The Crystal Biotech Doppler Master Model DM-1 Modular used for measuring blood flow velocity by providing the timing for all the Doppler devices in a chassis. This includes all variations of Doppler blood flow meters, Doppler Wall thickening modules. Contains the audio amplification circuits to drive the chassis mounted loudspeaker with the Doppler shifted audio signals. A 6-position front panel switch selects which Doppler channel is being monitored on the speaker and a volume control determines the speaker level. A front panel power indicator light is provided to indicate that the chassis power turned-on.