Manufacturer's Specifications:
Power Output: 250 watts continuous per channel into 8 ohms, 20 Hz to 20 kHz.
Rated THD: 0.025%.
TIM: Less than 0.03% (3.18 kHz square wave and 15 kHz sine wave, 4:1, measured at full power output).
Frequency Response: 20 Hz to 20 kHz, +0,-0.2 dB; 3 Hz to 100 kHz, +0, -1dB.
SMPTE-IMD: Less than 0.02%.
Dynamic Headroom: 2.5 dB.
Damping Factor (Low Frequency): More than 100.
Hum and Noise: 85 dB, A weighted, below 1 watt.
Input Sensitivity: 110 mV for 1-watt output (1.75 V for rated output).
Input Impedance: 20 kilohms, shunted by 50 pF.
Power Requirements: 108 to 132 V, 60 Hz, 10A.
Power Consumption: 240 watts at 0 output, 1,200 watts at rated output.
Dimensions: 19 in. (48.07 cm) W x 7 3/8 in. (18.03 cm) H x 17 1/4 in. (43.64 cm) D.
Weight: 50 lbs. (22.72 kg).
Price: $599.95 (kit only).
If you think that the audio industry has rejected the idea of ultra-high powered amplifiers for use in home audio systems, consider this power amp from those innovative creators of Heathkits in Benton Harbor, Michigan. By far the most powerful amplifier ever offered to audiophile kit builders by Heath, the AA-1800 not only boasts a rated power output of 250 watts per channel into 8-ohm loads, but upon testing dynamically, it shows a dynamic headroom of just about 3 dB. This means under short-term musical signal inputs it can deliver around 500 watts per channel, or a total of a kilowatt or more. This amplifier is available in kit form only, though I was supplied with a unit assembled by Heath personnel. Interestingly, once it is fully assembled, Heath does not recommend shipping the unit anywhere with its power transformer installed. Instead, the massive power transformer is equipped with locking-type multiple pin connectors which can be easily disconnected should it ever become necessary to transport the unit. The amplifier, therefore, arrived in two cartons! To protect the preamp's on-off switch, the AA-1800 can be turned on by a built-in relay, activated by a second, low current power cord connected to the pre-amp's switched, a.c. convenience receptacle, as well as by its own, rear panel power switch.
The black front panel is a single, wide expanse of metal interrupted only by four tiny LED indicators. One of these lights up when power is applied. Another is a protection circuit indicator, while the remaining two are peak power LED indicators which are activated when a full 250-watt output level is reached.
The rear apron of the AA-1800 is equipped with individual input level controls (one per channel), left and right phono-tip input jacks, five-way color-coded speaker connection binding posts, a fuse-holder which houses the main 10-A line fuse, and a three-position switch. The two required power cords also emanate from the rear apron.
Circuit Highlights
Aside from the major power supply parts, such as the aforementioned power transformer and two large 13,000-µF electrolytic capacitors, most of the other electronic parts of this amplifier are mounted on three major circuit boards, two identical output amplifier boards and a circuit protection board.
The output stage for the positive-going signal consists of multiple emitter-follower transistors and a parallel-connected pair of transistors. Another pair of transistors completes the series-parallel output configuration, which is biased by a pair of common emitter-follower transistors. This configuration has each of the main output transistors sharing half the output current and half the power supply voltage. The output stages for the negative half of the signal are identical in operation to those of the positive half and are symmetrical and 180° out of phase. Additional circuit blocks handle speaker turn-on delay, peak power indication, and speaker protection.
An interesting method is used to protect speakers from potentially damaging signals, whether due to amplifier malfunction or improper input signals, and is based on one transistor serving as a safe voltage detector in conjunction with an associated integrated circuit. If voltage remains outside a "safe region" for more than 50 mS, the complex circuitry disconnects the outputs from the speakers, and in addition, two circuit breakers function as temperature sensitive switches. At normal operating temperatures, these switches are closed. If the temperature of the heat sink ever exceeds 90° C, outputs again automatically disconnect from the speakers.
Measurements
With 8-ohm loads in place, the amplifier delivered over 300 watts per channel before rated harmonic distortion of 0.025% was observed, using a 1-kHz test signal. At the 20 Hz test frequency, power output for rated THD reached 287 watts; at 20 kHz the reading was 265 watts. A plot of power output versus THD is shown in Fig. 1. When input levels were reduced to produce exactly the rated 250 watts per channel (again into 8-ohm loads), I observed what amounted to test-equipment residual distortion levels at 1 kHz and 20 Hz of just under 0.003% and approximately 0.02% at 20 kHz. SMPTE-IM distortion measured 0.014% at the equivalent of rated output. Damping factor at 50 Hz measured 185, well above the 100 claimed by Heath. Using the Otalla method of TIM measurement, I was unable to detect any amplifier-produced IM products over a dynamic range of 75 dB on a spectrum analyzer, which means that if there were any TIM it had to be less than 0.018%! Dynamic headroom was amazingly high, measuring a full 3 dB. This, as said earlier, means that when driving the amplifier with music program signals, you can expect to reach peaks of 500 watts per channel or more before encountering noticeable clipping of the waveforms. Distortion versus frequency, for rated output, is plotted in Fig. 2. Frequency response for a -1.0 dB roll-off extended from around 3 Hz to 110 kHz.
Signal-to-noise ratio, as measured in accordance with the new IHF/EIA Amplifier Measurement Standards (0.5 V in and level controls adjusted for 1-watt output, with an A weighting network in series with the measuring instrument), was 88 dB. Worst-case channel separation over the entire audio band was never poorer than 65 dB.
Fig. 1--Distortion vs power output, Heath AA-1800 power amplifier.
Fig. 2--THD vs. frequency at rated output.
Use and Listening Tests
Heath makes a point in their well-written instruction manual about never connecting input grounds to output grounds when using this amplifier. This caused some grief when testing the unit on the lab bench (I had to "float" much of the equipment that ordinarily shares a common ground), but once that was worked out things went quite smoothly.
When I transferred the unit to a listening installation, I encountered no such problem, of course, since my reference loudspeakers are hard-wired individually to the test setup in the listening location. I was a bit fearful at first about really opening up the amplifier while driving my fairly efficient KEF 105 II reference loudspeakers, but the latter have their own protection features with a control that can be set as high as 200 watts per speaker. That's the way I ran the amplifier during extended listening tests, and I can report the sound was clean at those peak levels and that with many musical test records, including some wide dynamic range dbx-encoded discs, it is not at all unusual to come up to such peaks, however briefly.
The amplifier's sound might best be described as on the tight and bright side, delivering the kind of tight bass that characteristically occurs with amps having high damping factors and traditionally high overall loop feedback. The somewhat rough or edgy kind of high response that one also associates with this kind of solid-state circuitry seemed absent, providing further correlation between low TIM and absence of irritation when listening to high-frequency transients or other high-frequency program material. Still, some adherents of vacuum-tube sound may find the highs from this amplifier to be just a bit too brilliant, however clean they may also seem.
Physically, the AA-1800 needs a lot of space and adequate ventilation, but given those things it operated without even getting warm to the touch when reproducing music programs at the levels previously mentioned. Although I did not construct the amp, I did read the construction manual, and as usual it is written, in the typical Heath manner--meaning just about anyone who can hold and use a soldering iron and can read English should have no real trouble assembling this power amplifier. They may, however, have a bit of trouble lifting it into its final resting place, in which case that plug-in power transformer can always be dropped into place last.
-Leonard Feldman
(adapted from Audio magazine, Sept. 1982)
Also see:
Heathkit Model AA-1640 Stereo Amplifier (Nov. 1976)
Heath AJ-2520 Tuner (Mar. 1990)
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