Taken from High Fidelity Magazine, January 1961

AT A GLANCE: The Heathkit AA-40 is a basic stereo power amplifier rated at 40 watts output per channel, or 80 watts monophonically. It contains bias voltage and balancing adjustments, requiring the use of an external voltmeter.

The AA-40 met or exceeded its specifications in every important respect. It proved to be rock-stable under a variety of load conditions, with a clean, solid sound that places it in a select but widening group of outstanding power amplifiers. Its price, in kit form, is $79.95.

IN DETAIL: The Heathkit AA-40 consists of a pair of power amplifiers, each of which uses a pair of EL34 output tubes driven by a 6AN8. The power supply uses four silicon diodes in a voltage-doubling rectifier circuit, and a selenium rectifier to supply bias to the output tubes. The components are assembled and wired in an uncrowded manner, but the over-all dimensions of 11 in. deep by 15 in. wide by 7¼ in. high permit much leeway in installing the amplifier.

Each channel has a pair of meter jacks for bias and balance adjustment. To use them you connect a voltmeter across terminals and set the balance adjustment for zero reading. The voltmeter is then connected from one terminal to ground and the bias adjusted for a 1.5 volt reading. The two adjustments interact slightly, so they must be repeated once or twice until the correct settings have been obtained. Protective caps are provided to prevent accidental changes of these adjustments.

Each channel has its own level control. A slide switch marked STEREO- MON connects both channel inputs together for mono operation, with the channel A volume control affecting both channels. A phase-reversing switch in channel A assists in setting phase relations when the system is installed.

The output impedances are 4, 8, 16, and 32 ohms. When mono operation of the amplifier is desired, the two outputs are paralleled, with resulting impedances of 2, 4, 8, and 16 ohms. The unit also has a centre channel output with the sum of the two channel signals fed to these terminals. This output may be used for a centre-fill speaker. If so, an external L-pad is needed to adjust the volume on the centre speaker.

A common problem in power amplifiers, particularly those with silicon rectifiers, is the high surge voltage which can appear on the filter capacitors and other components before the tubes have warmed up fully. This problem is solved in the Heath AA-40 with "surgistor," or current-limiting resistor, used in the primary power lead to the amplifier. The surgistor acts as a time delay relay, which applies reduced voltage to the amplifier until sufficient current is drawn by the amplifier tubes, after which the relay closes and applies full voltage. Thus, full ratings of tubes and components can be utilized, without danger of shortening their lives by overload during warm-up.

In our tests, both channels proved identical in performance. Power output at mid-frequencies was 48 watts per channel at 1% harmonic distortion. Power bandwidth (the frequency limits at which half power, or 24 watts, could be obtained with 1% distortion) was from below 20 cps to over 20 kc. Intermodulation distortion, only a few tenths of a per cent at listening levels under 10 watts, reached 1% at 40 watts. A hint of the high quality of this amplifier can be seen in the 20-cycle harmonic distortion curve, which closely parallels the IM curve. Some 30 watts can be obtained at 20 cps with only 1% distortion, and the distortion at normal levels is very low. The frequency response of the AA-40 is within a few tenths of a db from 20 cps to 10 kc, and drops off slightly to about -2.5 db at 20 kc.

The hum level of this amplifier is entirely inaudible, being in the vicinity of 75 to 80 db below 10 watts. The amplifier is completely stable under any type of capacitive load we could devise. One rather severe test we apply to amplifiers is a measurement of output power at 10 kc with a 3-mfd capacitive load on their 8-ohm output terminals. This is a far more severe load condition than driving any commercial electrostatic speaker. As a rule, power amplifiers cannot deliver more than 20 or 30% of their normal output under this condition. The Heath AA-40 set a new record, with 37 to 38 watts output.

In listening tests, the amplifier sounded as good as it tested, clean and effortless at any level. One property it shares with a very few fine amplifiers is that of refusing to break up or sound mushy at the highest levels our ears could stand. This requires not only high power output, but a freedom from transient overload effects not found in all high-powered amplifiers. Much of the distortion commonly attributed to speakers at very high volume levels is actually caused by the amplifier. With the AA-40, even moderately rated speaker systems can give very clean sound at high levels.
H. H. LABS.

Building the AA-40

Although this kit, with its approximately 470 parts, poses something of a challenge to the novice at assembling electronic components, the process should present no insurmountable obstacles. The instruction booklet is extremely easy to follow, and the large pictorials, each devoted to a single stage of construction, guide the builder from beginning to end. With careful reading of the manual and unhurried attention to each step, construction time should run about twelve hours.