A15AS - Switchable Microphone Attenuator prevents preamp overload in applications where very strong signals are applied to a mic input. Provides 15, 20 or 25 dB attenuation. A15BT - Bridging Transformer matches balanced or unbalanced devices of different impedances.(33 kilohm primary, 600 or 7,500M ohm secondary). A15HP - High Pass Filter provides a low frequency cutoff to reduce unwanted low-frequency noise in mic signals. A15LA - Line Adaptor provides 50 dB attenuation, allowing a balanced line-level source to be connected to a balanced microphone input. A15PRS - Switchable Phase Reverser provides instant switch selection of normal or reversed phase of a balanced line without modification of equipment. A15TG - Tone Generator produces a continuous 700 Hz mic-level signal capable of driving low-impedance balanced lines. Extremely useful in setting up and trouble shooting audio equipment. Battery operated.