The Van Scoyoc circuit, invented in 1948, was mentioned previously (“Improving the Cross-Coupled Inverter”). You can replace the first tube pair in a Van Scoyoc circuit with junction field-effect transistors (JFETs) without deterioration of sound quality.
---Above: Parts List
FIGURE 1: Van Scoyoc input and driver circuit.
The combined input and driver circuit for a power amplifier is shown in Fig. 1. Transistors Q and Q2 are JFET transistors, and should have an Idss of 12-50mA and be matched for maximum 10% difference in Idss. The value of R3 and R4 depends on both the transistor type and Idss of the transistor used for Q1 and Q2 (Table 1). Resistors R9 and R10 should be matched to maximum 1% difference.
The circuit has four input terminals A, B, C, D, and two output terminals X and Y. A and C are inverted inputs, B and D are non-inverted inputs. Y is an inverted input. The A and B inputs have an input impedance of 100k-Ohm. The C and D inputs have an input impedance of 50-100 ohm, depending on the transistors used.
You should set the potentiometer P1 to 0V DC between terminals X and Y. The bandwidth of the circuit without C3 and R15 is 0-100kHz, and can be limited by C3 and R15. Bandwidth limiting should be used in conjunction with the use of high feedback (Table 2).
TYPE OF AMPLIFIERS
You can use the driver circuit for many types of amplifiers: single-ended, push-pull transformer coupled amps, and output transformerless amps. The accompanying figures show the various configurations.
Figure 2 is a push-pull amp with unbalanced input and unbalanced feed back, while Fig. 3 shows the push-pull amp with balanced input and balanced feedback. Figure 4 is a single-ended amp with balanced input and balanced feedback, and Fig. 5 shows a push-pull bridge amp with unbalanced input.
Figure 6 is an OTL amp with unbalanced input. With this OTL configuration, you’ll probably choose to make the following modifications: change V2 to 12BH7, R11 to 4.7k-ohm 4W, R10 to 18k-ohm 4W, and R9 to 15k-ohm 4W.
FIGURE 2: A push-pull amp with unbalanced input and unbalanced feedback.
FIGURE 3: A push-pull amp with balanced input and balanced feedback.
FIGURE 4: A single-ended amp with balanced input and balanced feedback.
FIGURE 5: A push-pull bridge amp with unbalanced input.
FIGURE 6: An OTL amp with unbalanced input.