Audio L → Amp L → 1 kΩ resistor → Transformer primary → Electrode L1 Audio R → Amp R → 1 kΩ resistor → Transformer primary → Electrode R2 Secondary center tap to ground or floating.
"Transcutaneous electrical nerve stimulation (TENS) : a review" – Sluka & Walsh (2003) – for physiological effects. "Electrical stimulation of nerves" – Mortimer (2004) – for safety and waveform considerations. estim audio generator