The Orban 674A Stereo Equaliser was a professional-grade audio processing unit designed to offer the versatility of a parametric equaliser with the operational convenience of a graphic equaliser. It functioned as a quasi-parametric device, providing eight independent equalisation bands per channel. Each band featured separate controls for tuning and bandwidth, allowing the user to adjust the centre frequency and the sharpness of the response curve. The unit was constructed to be a dedicated stereo device, with controls arranged to facilitate the maintenance of stereo balance. The design philosophy aimed to provide high-quality equalisation at a cost-effective price point, making it suitable for both professional applications and advanced audiophile use.
The equalisation section of the 674A comprised eight bands per channel, each capable of tuning over a 3:1 frequency range. The quality factor, or Q, was typically variable between 0.3 and 20, depending on the centre tuning position. The unit offered an equalisation range of ±16dB, utilising reciprocal curves where the maximum cut was equal to the maximum boost. This characteristic meant that while the unit could effectively notch out ring modes in sound reinforcement systems, it might not provide infinite-depth notches for removing fixed-frequency interference such as hum. The controls for tuning and bandwidth were long-throw, dust-shielded slide potentiometers, which provided good resolution for precise adjustments. Ticks on the dial calibrations indicated typical settings; when set to these positions, the 674A behaved like a standard octave-band graphic equaliser with bands aligned to ISO frequencies from 63Hz to 8000Hz.
The 674A included high-pass and low-pass filter sections that added significant versatility to the unit. Each filter section was continuously tunable over a 100:1 range across two decades and featured 12dB/octave Butterworth slopes. These filters were independently switchable and followed the equalisation section in the signal path. The unit provided separate main, low-pass, and high-pass outputs, allowing the 674A to be used as a full electronic crossover. This configuration enabled the filters to split the audio signal for bi-amped speaker systems, replacing the need for a separate crossover unit. The filters could also be used independently to restrict the frequency response of a system in the extreme high and low frequencies, which was particularly useful in stage monitor applications.
In terms of general performance, the 674A was designed to exhibit very low noise and distortion levels. It featured a high slew rate to minimise transient intermodulation distortion, ensuring that the unit could handle fast-attacking signals without introducing artifacts. The front panel included gain controls for each channel, providing up to 12dB of gain, which could be adjusted down to negative infinity. Overload protection was indicated by “peak-stretching” lamps that warned of clipping anywhere in the equaliser chain. The inputs were active and balanced, while the standard outputs were unbalanced. Optional transformer-balanced outputs were available for installations requiring balanced connections. The unit incorporated RF suppression on inputs, outputs, and power leads to ensure reliable operation in high-RF environments.
The internal circuitry of the 674A consisted of a balanced input buffer amplifier, eight main equalisation amplifiers connected in series, and tunable low-pass and high-pass positive feedback Butterworth filters. Each amplifier in the equalisation section handled one band only, ensuring that the bands were totally non-interacting. The total equalisation was the sum of the equalisations provided by each section. Peak boost was achieved by adding the output of a two-pole bandpass resonator to the main signal, while reciprocal dip occurred when this resonator was connected as a feedback element. The EQ IN/OUT switch bypassed the last seven main amplifiers and defeated equalisation in the first amplifier, maintaining equal gain and signal polarity in both modes.
The quasi-parametric nature of the 674A meant that the Q changed when the tuning or equalisation controls were adjusted, unlike full parametric equalisers where all three parameters are independent. However, careful circuit design allowed the 674A to produce curves in boost mode that were essentially identical to the desirable constant-Q curves found in true parametric equalisers. The equalisation controls produced peaking curves, but shelving characteristics could be approximated by tuning the lowest band to 20Hz and the highest band to 20kHz, with the breakpoint adjusted via the bandwidth control. The unit was built with industrial-grade parts and construction, including socketed integrated circuits, to ensure reliability and ease of maintenance.
The 674A was widely used in sound reinforcement and monitor tuning applications. In economy bi-amped installations, it could replace both the third-octave equaliser and the electronic crossover. Its narrowband, tunable notches were effective for dealing with ring modes, while the remaining bands could be used for wideband equalisation. In higher-budget systems, it served as an electronic crossover and notch filter, working alongside a separate third-octave equaliser for house curve correction. For non-biamped systems like stage monitors, the unit equalised the monitor and used its filters to restrict extreme frequency response. In mono reinforcement applications, both channels could be connected in series to provide sixteen EQ/notch filter bands and an electronic crossover.
In recording studios, the 674A was utilised to handle tasks that internal console equalisers could not effectively manage. Its fine-tuning ability allowed engineers to clean up tracks more effectively than with graphic or three-knob console equalisers. The tunable filters helped eliminate rumble, cymbal splash, and kick drum leakage. The unit was also suitable for correcting equalisation during or after mixing, offering broad, non-ringing boosts that were less coloured than those from third-octave graphics. Additionally, it served as an ideal adjunct to electronic music synthesisers, allowing for the creation of high-Q formats and spectral shaping.
The 674A found application in motion picture sound, where it replaced graphic equalisers for dialogue equalisation. When set to the tick marks, it functioned as a familiar graphic equaliser, but its parametric capabilities allowed for the notching out of extraneous sounds in location recordings. The high-pass and low-pass filters were invaluable for cleaning up noise and rumble without affecting dialogue. The unit could also simulate the “Academy Rolloff” in re-recording theaters using its low-pass filter. In stereo broadcasting, the 674A was used to enhance announce microphones, sweeten stereo music, and equalise phone or remote lines for flat response. Its RF suppression and optional output transformers facilitated problem-free installation in high-RF environments.
For dance bars and disco applications, the 674A provided a stereo equaliser that offered solid, punchy bass free from muddiness and an aggressive top free from the ringing typical of full-octave graphic equalisers. The eight bands allowed for substantial correction of response deviations in the upper bass and midrange, while narrowband notches addressed resonances in high-efficiency horn-type loudspeakers. In bi-amped installations, the separate low-pass and high-pass filter outputs served as a complete electronic crossover. The unit was positioned as a cost-effective alternative to more expensive parametric equalisers, offering rigorous quality for applications that would otherwise default to less capable graphic equalisers.
Features
- Eight bands per channel with independent tuning and bandwidth controls
- Quasi-parametric equalisation with ±16dB reciprocal peaking range
- Tunable high-pass and low-pass filters with 12dB/octave Butterworth slopes
- Separate main, low-pass, and high-pass outputs for electronic crossover use
- Long-throw dust-shielded slide potentiometers for precise control resolution
- Front-panel gain controls with 12dB available gain
- Peak-stretching overload lamps to indicate clipping
- Active balanced inputs and unbalanced outputs with optional transformer balancing
- RF suppression on inputs, outputs, and power leads
- Industrial-grade construction with socketed integrated circuits