Audio Developments Co. Passive Filter Vintage 60's *RARE* like Pultec HLF3

Exceptionally rare studio piece. Audio Development Co. (ADC) Model 706A Variable Filter. Manufactured in the 1960s for high-end broadcast and studio tracking, this mono, completely passive custom made high-pass/low-pass filter network delivers an unmistakable, thick vintage iron character. thought to be Warmer than a Pultec HF3C vintage. If you are looking for a unique tone-shaping tool to inject authentic analog soul into your tracks, this is it. Because it is a true passive design utilizing premium mid-century transformers and inductors, it shapes sound with an organic musicality that modern digital plugins or active equalizers cannot match. Running audio through it—even with the filters set to "Off"—passes your signal through heavy vintage iron, adding a gorgeous harmonic depth, weight, and subtle saturation often compared to classic Pultec-style circuitry. Smooth, Organic Filter Slopes unlike modern digital "brick-wall" filters that can sound harsh or clinical, the 706A features an incredibly smooth 12 dB per octave slope (2nd-order). It utilizes a gentle, progressive knee that rolls off naturally around the cutoff points. This allows you to aggressively sculpt frequencies while retaining the core warmth and natural transients of your audio signal.Precise Stepped ControlThe unit is equipped with independent, satisfyingly clicky stepped rotary switches for both bands, making settings perfectly repeatable: High-Pass Filter (Low Cut): Off, 50 Hz, 100 Hz, 250 Hz, 500 Hz, and 1,000 Hz. Perfect for cleaning up low-end rumble or muddy sub-frequencies while adding lower-mid warmth. Low-Pass Filter (High Cut): Off, 2,000 Hz, 3,000 Hz, 5,000 Hz, 8,000 Hz, and 10,000 Hz (up to 15,000 Hz depending on the exact series). Ideal for taming harsh digital high-end, smoothing out overheads, or creating a vintage broadcast vibe. (Note: As a true passive hardware unit, it performs best when matched to a 600-ohm impedance load via a high-quality studio preamp or utility amplifier to ensure the faceplate frequency markings align exactly with the cutoff points.)Condition: Cosmetic: Typical vintage condition. Minor rack wear and light scuffs consistent with age (please review the high-resolution photos closely). Faceplate lettering is incredibly crisp and legible. It does have several holes drioled through fron panel (see photos) Functional: 100% operational. All stepped rotary pots switch cleanly into place with no signal drop. Specifications: Manufacturer: Audio Development Co. (ADC) Model: 706A Type: Passive Variable High-Pass / Low-Pass Filter Era: 1960s I/O: Vintage terminal/pigtail strip connections (standard for professional gear of this era). Unit will be meticulously packed, double-boxed, and shipped fully insured