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Audio Drive Circuit Provides Balanced/Unbalanced Stereo/Mono

IP.com Disclosure Number: IPCOM000117568D
Original Publication Date: 1996-Apr-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

Sylivant, JW: AUTHOR

Abstract

With the availability of large voltage swing op-amps capable of driving low impedances, a balanced audio driver can be implemented without a transformer.

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Audio Drive Circuit Provides Balanced/Unbalanced Stereo/Mono

      With the availability of large voltage swing op-amps capable of
driving low impedances, a balanced audio driver can be implemented
without a transformer.

      The circuit illustrated in Fig. 1 shows two operational
amplifiers connected in the inverting gain configuration.  A
capacitor across the feedback of the first stage can be used to
provide a low-pass filter function, typically required if the signal
source is a Digital-to-Analog converter.  If the signal source is a
current output DAC, its output can be fed directly into the inverting
terminal of U1.

      By inserting 300 ohm resistors in series with the outputs, a
balanced driver is formed, suitable for connecting to the 600 ohm
impedance inputs of professional audio equipment.  The signal level
at the output of each operational amplifier should be set to the
level desired to be delivered to the load; i.e., 0.775 volts rms for
0 dbm.  The differential drive will be twice this value or +6 dbm,
but the voltage divider action of the series output resistors and the
600 ohm load impedance will reduce the level by a factor of 2 to give
a 0 dbm level.

      An additional advantage of this circuit topology is that it can
connect to high-impedance, unbalanced inputs of commercial equipment
and still deliver 0 dbm.  Output for this type of interface would be
taken from +Vout and ground.  Since the input impedance is high,
typically 10...