Vintage

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.