The Philips Motional Feedback (MFB) loudspeaker has been mentioned a number of times in these pages over recent years. The company achieved considerable success with both its first- and second-generation models, including the 22RH544, but in the UK at least, the third generation is less commonly encountered. The AH585 seen here is the smallest of three consumer speakers, the others being the similar but larger AH586 and the three-way AH587.

These new designs saw Philips work hard to apply its MFB techniques to a series of more conventional-looking speakers. They were also easier to hook up to non-Philips equipment as they used standard RCA connections operating at signal levels and impedances that suited all preamps, rather than just the few made by Philips itself.

Petal To The Metal

The two larger models appeared in 1978 but the AH585 did not arrive until the following year. Previous Philips MFB loudspeakers had been marketed as part of the company's 'High Fidelity International' line but this was beginning to look a little old fashioned as the 1980s approached. To freshen up the offer, the new MFB designs were launched as part of the Black Tulip series and were finished in simulated black ash sporting a bright metal trim to give them a modern, 'high-tech' look.

The AH585 was a small two-way model featuring a 180mm MFB woofer and a 25mm soft dome tweeter of a type popular across the industry at the time. Unlike the previous small MFB model (the RH541) it employed two power amplifiers in each enclosure and electronic crossover circuits. The construction of the cabinet was in line with established practices for quality loudspeakers of the period. The outer shell was constructed from 12mm-thick chipboard with the baffle being made as a heavy resin moulding with a deep internal lattice structure to enhance stiffness.

In the original MFB models the electronics had been placed in a sealed-off chamber in the rear of the cabinet, but in the third-generation designs the partition had been removed and replaced with a thick foam rubber block [see pic, p123]. The rear panel of the amplifier thus formed the back of the loudspeaker pressure chamber and had to be sealed accordingly.

A truly active loudspeaker offers the designer much greater freedom when it comes to the specification of the crossover characteristics. There is no need to worry about efficiency or losses in the components as this is easily corrected with further amplification. Also, filters with much steeper cut-offs and far greater rejection outside the desired band of frequencies can be realised. Phase linear techniques are also easier to employ in active crossovers than in passive ones.

The design of the crossover circuit in each AH585 unit has a number of features not found in conventional loudspeakers. The first is the use of a global 'rumble' type filter that has its corner frequency set at 40Hz and employs a sharp 12dB/octave slope. This effectively defines the lowest frequency the loudspeaker will play down to and prevents the MFB system from having to work too hard. It should not be considered as a serious limitation; conventional loudspeakers of this size begin to show significantly reduced sensitivity at a significantly higher frequency, typically around 100Hz. After this the signal path splits two ways, one going to the woofer and the other to the tweeter.

Meanwhile, the tweeter circuit contains a simple electronic filter that removes all but the treble frequencies, the intriguing part being that this tweeter signal is also electronically subtracted from the signal going into the filter for the woofer. This process performs much of the necessary bass filtering work and ensures that the phase relationship between the two signals is maintained. Nothing like this is possible in a standard passive crossover. After this, MFB correction is applied to the bass signal and in this form it passes to the power amps. This ensures the woofer cone follows the incoming programme signal accurately.

The Odd Couple

These third-generation Philips MFB speakers were the first complete range to employ DC-coupling between the power amps and the speaker drivers. The advantages this gives are less critical in an MFB system where the amplifier's output impedance is already close to zero (if not slightly negative) over the operating bandpass. It is, however, a further improvement that means the corrective mechanisms are required to work with less effort.

In the AH585 the bass amplifier is rated at 30W and the treble one at 5W [see PM's Lab Report, p125]. The output level of both amps is monitored and the input signal automatically limited if too much drive is applied. This makes the system as a whole practically impossible to overload. Conventional protection is included to switch the units off should a large DC voltage become present at the output of either power amp, perhaps as the result of the failure of one of the output transistors.

In the original MFB models each enclosure defaulted to standby in the absence of an input signal. Once programme material of a sufficient level was applied, the speaker came to life – often with some initial distortion as the amplifiers and MFB loop stabilised. Although the AH585 worked in superficially the same way, the internal arrangements were different. All the amplifiers remained powered all the time and when activated the drive units were connected to them via a relay. This could be seen as wasteful in some ways but it did mean that full performance was available immediately.

The AH585 is certainly easier to use than the older models (RH532, etc). In order to keep the cable lengths (and therefore their capacitance) the same, both units should still be daisy-chained together – there is a switch on each to select which plays the left channel and which plays the right.

Light Fixtures

Meanwhile, to the rear of the speaker is a knob that sets the input sensitivity and impedance, lower sensitivities being accompanied with a fixed load of 1kohm to suit the speaker outputs of integrated amps. For sensitivities less than about 3V the loading is 100kohm, which any modern preamp should be able to work with.

The version of the familiar Philips soft-dome tweeter used is mounted on an integral alloy flange with moulded-in concentric rings to break up surface waves. The woofer has a matching alloy trim ring, but this is not part of the drive unit itself. Two LED indicators show the power status, the green one being lit whenever power is present and the red one while the loudspeaker is in operation. Each enclosure comes with a 7m-long captured mains cable.

Tim Listens

If the intention here was to produce a speaker that combined the advantages of the MFB process with a more 'normal' sound then it can be considered a success. The older RH532 sounds startlingly different to everything else (with the possible exception of real live music) but while the AH585 is less of a shock, it sounds more familiar. This is not to downplay its qualities – another key virtue is stereo depth and imaging, both of which rapidly coalesce without the need for intense concentration by the listener.

Clever though MFB is, it is not magic, and one must remember that these are small loudspeakers in small sealed cabinets – they need to be located reasonably close to the wall behind them to help get the bass out into the room. Care here is vital as it's easy to smother the tight, tuneful 'MFB sound' in thick, reinforcement-originated boom. The other end of the scale is far simpler as Philips' tweeter simply shines in this application. It is easy to forget how normal crossovers and cables can blunt the performance of a tweeter, but having no coupling components and only about 30cm of wiring between the AH585's driver and its dedicated amplifier makes the treble extremely vivid and super-fast. Stands that see the tweeters positioned at near ear-level height are ideal.

An appropriate musical challenge for the AH585s was Kraftwerk's 'Trans-Europe Express' [Capitol CDP 7 46473 2], a piece which normally only makes any sense at all when played pretty loudly through large loudspeakers. Of course, it was the MFB-reinforced bassline I was listening for, but what actually grabbed my attention was the sharpness of the electronic percussion, even at low listening. The bass had its chance in the following track, 'Metal On Metal', and yes, it was still deep, rich and well defined. I felt that the 30+5W rating was well chosen in this design, although it was odd that the treble seemed to run out of headroom before the bass (albeit at a point where the sound as a whole was deafeningly loud). Typically, it's the other way around.

Acoustic material showed the AH585's tonal balance to be essentially neutral but overlaid with the distinctive MFB signature of bass that is both a little 'tubby' but very well defined. Each note was rendered as subtly different rather than gathered around a fixed port resonance. 'On Every Street' by Dire Straits [Vertigo 510160-2] sounded slightly warm, but it was elevated by that odd MFB trick of making it sound as if one is listening at the desk in the recording studio rather than to a finished recording at home.

No Return

The final proof of the 'rightness' of this design is that it is difficult to go back to listening to conventional loudspeakers. At any price, I always felt I was missing something. Unlike the early Philips MFB designs, these late models are less a means of demonstrating the possibilities of advanced technology and more a practical packaging of the techniques to create a more 'normal' loudspeaker of exceptional quality. It works.

Buying Secondhand

While plentiful on the Continent, all third-generation Philips MFB speakers are hard to find in the UK. Reliability is good, but this period of Philips production is well known for problems with soldering, so any fault investigation should start with a search for poor joints. The speaker relay is another common trouble spot – it is not easy to access but once removed it can be dismantled for cleaning. The quality of all the passive components is top rate so there is no need to replace any of these simply for the sake of it.

One thing that does seem to deteriorate is the MFB sensor itself. Reduced output means the system becomes less effective, resulting in bloated bass and little in the way of MFB action. There is a preset control at the rear of the unit which will compensate for this, the correction normally being well within the adjustment range. Even though the amplifiers are DC-coupled there is no need to zero the midpoint voltage as this is done automatically. The bias does need to be set though, and it is worth checking, as many of these units will have received no attention since they were made.

Hi-Fi News Verdict

While not as quirky as the original incarnations of Philips' MFB, the AH585 is still a special loudspeaker that is well worth hearing if you get the chance. MFB is only part of the story, of course, because the well resolved electronics, well-made cabinets and high-quality drive units all contribute to a fine level of performance. Easy system matching makes trying a pair relatively straightforward.

Sound Quality: 85%

Source: www.hifinews.com