[nwavguy]

I know there are plenty of Clip+ reviews that describe the player, how the reviewer thinks it sounds, etc. And I've found some RMAA measurements as well. But that still left a lot of questions about its true audio performance. So, to kick off the first hardcore objective review on my new blog, I chose the Clip+ as it's very popular and well regarded and seems to be something of a benchmark.
 
I compare the Clip+ directly to the iPod Touch 3G. So even if you don't care about the Clip+, but are curious about how the iPod stacks up, you also might want to check it out. I tested several things you can't learn from RMAA results including maximum output, output impedance, square wave performance, jitter, etc.
 
And, finally, because this is the first serious objective review on the blog, I've tried to provide some background as to how the measurements are made, why they're made, and what impact they may have on sound quality.
 
As with everything on my blog, comments are encouraged (here, on the blog, or contact me privately).
 

 

[Mad Max]

Would like to see some of this.
Or harmonic spectrum(a).

 

[Inks]

very very nice, I like you methods of review. 

 

[Br777]

wow.. totally awesome contribution.  I have to admit i dont understand a lot of the technical info, but an awesome read none the less.
 
this is probably an uneducated question, and may not even make sense, but is it possible to do these tests but using an LOD on the ipod?  would there be major differences expected?
again i appologize  if this question doesnt make sense.. i just know that its widely accepted that using an LOD on an ipod makes it sound better b/c it bypases some of the "headphone out" circuitry.
 
thanks.

 

[Trysaeder]

The number of times you say '5 times the price and weight' is really annoying. Otherwise I like your explanations for all aspects of the tests.
 
I've used a clip+ for my entire portable life and it's nice to see I made the right choice.

 

[elfary]

I think we are all really intrigued about the iPod measurements plugged to a portable amp through a line out dock. It seems that the only real weaknesses of the iPod are the output impedance and the maximum output and those likely could by supressed by going the lod route. (Which many guys including me do anyway).
 
I'm assuming that the frequency response graph showed in the FR epigraph is under a 15 Ohm resistive load and the fr graph showed in the output impedance test is gotten with an actual balanced armature iem instead of a mere resistive load. Is that correct ?
 
And the other big question that i have is if there even exist a headphone out that can drive multi driver BA iems without introducing any deviation on the frequency response. Even the Clip rolls off a tiny bit with the balanced armature iem used on these great tests conducted by the helpful nwavguy.
 
What i find annoying is that the Clip price to perfomance ratio is actually that incredible. In that regard i'd say that it puts to shame every other dap around.

 

[spickerish]

Nice review. Did you use Rockbox in your review?

 

[tlniec]

Excellent review -- and not just because it reaffirms my love for the Clip+!   Very cool to see relevant measurements depicted graphically, and it was nice that you included some extra explanatory text to help readers understand what some of the values mean/what numbers may represent an audible change/etc. 
 
I also really enjoyed the DAC performance measurements with a 1KHz square wave -- on another thread folks have been discussing a different piece of equipment and its response to a 10KHz square wave.  I had proposed they measure a 1KHz square wave instead, in order to bring several harmonics down to (potential) audibility.  Your square wave test on the Clip+ and Ipod helped reassure me I'm not out in left field, and that there is indeed probably more value in a 1KHz square wave test than a 10KHz.

 

[nwavguy]

Thanks, I'm glad you found the review useful. And yes, a 1 Khz square wave is generally considered the most revealing if you're only going to use one square wave test.
 
Time permitting, I might also get the Cowon i9 review up today.

 

[yuriv]

[size=10pt]First of all, that's a really nice report. It confirms what we already know about the Clip+ from others' measurements. I wrote a few notes as I was reading your blog entry. I'll use Pascal's excuse: this post is long because I didn't have time to make it shorter.[/size]
 
[size=10pt]1. I'm skeptical about the Clip+ having a class D output stage. Devices that have them typically have a rising source impedance at higher frequencies, due to the inductors in the filter. When I measured the voltage drop of a 20 kHz signal across a 16-ohm load, I pretty much got the same result as I did at 1 kHz. Plus your graphs don't show too much ultrasonic gabage. [/size]
 
[size=10pt]2. Older iPods have a flat response to 20 kHz. The iPhone 4 and the iPad have a slightly softer top end, even with no load--about 0.5 dB down at 20 k. Several CD players and DACs have a similar response. It seems that they're using a different reconstruction filter this time around. More on this later.[/size]
[size=10pt]EDIT 2/25/2011: This result had always bothered me, and I got it a long time ago, when the iPhone 4 came out and the iPad still had iOS 3. Since then, gsmarena and Anandtech have measured the iPhone 4 and the iPod Touch 4 and both of them got ruler-flat responses. So I borrowed the iPhone 4 again and redid the tests. I'm happy to confirm their results. The iPad too, measures now flat. It might have been a human error back then--overwriting the wrong files, etc.[/size]
 
[size=10pt]3. It's a little strange that the iPod Touch 3G has Rs = 7 ohms. Stereophile measured 5.5 ohms for a very old model. I'm also getting 5.5 ohms for my 2005 iPod Video. Additionally, I calculate 0.7, 1.4, and 0.9 ohms source resistance for the original Clip, the iPad, and the iPhone 4, respectively. But keep in mind that I'm also getting 0.55-0.6 ohms for the Clip+ compared to your 1-ohm result. It's in the same ballpark.[/size]
 
[size=10pt]4. There isn't a definitive standard on what the output impedance should be for headphone amplifiers. It's not like the situation for loudspeaker amplifiers, where manufacturers try to get as close as possible to an ideal voltage source with vanishing source impedance (the damping factor spec wars). Back in 1996 the IEC called for 120 ohms output resistance (IEC 61938, formerly IEC268- 15). It's hard to tell which headphones were designed to operate with such an amp. If the headphone's impedance is uniform with frequency, then they will sound roughly the same with a low-impedance amp. Otherwise the frequency balance will be different.[/size]
 
[size=10pt]The IEC has since changed its spec. I don't remember what the new guidelines are. As you can imagine, the current situation is rather chaotic. The Wikipedia entry for headphone amplifier says that most headphone amplifiers have Rs = 20-50 ohms. IEC 61938-type units are practically a current source for very-low-impedance IEMs. Early iPods have Rs around 5-6 ohms. It gets worse: except for the many "Designed for iPod" portable headphones, it's not easy to tell what source impedance a manufacturer had in mind when they designed and tested their headphones.[/size]
 
[size=10pt]So, is output impedance still a good figure of merit for headphone amplifiers? Personally, I like my amps to be close to ideal voltage sources. (You can always stick resistors in series, just in case). Some examples: Benchmark DAC1, Grace Designs m902, β22, and now the Clip/Clip+ and new iDevices. Their Rs is still not as low as loudspeaker amplifiers, maybe by design. Speakers, after all, usually stay connected to the amp. Headphones are occassionally inserted and removed when music is still playing, so if there's a temporary short, the extra series resistance can limit the current draw.[/size]
 
[size=10pt]5. The maximum volume possible into a 16-ohm load is now higher for the iPhone 4 and the iPad, thanks to the lower source resistance. For example, I'm getting 0.63V for 1% distortion on the iPhone 4:[/size]

 
[size=10pt]But if you just reduce the volume by a hair, 0.57V, THD for a 1 kHz tone is now 0.0035%:[/size]

 
[size=10pt]I suspect the same for your iPod Touch--you've reached the onset of clipping.[/size][size=10pt] But low-impedance earbuds and IEMs don't need to be driven to these levels. Most users won't even get close to clipping before it gets unbearably loud. With higher-impedance 'phones, the iPod can play a clean signal louder, so it all works out.[/size]
 
[size=10pt]6. Julian Dunn's J-Test signal is a little different from a pure tone at Fs/4. Its job is to attempt to introduce data-induced jitter. The .wav file is floating around somewhere online. I wonder if using that will make a difference in your tests. Even with these devices the jitter isn't high enough to make a difference when listening to music normally.[/size]
 
[size=10pt]7. Apple didn't get lazy with the square wave performance of the new iPods. In fact the old iPods play square waves just like the Clip and the Clip+. Apple really opted for different compromises. Here's what the impulse response looks like for these devices:[/size]

Impulse response for Sandisk Sansa Clip+ (above left) and Apple iPad (above right) and Apple iPod Video (below left). iPad 1 kHz square wave (below right)

 
[size=10pt]It's the difference between a linear-phase and a minimum-phase filter. The ringing looks bad for minimum phase, but some mastering engineers find this behavior to be less objectionable than the pre-ringing in the linear-phase filter's response. There's a recent discussion on Gearslutz where they all say that it's more damaging to music. For me, it's very hard to tell which unit is which when listening to music normally. The differences might just be of academic interest.[/size]
 
[size=10pt]Compare my pics with some of Stereophile's measurements for these two units:[/size]
 
[size=10pt]The Meridian 808.2/808i.2 can actually select between the two reconstruction filters: http://www.stereophile.com/content/meridian-8082808i2-signature-reference-cd-playerpreamplifier-measurements[/size]
 
[size=10pt]The Ayre QB9 has minimum-phase. It has a setting that reduces the impule response's decay time, but there's no free lunch: it has to roll off the top end even more:[/size]
[size=10pt]http://www.stereophile.com/content/ayre-acoustics-qb-9-usb-dac-measurements[/size]
 
[size=10pt]I can do that too with the iPad:[/size]

 
[size=10pt]The latest news is that Apple will be switching back to Wolfson. The time-domain behavior of future iPods, iPhones, and iPads may change again. I didn't have a problem when they were using Cirrus.[/size]
 
[size=10pt]You might not like how your iPod plays square waves and count it as one of its disadvantages. In practice, I really don't notice much of a difference when listening to music. (But maybe that's the catch--like 99% of the folks out there, I'm listening to the music instead of the gear.) [/size]
 
 
[size=10pt]BTW, I had a chance to compare my Clip and Clip+ to a Benchmark DAC1. I thought they sounded very similar, even with multi-armature IEMs. Maybe you can publish the results of a level-matched blind test between your DAC1 and the Clip+. If you don't crank up the level, it might be very hard to spot the differences; they have a few similarities: low-enough source impedance, impulse response, square wave (DAC1 might be flatter), etc.[/size]
 
[size=10pt]Keep up the good work![/size]

 

[nwavguy]

@Yuriv Great post! You answered some key questions about the 4G products from Apple and they're mostly encouraging. So that's good news. The output impedance issue is my biggest complaint about the Touch 3G (well ignoring iTunes at least).
 
As for the square wave response I agree with you and tried to make it clear in the review it's about balancing compromises and there are different opinions as to which way is "best". I've done ABX comparisons of different filter options and can confirm at least some of them do indeed sound different from each other. In practice, the differences between the Clip+ and the Touch 3G probably are not audible, or even if they are, likely don't make enough difference to get very excited about. I'll try and revise my reviews to be a bit more "neutral" on the topic.
 
But the output impedance is way more likely to cause audible differences. My UE SuperFi's sound different on the iPod vs the Clip+. As I tried to explain on my blog, some might prefer the frequency response variations induced by the iPod's high output impedance (see below) but I don't.
 
Benchmark, like many headphone/DAC manufactures, took the low impedance approach to their design (they call it "zero ohm") and I agree with that approach as it minimizes the interaction with various different headphones like you see in the graph below. However, as I point out in my blog, some headphones are designed with a higher output impedance in mind. And, with those headphones, one can argue a low impedance source will change their sound from what the manufacture was targeting. I agree there's no well accepted standard these days.
 
Fortunately, most of the headphones with the really wild impedance swings are higher-end models and are likely designed for a low impedance source. So, in practice, I'm not sure there's much of a problem with low impedance sources with most any headphone except for perhaps a few very esoteric ones. But, as shown below, there are problems with higher impedance sources.
 
As for the small differences in our output impedance measurements, I can do some more research there. The should be done at a frequency where there are not frequency response issues driving the load--I use 1 Khz. Ideally, they should be done with two different loads--say 15 ohms and 150 ohms--rather than a single load and no load. The dScope has selectable input impedance, and one of them is 150 ohms. So I'll try using that instead of no load (100 K ohms). It just makes the math slightly more complicated
 
The Class D opinion is hard to call from what I've seen so far. You bring up a good point about the output inductor and I'd have to make more measurements there. But there actually is a fair amount of ultrasonic hash on the Clip+. It has way more than the Touch 3G. The other ways to get response down close to DC with a single ended low voltage power supply involve likely more costly and power hungry designs. Given the Clip's obviously tiny battery and low cost, Class D (or similar) seems to make more sense.
 
As for clipping levels, I did the best I could with a 0 dBFS signal and the discrete volume steps available on the devices. As I point out in the review, you often have the choice between "well under clipping" and "well over clipping" when you really want a volume setting in the middle. So I get as close as I can to 1% THD without going over. I may start publishing a gain figure for the maximum volume setting to compare the max volume setting possible between players. Classical music lovers with inefficient headphones might, for example, want more gain available. The downside is this confuses some people into thinking max gain = max loudness when it doesn't. It's like the guitar amplifiers in Spinal Tap going up to "11" instead of "10".
 
Finally, I haven't tried any formal showdown between the DAC1 Pre and the little Clip+. At 40 times the price ($1600 vs $40) it seems a bit unrealistic in some ways. But I agree it could be rather interesting! The DAC1 is my main source for listening with both speakers and headphones when it's not being recruited for "bench test duty".
 
Here's the plot showing what the iPod's output impedance does to the frequency response:
 

 

[Deep Funk]

Thanks for this review...

 

[Br777]

this thread really is a must read.
 
its funny b/c i have tried several times to no avail to find a noticable difference in sound quality between my ipod touch 4g and my clip+  when both have no "effects" turned on.  
 
I always suspected that people that CAN tell a difference have headphones that are "driven" better on one player than the other.  I guess that is validated here?
 
still curious about the ipod w/ LOD testing  too.
 

 

[shigzeo]

The new touch 4G is quite a step up from my 2G, which might be similar to the 3G in hardware. Certainly, I've noticed the 2G';s behaviour when driving certain BA earphones. The only real area I prefer the touch 4G to the clip+ is that its background noise is lower with the same crazy BA earphones. The better separation is nice, I suppose, but not that big of a deal. Of course, both have crap EQ's unless you install EQu or Equalizer on the touch, or RB on the clip, and stock, the touch does real gapless.
 
If all I did with the touch was music, I'd find it sorely matched. But it does everything from mail, to books, to dictionary, to games, for me.
 

 

[LFF]

Awesome job! This just confirms what I always heard with the Clip+. Many people called me stupid when I said the Clip+ was very accurate.