JFET Design Spotlight: A Single JFET Phase Splitter for audio applications

“There is sound and then there is the sound of sound.”™ In designing discrete element electronic audio circuitry, there will inevitably be a need to split a signal to a 0° and 180° phase differential. Either to drive a balanced signal to a transformer, a line signal to an XLR connector, or a differential voltage to drive a push-pull stage within a power amplifier stage, or on and on and on. Though not always needed for the task, a phase splitter will often require a zero volt DC offset, particularly for transformers to eliminate core bias and XLR drives where a transformer is the terminating end. Many high end audio systems seem also to need a zero offset differential input. When designing a differential Bipolar or JFET stage with current sourced emitters or sources, the attendant circuitry to make it a zero bias differential output can become a concern, “I’m being nice”. The single transistor splitter having a source/emitter resistor and a drain/collector resistor works but has the additional circuitry to reference outputs to ground. In the effort to minimize active components in the audio path and retain as high a linearity/fidelity for my designs, the circuit here is was what became a solution for many applications.

Though the signal output of each arm of this differential splitter is -4db down from the input signal it is flawlessly the same, though inverted, and referenced to ground by an offset potentiometer. The POT places the source voltage at exactly -5V and the collector at +5V where the resistor string for both terminations have 1mA as does the JFET current. R8 & R9 combine R string current with the FET current to set 5 V from each rail for the JFET S&D. Choice of JFET depends on the load impedance and Gm required for an application of the design. Those choices determine linearity/fidelity for a signal range needed for the use. VDD&VSS can be anything up to the S-D compliance limit but I’ve found the sweet spot for a LSK170A to be 10 V. the resistor string for both arms is a 3:2:1: Ratio making the choice of Id simple. For lower Noise, Parallel JFETS such as Dual LS parts work well and offer greater drive current, such as the LS844s. An earlier circuit in these posts uses this configuration for the zero DC pass through audio application. This circuit and a traditional splitter are shown. Frequency and voltage response of input/output and gate to source voltage are shown. For this configuration, Gm shift/input voltage should be noted for the even order harmonic content on the output signal giving it a Vacuum tube like influence.” - Kirkwood Rough

Low distortion JFET phase splitter using LSK170A, generating clean ±phase outputs with zero DC offset for high fidelity audio designs

Input vs. ±phase output showing precise 180° phase splitting with matched amplitude and low distortion

Frequency response showing consistent gain and phase behavior across bandwidth with well matched ±phase outputs