Australian Hi-Fi

Laboratory Test Report

- Steve Holding

Channel balance was outstandin­gly good at just 0.005dB, which we think is the best we have ever seen

Readers interested in a full technical appraisal of the performanc­e of the OAD Padma preamplifi­er should continue on and read the LABORATORY REPORT published on the following pages. Readers should note that the results mentioned in the report, tabulated in performanc­e charts and/or displayed using graphs and/or photograph­s should be construed as applying only to the specific sample tested.

Graph 1 shows the THD of the OAD Padma when it’s delivering one volt out (at its balanced outputs). As you can see for yourself, all distortion components except for the third harmonic, which is sitting at –99dB (0.0011%), are more than 100dB down (0.001%). Indeed, the second harmonic is at –111dB (0.0002%), the fourth harmonic is at –115dB (0.00017%), and all other harmonic distortion components are more than 120dB down (0.0001%).

The other ‘spikes’ (signals) you can see on the graph are not related to the test signal and appear to be sub-bands from a signal outside of the audio band — you can see the pattern repetition most obviously in the way groups of three equally spaced signals appear on the graph between 4kHz and 6kHz, then again between 6kHz and 8kHz, and again between 8kHz and 10kHz, and between 10kHz and 12kHz. None of these would be audible as they’re too low in level to be above the noise floor of your listening room. And, as you can see from the tabulated figures, the overall THD was measured at 0.008% (again, at one volt).

Graph 2 shows CCIF intermodul­ation distortion, with the two test signals (at 19kHz and 20kHz) as the peaks to the right of the centre of the graph. All intermodul­ation distortion products are more than 100dB down (0.001%), and the di˜fference signal at 1kHz is down at –116dB (0.00015%) which is much lower than usual. Again, there are sub-band signals present in the output; here they’re almost entirely more than 110dB down (0.0003%) and so also would not be audible above the noise floor of your room.

The frequency response of the OAD Padma preamplifi­er is shown in Graph 3. You can see that it’s ruler-flat from 10Hz out to 4kHz, even with the graph’s scale set at just 0.5dB per horizontal division. Above 4kHz it rolls o˜ so slowly that the response is just 0.5dB down at 40kHz. This graph shows that the normalised tabulated audio band response measured by Newport Test Labs is 20Hz to 20kHz ±0.1dB, and the 5Hz to 40kHz response is ±0.25dB. These upper and lower frequency limits are the limits of the analyser used to make the measuremen­t and create the graph.

You can see from the tabulated results that the overall frequency response of the OAD Padma measured by the lab is <0.5Hz to 50kHz –1dB and <0.5Hz to 102kHz –3dB. All of these frequency responses are excellent, being far more linear and extended than would ever be required, even for hi-res audio sources.

Graph 4 shows the Padma’s response to a multi-tone test signal, where it’s required to reproduce 20 di˜fferent frequencie­s at exactly the same level, and you can see that it sailed though this difficult test. Note that the individual tones are not supposed to reach the 0dB point on the graph — they’re all reproduced at the correct level. As you can see, although there are some unwanted sub-band signals, they are all higher than 2kHz in frequency and more than 100dB below the reference level.

It wasn’t only the Padma’s frequency responses that were excellent, either. The

channel balance was outstandin­gly good at just 0.005dB, which we think is the best we have ever seen. The channel separation was also outstandin­g, with the OAD Padma returning results of 125dB at 20Hz, 123dB at 1kHz and 112dB at 20kHz. These results are all good, but the one at 20kHz is exceptiona­lly good. Inter-channel phase errors were very, very low, being virtually non-existent at 20Hz (0.01°) and just 1.13° at 20kHz.

Newport Test Labs’ results for signal-to-noise ratios are well below OAD’s claim of 110dB, most likely because the company is referencin­g its measuremen­t to the Padma’s maximum output voltage, whereas Newport Test Labs’ results of 84dB unweighted and 90dB A-weighted were obtained with a one-volt input and a one-volt output (unity gain), while its results of 90dB unweighted and 96dB A-weighted were obtained with a two-volt input and a two-volt output. If Newport Test Labs had referenced to the maximum output voltage it measured the Padma as being capable of delivering (17.2 volts), the A-weighted signal-to-noise ratio would have been 114.6dB.

The input sensitivit­y of the OAD Padma for an output of one volt was measured as being 129mV using the balanced inputs and outputs, which means overall gain was 17.8dB.

Power consumptio­n during operation was low, with Newport Test Labs measuring it as 9.49 watts. Power factor was +0.707.

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