Measured amplifier specifications translating to non-ideal loads - AVS Forum
2-Channel Audio > Measured amplifier specifications translating to non-ideal loads
friend'scatdied's Avatar friend'scatdied 11:36 AM 03-14-2014
I read an article recently from Benchmark:
http://benchmarkmedia.com/blogs/news/12838141-headphone-amplifiers-specifications-are-only-part-of-the-story

What's the opinion on this? Do measurements really deviate wildly for different well-designed upstream components under non-ideal loads?

It doesn't seem like they're measuring the right things.

A9X-308's Avatar A9X-308 12:27 PM 03-14-2014
It's basically advertising for benchmark in the form of a 'white paper'. With no detail as to what the other 2 units are and independent measurements to confirm, it may be correct or it may be a bunch of BS. Or they may have simply hand picked the two worst units they could find to compare with theirs; after all it is in their best interest to chose competitor devices that allow their product to look it's best.

Take it with a large grain of salt.
FMW's Avatar FMW 12:48 PM 03-14-2014
Take all claims made by the audio equipment industry with a grain of salt.
arnyk's Avatar arnyk 12:49 PM 03-14-2014
Quote:
Originally Posted by friend'scatdied View Post

I read an article recently from Benchmark:
http://benchmarkmedia.com/blogs/news/12838141-headphone-amplifiers-specifications-are-only-part-of-the-story

What's the opinion on this?

Old news.
Quote:
Do measurements really deviate wildly for different well-designed upstream components under non-ideal loads?\

Not at all. As usual, you have to design the measurement to consider actual use.
Quote:
It doesn't seem like they're measuring the right things.

Exactly. The problem is that modern headphones and earphones often have very low impedances (for traditional headphones), such as 8 or 16 ohms. Amplifiers that drive them need to (and often do) provide relatively low source impedances, generally well under 1 ohm. This sort of thing is well known to designers and testers of portable digital players. For example many vendors have been supplying portable digital players such as later generation iPods and the Sansa Fuze that provide source impedances of well under 1 ohm.

The Benchmark Media white paper http://test.benchmarkmedia.com/discuss/sites/default/files/Headphone-Amplifier-Performance-Part-1.pdf mentions the Sony MDR-V6. Here is its impedance curve from http://www.innerfidelity.com/images/SonyMDRV6.pdf



The lowest point of this curve is 60 ohms, but it goes as high as 120 ohms at 40 Hz. This means that a headphone amp that provides a source impedance of 60 ohms is for example going to produce excess output at 40 Hz.

You can do an adequate job of measuring the source impedance of this headphone amp with a 60 ohm resistor, but you have to do the appropriate calculations:

http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm.

A common rule of thumb is that an amplifier needs to provide a source impedance of no more than 1/50th to as little 1/100th or less of the load impedance, when the load impedance varies with frequency over the operational range. This ratio is the same as the inverse of damping factor.

http://en.wikipedia.org/wiki/Headphone_amplifier

"
Output Impedance

Many headphone amplifiers have an output impedance in the range of 0.5 - 50 Ohms. The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms, but in practice this is rarely used and not recommended with modern headphones. High output impedance can result in frequency response fluctuations, due to varying load impedance at different frequencies. In 2008 Stereophile Magazine published an article that showed that a 120-Ohm output impedance could cause a 5-dB error in frequency response with certain types of headphones. However, the author of the article also states: ″The ramifications for subjective assessment of headphones are more troublesome because it is usually unclear what assumptions the manufacturer has made regarding source impedance. ″ [1]

More importantly, low output impedance can reduce distortion by improving the control that the source has over the transducer. This is often expressed as damping factor, with higher damping factors greatly reducing distortion.[2] One company shows a 45 dB improvement in THD+N at 30 Hz for their low-impedance amplifier compared to a 30-ohm amplifier.[3] For example, a 32 Ω headphone driven by a headphone amp with a <1 Ω output impedance would have a damping factor of >32, whereas the same headphone driven with an iPod Touch (7 Ω output impedance) [4] would have a damping factor of just 4.6. If the 120 ohms recommendation is applied, the damping factor would be an unacceptably low 0.26 and consequently distortion would be significantly higher.
"
commsysman's Avatar commsysman 08:48 AM 03-15-2014
Your big problem with headphones is getting a good impedance match between the headphones and the amplifier, especially if the headphones are less than 50 ohms.
A9X-308's Avatar A9X-308 11:28 AM 03-15-2014
Quote:
Originally Posted by commsysman View Post

Your big problem with headphones is getting a good impedance match between the headphones and the amplifier, especially if the headphones are less than 50 ohms.
Impedance match? You simply drive them, same as with speakers, with the lowest source impedance possible. It's not even remotely difficult.
MBentz's Avatar MBentz 10:27 PM 03-15-2014
I've seen these kinds of measurements firsthand and it's not just marketing fluff. The headphone load always affects the amplifier performance.

I would have liked to see more information regarding the gain structure....those competitor THD+N plots are dominated by noise, which tells me something is weird in the setup.
arnyk's Avatar arnyk 06:30 AM 03-17-2014
Quote:
Originally Posted by A9X-308 View Post

Quote:
Originally Posted by commsysman View Post

Your big problem with headphones is getting a good impedance match between the headphones and the amplifier, especially if the headphones are less than 50 ohms.
Impedance match? You simply drive them, same as with speakers, with the lowest source impedance possible. It's not even remotely difficult.

+1

The above comment, including the fact that its not difficult but not that common to provide a very low impedance source for a headphone jack, is reinforced by the fact that inexpensive headphone amps such as the ones in the Sansa Clip and Fuze (about $40) and the Fiio E5 (about $20) meet this criteria. Their source impedance have been tested and found to be less than 1 ohm over the audio band.

One can always increase the source impedance of a headphone amp with a simple pair of series resistors, but decreasing it is tough. The Sansa Clip and Fuse have 6 band octave equalizers built in which accomplish the possible positive results of any attempt at impedance matching, and are far more flexible. convenient and powerful.
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