Heathkit AP-1615
A preamp that promised the specs of the expensive stuff, if you were willing to solder it yourself.
Overview
The AP-1615 was Heathkit's stereo preamp and control center, sold as the natural front end for the company's own AA-1640 "Superamp." Heathkit's catalog copy made the pitch plainly: the AP-1615 "offers the kind of specifications and features normally found on preamps costing much more." That's marketing, and Heathkit was very good at marketing. The numbers printed alongside it hold up to a second look.
Start with the phono stage, because that's where a preamp of this era either earns its keep or doesn't. Heathkit rated input sensitivity at 2 mV for phono and 180 mV for the high-level inputs - tuner, tape, aux, tape monitor, and tape dubbing. Phono overload is specified at over 100 mV. That mattered in 1976, when high-output cartridges and hot pressings could swamp a marginal phono section, and Heathkit's figure leaves real headroom. Hum and noise are quoted at -65 dB for phono at 2 mV input and -85 dB for high-level at 180 mV. Those aren't spectacular figures by the standards of the costliest separate preamps, but they're honest ones, measured at realistic input levels rather than massaged reference points.
Distortion is where the AP-1615 makes its strongest claim. Heathkit specified harmonic and IM distortion at less than 0.05% from 20 Hz to 20 kHz at 1.5 V output. That's a full-bandwidth number, not a single-frequency cherry-pick. Rated output is 1.5 V, with tape output at 180 mV. Channel separation is quoted at 50 dB at 1 kHz, which is adequate rather than exceptional. Collectors who've lived with these units tend to describe the stereo image as solid rather than spectacular.
Frequency response is where the spec sheet gets a little uneven. The phono section is rated ±0.5 dB from 30 Hz to 15 kHz. The high-level inputs are rated ±2 dB from 20 Hz to 20 kHz. That's a looser tolerance on the line stage than on the phono stage, the reverse of what you'd expect. Keep it in mind if you're comparing this against period preamps with tighter line-stage specs. Neither figure is bad, but neither makes the AP-1615 a spec-for-spec giant-killer.
The feature set is where the AP-1615 justifies its place in a system. It has two phono inputs, aux and tuner inputs, tape monitor, a mono switch, a headphone jack, and front-panel dubbing inputs and outputs. The dubbing jacks let you patch a tape deck in without crawling behind the rack. There's a high-frequency filter, and a sub-sonic filter that Heathkit specifically flagged as important with high-power amplifiers. Turntable rumble and record warp, amplified by something like the AA-1640, can waste enormous amounts of amplifier power and push woofers into distress.
Heathkit also built in a relay that instantly mutes the output when power is switched on or off, preventing the turn-on and turn-off thumps that could otherwise reach the speakers through a big amplifier. It's a small thing until you've heard a 200-watt amp thump a woofer.
The AP-1615 came in a case with solid walnut end panels, Heathkit's house style for this generation of components. Shipping weight was 12 pounds. Power requirement is 120/240 VAC at 60/50 Hz, so the same unit could be configured for either mains standard. Dimensions are 4½" H x 17½" W x 8¾" D, which is standard component width and shallow enough to sit on most shelves.
Two optional accessories expanded the system. The AD-1305 is a five-band, two-channel equalizer with slide controls for each channel, designed to work with the AP-1615 or any receiver or amplifier with a preamp output and power amp input, or a tape monitor loop. Each band offers a 12 dB per octave slope, with center detents for an exact 0 dB setting and a tone-flat switch that bypasses the equalizer entirely. Heathkit specified its input impedance at 100k ohms and output impedance at 100 ohms, rated output 1.5 V rms with 5 V rms overload, signal-to-noise at 90 dB below 1.5 V, and THD and IM distortion both at 0.05% from 20 Hz to 20 kHz at 1.5 V output. Overall gain flat is unity. The active bandpass filters use large, fully shielded coils, a construction choice aimed at keeping distortion low. It also has its own tape monitor, so it can be inserted into a receiver's tape loop without costing you monitoring capability. Like the AP-1615, it came in a case with solid walnut end panels, and shipped at 11 pounds.
The other companion is the AD-1307 deluxe audio control center, which is a different animal: a switching and monitoring hub rather than a preamp. It provides complete AC power and speaker switching, lets you run one stereo amplifier to two pairs of speakers or two stereo amplifiers to four speakers, and includes four switched and four unswitched outlets plus a main power switch and a line cord rated to carry up to 13 amps. That's enough to power-switch an entire system from one point. It uses the same peak-responding output meters as the AA-1640, calibrated from -30 to +3 dB and 0.2 to 200 watts into 8 ohms, with a response time under 50 microseconds and a fall time greater than 0.5 seconds. There's a front-panel headphone jack for conventional phones and a barrier-type terminal strip on the rear for electrostatic headphones. Heathkit claimed it could be assembled in a couple of evenings. It shipped at 11 pounds and measures 17½" W x 8½" D x 4½" H.
On the meters, Heathkit leaned hard on a period review. Julian Hirsch, writing in Stereo Review (May 1975), described the AA-1640's meters, the same ones used in the AD-1307, as "by far the most useful we have ever seen on an audio amplifier," adding that they were the only ones with any practical utility for the listener. Hirsch's assessment of the AA-1640 itself, in the same issue, was that its electrical performance "makes comment superfluous" and that it ranked with the finest high-power amplifiers on the market. Heathkit reprinted both lines in its own catalog.
The AP-1615's own reception is harder to pin down from the available material. Heathkit's catalog positioned it as the correct partner for the AA-1640 and for any high-quality power amplifier, and the feature set - two phono inputs, sub-sonic filter, relay muting, front-panel dubbing - supports that positioning. The available documentation doesn't provide a full independent bench test of the AP-1615 itself, or any production-run figures.
Collectors chasing a Heathkit preamp today are usually chasing a specific combination: the AP-1615 with the AA-1640, built as a matched pair, often with the AD-1305 equalizer in the chain. The AP-1615 is a kit-built component, which cuts both ways. A well-assembled example with clean solder joints and correctly dressed wiring is a capable preamp. A poorly assembled one is a troubleshooting project, and the wiring density in a preamp of this era means tracing a cold joint or a misrouted ground can eat a weekend. The walnut end panels are a plus for anyone who cares how the rack looks, and the 120/240 VAC capability means units turn up on both sides of the Atlantic.
Based on the documented specs, the AP-1615 is not a line-stage champion. The ±2 dB high-level tolerance is the number to weigh against whatever it's being compared to. What it is, is a flexible control center with a strong phono section, real protection circuitry for high-power systems, and a documented family of accessories that let an owner build out a complete Heathkit front end.
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