considered an undesired zero of the circuit's transfer function. This zero causes a break frequency that's above the audio band in a proper design. The idea could be used to implement the controversial "Neumann time constant", or the frequency of the zero could be chosen so as to force the capacitor ratio to correspond to actual standard values. Methods for achieving the best possible equalization accuracy are presented. The impact of finite op-amp gain-bandwidth product and DC open-loop gain on RIAA equalization errors is quantified. A SPICE technique for isolating the effects of op-amp-induced errors from the total equalization errors is shown. A complete derivation of the design equations used is provided in an appendix. This derivation uses a Foster expansion approach not covered elsewhere in the literature.

5. Improved SPICE Models for MJL3281A and MJL1302A

These bipolar power transistors are very popular as output stage transistors for audio power amplifiers. The SPICE models currently available from ON Semiconductor for these devices do not yield simulated performance that matches the datasheet measurements very well for some parameters. This leads to misleading simulation results in many cases. At a typical quiescent current one would use for a power amplifier output stage design, the SPICE model for the MJL3281A shows a simulated fT that's low by a factor of six. The MJL1302A model shows a simulated β that's low by a factor of two to three, depending on collector current. The article shows graphs of the discrepancies between simulated and measured data. New models are developed in the article and can be downloaded here. Detailed information of the model extraction process is presented. Graphs of the simulated fT, β and other parameters for the newly-developed model are shown, demonstrating improved correlation with measured device parameters.

A Note About SPICE

The power transistor SPICE article borrows heavily from the book Semiconductor Device Modeling with SPICE by Massobrio and Antognetti. That text contains a wealth of information about the equations used internally by SPICE to simulate device behavior. Such information is probably of most use to those writing simulator software or support tools. But there is also much insight to be gained by the end user in studying some of the SPICE modeling equations. Much of SPICE bipolar transistor modeling is related directly to device physics. But in its attempt to provide the most detailed information possible to software developers, Massobrio and Antognetti miss some opportunities to provide insight to the end user. I found that reading the text gave me a fuller understanding of the Early effect and the variation of fT with collector current, but not until I had filled in some missing pieces. In the section of this site that describes how the model parameters were extracted, I have tried to pass on what I've learned about these two subjects to the reader. Looking at the Early effect from the perspective of the Gummel plots is especially thought-provoking. I have also attempted to fill in some missing information in the study of fT behavior vs. collector current by first relating the behavior of hFE vs. frequency to the hybrid-π model parameters. Then the hybrid-π model parameters are connected to the SPICE BJT model parameters to complete the picture.