NwAvGuy's Heaphone Amp Measurement Recommendations
Editor's Note: NwAvGuy is an audio engineer who showed up recently in the headphone hobby scene. He made a really big splash with measurements and criticisms of commercial products and DIY designs on his blog, and with his "politically incorrect" postings on some of the headphone forums. If the quality of investigative journalism is measured in ones ability to create a stir, NwAvGuy should win a Pulitzer.
I must have a thick skin: I found his posts interesting ... a little too long maybe, but quite interesting. Let's put all that aside for the moment, however, that's not what we're here for today.
I've been struggling to find just the right set of measurements to implement for testing headphone amps at InnerFidelity, and thought it might be cool to get some advice from NwAvGuy. He obviously has the chops, so I asked him if he would like to write his recommendations for my test routines in the form of an article to introduce me, and InnerFidelity readers, to the various sets of measurements he would recommend. I did place some constraints on the article: it needed to be around 2000 words, and I had room for only 7 graphs and maybe a dozen or so single point measurements.
He's done that ... and even more. The following is a very good and nicely concise introduction to the important headphone amp measurements. It also includes links to his blog to provide a more in-depth understanding of the information. All told it's likely in excess of 10,000 words. A mighty large chunk of work, written largely at my request to help headphone hobbyists understand the numbers, all completely voluntary and uncompensated. I'm grateful, and hope InnerFidelity readers will set aside the drama for a moment and just absorb some good info.
STIG’s COUSIN: Top Gear is one of the BBC’s biggest shows. They evaluate expensive cars in entertaining ways then turn them over to their anonymous racing driver, known only as The Stig, to find out how fast they’ll lap their track. Some say NwAvGuy is Stig’s geeky cousin. Instead of testing cars, as an Electrical Engineer, I design and test audio gear. I want to thank Tyll for requesting my thoughts on measuring electronics. I'm all for improving measurements
WHY MEASUREMENTS? Here are some key reasons:
- Compatibility - Measurements can predict if the UltraCans play loud enough with the UberAmp and more.
- Comparisons - Measurements allow comparing gear in fair and useful ways that are otherwise impossible.
- Objective - Measurements are unbiased and not affected by preferences, cost, aesthetics, etc.
- Many More - See: Music vs Sine Waves
BACKGROUND: Audio measurements are specified in the following units:
- dB SPL - Acoustic sound pressure
- dBr - Relative to some (hopefully) specified reference level
- dBFS - Relative to Full Scale (the maximum) digital signal
- dBv - Relative to 1 Vrms
- dBu - Relative to 0.775 Vrms
- Volt - A measure of signal level usually given in Vrms and you can convert between volts and dBu.
- Watt - A measure of energy usually in mW (0.001 Watt) for headphones
- Hz - A measure of frequency
NECESSARY NINE: The following nine measurements shine a spotlight on what matters most. An amp clearing all the hurdles should work well with your headphones and not get in the way of the music.
1 - OUTPUT IMPEDANCE: Guideline: 2 Ohms or less.This is perhaps the most common reason different headphone sources sound different and it's often not disclosed or measured:
- Frequency Response - Higher output impedances can create audible peaks and dips in the response.
- Bass Damping - Higher output impedance can reduce bass damping resulting in audibly less controlled bass.
- Guideline - With the exception of a few rare headphones designed for higher impedances, divide the headphone impedance by 8 to get the maximum recommended output impedance. For 16 ohm headphones it's 2 Ohms. For more see: Output Impedance
- Measurement - It's measured by comparing the 100hz output at 100 mV with no load to the voltage with a low impedance load and cranking the math: Zout = (Rload * (Vnoload - Vload)) / Vload
2 - OUTPUT POWER: Guideline: Ten times the headphone's 90dB SPL voltage. Tyll's headphone measurements make it easier to calculate necessary amplifier power. Headphones cover a huge range of sensitivities and impedances creating a wide range of power requirements. See More Power for a complete explanation but here are the basics:
- Interactions – Sources produce wildly different amounts of power depending on what headphones are used. All sources have a maximum output voltage that limits their power into higher impedances, some have limited current into lower impedances, and many have significant output impedance. All three can interact with the headphones in complex ways.
- Impedance – The headphone impedance determines the relative amounts of voltage and current required. Typically low impedance loads need more current and high impedance loads need more voltage. For math geeks, Power = (Voltage^2) / Impedance while Voltage = Current * Impedance.
- How Much Is Enough? – Most consider peaks of 110dB SPL sufficiently loud. Tyll measured the 300 ohm HD600 at 0.23V for 90dB so they need ten times more, or 2.3V, for 110dB. The power is (2.3 * 2.3) / 300 = 18 mW. This works for any headphone Tyll measured.
- Measurement - The maximum output level at 1Khz and 1% THD should ideally be specified in both voltage (Vrms) and power (mW) at several impedances. THD+N vs Output Voltage or Power can be graphed with a line for each load impedance.
3 - NOISE: Guideline: -100dBu/7uV. Many headphone amps have audible noise especially with sensitive headphones. For all the details, see Noise Explained, but here are the essentials:
- Guideline - Tests show 85dB below the maximum listening level will usually be inaudible. With sensitive IEMs that means noise below -100dBu or 7 uV. For a rule of thumb, divide Tyll's 90dB headphone sensitivity number by 2000 to find the maximum noise voltage.
- Absolute vs Relative - Noise can be measured in absolute terms as a voltage but it's most often specified as Signal to Noise Ratio (S/N or SNR) where the noise is compared to a (hopefully) specified reference level.
- Upstream/Downstream – Noise upstream of the volume control is mostly relative to the music--changing the volume doesn't change the SNR much. But downstream noise is fixed (absolute) and always present even when listening at low volumes. Some amps are noisier at half volume than full volume.
- Headphone Sensitivity – The more sensitive the headphones the louder the downstream noise.
- Reference Level – A noise measurement in just “dB” is nearly worthless without a reference. If you see such a number it’s probably referenced to the maximum output of the device. This makes the spec more impressive but is more accurately the dynamic range. For example -100 dBu (reference 0.775V) becomes -120dB referenced to a full output of 7.8 Vrms.
- Weighting - Noise is measured as the sum of all noise at audio frequencies and can be un-weighted (the raw number), or “A-Weighted” adjusting for the sensitivity of human hearing typically improving SNR by several dB. You can’t fairly compare A-Weighted to un-weighted measurements.
- Measurement – Noise should be measured at the worst case volume setting and, if applicable, the highest and lowest gain settings with inputs shorted. The A-Weighted results should be listed in microvolts and dBu. A graph showing the noise spectrum from 20hz - 20Khz is useful to distinguish hum or digital noise from broadband hiss. If the same reference is used, this graph can be compared to distortion graphs. Use an online calculator to convert between volts, dBv and dBu.