The Orban 245F Stereo Synthesizer was created for professional users who demanded the highest standards in audio processing, providing a means of transforming any monophonic source into a rich, convincing pseudo-stereo signal without degrading sound quality. Introduced in 1974, it was one of the earliest and most successful devices to create synthetic stereo that preserved tonal accuracy, phase integrity, and mono compatibility, while adding spatial depth and dimension.
Unlike simple delay-based or comb-filter pseudo-stereo devices of the period, the Orban 245F employed a sophisticated phase-cancellation synthesis technique using complementary comb filters derived from a patented Orban design. This process divided the input spectrum into five frequency bands of approximately two octaves each. These bands were alternately distributed between the left and right channels, giving the resulting output many of the perceptual characteristics of true stereo, including depth, space, directionality, and stability, while maintaining good channel balance and full mono compatibility.
The system’s operation was based on precise control of spectral placement. Each band could be adjusted using two DIMENSION controls, which allowed engineers to alter the relative positioning of the frequency bands to fine-tune the stereo field. This design provided great flexibility, enabling the user to achieve optimal subjective balance regardless of the tonal spectrum of the source material.
A recommended “best compromise” DIMENSION setting of LOW=3.0 and HIGH=7.5 was provided for typical programme use, though users could permanently fix their settings using Beckman 72PR25K trimmers for consistency in broadcast or installation environments. This approach meant that engineers could easily tailor the stereo effect for various material types, from speech and solo instruments to full orchestral or pop mixes.
In addition to its DIMENSION controls, the Orban 245F featured a SEPARATION control allowing variable adjustment of the degree of stereo separation. The control worked by partially summing the left and right channels to reduce or expand the stereo width. This made the 245F extremely versatile: users could reduce separation for natural headphone monitoring, control vertical cutter excursion during vinyl mastering, or improve mono loudness and compatibility in FM stereo broadcasting.
For example, increasing the SEPARATION setting to “10” raised the difference signal (L–R) level to match the mono sum signal (L+R), effectively increasing the perceived stereo width by up to 6 dB on each channel relative to a zero setting. This capability proved valuable in competitive FM broadcasting environments where perceived loudness and clarity were crucial.
A STEREO/MONO switch on the front panel enabled instant A/B comparison between the unprocessed mono input and the stereo-synthesised output, while a GAIN control provided up to 9 dB of boost to optimise operating levels. Importantly, the gain stage was located between the active-balanced input circuitry and the signal processing section, preventing overload of the input stage even at high drive levels. The electrical sum of the two output channels was always proportional to the mono input, ensuring complete mono compatibility and preventing tonal coloration.
This design meant that if the stereo output was summed back to mono, the signal was identical to the original mono source, a key advantage for FM broadcast and disc mastering applications where phase accuracy was essential. The sum of left and right output powers was also proportional to the input power, ensuring consistent acoustic energy and maintaining identical frequency balance to the mono signal.
The Orban 245F’s performance was highly stable across all settings. Its frequency response of the stereo sum signal was ±1 dB from 20 Hz to 20 kHz, while total harmonic distortion measured less than 0.1% (typically 0.02%) at +19 dBm, and noise performance was outstanding at less than –80 dBm unweighted, typically –83 dBm. The unit’s output could drive loads greater than +19 dBm into 500 ohms or higher, and optional transformer-balanced outputs were available for full isolation in professional installations. Inputs were active-balanced, bridging at over 100 kΩ, with absolute overload occurring at +26 dBm. The 245F maintained phase integrity throughout, operating in-phase from input to output.
In operation, the Orban 245F offered a straightforward calibration process. After connecting the unit, users would feed a mono test programme into the input, set the MODE switch to MONO, and adjust the GAIN until both output channels peaked evenly around –4 VU. The MODE switch would then be set to STEREO, allowing the engineer to fine-tune the DIMENSION and SEPARATION controls by ear for optimal spatial balance and tonal stability. When correctly adjusted, the pseudo-stereo image would appear well centred with no undesirable frequency imbalance or phasing artefacts. The SEPARATION control could then be used to fine-tune stereo spread and loudness for specific playback conditions, such as FM transmission or disc mastering.
The Orban 245F proved particularly useful in FM stereo broadcasting, where phase errors between the stereo channels could cause severe mono compatibility issues. By using the patented phase-cancellation synthesis, Orban reduced this problem significantly, offering FM stations a method to process mono content into stereo while avoiding degradation in mono receivers. It also became invaluable in record mastering and reissue work, allowing engineers to reprocess old mono recordings and give them new life in stereo-compatible formats. The 245F was also employed in broadcast automation systems and early cable and satellite setups, where mono sources could be converted into stereo to fill the full bandwidth of stereo transmission systems.
The internal design was robust and engineered for professional use. Inputs and outputs were provided on a Type 140-Y barrier strip with #5 screw terminals, and the power supply accepted 115–230 V AC at 50–60 Hz with ±10% tolerance, consuming only 2 VA. For installations requiring full transformer-balanced outputs, Orban offered a retrofit kit that could be easily installed using pre-drilled chassis holes. The device required 1¾ inches (4.5 cm) of rack space in a standard 48.3 cm rack and weighed approximately 3.2 kg, making it compact yet sturdy enough for studio and broadcast rack systems.
In use, the Orban 245F was praised for its ability to produce an exceptionally natural and phase-coherent stereo image, far superior to the crude delay-based pseudo-stereo units of its time. It was used in mastering facilities, broadcast studios, and reissue production lines to expand the perceived width of recordings while maintaining clarity and mono integrity. Its reliability, low noise, and technical sophistication made it one of the finest pseudo-stereo processors ever produced.
Features
- Converts mono sources into convincing pseudo-stereo without tonal degradation
- Patented phase-cancellation stereo synthesis for realistic spatial imaging
- Two DIMENSION controls for adjusting stereo image balance and depth
- SEPARATION control for varying stereo width and mono compatibility
- STEREO/MONO switch for instant comparison between processed and unprocessed signals
- Up to 9dB of adjustable gain for optimising output levels
- Ideal for FM broadcasting, mastering, and reissuing mono recordings
- Optional transformer-balanced outputs for professional installations
- Compact rackmount design with rugged construction