Philips RH 551 Preamplifier Page 2

If not in use for its intended purpose, this forms an ideal input for a CD player as it bypasses the first active gain stage and routes the signal directly to the top of the volume control. Noise performance is improved still further, and so this was the input I chose for my listening tests.

sqnote Tim Listens
While the output of the RH 551 is intended for MFB loudspeakers, it can also drive a conventional power amplifier provided a reasonably sensitive model is chosen. However, I chose to partner it with a pair of RH 532 MFB enclosures (grilles removed) since this is the way most of the surviving examples are likely to be used.

This set-up does raise the question of why so many sound-shaping devices were fitted to the amp as the goal of MFB was to provide a flat and extended frequency response without such measures. On the other hand, one could argue that since the loudspeakers employ intricate electronic signal processing to deliver a subjectively accurate sound there is no harm in adding a little more further up the chain. Whatever one's point of view on this matter, the various filters fitted to the RH 551 certainly work well and are intelligently scaled, the bass and treble controls having a wide range and the 'presence' (midband) one being rather more subtle.


DIN connectors and outputs for use with MFB speakers are on the left while the sunshine logo on the far right indicates the design is suitable for use in all climates

I felt the loudness contours to be superfluous in my set-up, most notably the deepest one which made the sound unbearably thick and bass heavy. The high- and low-cut filters also seemed of little value since (like every HFN reader, I'm sure) I don't own any dirty or warped LPs.

Without the RH 551, one way of running RH 532 speakers is from the headphone socket of a quality CD player such as the Technics SL-P1200 [HFN Aug '13]. I was surprised that when using the preamplifier in the chain a tighter, more focused sound resulted. To my ear there was a little less bass, but this seemed to give the MFB system a bit more headroom to work with.

Power Shots
The net result was to transform the RH 532s from being a clever party trick into a top-flight loudspeaker whose performance in certain areas would be a tough act to follow. 'Calling Elvis' by Dire Straits [On Every Street; Vertigo 510160-2] at first seemed to lack some of its familiar warmth, but the characteristic staccato rim-shots that punctuate the track were rendered in the powerful and fast way only Philips' MFB loudspeakers seem able to deliver.

Playing the disc directly from the CD player did result in a touch more drama at times, but with the RH 551 in the system the sound opened up and inhabited a larger space, both in terms of width and depth. The ability to project vocal texture also appeared optimised with the RH 551 bridging the gap between the source and the speakers. I've found vocals to be a strong suit of well designed truly active (as opposed to merely amplified) speakers, but as one of the few preamps purpose-designed to suit the RH 532, the RH 551 seemed to raise the bar here another notch or two.


Mostly Philips-made components are used in the RH 551. The space [top left] would have accommodated the power amplifier section of the related RH 521 integrated model

I'll confess here to finding the mixture of singing styles in 'Suspended In Gaffa' from Kate Bush's The Dreaming [EMI CDP 7463612] at best a distraction, at worst an irritation, but here it became a source of fascination, with the piece as a whole now making a little more sense.

Of course, when this preamp made its way onto the market the phono stage would have been the main input used by listeners and I found it still to be effective today. The DIN connector contains both chassis and signal grounds and, adapter in hand, I found the RH 551's RIAA input proved to be both lively and detailed-sounding. Particularly, I might add, with modern pick-ups like the Ortofon 2M Black [HFN Mar '11] employed for the majority of the listening.

Being critical, one could describe the presentation as a tad hard and brittle when compared to the unit's performance with line-level sources. But this wasn't to an extent that would rule it out of consideration for use in a vinyl-based system.

Buying Secondhand
The RH 551 was available in the UK but only a few were sold here. Most found homes on the continent, making this the best place to go looking today. These were well-made units constructed mainly from parts produced within the Philips organisation so reliability is good. However, there are a lot of switch contacts through which the signal has to pass and most faults are caused by dirt and tarnish on these.



Preamps can be designed to be simple, but the service sheet for the RH 551 reveals it to be a complex and intricately configured product

Sticky Notes
Another common issue is that the fusible resistors used to protect the power supply section can change value or go open-circuit, often without obvious symptoms. Performance does suffer though, albeit in a subtle way. Any overhaul should therefore include a check that all the supply voltages are at the correct level and free from excessive hum and noise.

As for those fish-eye meters, unfortunately those can jam up or stop working when the glue inside that holds their delicate bearings in place loses its integrity over the passing years. In spite of their appearance, the meters themselves are conventional rectangular types, so a pair salvaged from a scrap cassette deck can often be used to save the day. Internal adjustments are provided to calibrate the scales if this has to be done.

Hi-Fi News Verdict
This was the best purpose-built product Philips made to drive its MFB speakers, so if you have a pair and are serious about achieving the best performance from them, then the RH 551 is worth seeking out. It is also worth considering as a versatile preamp for use with other active speakers or in a conventional system, the DIN-only connectors and slightly quirky appearance being the only real drawbacks.