Magnum British Class A Pre-Amplifier MP-300 (Used)

Made In England


The MP300 preamplifier is a black box some 17 x 11 x 2.5in.

 On the front panel, looking from the left, is a power on/off switch with associated LED indicator and next to this is a mute button.

 To the right is the source selector switch offering tape, tuner, video, auxiliary, CD and phono inputs. Next in line come mono and monitor push-switches. A balance control (which irritatingly has no centre detent) and a good-quality ALPS volume control complete the line-up.

 On the rear drop at extreme left are a ground terminal and a push-button switch giving MM or MC facilities on the phono input. Then comes a long row of gold-plated RCA input sockets, with L and R outputs also appearing on similar sockets.

At extreme right is an IEC mains input socket with associated mains fuse. There is no headphone socket, and at this price point I would have expected one. Removing the preamplifier lid reveals that the circuitry is constructed on a double-sided printed-circuit board; the only component not carried on the board is the toroidal mains transformer together with its associated mains and LV distribution wiring. The transformer secondary feeds a bridge rectifier and four 4700μF 35V reservoir capacitors, from which in turn are derived +24V and -24V rails via 1A 78- and 79-series three-terminal regulators. On a small point of detail, both these devices have incorrect values of input and output capacitors fitted to them. Positive regulators in the 78xx/LM340 family require 0.22μF between input and earth if - to quote the National Semiconductor databook - "the regulator is located far from the power supply filter".

Equally, a 0.1μF ceramic disk across the output of the regulator is recommended "to help transient response".

In the MP300, capacitors with a value of 0.15μF are fitted in both positions. More importantly, the same values are used on the input and output sides of the negative regulator, whereas the manufacturer states that a 2.2μF tantalum capacitor is required on the input of a 79xx/LM320-series device and a 1μF component on the output. Oscilloscope checks showed that both positive and negative rails in the MP300 were stable, but nevertheless it would have been preferable for the designer to have specified components of the recommended values. 


 The phono stage makes use of four PMI OP27 op-amps in what appears to be a fairly conventional circuit. The output from this is fed to a gain stage employing discrete devices. There is no user-variable loading for MC cartridges; the specification quotes 100 ohms. All the integrated circuits are mounted in ordinary commercial-quality (i.e. not gold-flashed or otherwise treated) IC sockets, which is arguably not a good design feature - especially in circuits handling very low voltages such as those found at the input of an MC amplifier. Amongst other things, the ohmic qualities of the physical connection between the socket and the IC will inevitably degrade with time, which will almost certainly cause a deterioration in sonic performance. The only advantage of mounting an IC in a socket is that it becomes very easy to replace in the event of failure. However, an eight-legged op-amp is hardly difficult to remove by desoldering in the usual way if it does fail, and the odds against well-proven devices such as OP27s failing in this type of application must be fairly remote.


Apart from these items there is little to criticize from the constructional point of view, although I mildly disliked the somewhat untidy cabling from the transformer. A long bolt protruding through the rear of the PCB was presumably intended as a mounting for the board but lacked any nut or washer and was decidedly bent. I also thought the single sheet of paper doing duty as a manual, with its rather fractured-English text and random capitalizations and punctuation, was somewhat less than comprehensive.







Note : the capacitor inside this pre-amplifier had been upgraded to Audiophile grade
          


Price : RM 1650

Contact : Simon Ting 012-3612507