Australian HiFi - Lab test RGM175

This is the first amplifier I think I have ever reported on that does not have a flat response. There is a considerable boost at the bottom end of the frequency response, starting at +9dB at 10Hz, decreasing to +5.5d13 at 20Hz then +0.4d13 at 100Hz. There's also a slight boost at the high frequency end of the response that starts at 6kHz to become +0.2d13 at 1OkHz and increases to be +0.8dB at 20kHz before shelving at around +3dB at 200kHz.
The output impedance of the Redgum is exceptionally low, with the result that this frequency response is identical when it's driving a loudspeaker --something that can't be said for the amplifiers we've recently tested using Australian HI-FI Test Laboratories' new 'simulated loudspeaker' load. This imperviousness to load is shown on the second of the frequency response graphs, where the scale has been expanded to 0.4dB per vertical division. The lower of the two traces is the response into an 8-ohm load; the other trace is the response into the simulated loudspeaker load.
Australian HI-FI Test Laboratories measured the power output of the Redgum RGM175 power amplifier (since the amplifiers are monoblocs, only one channel was tested) as being slightly under the 175-watts claimed in the brochure, as you can see from the figures tabulated in Table 1, which showed an output at lkHz of 126watts (21.0dBf) into 8-ohms, 170-watts (22.3dBf) into 4-ohms and 192-watts (22.8dBf) into 2-ohms.

Closer examination of Redgum's published specifications showed that its power output results were obtained whilst powering the RGM175 with a mains voltage of 260-volts, some 20volts higher than the Australian standard of 240-volts (though a ±10% variation is permitted within the standard), so the amplifier was re-tested at this higher mains voltage, the results of which are tabulated in Table 2.

Although power output increased significantly with the higher mains voltage, the amplifier still fell somewhat short of delivering the continuous power output claimed, returning 148-watts. (21.7dBf into 8-ohms, 200watts (23.0dBf into 4-ohms and 227watts (23.5dBf into 2-ohms (lkHz).

Lab staff then noted that individual test report supplied with the RGM175 showed that Redgum's own QC department had measured the maximum output of this very review sample at 165.7-watts continuous into 8ohms, some 10-watts (0.2dB) short of its own brochure claim. The lab rang Australian HI-FI's editor, Greg Borrowman, who in turn put in a call to Redgum and it transpired that the expensive glossy brochures are 'old stock', designed and printed when the RGM175 was fitted with a different power transformer. This particular transformer proved to be unsuitable for export, because it could not easily be adapted for mains voltages in other countries (primarily the US market). In the course of redesigning the power supply section, Robinson tried three other power transformers before finally designing and manufacturing a special low-regulation model himself.
During the re-design process, Robinson said he was focusing primarily on ensuring the amplifier could deliver the highest possible power output on transients, irrespective of the line voltage on which it was operating, a design aim in which he says he was successful. It was only later when testing the first production models that he realised the continuous power output of the amplifiers using the new transformers had dropped slightly, but that since the drop amounted to less than a single decibel, he considered the gains in dynamic power output outweighed the loss in continuous power output.

As for the power output being measured with a 260-volt supply voltage, Robinson says the mains voltage at Redgum's factory in the Box Hill South suburb of Victoria averages 262volts, and that's where the final QC testing takes place. As for the numerals in the model number of the amplifier itself, Robinson says the 175' is still appropriate because the amplifier will deliver more than 300watts into 8-ohms under the US IHF A-202 amplifier standard and more than 175-watts continuously into 4-ohm loads.
Just before we went to press with this review, lan Robinson e-mailed to tell us he had redesigned the power transformer yet again, gaining a small increase in continuous power output whilst still maintaining very high dynamic power. This new transformer will be fitted to all models with a serial starting with S.N. #### #### ##0575. He sent us a sample RGM175 fitted with the new transformer for testing at 250volts. This amplifier's power output came in at more than 175-watts continuous at all three test frequencies into 8-ohms. The full results for all other load impedances are tabulated in the third (lowest) power output table on page 42.

The frequency response was extended, stretching from 2.5Hz to 207kHz -ldB and from 1.8Hz to 271kHz -3dB. Note that these are only down-points (not ± limits) because of the non-flat nature of the frequency response within the audio band, as noted in the first paragraph of this test report. The 'normalised' ±dB response across the audio band is 20Hz-20kHz ±2.75dB.

Signal-to-noise ratio was high, even though it was the result for the complete signal chain, including the passive pre-amplifier and the necessary interconnects between the two. Referenced to one-watt output, the lab measured –73dB unweighted improving to -85dB with IHF-A weighting. Under the same conditions, but this time referenced to rated output, as this specification is usually reported, the Redgums returned signal-to-noise ratios of -9ldB (unweighted) and -105dB (IHF-A).

Input sensitivity was measured at 68.6mV for one-watt output, and 764mV for 126watts output. This will be more than sufficient for most sources, but some cassette decks and FM tuners may not be able to produce much more than 500mV or so, in which case they would not be able to push the Redgum to its full rated output-though they'd get close.

THD+N was measured at a low 0.02% for one-watt output, decreasing to just 0.007% at rated output. The spectrum of distortion is shown in the accompanying spectrograms. At one watt into 8ohms, all harmonics were below -90dB with the sole exception of the third, at -86d13 (0.005%). All harmonic above the eighth were more than 100dB down (0.001%). Distortion increases very slightly when the amplifier is driven into 4-ohm loads, with a second harmonic at -85dB (0.005%) a third at -82dB (0.007%), a fourth at -94dB and a fifth at -86dB. All others are below -90dB.

Distortion decreases at higher power levels, something that is not overly clear on the graphs because for some reason the graphs showing distortion at the higher power levels were re-scaled, so that the bottom line is at -140dB, rather than the –120dB used for the one-watt graphs [Australian HI-FI Test Laboratories has assured us it will provide us with similarly-scaled graphs for future reviews... Editor]. At 100-watts into 8-ohms, the second harmonic is at -85dB (0.005%), but all higher harmonics are below -100dB with the exception of the sixth at -94dB and ninth at -98dB. At 160watts into 4-ohms, the second was at -82dB (0.007%), but most other components were more than 90dB down and, in most cases, at or better than 100dB down. The reduction in noise and distortion is also obvious on the Audio Precision graph showing THD+N vs Power Output. The steep rise at the end is the point at which the amplifier starts to clip (see power output table). Distortion and noise vs frequency is shown in another graph from the Audio Precision System Two, and is an excellent result.
The Redgum's phase accuracy was nigh-on perfect, as you can see, and the amplifier doesn't invert polarity. The very low output impedance also means the amplifier has a superb damping factor, which was measured at 600 at IkHz. AHF

Steve Holding

(click on graph to enlarge...)