The α24 fully-differential line amplifier

Technical highlights

Multiple applications

The α24 was conceived to have multiple uses, providing a flexible platform on which to build balanced analog circuits.
  • Balanced pre-amplifier line stage or buffer
  • Voltage DAC analog output stage with low-pass filter (LPF)
  • Balanced-to-unbalanced converter
  • Unbalanced-to-balanced converter
  • Pro-audio applications

Inputs, outputs, features and performance

The α24 was designed to achieve the following characteristics and performance:
  • Fully-differential input (balanced or unbalanced)
  • Fully-differential (balanced) and unbalanced outputs
  • Ultra low noise
  • Ultra low distortion
  • High common-mode rejection ratio (CMRR)
  • Low DC offset
  • Low drift
  • High stability/accuracy
  • Optional differential 3rd-order (18dB/octave) analog LPF
  • Several board connector options

Fully-differential

The terms "fully-differential" and "balanced" are not equivalent. Many balanced systems are not fully-differential, and fully-differential circuits can support unbalanced input signals. α24 is a true fully-differential circuit. Please read the Fully differential explained forum post, and the Wiring & ground section for more details.

Topology

The α24 topology is a modified instrumentation amplifier. This topology, as the name suggests, possess the desired performance characteristics, and is often used for laboratory-grade measurement/test equipment. The standard instrumentation amplifier has a two-stage topology: a high-CMRR input buffer/gain stage, followed by a balanced-to-unbalanced converter with optional gain or loss:



The standard instrumentation amplifier topology above provides only an unbalanced output, and does not have a LPF function.

In the α24, the modified instrument amplifier topology has three stages. The first stage is the same as a standard instrumentation amplifier, and is implemented with a OPA1612 dual opamp. The second stage is a fully-differential opamp, using the OPA1632, with an optional 3rd-order LPF. The output from this stage is used to drive the balanced XLR analog output. Then, the third stage resembling the standard instrumentation amplifier's second stage, implemented with the OPA1611, is used to convert from balanced to unbalanced for the RCA analog output.



Of particular interest is the Vocm pin on the OPA1632. By connecting it to ground, the differential output common mode voltage is now referenced to ground. It shifts the output common mode "zero" reference to ground potential (even when there is a large input common mode DC offset), and thus avoids the use of coupling capacitors. Due to tight-matching of devices within the opamps, and due to balanced effective resistances on all opamp input pins, it doesn't even need a DC servo circuit to maintain very low offsets. This functionality is particularly useful when the α24 is used as a DAC analog output stage, where the DAC chip has a large common-mode output DC offset (relative to ground) due to its single-rail power supply.

A benefit of the α24 topology is that you could connect either input to ground and drive the other input from an unbalanced source, and yet the balanced output will provide a fully-differential signal. In effect, it also performs unbalanced-to-balanced conversion, so that when used in a preamp (and driving a fully-differential power amp), the signal would be differential from the preamp to the speaker terminals. No additional conversion circuitry or transformer is needed. Of course, with a fully-differential source, the signal will then be fully-differential from the source to the speaker terminals.

Opamp choices

The OPA1611, OPA1612 and OPA1632 opamps were chosen for their combination of ultra low noise, ultra low distortion, high output current capability, rail-to-rail voltage swing (OPA1611/OPA1612), fully-differential architecture with high CMRR (OPA1632) and wide supply voltage range characteristics. These opamps are also specifically designed for audio, and are superb sounding devices off the test bench.


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