by Fred Nachbaur, Dogstar Music ©1998, 2000


The design philosophy was to improve on "vintage" tube designs by incorporating refinements normally only associated with solid-state gear. Prime among these is the use of differential input stages, in which the inverting input is used strictly for feedback. Another is the use of direct coupling between stages, a technique once quite common in oscilloscopes, but rare in audio gear. The resulting circuits are thus closely related to the operational amplifier, and used in a similar fashion.

In addition to the quasi-opamp idea, some other design concepts used in the project are:

  1. Use of readily available parts wherever possible. For this reason, tube types such as 6L6GC, 12AX7A and 12AT7A were employed.

  2. The power amplifier was designed to be general enough to allow the use of more exotic beam pentodes (and even triodes) by suitably modifying the sockets and/or pinouts as required.

  3. Direct coupling was used in crucial parts of the circuitry. More about this later.

  4. Flexibility in use was a prime consideration. In addition to the "usual" amenities expected on an integrated amplifier (line-level inputs, phono inputs, tone controls, tape monitors, etc.) some additional features include: a mic/line input with gain trim for stereo instruments, etc.; a "PA mode" switch that allows bypassing the negative feedback employed in the driver/PA module for that "classic tube" sound; full, independent tone controls, including midrange; a tone bypass switch; a stereo/mono switch to allow for bridged mono operation; and an effects loop for inserting graphic equalizers, etc.

  5. A Word About Negative Feedback

    It is perhaps an unfortunate accident of history that the term "negative feedback" was used to describe the linearization technique of applying a portion of the output signal back to the input, in opposite phase to the applied signal. It gives the impression to audiophiles with enough knowledge to be dangerous that "negative" feedback must somehow be a "bad" thing. I've seen diatribes against the evils of negative feedback, yet in the same breath such critics extol the virtues of "ultra-linear output transformers." The joke is that such transformers (using screen-grid taps on the primary) achieve this linearization by applying local negative feedback to the screen grids!

    Perhaps a better moniker for "negative feedback" would be "dynamic compensation" or something on that order. Consider the following characteristics of well-designed negative feedback systems:

    The completed RA-100 Prototype
    The Completed, operational RA-100 Design Prototype

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