Shure A15BT Bridging Transformer

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The Shure A15BT Bridging Transformer can be used to connect balanced and unbalanced devices having different signal levels or impedances. Depending on the configuration, the A15BT provides a minor increase or decrease in signal level. The A15BT can be used to:

  • Connect a mono output from an FM tuner or receiver to the input of a microphone mixer
  • Connect a high-impedance microphone to a medium-impedance input on an audio recorder
  • Connect a balanced line-level output from a mixer to an unbalanced Aux-level input on an amplifier or recorder
  • Convert an unbalanced Aux-level output to a balanced 600 Ohm line output (example: Shure SCM268 Mixer)

Typical Applications Examples

To connect the 600 line output of a preamplifier, mixer, or recorder to a high-level, high-impedance, unbalanced (Aux) input:

Connect the 600Ω output to the 33,000Ω XLR socket of the A15BT. Unbalance the XLR plug on the A15BT and convert it to 7500Ω or 600Ω to provide the proper level and best signal-to-noise ratio. Connect the A15BT to the Aux input. In this configuration, the A15BT “bridges” the 600Ω line output. Refer to the table on the back page for the maximum input level to the A15BT.

To match a high-impedance microphone output to a medium-impedance (approximately 10,000 Ω) input on a reel-to-reel or cassette recorder: Unbalance both ends of the A15BT. Connect the microphone output to the 33,000Ω XLR socket. Connect the 7,500Ω XLR plug to the recorder.

For an unbalanced Aux input to accept a balanced low-impedance output: Unbalance the 33,000Ω XLR socket on the A15BT and connect it to the Aux input. Convert the XLR plug to 7500Ω or 600Ω and connect it to the balanced low-impedance device.

When a balanced 600 line output is needed on a unit with an Aux output (such as the Shure SCM268):

Unbalance the A15BT 33,000Ω XLR socket and connect it to the Aux output. The A15BT XLR plug offers a balanced line output at both 600Ω (at about a –20 dBm level) and 7500Ω.

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  • XLR Pin Connector (M): 7500Ω or 600Ω
  • XLR Socket Connector (F): 33,000Ω

Voltage Ratio 7500 Ω

  • Medium-impedance to high-impedance: +6.4 dB
  • High-impedance to medium-impedance: –6.4 dB

Voltage Ratio  600 Ω

  • Medium-impedance to high-impedance: +17.4 dB
  • High-impedance to medium-impedance: –17.4 dB

Driving Source Impedance (ohms)

  • 600
  • 600
  • 600
  • 7,500
  • 7,500
  • 33,000
  • 33,000

Winding Being Driven (ohms)

  • 600
  • 7,500
  • 33,000
  • 7,500
  • 33,000
  • 33,000

Maximum Level (Volts)

  • 0.5
  • 2.5
  • 6.0
  • 1.5
  • 5.0
  • 5.0

Case Full magnetic shield, steel with gray enamel finish
Dimensions 19 mm (3/4 in.) diameter; 89 mm (3 1/2 in.) long

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