This project is a ‘hybrid’, combining integrated-circuit and tube technology in one circuit. It was developed by the author as a case study that’s elaborately described in his book The State of Hollow State Audio — in the Second Decade of the 21st Century, published by Elektor. The 12EL6 tubes used here are low-voltage “space charge’ types discussed at length in the book.

Untitled design


The IC seen in the circuit diagram in Figure 1 is a type OP27 op amp. The type LTC6090 IC originally used in the design proved hard to get in experimenter quantities. Since the input resistance of the op-amp stage must be about 1 MΩ, an op amp is needed that has very low bias current, offset current, and offset voltage. Also, the op amp must have very low noise. The OP27 meets these requirements and is readily available in small quantities. The LTC6090 boasted rail-to-rail input and output voltage ranges, whereas for a 2 kΩ or higher load the OP27 has a typical span of 1.5 V (maximum 3.5 V) between maximum...