Philips RH 551 Preamplifier Lab Report

Lab Report

Launched in 1976 to partner Philips' MFB (Motional Feedback) loudspeakers [HFN Jul '13], the RH 551 preamplifier could also be pressed into service alongside a conventional power amplifier from the same or, indeed, current era. Maximum output is just shy of 3V from a respectable 50ohm source impedance while the +21.4dB overall gain is very generous. Neither did this compromise the A-wtd S/N ratio which is equally generous at 90dB (re. 0dBV) even though the residual noise shows some hum at –75dBV (178µV). Distortion is impressively consistent with frequency at 0.05%/20Hz, falling gently to 0.02%/10kHz and 0.03%/20kHz [see Graph 2, below], and increasing equally gently with output level from 0.009%/200mV to 0.02%/1V and 0.05%/2V (all re. 1kHz). Nevertheless, the 'story' of the RH 551 arguably resides in its comprehensive response-shaping features.

At its 'flattest' the RH 551's frequency response reaches out from 8Hz-76kHz [–3dB; black trace, Graph 1 below] with the bass control offering a max/min adjustment at 20Hz of +14.8dB/–17.6dB [red traces, Graph 1] and the treble control matching this at 20kHz with +16dB/–21dB [blue traces]. Philips' additional 'presence' control was subtler in operation, offering a +2dB lift at 2kHz with a roll-off to –4.4dB/20kHz, and a –2.8dB cut at 2kHz with a boost thereafter to +3dB/20kHz [green traces, Graph 1]. In addition to this tone-shaping, Philips also included a four-position 'Contour' facility – a loudness control by another name – with position '0' representing the flattest response. Contour 1 delivered a 'shelf' climbing to +1.3dB/20Hz and +0.95dB/20kHz, Contour 2 at +1.4dB/20Hz and +1.2dB/20kHz while Contour 3 offered a wilder +5.5dB/20Hz and +2.8kHz/20kHz. PM


Freq. resp. (Contour 0, black; max/min Bass, red; max/min Presence, green; max/min Treble, blue)


Distortion versus extended frequency at 0dBV output (left channel, black; right channel; red)

Maximum output/input (<1% THD) 2900mV / 6900mV
Output imp. (20Hz–20kHz/100kHz) 56–47ohm / 46ohm
Input sensitivity (re. 0dBV) 85mV
Frequency resp. (20Hz–20kHz/100kHz) –1.10dB to –0.35dB / –4.7dB
Stereo separation (re. 0dBV) 71dB to 32dB
A-wtd S/N ratio (re. 0dBV) 89.8dB
Distortion (20Hz-20kHz, 0dBV) 0.020–0.050%
Power consumption 10W
Dimensions (WHD) / Weight 470x117x280mm / 5kg