Stereo Amplifier

Review: McIntosh MA5100 Stereo Amplifier

McIntosh give a guarantee that the performance limits are the maximum deviation permitted: "We promise you that the MA5100 you buy must be capable of performance at or exceeding these limits or you get your money back. McIntosh is the only manufacturer that makes this guarantee". They also offer a free three-year factory service contract covering parts and labour, but I would suspect from the excellence of the components used that this department is the least busy in the McIntosh organization.

The appearance of the MA5100 is most impressive with a front panel measuring 15 by 4-1/2 inches, finished in black and brushed silver, and the chassis finished in chromium and black. The legend on the black section of the front panel is rear-illuminated when the amplifier is switched on, and it carries the following controls: on the left is the input selector with positions for auxiliary, tape, tuner, phono 1, phono 2 and tape head. Next is a dual-concentric bass control, dual-concentric treble control and main volume control. Below this panel is the mode selector switch. It has seven positions: L to L & R, R to L & R, stereo reverse, stereo, mono (L plus R), L + R to L, and L + R to R.

There are eight toggle switches covering RIAA or LP phono equalization, tape monitor, phase reversal, speakers on/off, power on/off, LF filter, HF filter and loudness. There are two headphone sockets and a balance control.

On the rear of the chromium-plated chassis is the captive mains lead and mains fuse. The legend on the mains transformer refers only to a 117 volts, AC 50-60 Hz supply, and the back panel refers to a 2.5 amp slow-blow fuse. The power demand varies with the output over a range of 70 watts for zero signal to 200 watts for 45 watts output, both channels driven. As the 2.5A fuse is correct for an 117V supply, it would be safer to have a 1.25A rating when used on 240V supplies. Next are three AC power outlets for use with USA flat-pin plugs, left and right channel screw-terminal loudspeaker outputs, gain control for L + R signals, earth terminal, and a group of phono sockets for tape out, tape in, auxiliary, tuner, phono 1, phono 2 and tape head. For unused high sensitivity inputs, earthed phono plugs are supplied.

As one might anticipate, the circuit arrangement is complex but logical and makes use of 34 silicon transistors, 23 silicon rectifiers and diodes, 2 silicon bilateral switches and 2 Triacs. The pickup and tape head pre-amplifiers use three silicon transistors in each channel, comprising a very low noise input transistor followed by an emitter follower stage directly coupled to the third transistor. Frequency correction for pickup and tape head is achieved through a precise negative feedback circuit, and the design is such that an input signal of 125mV will not overload the pre-amplifier, which is a considerable safety margin.

The output of the pre-amplifiers is connected to the input selector switch and the full-range balance control. An unusual feature is the use of a four-section ganged volume control, the first section being placed before the three-transistor tone control stage and also coupled to the loudness switch. The second section is placed at the junction of the tone control and filter amplifier stages. By using this split system, there is no danger of overloading the tone control amplifier section, and one maintains the beat possible signal-to-noise ratio and minimum distortion. Another useful feature is incorporated in the left channel filter amplifier: the output signal can be selected to be in phase with the right channel or 180° out of phase.

The power amplifier uses nine silicon transistors in each channel and there is both AC and DC feedback around the whole amplifier. The output transistors are mounted on massive, black anodized heat sinks and, as they are driven from a 115V DC supply rail, they are enclosed in a perforated black box to prevent accidental shock. Very elaborate precautions are taken to prevent damage to the output transistors due to overloading or short-circuiting of the loudspeaker terminals. First there is a temperature sensing circuit which immediately switches off the drive to the output transistors i f the temperature of the transistors rises above a pre-set value. There is also a special power output Sentry monitor circuit using Triacs, which are very high speed electronic switches. If the power output rises above a certain level or a short circuit takes place, the Triacs operate in a few micro-seconds and reduce the drive to the output transistors. McIntosh can thus claim complete reliability of the protection circuits.

The power supply consists of four separate sections. The fully screened mains transformer has two identical primary windings which are in parallel for a 117V AC supply or in series for 234V. The two secondary windings feed fullwave silicon rectifiers followed by a highly smoothed RC network using 9,300μF electrolytic capacitors for the pre-amplifier and penultimate power amplifier stages.

The whole of the input, tone-control and filter circuitry is screened, together with all the controls on the front panel. Finely printed on the screening box is a block diagram giving the position of each control in the circuit and the gain of each section. The accompanying booklet lucidly explains the functions of the controls, includes a comprehensive specification, typical response curves, harmonic distortion and intermodulation distortion plotted against power output.

How it performed

Our response curves confirm the claims of the McIntosh company and show the excellent frequency response and the remarkably flat curve for phono or tape head input, indicating the exactness of the frequency equalization network. The oscillograph traces which were examined at various frequencies also confirmed the performance for sine and square wave signals. Overdriving the amplifier to approximately 70 watts per channel showed symmetrical flattening of a sine wave input, a good feature.

A more extensive analysis was made at 40 watts by substituting a Marconi heterodyne analyzer, which measures the amplitude of each individual harmonic. Having measured each harmonic separately up to the fifth, one then performs a mathematical calculation which gives a harmonic distortion figure excluding hum and most of the noise. Thus, at the 40 watt level, the total harmonic distortion gave a figure of 0.07% whilst taking the individual harmonics gave only 0.041 %, the difference being the slight (and inaudible) hum and background hiss.

For the gramophone record enthusiast with a collection spread over the years, the disc compensation switch is a useful feature. Most modern amplifiers are designed for use with stereo recordings and ignore the earlier and often excellent mono LP records. The McIntosh MA5100 has a two-position switch which enables one to select the correct equalization required by modern stereo recordings cut to the RIAA equalization curve, or early mono LP recordings made before standardization came in. Both the bass and treble controls are of the dual-concentric type, so that one can adjust the response separately for each channel and once suitably adjusted, a built-in friction clutch drives both sections in step.

The bass and treble switched filter controls perform according to the designer's specification but differ somewhat from normal BrItish practice. This applies in particular to the bass filter. The specification suggests that there is a rate of 12 dB/octave. In practice, as the B & K filter curve shows, attenuation starts gently at 100 Hz: at 50 Hz the attenuation is only 4 dB and at 25 Hz (octave steps) a further 5 dB. As the main use of the bass filter is to reduce turntable rumble, which only occurs normally below about 30 Hz, one feels that a steeper cut starting at a lower frequency would be advantageous.

The excellent operator's manual has a section on the loudness switch in which it correctly states that, when the volume is reduced, it is a characteristic of human hearing that the bass and treble decrease more rapidly than the midtonal range. Thus the loudness control automatically provides the bass and treble boost required to compensate for the change in response of the human ear at low loudness levels. In fact, as the loudness curves show, the circuit provides the appropriate amount of bass boost but it does not give any treble boost at any volume level. One can however correct the situation by slightly increasing the treble controls.

Behind the front translucent panel are several lamps, some fed with rectified AC to reduce the possibility of hum pickup. Naturally, as the lamps are in a sealed cavity, the heat generated warms the front panel slightly after the amplifier has been in operation for a few hours. It does appear that, if any of the lamps fail, it is necessary to remove the whole front panel whereas a small mechanical modification would allow replacement by removing only the screening cover. Whilst this is a minor point, the manufacturer does state on the legend printed on the screening cover that there are no user serviceable parts inside and it should only be removed by qualified personnel.

With these few minor criticisms, the McIntosh MA5100 is a superb design both electrically and mechanically. It was in use for nearly two months and every control operated with smoothness and complete absence of background noise. It is one of the few amplifiers on the market which win take a signal direct from a tape head. It meets all the claims of the designers and I would place it in the top group of amplifiers. It is certainly one of the best to come to us from across the Atlantic.