Handset audio levels from most phone sets is loud enough to be satisfactorily recorded with most digital audio recorders (including all recorders manufactured and distributed by Omnicron Electronics). However, some phones may have weak audio that can be boosted by +15db by using the TSB-15L.
Instead of connecting the handset audio directly to the recorder, connect it to the modular input jack on the TSB-15L, and connect its 10″ output cable to the recorder’s input jack. Audio levels are increased by a transformer. No external power is required.