by Fred Nachbaur, Dogstar Music ©1998, 2001


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 12AX7A and 12AT7A were employed.

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

  3. Flexibility in use was a prime consideration. While this articles details the use of this circuit design as a magnetic (RIAA) and ceramic phono pre-amplifier, there is virtually no limit to the number of possible applications with suitable modification of the feedback networks.

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 Preamp Prototype
The Completed, operational Standalone Preamp Prototype

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