Audio Merge CF67B Variable Filter

The Audio Merge CF67B is a fully analogue, single-channel passive LC high- and low-pass filter unit designed for professional audio systems, featuring stepped cutoff control, selectable filtering modes, DSP-validated vintage Altec tonal characteristics, customised multi-tap inductors and a rackmount 19" 2U format optimised for modern line-level studio environments.

Vintage Filtering Heritage with Modern Precision

The Audio Merge CF67B draws inspiration from the classic Altec 9067B and Langevin 255 filter designs, combining their recognised tonal character with modern engineering suited to contemporary studio workflows. Engineers designed the unit to recreate the distinctive behaviour of vintage broadcast filters while adapting the circuitry for modern impedance-bridging systems rather than traditional 600Ω power-matching environments.

The CF67B preserves the sonic behaviour of the original circuits through DSP validation during the design stage. This process ensures that the analogue filter response maintains the characteristic curves associated with the historic hardware while benefiting from precise component calibration and contemporary manufacturing standards.

Passive LC Architecture and Pure Analogue Signal Path

The CF67B uses a passive LC circuit topology that relies exclusively on inductors, capacitors and resistors to shape frequency response. This architecture avoids active amplification stages and therefore requires no external power supply. The signal passes through a fully analogue path that introduces no additional electronic noise beyond the source device.

Passive operation means that the signal-to-noise ratio and noise floor depend entirely on the connected equipment. The design prioritises signal purity and reliability, which makes the unit suitable for demanding recording, broadcast and mastering environments where stable analogue processing remains essential.

Variable High-Pass and Low-Pass Filtering

The CF67B provides separate stepped controls for both high-pass and low-pass filtering. Each filter section includes eleven positions including an OFF setting and ten selectable cutoff frequencies. The high-pass filter ranges from 70 Hz to 7.5 kHz, while the low-pass filter ranges from 250 Hz to 10 kHz.

Independent filter operation allows engineers to restrict specific portions of the audio spectrum instantly. Users can also overlap the two filters to isolate or sculpt narrow frequency bands, which enables creative sound design and precise tonal shaping during recording, mixing or broadcast processing.

Flexible Filtering Modes and Resonance Control

The CF67B includes a function selector that determines how the filter stages operate. Engineers can apply only the high-pass filter, only the low-pass filter or combine both simultaneously. This flexible routing expands the number of available tonal configurations and supports a wide range of corrective and creative filtering tasks.

The BELL mode introduces six selectable damping levels that control the resonance behaviour around the cutoff frequency. This capability allows precise emphasis or attenuation of frequencies near the filter point, enabling subtle tonal sculpting or more pronounced filtering effects depending on the chosen setting.

600Ω Mode for Vintage Behaviour

Modern studio equipment normally uses impedance bridging rather than impedance matching. The CF67B adapts to this environment while also offering a dedicated 600 mode that recreates the electrical behaviour of classic 600Ω broadcast systems.

When users engage the 600 mode, the filter responds as though it connects within a traditional matched-impedance signal chain. This mode introduces a 6 dB insertion loss that mimics the behaviour of historic hardware and produces the familiar tonal interaction associated with vintage studio filters.

Precision Components and DSP Validation

Audio Merge equips the CF67B with customised multi-tap inductors that form the core of the passive LC network. These components provide the stepped cutoff positions that define the filter curves.

During development, engineers analysed the analogue responses using DSP-based calibration tools to ensure that the curves accurately replicate the classic Altec filter behaviour. This approach allows the unit to maintain vintage tonal authenticity while achieving precise and repeatable frequency responses.

Connectivity and Studio Integration

The CF67B supports parallel input and output connections to accommodate flexible studio routing. The front panel includes a ¼-inch TS connection, while the rear panel provides both ¼-inch TS and XLR connectors. These options allow quick integration with patchbays, consoles and outboard processors.

True bypass switching routes the signal directly from input to output when filtering is not required, ensuring complete transparency within the signal chain. The unit handles line-level signals up to +38 dBu with extremely low distortion, maintaining professional studio performance across a wide dynamic range.

Professional Studio Construction

Audio Merge builds the CF67B as a stereo-paired analogue device housed in a standard 19-inch rack enclosure occupying two rack units. The unit measures 88 mm in height, 483 mm in width and 110 mm in depth, with a weight of 2.7 kg.

Each unit undergoes hand assembly in Barcelona and features carefully selected passive components that guarantee consistent performance across the full frequency spectrum. The design supports 260 different filter combinations, allowing engineers to create highly specific spectral shaping during music production or broadcast processing.

Why Choose the Audio Merge CF67B

• Delivers authentic vintage filter character while integrating seamlessly into modern studio systems

• Enables precise spectral shaping for recording, mixing, mastering and broadcast work

• Provides a wide range of creative sound design possibilities through overlapping high- and low-pass filtering

• Maintains a completely analogue signal path that preserves tonal integrity and avoids digital artefacts

• Offers flexible connectivity and true bypass operation for effortless integration into professional signal chains

• Recreates classic 600Ω filter behaviour for engineers seeking genuine vintage tonal interaction

• Passive LC filter design inspired by Altec 9067B and Langevin 255
• Fully analogue signal path with no active components
• Variable high-pass and low-pass filtering
• Ten stepped cutoff frequencies for each filter section
• Independent high-pass and low-pass filter controls
• Function selector for HPF, LPF or combined HPF and LPF operation
• BELL mode with six selectable damping levels
• 600Ω mode for vintage impedance behaviour
• Custom multi-tap inductors
• DSP-validated filter response
• True bypass switching
• Line-level signal operation
• Maximum input level of +38 dBu
• Frequency response of ±0.5 dB from 10 Hz to 70 kHz in OFF mode
• Parallel input and output connections
• Front ¼-inch TS connection
• Rear ¼-inch TS and XLR connections
• Stereo-paired design
• 19-inch 2U rackmount format
• Passive operation requiring no power supply
• 260 possible filter combinations

This item, like everything we supply, comes with the complete support of our expert team. Our excellent technicians have been specifying, building and supporting studios for over twenty years. If you have any problems with installation, setup or everyday use, they’re never more than a click away, and always happy to help.

You can contact them here.