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A Beginners Guide and Other Thoughts on Audio - Chapter 4: Preamplifiers

This month in Roger’s Corner we present Chapter 4 of A Beginners Guide and Other Thoughts on Audio. This chapter is dedicated to premplifiers. In this chapter you can read Roger’s thoughts on preamplifier design, the importance of impedance and capacitance, line stage noise, cable influences, and a continued discussion from Chapter 3 on the use of feedback in audio circuit design. What follows is a little history on Roger’s preamplifier experience.

Even a great designer like Roger A. Modjeski had to get his start somewhere before he could master audio circuit design. While Roger received his Electrical Engineering (EE) degree (Magnum Cum Laude) from the University of Virginia and had already learned more at his age about audio circuits than a vast majority of his student peers, he knew that there was much more to learn as an EE curriculum does not prepare one for a career in audio circuit design and production. Roger understood that to expand his knowledge he had to be mentored by someone who he could respect and learn from. That person was Harold Beveridge. Roger began apprenticing at Harold Beveridge, Inc. (HBI) in 1978 and later became HBI’s Chief Engineer.

When Roger joined HBI, Harold Beveridge needed someone to design electronics for the next iteration of the Beveridge 2S speaker, known as the 2SW. This included a solid-state amplifier to drive the woofers, an electronic crossover to facilitate bi-amping, and a preamplifier. Roger performed all the research, development, and testing to complete the designs. Undoubtedly, the tour de force was the Beveridge RM-1 preamplifier. It was a revolutionary product for its time and the first direct coupled servo controlled all tube preamplifier (including two phono sections and several useful controls) designed and manufactured for consumer audio.

After starting Music Reference Roger designed the RM-4, a dedicated all tube “pre” preamplifier for moving coil phono cartridges. The Rm-4 was the first such component designed to step up the voltage of a moving coil cartridge using all tube circuitry allowing it to be used with moving magnet phono inputs on preamplifiers and receivers of that time. Years later Roger went on to design the well received RM-5 preamplifier. However, one of Roger’s favorite preamplifiers was the DIY kit that he offered known as “pot in the box”, a simple passive preamplifier using a high quality Noble attenuator. You may especially enjoy the section of the chapter where Roger tries to put to rest the silly audiophile notion that passive preamplifiers restrict dynamics.

A Beginners Guide and Other Thoughts on Audio
Roger A. Modjeski

Chapter 4: Preamplifiers

In the past, a preamplifier was thought to be comprised of a phono and line section together in a single chassis. When vinyl playback went by the wayside in favor of digital, we stopped putting phono sections in preamplifiers, and started calling them line stages. Now with the return of vinyl playback phono sections have come back. However, we have done it a bit differently this time around where in some cases people want to have the phono section and line section in a separate chassis. Again, we can still put these two components together in a single chassis, and frankly, there is no downside to doing that. It is not like when you are integrating a preamplifier and a power amplifier together, where there might be some concerns about power supply interference. Putting two preamplifier components together in one box is actually a good idea.

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