Heathkit AD-1305

Name: Heathkit AD-1305
Brand: Heathkit

Five slide controls per channel, 12 dB per octave of surgical correction, and a bypass switch for when you trust the recording more than the room.

Overview

The AD-1305 is a five-band, two-channel audio equalizer, sold by Heathkit as a kit and documented in the company's 1976 catalog material. A receiver gives you bass and treble and calls it tone control. The AD-1305 gives you five separate bands per channel - 30-125 Hz, 125-500 Hz, 500-2000 Hz, 2000 Hz-8 kHz, and 8 kHz and up - each on its own slide control. Broad shelving drags the whole bottom end with it. Here each slider works a defined slice of the spectrum and leaves the neighbors mostly alone. Heathkit's own catalog copy described the AD-1305 as offering "more complete control than conventional bass and treble controls," and for once the marketing language holds up.

The design intent is correction, not coloration. The catalog material frames the AD-1305 around fixing real-world problems: room acoustics, cartridge and speaker differences, and deficiencies baked into the program source itself. A five-band equalizer with a tape monitor loop is a restoration tool as much as a listening tool. Heathkit's literature explicitly notes it can be patched with tape recorders to filter old or scratchy records or broadcasts before recording.

The numbers behind the sliders are respectable for the price class. Each band moves on a 12 dB per octave slope, and the bands are spaced so they do not overlap, so the correction stays where you put it. A center detent gives an exact 0 dB setting on every band, so you can zero the thing by feel rather than by meter. When you want the equalizer out of the signal path entirely, a tone flat switch bypasses it.

The circuit is built around active bandpass filters using large, fully-shielded coils, which Heathkit credited with keeping distortion extremely low. The published figures back that up: total harmonic distortion of 0.05% from 20 Hz to 20 kHz at 1.5 V output, and IM distortion of 0.05% at 1.5 volts. Signal-to-noise is quoted at 90 dB below 1.5 V. Input impedance is 100k ohms and output impedance is a low 100 ohms. The unit is rated for 1.5 V rms output with 5 V rms of overload headroom, enough margin that a hot source will not clip the equalizer before it clips the power amp. Overall gain in the flat setting is zero dB, unity, which is the right choice for a processor that sits in a tape monitor loop.

One detail separates the AD-1305 from a lot of budget equalizers: it carries its own tape monitor. That means it can be inserted into a receiver's tape monitor circuit without costing you the receiver's own monitoring capability. The chassis provides one switched and one unswitched AC outlet, and the power supply is switchable between 120 and 240 VAC at 60 or 50 Hz, so the kit was not limited to one market. Shipping weight was 11 lbs. Dimensions are 4¼" H x 17½" W x 8" D. The case is finished with solid walnut end panels, matching the AP-1615 preamp Heathkit positioned it alongside.

That pairing explains who the AD-1305 was built for. Heathkit's catalog copy presents the AD-1305 as the companion piece to the AP-1615 preamplifier - "the optional AD-1305 (right)" - and also notes that the AR-1515 receiver's preamp-out/amp-in jacks let it be used with any high-quality audio equalizer, the AD-1305 named among them. Heathkit sold the AD-1305 as the tone-control solution for buyers who had deliberately chosen components without elaborate tone controls. The AP-1615's own tone section was basic by design.

That is the honest case for the AD-1305, and it is a good one. It is not a graphic equalizer in the modern sense — five bands per channel is coarse compared to the ten- and twelve-band units that came later, and anyone expecting to notch out a specific room mode will find the band spacing too wide for surgical work. What it does offer is broad, predictable, non-overlapping correction across the audible range, with a clean bypass and a genuinely low noise floor for its era. For a kit-built component from 1976, that is a lot of control for the money.

The AD-1305 is a Heathkit, which means two things in practice. First, condition varies enormously - a carefully assembled example and a rushed one can look identical from the outside. Second, the assembly documentation is well organized: parts are numbered in three groups (1-99 on the equalizer circuit board, 101-199 on the control circuit board, 201-299 on the chassis), resistors are 1/2-watt 5% unless marked otherwise, and voltages are specified to within ±20%. Keep that ±20% in mind when troubleshooting, since a reading that looks slightly off may be entirely within spec. Manuals and schematic sheets survive in circulation, which makes restoration realistic for anyone comfortable with a soldering iron.

The AD-1305 is not the famous piece in the Heathkit line. That status belongs to the amplifiers and receivers. It is the piece that makes a stripped-down preamp or receiver behave like a fully equipped one. Buyers who already own an AP-1615 or an AR-1515 and want tone control beyond bass and treble are the natural audience.