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Distortion


x-over

Classic 'spiky' crossover distortion is the reason most AV receivers have a sound lacking smoothness, our tests show.


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Spectral analysis of the signal above shows a typical 0.3% figure for distortion at 10kHz, 1W, into 4 Ohms, with strong odd order harmonics. This is typical of a low budget receiver amplifier. Compare this with the Onkyo receiver below.


WHAT IT TELLS US

A measurement of distortion is supposed to tell us about the purity, or lack of it,  of the sound. How distortion “sounds” is quite complex and below about 0.2% it becomes difficult to be certain distortion is overtly, rather than subliminally audible. Digital distortions, where harmonics far from the fundamental exist and are totally uncorrelated to it, the ear detects as a coarseness or roughness in the sound.

In receiver amplifier output stages dynamic skewing of the transfer function as high frequency signals rise in amplitude causes the distortion structure to change. This produces a pattern of high order harmonics changing in uncorrelated fashion with the music signal, an effect the ear readily detects. We look for this whilst testing by observing how the distortion spectrum changes with level. Because receivers often use power amplifier stages of mediocre quality this is a not an uncommon problem. However, it is also a differentiating factor between budget and audiophile designs. Some modern receivers, notably from Onkyo and Marantz, measure was well as hi-fi amplifiers in this respect, maintaining a steady distortion pattern.


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Distortion pattern of a quality receiver, an Onkyo TX-NR906. Distortion level is low at 0.02% and – better – harmonics are just second and third. Output is 1 Watt at 10kHz into 4 Ohm load, revealing of crossover distortion.


With AV receivers analogue distortions occur through the analogue sections, meaning via the Aux (Direct mode) and Multichannel inputs. Digital distortions arise where Analogue-to-Digital conversion occurs through the Aux inputs (via the processors) and Digital-to-Analogue conversion occurs before the power amplifiers. We measure both.



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HOW WE MEASURE IT

Distortion is measured with the amplifier connected to both 8 Ohm and 4 Ohm resistive loads, the latter producing higher distortion due to the higher current draw.

The load resistors, custom built to our specification, use zero hysteresis (iron free) wire to avoid high frequency distortion from magnetic effects. This ensures our high frequency (10kHz) distortion measurements are representative of the amplifier under test, and are not influenced by the load. No test equipment earth connections are made to the loads; they are fully balanced.

We measure harmonic distortion at a low output of 1 Watt and just below full output (-1dBV), at 1kHz and 10kHz, into 8 Ohm and 4 Ohm loads. This is a spot measurement scheme that reveals an amplifier’s basic behaviour. The result published in the magazine shows distortion at 1 Watt, 10kHz into a 4 Ohm load, a realistic test yet one that yields the highest distortion figure and shows the presence of ‘crossover distortion’. It is also our quoted distortion figure and it can be up to ten times greater than the result at 1kHz that manufacturers commonly quote. Spectral content is shown by our Rohde & Schwarz UPL analyser and this also provides all distortion figures. A Hewlett Packard 8903B provides an interesting time domain picture of the distortion residual.

Digital signals are input from both our Rohde & Schwarz digital generator via S/PDIF, and Burosch and Philips (SACD) test discs, played on an Oppo DVD player connected via HDMI. Distortion is measured using 16bit and 24bit PCM signals, as well as DSD from Philips SACD test disc.

Distortion with the VHF/FM tuner section is measured at 100% and 50% modulation levels, as set on our Leader signal generator. Distortion levels of 0.4% and 0.2% respectively, comprising mostly second and third harmonics, are common.

 
Comments (1)
distortion or crackle center channel
1Sunday, 24 November 2013 16:00
freek
i have found that on an onkyo tx nr 818 receiver, also a crackle exists in center channel and on a moderate volume level.
it´s a mostly male or a female with lower voice always in speaking only. mostly with speaking a 's' or 'g' in a word. better said when a word is not spoken 'flat' but with a 'curve' or more articulated spoken.

question 1: is my founding true?

guestion 2: are there adjustments in the receiver options to resolve this or a separate device?

question 3: do arcam or rotel receivers not have this problem?

There is no reason for this other than a fault in the Centre channel. It could the the centre speaker: try moving it to Left or Right and see if crackle follows the speaker. If it does, then it is the speaker. If it does not then it may well be the amplifier and you need to get it checked. NK

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