Headphone Amp Measurements Done...Almost (Aaaaarghh!)
Today, April 1st, is InnerFidelity's second birthday. Not going to make a big deal out of it because what I really wanted to do is get this darn amp measurement routine done. So, about five days ago I just ignored all else and started cranking away at code. Oh boy, that first day was murder. I was getting all sorts of weird readings; I though I was going crazy. Turns out it was a bad solder joint in one of my cables...made by me, argh!
I'm not going to waste a bunch of time like that again, and I remembered Ted Paisley of The CablePro had offered to hook me up with some cables for my tester when we chatted at RMAF last year. I gave him a call---with much panic in my voice---and he did indeed get me set up with two Freedom UP-OCC Mini to RCA cables, and two sets of Freedom UP-OCC RCA to RCA interconnects. Not only did I need them overnight, but they had to be built as well. The box arrived on my doorstep the next morning; the cables are sweet and work dandy; Ted, you're a cable stud! Thanks man! (FWIW, both HeadRoom and TTVJ Audio carry CablePro cables. There's a reason for that: Ted and crew do very nice work.)
New cables in hand, measurements now seemed to be moving along nicely, so I began modifying the previous test. The biggest changes since last go around was that I changed the loads I was using, and the new test now uses 150, 32, and 16 Ohm loads. I've gotten rid of the THD vs Frequency plot because it didn't seem to indicate much, and added a 500Hz tone spectrum for folks who just want to see the harmonic series. Let's have a look at one of the new data sheets.
I've shown the HeadAmp Pico in the plot above because it measures so darn well; better, in my opinion, than the JDS Labs O2, which is no slouch. I love the Pico...I wonder how much that has to do with its outstanding measurements. In total, I measured 23 unbalanced solid-state amps of a variety of types. You can download the headphone amp measurements .PDF here.
All the graphs on the sheet are measured with the 32 Ohm load except for the THD+noise Vs. Output Vrms which has plots for all three loads. The output from the AP tester was set to 0dBu (0.7745Vrms), with the exception of the 'Noise Spectrum' plot where shorting plugs are inserted into the input of the amp. I used the lowest gain setting on the amp that allowed me to get to a unity gain setting with the 32 Ohm load. Single point measurements of gain and noise were measured with the 32 Ohm load and the volume control at maximum.
Some of the amps measured very well (like the Pico and O2) and some did not. The problem is figuring out if an amp was measuring poorly because it's just poorly designed, or because of some problem with my test equipment or procedure. Sadly, I think there's a bit of both going on. Let's step through some of the amps in the .pdf (Headphone Amps .PDF here), I'll explain some things about the measurement interpretation as we go along.
Frequency response is nice and flat, but something went sideways with the phase response. I'll remeasure this amp sometime soon, but the thing that really caught my eye on this sheet is all the steps in the THD Vs Output Volts plot at the top right. There are a couple amps with strange features on this plot. The tech support at Audio Precision is second to none, I gave them a call this morning to suss out what was up with the 'steps' on the THD vs Volts plot. It turns out this may be just one of those facts of life that we'll have to live with. This is a very sensitive measurement, and when conditions are just right, small calibration differences in the various ranges the the tester goes through can cause these steps to appear. The engineer at AP said there may be some things I can do to make them go away or minimize them (mainly putting attenuators on the output of the AP's generator), but those work arounds can't be automated. So, I think we might be stuck with these features at times.
The four plots in the middle of the page are somewhat similar. All are amplitude vs. frequency plots. For the 'Noise Spectrum" plot, I insert a shorting plug into the input and simply look at the noise spectra of the output. You'll notice the top of this plot is -50dBu and the bottom -160 dBu; this allows us to see as much information as possible in this chart. Mostly what you will see in this spectra is noise from 60Hz AC sources getting into the amp. This may be from poor regulation, power transformer radiation, and/or radiated noise from fluorescent lights. You can tell it's from the AC because of the frequencies involved: 60, 120, 180, 240, 300, and 360Hz, etc in multiples of 60Hz. This plot also allows you to observe the general noise level of the amplifier, in this case the baseline for noise is about -135dB below 0dBu.
The '500Hz Spectrum' plot is simply the spectra of a 500Hz tone. You'll see in the ALO International plot three small peaks to the right of the main 500Hz tone, these are the harmonics of the 50Hz tone at 1kHz, 1.5kHz, and 2kHz. The sum of these peaks would be the 'Harmonic Distortion' of the amp, and the baseline of the curve is the noise level. What you're seeing here is a visual representation of the THD+noise of the amp. To the left of the 500Hz peak you'll see some features. If you look carefully you can see that these are the same 60Hz noise spikes seen in the 'Noise Spectrum' plot.
The next two plots are the 'Intermodulation Distortion' (IMD) graphs. In both, two signals are present, and the idea is to see how these two signals mix and create sum and difference signals---which is their intermodulation distortion products. In the SIMPTE IMD plot the two signals are at 60Hz and 7kHz. It's a little hard to pick out the intermodulation products on this amp (because they're small and are hiding in the 60Hz noise peaks), so let's move on to the next page in the .pdf.
Alpha Design Labs Cruise
This is a fairly quiet battery powered amp, so the noise plot doesn't have any 60Hz appearing. But as you can see, in the SIMPTE IMD plot you can see numerous bumps to the right of each primary tone (at 60Hz and 7kHz). The CCIF IMD plot has two peaks on either side of 20kHz spaced 1kHz apart (19.5kHz and 20.5kHz). What this will give is a signal at 1kHz (called the difference signal) and one at 40kHz (called the sum signal), and in serious cases, peaks every 1kHz from 1kHz up to the two probe tones.
Now, here's where I have a problem: It looks like this amp is measuring poorly. Not only is there a lot of signals generated in the IMD plots, but you can also see a strong series of ongoing peaks in the '500Hz Harmonic Spectrum' plot. This type of result will often come from 'clipping' of a sine wave signal. After talking with the engineers at AP this morning, I think a 0dBu signal may be to hot for some of the portable amps. Most portable device line outputs evidently run in the few hundred millivolt region and the 773mV of the 0dBu signal may simply be overdriving the input stage of the amplifier. This could easily account for the strong harmonic series seen in the Cruise. The engineers at AP suggested I test the amps at -10dBu, which, according to them, is a common test level for consumer audio gear. The downside of testing at this level is that noise in some measurements will become more dominant, and, of course, I'd have to do all these measurements over again. (I've pretty much resigned myself to that.)
At the bottom left of the page is the 'Crosstalk' plot. Here one channel is swept through the frequency range, while the other channel gets no signal. The tester listens to the un-driven channel for any signal that's bleeding over from the driven channel. The rising straight line of the Cruise is most common as the capacitive and inductive coupling that's often the source of crosstalk gets more and more efficient as frequency increases. The most common place for crosstalk to occur is in the volume control.
Apex HiFi Butte
Let's see if we can start quickly going through amps for a good snap-shot view of performance.
The Butte is flat from 10Hz to 40kHz. THD+noise vs. Output V shows an amp with ample headroom. (Vertical parts of curves at right hand edges are 3V@16Ohm, 6V@32Ohm, and 7V@150Ohm.) The Butte has fairly low distortion (below .01% above 0.3Vrms output). It has a moderate amount of AC noise as seen in the 'Noise Spectrum' plot, and little harmonic and IM distortion as seen in the other three frequency spectra. This can also be seen in the THD vs. Output V plot: If the left hand side of the curve is going downward and is a straight line, then it is mostly a measure of noise, not distortion. Crosstalk is about average ending at about -50dB@20kHz. Output impedance of 1.8 Ohms is acceptable.
Apex HiFi Glacier
Dead flat in both frequency and phase. THD Vs. Output Volts shows good headroom for a portable; curves are mostly noise limited, but upper third or so of curves bend upward showing increasing distortion products. The amp is fairly low in noise at between -130 and -140dBu. 500Hz harmonics show a moderate amount of THD, but if you look closely you'll see the even order harmonics are slightly more emphasized than the odd, likely leading to the comfy sound of this amp. SIMPTE and CCIF IMD shows mild IMD distortion. Crosstalk in this case is a flat line to 400Hz. I'll need to verify this with some engineers, but I believe a flat crosstalk line indicates that most crosstalk is being directly coupled through the power supply. 2.3 Ohm output impedance is acceptable.
Flat in frequency and phase to 30kHz. Good headroom, and mostly noise dominated THD. Moderate AC noise and -130dB noise baseline. Very little harmonic distortion in 500Hz spectra. Very good IMD results with little showing but AC noise. Good crosstalk numbers, right channel worse than left. 99 Ohm output impedance!!! Not good at all.
Flat in frequency and phase. Good headroom, but has some significant distortion issues. Very little AC noise, and this is not a battery powered unit. Pretty strong harmonic series on 500Hz plot, looks like borderline clipping, though this being a home unit should be able to deal with the 0dBu levels. Fairly strong SIMPTE IMD products especially in the mid-frequencies. Noise level in the bass increases significantly with CCIF IMD signal. Crosstalk mostly power supply related. 1.4 Ohm output impedance is groovy.
I'm going to let you look over the rest, but I will draw your attention to the HeadAmp Pico and the LDS Labs O2. These are the two best measuring amps of the group. Both are battery powered. Generally, the O2 is lower noise, but the Pico is lower distortion.
Dag nubit! I really wanted to be done with this today, but I think it's going to take looking at these tests using a -10dBu standard to tell whether it should be changed or not. A buddy who I sent these to this morning also pointed out that the first line in the single point measurements is likely in Watts, not milliWatts. So, a bit more to do, I'll try to redo the measurements at the lower voltage this month so we can move forward one way or the other.
I have to say it's also very helpful to get comments from folks, sometimes I use them and sometimes not, but I always read them. Please feel free to one last time lobby for your desires on this sheet. It'll be frozen pretty hard in place next time through.