InnerFidelity November 2012 Update
Okay, they're not complete yet, but it is automated! Brian (NA Blur on Head-Fi) finished his first pass as best he could without a running system in front of him, and we met Wednesday of last week to go over it. He structured the code and subroutines of the program that runs the test, and built the spreadsheet template. Now it was handed off to me to get it to run on the Audio Precision tester.
Brian did a great job, but there were all sorts of limits to what he could do without the hardware in his home to run the test, so there were plenty of bugs to fix. But there were two real problems facing me as I got it up and running. The first was getting the THD+Noise vs. Output Voltage to stop when it reached 1%. Try as I might, I just couldn't figure it out, so I called Audio Precision tech support for help. OMG! If only everybody had tech support this good! First, you don't need to pay for it, and second, the guys answering the phone are amazingly good at helping...AMAZINGLY GOOD. They told me how to go about it, and in 20 minutes I had the test up and running. The second problem was a bit weirder having to do with the FFT sampling rates. The tech support guys again came to the rescue and talked me through panels that I didn't even know were on the machine, and again, the problem was solved within minutes.
I won't bore you with the minute details, but by Saturday evening I had a running test. Now, this isn't the complete test. My goal was to get the program running well enough to take all the graphs and spit them into the Excel spreadsheet with one load resistance, and then to measure a sampling of solid state amps to determine if the graph ranges were reasonably set. By Sunday afternoon I had measured seven amps, and then last night and this morning I measured them again to determine if it was reliably getting the same results. The bottom line? I think it looks great.
So, for your viewing pleasure, I've created this .pdf booklet of all fourteen datasheets, two for each amp. I'm somewhat familiar with the various measurements, so I'll talk you through a few of them. I must admit I'm not nearly as fluent with interpreting amp measurements as I am with headphone measurements. I can help you with the basics, but I'd love to hear from some of our more technical readers in the comments about what you see in these data. As a refresher and guide, you may want to go back to this article NwAvGuy wrote for InnerFidelity to help me develop the tests. He's got some in-depth information there.
Basic Parameters - NwAvGuy suggested to me that all measurements should either be at unity gain or with the volume control at the 12 o'clock position. I decided to use unity gain as it would make the voltage levels the same on all measurements and would aid in direct comparisons. I picked 0dBu (0.774Vrms) as the reference input level as it's a voltage that should work on most amps, and is a hot enough signal to get rational noise ratios. I may have problems with some portable amps at that level. If so, I'll may make a second test routine for portable amps that uses a lower voltage standard.
Frequency Response - FR is measured from 10Hz to 100kHz. I'm thinking about extending this to maybe 300kHz, but unfortunately Excel will only do log scales in powers of ten and would cause the graph to go out to 1mHz, which would reduce the size of the plot in the graph. Quite a few of the amps measured were flat to 100kHz, so it would be nice to extend this plot.
THD+Noise vs. Frequency - This is a measure of harmonic distortion at various frequencies. The AP starts this measurement at 20Hz, and once it reaches 7kHz, the third harmonic is at 21kHz and needs to stop as the AP has a filter to limit distortion products above 20kHz, so measurements beyond 7kHz become misleading as it filters out the third harmonic.
Noise Spectrum - This test is taken with no signal present and shorting plugs inserted into the inputs of the amp. It shows the spectral energy content of the amp's self generated noise.
THD+Noise vs Output Voltage - This shows the amount of distortion present as the output voltage rises. Solid state amps typically clip hard at some point and the rapid rise at the right-hand side of the plot is an indication of this clipping distorting the signal. You can look at the right-hand near vertical clipping point and basically see how much voltage the amp can drive. When you see the left-hand part of the curve descending linearly, you can assume the measurement is limited by system noise. As it begins to curve, you can assume distortion is beginning to become more dominant.
SMPTE IMD - (Oops, sorry for the misspelling on the graph, still lots of things to tidy up.) With this test, two tones are played into the amp: 60Hz at 0dBu and 7kHz at 1/4 the level. You can see these as two spikes in the curve at those frequencies. These tests are used to look at non-linearities in the higher frequencies of the amp that cannot be seen in the THD+noise measurement due to the bandwidth limitations I mentioned in the THD+Noise vs. Frequency section above. Basically, what you're looking for is a clean graph with as few spikes as possible and for the noise floor to remain constant. Remember, what you see in the "Noise Spectrum" plot will also appear in the SMPTE IMD plot.
CCIF IMD - similar to the SMPTE IMD plot, but with more sensitivity to errors in the higher frequencies. Here again, you're looking for the noise floor to remain flat, and for intermodulation products to be as narrow and short as possible.
Single Point Measurements - The small table in the bottom right corner are for single point measurements and are fake. This table will be filled in on the next iteration with data measured with four different loads (16, 32, 150, and 470 Ohms).
I've started reading seriously John Atkinson's pre-amp measurement sections and NwAvGuy's site to try to become more expert at interpreting the measurements, so I'm not going to attempt a blow-by-blow analysis of the measurements here, but I will make a few comments. I'm going to preface them, though, with the observation that almost all these amps are ruler flat from 20Hz to 20kHz. Headphone amps are MUCH closer to each other in sound quality than headphones. The best measuring amp is not necessarily the best sounding amp. I think the most important thing amp measurements can do is isolate amps that might have glaring issues. After that, I reckon we will all be on a learning curve here to find some correlations between measurements and sound quality.
NwAvGuy's O2 as built by JPS Labs is clearly a top performer. This amp was measured with AC power to put it on a more even keel with the other amps, but it still showed an extraordinarily low noise floor and very good THD measures. THD+noise and Noise Spectrum plots are stunningly low. The O2 did suffer some spreading of the CCIF IMD peak at 20kHz, where Meier Corda Rock had the best results of the group to my eyes.
I felt the repeatability of the test was pretty good--I'm very happy to see that--so I thought I'd test the sensitivity a bit with the HeadRoom Desktop. I was using it with the Desktop Power Supply on which it stacks. I knew that stacking them does allow a little AC cross-talk into the amp as the transformer radiates some AC into the amp. So for the second test I moved the Desktop Amp from atop the power supply and separated them by a foot or two. You can see in the two Noise Spectrum plots that the second test has virtually no power supply 60Hz harmonics. You will note a little blip at about 43Hz. This blip can be seen in all plots with somewhat differing amplitude. I think this is something coming from the measurement system and I'm going to have to do some gremlin hunting to see if I can find its source. Ah, well.
The Lake People G109 seemed to have quite a bit of CCIF IMD distortion around the 20kHz peak. The Burson HA-160D had considerable noise. It seemed to me the O2, Corda Rock, and Butte fared very well indeed.
What do you see?
I did measure some headphones this month, but felt I should get this post done and up as quickly as I could, so headphones measured this month will appear in the December update.
Upcoming headphone reviews will be the V-Moda M-100, Sennheiser Momentum, and a review of the Sennheiser HD580/HD600/HD650 family. I've also started receiving a bunch of high-end electrostatic amps and headphones for a large comparative review I'm planning for right before Christmas.
Have a great turkey day!