Review from Hi Fi Choice 55 August 1990
Magnum MP150 / MF150 Pre - Power Amplifier
After a brief spell of success in the UK under the auspices of Pure Gain, the production of Magnum amplifiers was taken up by a new company called rather confusingly, A. C. Magnum. Some two years later the availability of Magnum amplifiers has been bolstered with the appointment of Hailey Audio as official UK distributors, a move that leaves Magnum's overseas business quite unaffected.
The Magnum amplifier range is quite comprehensive, encompassing two integrated models and no less than four pre-amps and seven power amps, the largest of which is a 320W monoblock costing just under £2000. By contrast the MP150 / MF150 represents Magnum's entry-level pre / power combination.
Four milled alloy knobs grace the slim (although rather deep) pre-amp, catering for 5-way input selection, tape monitoring, volume and balance - though the latter could do with a centredetent if only to ensure correct channel matching. Interestingly, Magnum has facilitated the independant selection of either MM or MC cartridges via the front panel, instead of fitting a gain switch on the rear of the amp as tends to be common practice. All input selection is effected via remote 4052 CMOS switches, one provided for each channel to minimise crosstalk. Further to this the phono selection is distinguished via a bank of encapsulated relays, Magnum employing two 741 op-amps in the form of a headlamp (with additional op-amp based regulation) prior to the actual RIAA eq section. An Hitachi HA12071 three-stage op-amp is used for active disc equalisation, an option also exercised by Kelvin.
The output / line stage of the MP150 pre-amp is rather unusual, utilising a single Motorola 2N3906 transistor as a driver with a BC182B providing a constant current source. The latter covers the negative excursion of the waveform while the driver covers its positive excursion. The transistor never switches off and so is effectively operating in single-ended Class A mode!
Magnum's MF150 power amplifier is altogether simpler though no less elegant; the recessed heatsinking near the rear of the case is just one example of its sensible construction. The guts are based on Hitachi's original power MOSFET application circuit though the MF150 features newer 25K1058 / 2SJ162 devices which, though not super-fast, are perhaps better suited for duty in an audio amplifier. A full Zobel network is incorporated but Magnum has eschewed any form of output protection circuitry.
Magnum's MP150 pre-amp is an interesting blend of compromises. On the one hand it offers a good midband separation of 80dB, very low A-wtd noise figures such as -84dB for MM disc, sensible input sensitivities and sufficient +29.5dB / +25,3dB headroom for MM / MC cartridges. On the other it suffers from a constant 0.7dB channel error because the balance knob has to be visually aligned, an unusual negative 20Hz phase shift on MM (note the 0.3dB bass boost below 20Hz on the RIAA plot), and very high levels of THD (typically 1.3 per cent all inputs).
THD falls to 0.11% via the internal TL072 tape buffer which offers a high 8.3V at just 1% distortion whereas the main pre-amp output is limited to 0.87V @ 1% THD rising to 6.89V @ 5% THD. This is due to the non-linearities inherent in the pre-amp's single-ended Class A output stage, giving rise to the strong 2nd-4th harmonics on the 3D plot in addition to some very characteristic IM routes.
Interestingly, the MF150 power amp offers a similar range of summation IM distortions (extending to 5th order) but with relatively innocuous 2nd and 3rd order IM routes extending into the audio band. Open-loop non-linearities are also reflected in this amp's sensitivity to RF noise in the 100, 180, 350 and 470MHz regions - Magnum should investigate and cure this breakthrough.
Power-wise the MF150 is healthy enough, offering 89W into 8 ohm with a +1.5dB rise into 4ohm and peak current of 12.5A. Noise is very low, stereo separation good and the output impedance held down to a creditable 0.014 ohm, equivalent to a notional 8ohm damping factor of 587 at 20Hz.
During and after the listening tests our panel commented on what they considered to be a marked discrepancy in performance between the line and disc stages of the pre-amp, despite being very impressed at the overall sound of the combo. Specifically, the line input was thought to sound smooth and very tidy with a balanced, strain-free and extended bandwidth. Furthermore it offered a good sense of pace and involvement and while there was a hint of graininess at the very top of its range this did little to dampen our listeners' enthusiasm for the emotive accuracy of the music.
Meanwhile the combo failed to reveal all the spatial qualities available from the PDM compact disc source which we used during the listening tests, even though this omission was traded for a degree of comfort and fluidity missed by many other amplifiers - a justifiable bargain in the view of the assembled listeners. On the downside there was an immediate narrowing of the soundstage, though no apparent loss in stereo depth via MM disc.
In tonal terms the sound of the MP150 / MF150 combination was smooth and even; strings were a little strained but were certainly not hard or edgy. However the music, though not totally lacklustre, was neither particularly vivid nor alive, and was described as 'languid' by one listener when compared to the radiance of the CD input.
Via the CD input it was the subtle captivation of the amp - its sheer lack of the obvious - that dawned on our listeners over a period. One erudite panelist implied that it homed-in on the intellectual centres of the brain rather than going for the throat!
If only from the subjective standpoint this pre / power combination is particularly suited to CD-based systems, with records as a secondary source. The MOSFET power amp is clearly the more successful of the duo, for the pre-amp's value - though still high - is inevitably compromised by its flexible but inferior disc stage. Still, with these provisos in hand, and bearing in mind the keen pricing of the amplifiers, a qualified recommendation seems most appropriate.
Please note that after switching on, there is an 8 - 12 second delay shorting - circuit incorporated at the output of the Pre-Amp. this eliminates power surges to the loudspeakers in the event of the Power Amplifier being "on" before the Pre-Amp, when the short circuit is realised, there will be a click from both speakers - the system will now function normally.