Altec Model 360 stereo integrated amplifier  (1964)
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Taken from 'High Fidelity Magazine' October 1964

THE EQUIPMENT: Altec Playback Three Sixty, an integrated stereo preamp—power amplifier. Dimensions: 5½ by 15 by 11¼ inches. Price, $366. Manufacturer: Altec Lansing Corp., 1515 S. Manchester Ave., Anaheim, California

COMMENT: The Playback Three Sixty is a completely solid-state control amplifier offering reliable performance in the medium-power class. The front panel contains six rotating controls and a row of nine keyboard-type switches. The rotating controls include: a six-position program selector (microphone, tape head, phono, radio, tape amplifier, and auxiliary); a volume control combined with the power off-on switch; separate friction-coupled concentric treble control knobs for each channel; similar type bass controls; a channel blend control; a channel balance control. The keyboard switches are for: rumble filter; stereo-mono; tape monitor; channel reversal; high or low gain; volume contour; scratch filter; phase reversal; headphones or speakers. A low-impedance headphones jack also is on the front panel.

The rear of the amplifier contains six pairs of stereo input jacks that correspond to the settings of the program selector, plus an output jack for feeding each channel to a tape recorder. There also is a slide switch to select suitable gain and equalization for either a magnetic or ceramic (crystal) cartridge. A separate jack provides an "A plus B" signal for driving an additional basic amplifier (for monophonic sound in another room, or a center channel in the main listening room). In addition, there are terminals for a center-channel speaker that may be hooked up directly to the Three Sixty. The left and right speakers are connected directly to two dual-terminal barrier strips; suitable output impedance (4, 8 or 16 ohms) is selected by a slide switch. The rear of the set also has two AC convenience outlets.

The circuitry of the Three Sixty is built around twenty-eight transistors, and three diode-rectifiers. To prevent damage to the transistors or amplifier failure owing to an extreme rise in heat (which may be caused by excessive current in the output stages due to a short circuit), the amplifier is equipped with three thermal-type, automatic-reset circuit breakers. Two of these devices are in the output signal channels, and the third in the AC power line to the amplifier. In tests conducted at United States Testing Company, Inc., these circuit breakers proved to be excellent safeguards against short circuits or severe overload. USTC regards their use as a definite design advantage as well as a convenience, inasmuch as they obviate the need to pull the amplifier out of its installation to replace a fuse.

In performance tests, the Altec Three Sixty exceeded its power claim of 25 watts per channel at about 1.5% total harmonic distortion, providing actually 25.9 watts on the left channel, and 26.2 watts on the right channel. Other related measurements are shown on the accompanying charts. The power bandwidth, extending from 13 cps to 30 kc, is especially good for an integrated amplifier. Harmonic distortion varied with the power the amplifier was called on to deliver, rising somewhat generally above 20 watts per channel and in the high-frequency region. The IM distortion was typical of many solid-state amplifiers, decreasing as audio power output was increased up to almost full power. The lowest distortion, in general, was measured with the amplifier driving an 8-ohm load, and at power output levels from about 10 to 20 watts per channel. This, along with the amplifier's fairly low damping factor, would suggest the Three Sixty's optimum use in driving well-damped speakers of moderate to high efficiency. Frequency response, RIAA disc equalization, and NAB tape equalization all were very good; the tape-head playback characteristic, in fact, was one of the finest yet measured. Tone control, loudness contour, and filter characteristics all conformed closely to the curves shown by the manufacturer and were quite satisfactory.

An unusual feature of the Three Sixty is its "high gain" switch for changing the amplifier's input sensitivity. The switch, when off, decreases the amplifier's gain and thus can prevent overloading from high-level sources that are supplying very strong signals. In its ON position, the switch permits the Three Sixty to operate at full gain. The OFF position of this switch, as expected, improved signal-to-noise ratio, although the S/N characteristic was very good in the ON position. There was no appreciable change in distortion from one position of the switch to the other.

Square-wave response was, in general, good for an integrated amplifier. The 10-kc photo showed some roll-off of the extreme highs, but no ringing. The 50-cps photo showed some low-frequency boost but no excessive phase distortion. The amplifier proved to be completely stable with all loads. All told, the Three Sixty shapes up as a very worthy contender in the medium-power class of new, all-transistor integrated amplifiers.