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A Beginners Guide and Other Thoughts on Audio - Chapter 3: Amplifiers

Roger Modjeski and Anthony Chipelo | Published on 1/29/2024

RM-9 Image 1

This month in Roger’s Corner we present Chapter 3 of A Beginners Guide and Other Thoughts on Audio. This chapter is dedicated to Amplifiers. When people think of Roger A. Modjeski as an audio designer the first thought that generally comes to mind is his experience in designing tube components, and for good reason. Roger was one of the most innovative tube circuit designers ever, especially when it came to amplifiers.

Starting with the Music Reference RM-9 that many felt was his finest achievement, a dual ultralinear amplifier utilizing innovative feedback (gain) circuits. Followed by the RM-10 amplifier that used the EL-84 tube in a manner that achieved 35 watts per channel from a single pair of tubes per channel, where previously half that was the norm. Then finally Roger’s last large production amplifier, the RM-200 with its differential circuit, hybrid design (solid-state input and tube output!), and possibly the only tube transformer coupled amplifier that increases power as the speaker load decreased while remaining stable down to a 1-ohm load.

Roger researched many amplifier types and topologies and even designed a few circuits to boot in support of his research. In this chapter he discusses everything from amplifier classifications, (including Class D), types (solid-state, single ended triode, push-pull, OTL, and others) and design elements including a fascinating segment on the technique of using feedback in audio design, as well as how to determine the appropriate power level when selecting an amplifier, and the role of output impedance in amplifier/speaker matching. It’s quite a long chapter so take your time because I can assure you there is a lot of great information worth digesting.

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

Chapter 3: Amplifiers

The most important thing to know about power level when selecting an amplifier is your speaker sensitivity and how loud you listen. Many people buy amplifiers that have far more power than what is needed. An amplifier with far more power than needed is not necessarily a good thing. I recommend spending $50 and buy a good sound pressure level meter. They have been confirmed to be very accurate, certainly accurate enough for this job and certainly better than guessing. As a second recommendation, download an SPL meter app from your smart phone. While we may not know how accurate it will be, as it depends on the app, at least it will be better than guessing. I notice when people guess their SPL, they usually guess at least 10 or 20 dB higher than it is and that would unfortunately lead to them using an amplifier that is much more powerful than needed. Therefore, I cannot stress how important it is to know within a reasonable degree of accuracy your sound pressure level listening.

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