Glensound GS-ET4 Single Channel Audio Transformer IsolationFour channel audio isolation using high specification Lundahl transformers Single Channel Audio Transformer Isolation. The Glensound External Transformer (ET) range was designed to help a specific outside broadcast customer who was having no luck in curing hum problems with cheap DI boxes. This customer was providing audio facilities in a venue away from the OB truck and there was sometimes hum being received at the OB truck on his output circuits.