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SINAD Measurements

sergeauckland

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#21
Hi,
A few brands display the THD for the range 20 Hz to 20 kHz for their amplifiers. How can it be compared to the THD measured at 1 kHz ?

Also, for those products, why is the THD value under @1W lower than the THD value @50W? On Amir's charts, the curve goes down with the Watts before clipping.
In answer to the first question, THD at 1kHz is usually lower than for the full bandwidth. A device conservatively specified will spec the distortion over the full bandwidth, in the knowledge that that is the worse-case. Marginally specced products quote at 1kHz only, and you can only wonder what they're hiding elsewhere.

As to the second question, Amir's measurements aren't pure THD, but THD+N, that is, including noise. As the power (and hence voltage) level falls, noise becomes a larger proportion of the total, so the THD+N figure gets worse. With many (analogue) products, pure THD, i.e. excluding noise, should fall as power levels fall, or at least not increase. If it does increase, that points to low level issues like crossover distortion.

S.
 

daftcombo

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#22
In answer to the first question, THD at 1kHz is usually lower than for the full bandwidth. A device conservatively specified will spec the distortion over the full bandwidth, in the knowledge that that is the worse-case. Marginally specced products quote at 1kHz only, and you can only wonder what they're hiding elsewhere.

As to the second question, Amir's measurements aren't pure THD, but THD+N, that is, including noise. As the power (and hence voltage) level falls, noise becomes a larger proportion of the total, so the THD+N figure gets worse. With many (analogue) products, pure THD, i.e. excluding noise, should fall as power levels fall, or at least not increase. If it does increase, that points to low level issues like crossover distortion.

S.
Thanks a lot.
What would you think of an amplifier which has 1% THD between 20 Hz and 20 kHz ? 0.1 % ? 0.01 ?
 

March Audio

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#23
For thd measurements you must know the bandwidth over which it is measured.

For a 1kHz fundamental if the measurement bw is 20kHz you could include 19 harmonics. However if your fundamental is 6 kHz you will include 2 harmonics, at 12 kHz no harmonics.
 

sergeauckland

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#24
Thanks a lot.
What would you think of an amplifier which has 1% THD between 20 Hz and 20 kHz ? 0.1 % ? 0.01 ?
If that's a tube amplifier, it's OK, for a Solid State amplifier very poor. As to what the 1kHz figure would be, it's impossible to tell. For a tube amplifier, possibly 0.1%, but that's a guess and it could be a lot worse. For a SS amplifier, however, output power is often quoted at 1% THD, as that's just into clipping. Come down a little on power so out of clipping and distortion should drop a lot lower, 0.01% isn't unusual these days.

When I measure amplifier distortion, I always measure just below clipping, and as long as that's low, and the power output close to (or exceeds) the rated power, then I'm happy the amp is performing adequately.

My THD analyser uses a 100kHz bandwidth so I can capture enough harmonics for the measurements to be meaningful.

S.
 

amirm

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#25
Hi,
A few brands display the THD for the range 20 Hz to 20 kHz for their amplifiers. How can it be compared to the THD measured at 1 kHz ?
Since their range includes 1 Khz, then our 1 Khz results can be compared to their number. I also run THD+N versus frequency although its bandwidth is much higher than the manufacturers tend to use (so shows more pessimistic results).
 

amirm

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#26
Also, for those products, why is the THD value under @1W lower than the THD value @50W? On Amir's charts, the curve goes down with the Watts before clipping.
They may be using filtering for the 1 Watt and/or letting the max power go well into clipping. I stop the moment distortion starts to rise in my power graphs.
 

DonH56

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#28
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#30
Headphone amplifier S/N should be referenced to their specified rated output level, not their maximum output achieved.
Agree to this statement. However I like to add that nobody in their right mind would want to listen to their headphones at 1W power, even 100mW will ensure deafness. Headphone amp should measure from 1mW to the max of 10mW; or power level predetermine by the target headphone's target SPL level.
 
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