Vintage

Lab Report
In addition to its corrective ‘motional feedback’ technology, the AH585 is, first and foremost, an early example of a fully active loudspeaker with a line-level input, two-way electronic crossover and separate power amplifiers for its bass and treble drivers. The motional feedback (~10dB) is applied within the LF amplifier to linearise the behaviour of the 180mm pulp-fibre coned bass/mid driver although it cannot accommodate structural modes in the cone substrate or cabinet – note resonance at ~600Hz in both the response [Graph 1] and CSD waterfall [Graph 2].

The input sensitivity of the AH585 is adjustable to match the output of any likely partnering preamplifier/amplifier via a variable attenuator at the rear [see opposite] operating over a notional 0-25V input range. In practice, with the AH585 set to a 1.2V input sensitivity, a 200mV line input raised a 1m axial response to the 85dB/1kHz level illustrated here [see Graph 1, below]. This response, optimised as far as possible, still shows errors of ±4.3dB largely due to a broad depression through the midband that announces a far brighter treble above 4kHz. There are what look like cancellation ripples in the dome’s HF response, the latter extending to 20kHz (–6dB re. 10kHz).

Very small adjustments of the LF and HF trimpots will effect relatively large swings in output, these acting below and above the 1.5kHz crossover, respectively. Positive and negative adjustments of just 10o on the trimpots are illustrated by the red (boost to LF) and blue (attenuation of HF) traces [Graph 1], these amounting to ±2dB. The MFB trimpot has no impact on harmonic distortion – this exceeding 1% above 86dB/1m output – but it does influence the AH585’s LF tuning. The idealised 43Hz-1.3kHz (–6dB) bandwidth of the bass/mid driver is achieved with the pot set at the ~11.30 position, this yielding a diffraction-corrected bass extension of 40Hz [–6dB re. 200Hz; green shaded area on Graph 1]. This is very impressive for a driver this size in a sealed cabinet of limited (9 litre) internal volume. PM

Response inc. nearfield bass driver [green], freefield corrected to 1m at 2.83V [yellow], ultrasonic [pink]. Slight +ve adj. to LF (red); –ve adj. to HF (blue)

A strong resonance at ~600Hz also punches through the response [above]. Cone modes are >3kHz