Meridian Explorer: A Case Study of the Effects of Output Impedance on Headphone Listening

Meridian Explorer ($299)
In early February 2013 Meridian released it's new USB powered DAC/headphone amp with much fanfare. By the end of the month, Head-Fi.org member Purrin had found its performance in listening tests underwhelming and had measured the output impedance of the Explorer at about 48 Ohms. This is not good.

Fortunately, Meridian had already received feedback from early units delivered to dealers and distributors, and had got wind of the problem when driving the low impedance headphones commonly used in portable applications. An engineering change was put into motion. Unfortunately, the wheels were already turning and the product introduction continued on schedule with the original product going into production. From what I could gather, a few thousand Explorers were shipped before the running change could be made. The first run of modified product has just finished production (about March 13th 2013), and is currently in shipment to dealers. I'm told the Explorer is selling very well, and dealers have little stock on-hand from previous production runs. I expect it would be safe to place an order now from any retailer that is currently out of stock, and am fairly certain that the unit received will include the changes.

I received the revised version a few days ago, and measured the output impedance:
With 0.1988Vrms out open circuit, I get 0.1922Vrms with a 150 Ohm Load for a calculated 4.99 Ohm Output Impedance. With 0.1988Vrms out open circuit, I get 0.1723Vrms with a 32 Ohm Load for a calculated 4.92 Ohm Output Impedance.

Bottom line: The revised Meridian Explorer now has a about a 5 Ohm output impedance.

What's the big deal? Let me explain...

Output Impedance, Frequency Response, and Damping Factor
Print
In the illustration above, the circuit to the left models the prior Explorer with a 50 Ohm output impedance, the circuit on the right shows the current version. The resistor shown as Rzout is the output impedance of the amplifier. The word 'impedance' indicates both resistive and reactive (capacitive and inductive) elements and is not actually a simple resistor, but for the moment we'll just think of it like a resistor inside the amp and in series with the output. The Rzload is the voice coil of the headphone. I used the Sennheiser Momentum in my listening tests, which has a 22 Ohm impedance, but I've shown a 25 Ohm load just to make the math easy. With an output impedance of 50 Ohms into a 25 Ohm load, 2/3 of the voltage (and power) in the amp is being lost in driving through the output impedance of the amp, with only 1/3 the power being delivered to the load. Power is lost inside amplifiers all the time, so inneficiency isn't really the problem here. Let's move on to the diagram on the right.

The circuit to the right models the new Explorer with 5 Ohms output impedance. Now the ratio of output impedance to load is much different. In this case only 1/6th the voltage is lost in the amp, and much more voltage is available for driving the load, so this is a more efficient configuration.

The important thing to observe here is that changing the ratio of load impedance to output impedance changes the voltage at the load. Lower the output impedance and more voltage is available at the load. But the converse is true as well: If you raise the resistance of the load, it will get more voltage. That's very important to know because most headphones have impedances that change with frequency. Let's talk about that.

Effects on Frequency Response with Headphone Impedance Changes
Let's look at the impedance curve for some headphones.

Sennheiser HD 600
Meridian_Explorer_Graph_SennheiserHD600Impedance

The purple trace in the graph above is the impedance curve of the Sennheiser HD 600. At 1kHz you can see it's just above 300 Ohms, but the primary driver resonance at 100Hz causes the impedance to rise to 550 Ohms. So at 1kHz the HD 600 looks like a 300 Ohm load, but at 100Hz it looks like a 550 Ohm load. That means that the HD 600 will get a little more drive voltage at 100Hz than at 1kHz, and will be somewhat louder at that frequency dependent on the output impedance of the amp.

In the case of the original Explorer with 50 Ohms output impedance with 1Vrms drive, the headphones will get 0.86Vrms at 1kHz, and 0.92Vrms at 100Hz. That calculates out to a 0.58dB increase at 100Hz. The point here is that the impedance curve of the headphones interacts with the output impedance of the amp causing a frequency response change of the headphones. But because these are fairly high impedance headphones the change doesn't amount to much. This is one of the reasons why it's important to use high impedance headphones with high output impedance amps (like OTL tube amps).

Let's look at another pair of headphones.

Audeze LCD-2
Meridian_Explorer_Graph_AudezeImpedance

Here's a pair of planar magnetic headphones (Audeze LCD-2). One of the unique characteristics of planar magnetic cans is that they are purely resistive in nature and the impedance doesn't change at all with frequency. Even though these cans have a low impedance and would strongly interact with the high output impedance of the early Explorer, the impedance doesn't change so there will be no frequency response change due to the high output impedance of the amp. The poor damping factor will have its effect on the sound though, we'll get to that in a bit.

Audeo PFE
Meridian_Explorer_Graph_AudeoImpedance

Here's the impedance plot of the Audeo PFE balanced armature in-ear headphone. Again, the purple line shows the impedance plotted against frequency. It starts out as a 32 Ohm headphone, but as frequency rises, so does its impedance. At 3kHz it has a driver resonance bump to 50 Ohms, and at 20kHz its impedance has risen to 80 Ohms. With the high 50 Ohm impedance of the original Explorer, and the low overall impedance of these cans, we can expect a significant coloration as the high output impedance interacts with the widely swinging impedance curve of these headphones.

Meridian_Explorer_Graph_AudeoPFEPlot

In the plot above I measured the frequency response of the Audeo PFE with both the old and new versions of the Meridian Explorer. I then plotted the difference (blue) between the two FR plots in dB on the left hand scale. I also plotted the impedance curve of the Audeo PFE on the right hand scale. Because of the step size of the digital volume control of the Explorer I couldn't exactly match the two levels and ended up with about a -1dB difference at the low frequencies. So the difference reference is at the -1dB line.

We can see that with the 50 Ohm output impedance of the old Explorer, we have a fairly strong effect on frequency response due to the interaction of the headphone impedance curve and the high output impedance of the amp. At the 3kHz impedance bump the Audeo PFE was 1.5dB louder with the old vs. new Explorer, at 20kHz it's nearly 4dB louder. This is a fairly significant change in frequency response and would be easily heard as the PFE being substantially brighter sounding on the old Explorer.

Frequency Response Summary
If you have one of the early Meridian Explorers you will find some headphones more colored than they normally would be due to the interaction between the impedance changes with frequency and the output impedance of the amp. Generally speaking, higher impedance headphones and headphones with very flat impedance curves will suffer less coloration due to the 50 Ohm output impedance of the early Explorer.

There's more to this story though, let's talk about Damping Factor...

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MERIDIAN AMERICA INC.
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ARTICLE CONTENTS

COMMENTS
lithium's picture

Hi Tyll,

Excellent technical article and really helps in understanding the problem. It seems several manufacturers releasing new products are having this problem. John Grandberg mentioned in the comments of the ZDAC article that they also had a problem in their initial production run and had an output impedance of 40 ohms from the headphone out. They have corrected it to 10 ohms now. This could also be a potential reason for John rating his ZDAC(10 ohms) better than the XDA-2, while Steve Guttenberg felt the ZDAC (40 ohms) to be inferior through headphones.

Can I also suggest more technical articles as I feel there is a lot of false unscientific information floating around. Thanks

John Grandberg's picture

I was thinking the same thing with regards to why I enjoyed the Zdac more than Steve. 

Impulse's picture

Second image of the article doesn't seem to be loading for me, above where it says "In the illustration above", the circuit comparison. Anyone else?

I've also got a question regarding damping factor... Does it apply equally on planar magnetics? The last line on the LCD2 impedance section seems to imply it does, but I've seen a lot of people stating the contrary (supposedly because they lack conventional drivers)... The concept just seems a little harder to understand and study than the way a swinging headphone impedance interacts with the output impedance.

Great article btw! Though Meridian's comments at the end seem a little disingenuous, maybe I'm just reading too much into it but the product IS a portable solution and they're making it seem like low impedance headphones or multi-driver IEM are a rarity in that realm rather than the norm. I imagine even a 5 ohm output impedance would still be an issue with a ton of IEM out there...

John Grandberg's picture

The image is missing for me when I use IE but works fine in Chrome. Weird. 

For LCD-2 and other planars, I notice a slight difference in bass control with a lower damping factor, but it's far less significant than with other types of headphones. 

Impulse's picture

It's actually missing for me on the stock Android browser. Thanks for the note about planars, seems like another good reason to go for a pair of them for use in the living room with my AVR.

Edit: They're showing up on Chrome for me now.

Seth195208's picture

You have a real gift when it comes to explaining technical subjects to laymen. Great article.

Tyll Hertsens's picture

Thanks man. 

You know, I'm not an engineer, at best I'm a decent technician. But I'm an awfully curious guy and have always been interested in the way things work. So in many ways I'm a layman myself...that's just figured a lot of stuff out.

I feel my job here at InnerFidelity is primarily to open up the eyes of hobbyists and enthusiasts to the field before them; to help them get off the ground and learn enough to find the parts of this activity that are of real interest. And to do that in the broadest possible way. Once folks involved in this activity are on a path towards some particular interest, there are a number of folks far more qualified than I to learn from. So your comment strikes deep in my personal sense of mission. 

I want to clearly make the statement that I encourage those who are very qualified----far more than I---on certain topics to reach out to me via PM or email (tyll@innerfidelity.com) if they wish to write articles for this publication.  There are LOTS of things I find myself not feeling qualified to comment on. 

I'll give you an example: I'd love to see a difinitive article on Fostex T50RP modification. I couldn't possibly wite that. I could help with some measurements, but I've very little experience modifying headphones.

The one thing I do want to stress though, is something I try very hard to do in my writing, which is to be authoritative and difinitive to enthusiasts at large. I want to create a publication that when someone asks, "Where do I start?" The knowledgable entusiast says, "First read the stuff at InnerFidelity." The good news here is that if the goal is to provide solid basic information for the beginning and intermediate level enthusiast it's not terribly hard...but you do have to put in the effort to make sure it's spot on information. 

I tell you something truly: I'm not as productive in word count as I'd like to be. When I look at other audio journalists I find myself saying to myself, "How the fuck do they write so many articles?" I'd like to do more, but I end up spending so much time reading the forums and various papers and such that it just takes way more time than I'd like to develop what I would consider 'The Full Story'. 

Uhhh...I've rambled off topic. Thanks for the complement, mate. I'll keep working on it.

ultrabike's picture

Being able to explain concepts in layman's terms is a gift and requires effort, all of which IMO translate into quality articles here at IF.

Seth195208's picture

The beauty of being a "subjectivist" leaning audio reviewer is that you can write all day about what you feel subjective about. Diarrhea of the hand is totally allowed and perfectly OK.

THE "objectivist" leaning audio reviewer is constrained by his inability to allow himself to write about things subjective because he deeply believes that his own personal opinion is just not a good enough reason to potentialy make or break a company's product(Or maybe even a company itself). Measurements at least tell certain truths about a product and he feels that writing about a product within the confines of those truth's is a more responsible aproach. This also means that ultimately, he's going to have much less to say compared to a subjectivist writer because good research and good measurements are  difficult and time consuming.

Readers of objectivist leaning magazines and websites also have less to say in response to these articles for the same reasons. Good research and good measurements speak for themselves. There just isn't much to add other than "thanks for the info!"

Readers of subjectivist leaning mags and websites usually have much more to say on forums because there are no objective truth's to abide by. Ultimately, anything goes..

Impulse's picture

Well said! I think it all points back to the blogger vs journalist argument... The blogger's just chiming in with his opinion, repercussions be damned.

Bloggers aren't held accountable to the same standards (or any standards) and they're not necessarily after the truth of the matter, just a story. The professional journalist or reviewer comes in with a more objective frame of mind because he believes that's precisely the service his readers seek...

Anyone can have an opinion, doesn't take any education or even experience in your field... to the heart of the matter and presenting readers with a properly researched and balanced perspective takes far more work.

Innerfidelity kinda reminds me of Anandtech in that regard (for all you computer geeks out)...

Seth195208's picture

..are kept in line when they know that a guy like John Atkinson is there to compare his measurements with their subjective opinion. 

HiFiGuy528's picture

Thanks for letting us know about the issue.  I appreciate your work. :)

 

Mike

OneMic's picture

I think it is great to see articles written on the interactions between amplifiers and moving coil speakers; so thanks for writing it. 

There is however a very large problem with this article.  It is the assumption that voltage is what drives a moving coil speaker, when infact it is the current from the amplifier which produces the sound.   The equation for the applied force to an electromotive system is F=B*l*i   (Force = Magnetic Flux Density * length of wire in magnetic field * current)    {It should be noted however that electrostatic systems like Stax headphones are voltage controled.}

Nelson Pass wrote a great article about current drive when he released his F1 (80 ohm Z-out) amplifier for single driver speakers which is definately worth the read 

http://www.firstwatt.com/pdf/art_cs_amps.pdf

 

xnor's picture

I = V/R ... Ohm's law

Near the resonant frequency efficiency usually rises causing the driver to need less current to output the same sound pressure. Forcing a fixed current across the driver will usually cause peaky, loose or kinda rolled-off bass and other frequency response oddities.

Tyll Hertsens's picture

You are exactly correct...problem is it's much easier to explain with dialog using the voltages. 

But I want bypass commenting on my post to say that I think current drive amplifiers will be the next big thing in headphone amps.  Transconductance amplifiers (voltage in/current out) are tricky beasts, but I think we'll find that the relatively easy load of most headphones will allow makers to design really good current amps with 15 Volt rails and off-the-shelf parts. 

I used to work on scanning electron microscopes, and the only way to get linear rastering of the electron beam is by driving the scan coils with current amplifiers. If you want linear movement of a voice coil, a transconductance amplifier is the way to go.

ultrabike's picture

Thank you for the article linky OneMic! Currently reading...

xnor's picture

A thing is that these full-range drivers are optimized for efficiency, have very low Qts (as low as ~0.2) and big magnets and therefore normally need a horn to produce decent amounts of bass. The low Qts also explains the bass roll-off.

Headphone drivers usually work in an extremely small (relative to driver size) more or less sealed chamber on the inner side. Measurements show most headphones are underdamped suggesting a high Q factor - the opposite of those loudspeakers in that article.

Current drive might work with some headphones where output impedance doesn't matter much in the first place, e.g. those with mostly resistive impedance like planar magnetics ... but for the majority of headphones it will just make frequency response and even distortion worse.

ultrabike's picture

First off all, thanks for the article (Nelson Pass). It made me think through things I have not done so before. Very interesting read.

It makes sense to me that the current and the voltage output of an amplifier will not  track each other well unless the load is resistive. If dealing with a voltage amplifier, the current will be both a function of the source and the reactive load. If dealing with a current amplifier, the voltage will be a function of the source and reactive the load. More than likely the amps will behave differently.

Like the article said, many (if not most) speakers, and perhaps headphones, assume a voltage amplifier and this may be reflected in the balance and smoothness of the Frequency Response (FR) SPL plot when using such amplifiers. If using a current amp, it is quite likely that the FR plot will look very different, and not necessarily for the better.

After a quick read of the Nelson Pass article (thanks OneMic!) I believe that the current amp was exploiting the impedance of the full range speakers to improve on some of their issues in FR. Maybe the sensitivity vs. frequency of some of this drivers (not necessarily all drivers in general) deliver better SPL FR when using a current amp. But that probably was not sufficient, and a parameterized analog equalizer optimized for a particular driver was developed (the RLC circuit in page 5.) Depending on the driver and enclosure, the circuit components are tuned.

It is discussed that the circuit can be reconfigured in series for voltage amplifier applications. Furthermore, some drivers (such as the Jordan J92S) did not require that much RLC circuit tweaking and did not perform much different when driven by a current or voltage amp. It seems its all very driver dependent and basically a full system optimization problem.

I tend to agree with xnor in that headphones (specially crazy impedance ones with decent FR when driven by typical voltage amp) might actually degrade tonality wise if using a current amplifier.

As far as full range drivers for speaker applications, I believe many of this gains from current amplification + analog eq can probably be achieved with the use of a parametric or multi band equalizer. The advantages here are perhaps that an RLC circuit is passive and perhaps cheaper/practical.

Sorry for being so far off topic.

Seth195208's picture

..is easy to hear with most iem's. In fact, in this day and age, I'm amazed that any manufacturer believes that anything above 1 ohm is acceptable for any headphone amp. It ain't like it used to be..

Impulse's picture

Yeah, it'd be one thing if this was a desk amp... Then you could argue (as they tried to) that it just isn't made for certain kinds of headphones. It's a very compact portable product tho!

I would assume the intended market is precisely IEM users and low impedance headphones like the Momentum and other portables. Maybe I'm just way off on that assumption tho...

Tyll Hertsens's picture

I think you're right.  But it's reasonable to cut Meridian a little slack as this is their first headphone product. Sure, we can say they should have known better, but a lot of what people know come from learning from mistakes. So, lesson learned for them.

Impulse's picture

At 'least they did react and implemented a change even as they're selling out on the product.

elmura's picture

As an audio designer, there are always trade offs depending upon the application, target market, and target price.

The lowest typical IEM phone impedances is 16-ohm, with most IEMs measuring much higher than that. Divide 16 by 8 and you get a 2-ohm output impedance as a maximum for those headphones. Therefore, above 1-ohm is acceptable.

The other end of the spectrum, 600-Ohm impedance phones, the output impedance can easily be up to 75 Ohms without having any colouration (assuming the amplifier can drive that sort of load)

The point is, do not dismiss an amplifier design off-hand because of the output impedance - it depends on the application.

lachlanlikesathing's picture

I'm so glad that you posted this. Tremendously great explanation with graphs as well!

At least as far as Head Fi I'm seeing more and more people understand what effect output impedance actually has. With the combination of impedance graphs and output impedance figures on amps we can start to make some educated guesses about the combination of amps. (Compared to when I joined a few years ago and the idea was basically: High impedance? Amp it, more bass! Multi-BA? Amp it, more bass!)

I hope we can see the industry move towards always specifying output impedances on amps.

I'm wondering if you would consider measuring the output impedances of some popular sources? There are bits of information here and there but nothing assembled into one nice place.

And one question: resistance adapters. (This has always confused me). Adding an impedance adapter in series between the headphone and the amp - does this just increase the Z-out of the amplifier and reduce the damping factor? Or does it increase the headphone's impedance as seen by the amplifier? What effect does this have on the sound?

xnor's picture

Both: for the headphone the output impedance increases (reducing the damping factor), for the headphone amp the load impedance increases.

mikeaj's picture

edit: nevermind xnor covered it already

Seth195208's picture

..goes a long way.  If I was designing a product (which takes a Lot of time), I would at least be a little curious as to what people need, especially in a small portable amplifier.  A couple hours on Innerfidelity and head fi is all it would take. What you would get in return for those two hours is priceless.

mikeaj's picture

How much of the difference in the 30 Hz square wave plots is a consequence of the different frequency response?

Tyll Hertsens's picture

Probably not much as the Momentum's impedance plot is fairly flat, and with the 30Hz square wave shape change is probably only important from, say, 150Hz and lower. So I think most of the change to the wave shape change was due to damping factor, but it is just an educated guess.

ultrabike's picture

I could be wrong, but think electrical damping factor might be lumped into the overall SPL FR (DAC+Amp+Headphone).

If I remember correctly, electrical damping factor is defined and derived for 2nd order RLC circuit Transfer Functions (which are related to the Frequency Response by simple variable substitution: s->jw.) Higher order circuits could be decomposed into several 1st and 2nd order TFs, each of which define their own damping factors (maybe sort of like having multiple Q factors in audio.)

In other words, I think there is always a relationship between electrical damping factor( s ) and elecrical impedance FR in electrical circuits.

Furthermore, if the output impedance of the amp is not zero, the coil in the headphone may result in some feedback. But I think that this should be captured by the Amp+headphone electrical impedance Transfer Function (which takes into account C and L storage and time constants)  which together with the Sensitivity vs. Frequency might yield the SPL FR of the whole system including Headphones.

If you notice Tyll, the L and R driver of the Momentums are fairly well matched, but there seems to be slight differences. Me thinks (or hallucinates) maybe enough to affect the 30Hz square wave...

KikassAssassin's picture

Great review! I was happy to see some blind listening tests, and I enjoyed the easy to understand explanations of how impedance affects the sound of headphones. Keep it up, Tyll!

br777's picture

I believe this product points to a larger problem that runs rampant in this hobby.

rant approaching...

I dont want to single out this company, they have simply shed light on the larger issue in a more public way.    The issue is that companies dont test their gear, or dare i say dont even know that they need to, or how to do so. They release products under the pretense of "this dac chip is pouplar in expenseive equipment so it must be the best"  or this amp configuration is the best because i say so, and so many other rediculous reasons, when it seems that many of these companies dont actually know what their end results of their component mashing has produced beyond what they "hear".   They dont seem to understand that certain components dont play well with others no matter how "good" they may be.  Some of them release very limited test info, some of them wont even release the component specs at all. They grossly ignore the fundamentals of bias, and sometimes even electronics.  Then NwAvGuy comes along and builds a "perfect" amp and dac for peanuts.  Its not surprise he and a very small handful of others are the only ones in this hobby that many of us trust.   bah humbug sad

 

Impulse's picture

From what I've read, his designs were neither perfect nor revolutionary... Good value for the money, sure, but he's not the only one doing that and the O2/ODAC are not even the absolute best bang for the buck within their price range.

NWAVGuy was certainly knowledgeable, honest, and outspoken; tho he kinda had his own axe to grind, but he's not the sole voice of reason out there... Just gotta know where to look. Seems he's gone into hibernation anyway.

Point is, there are plenty of companies big and small that do have a better grasp on what they're doing, at either end of the cost spectrum... Like Tyll said, this was one of Meridian's first efforts in this realm. Personally I don't think that excuses it, but I don't view it as a referendum on the industry...

There's far better (or worse, depending on PoV) examples of what you're describing, unfortunately.

ultrabike's picture

LOL! Impulse beat me to it!yes

Yes, the Meridian may not be the perfect product for every headphone. The high impedance makes it less "compatible" with low to mid impedance headphones. Even if this is fixed, it may still not be a dream come true.

The O2+ODAC combo indeed seems like a more headphone "compatible" DAC/Amp, and well regarded. But while one could definitively do worse, IMO $285 (http://www.jdslabs.com/item.php?fetchitem=48) is not peanuts.

Furthermore, the combo lacks coax and optical inputs which would be useful for certain applications, it's not really portable, and it's presentation is not necesarily to everyones liking. It's a good combo, but not necesarily "The One Amp To Rule Them All."

A simple Sansa Zip might have a better "perfect"/"peanuts" ratio (P/P) if used with certain IEMs (and pretty much in general when used with high efficiency, mid to low impedance headphones)

If one is going to venture in the $285 "peanuts" range and one digs the "objective" vibe, a Schitt Modi/Magni might yield a better (P/P) ratio if portability is not an issue. A Leckerton UHA-6s might yield better P/P ratio if portability is an issue.

Anyway, the above is just an opinion, and everyone has onecheeky

Tyll Hertsens's picture

....but many well healed headphone enthusiast find it a bit dry sounding. I think it is important to realize that at some point when approaching technical competence, matters of listening taste come into play. 

I've heard it said that in the process of engineering compromise (and I'll guarenty you that design engineers are always compromising as the produce a product) designers choose in what way they vary from perfect. And they do so (hopefully) with an ear toward the sound of the product. 

For example, I love the sound of my HD 600 with the Bottlehead Crack. But it's far from technically perfect. Similarly, some people will find themselves liking the Meridian Explorer with the high output impedance. Technical performance and personal satisfaction live in two different domains. While we can ascribe objective metrics to technical performance, we simply have to accept that the measure of personal preference is wholely subjective...and it's personal preference that sells product, not technical performance.

I think this is what makes this activity interesting: The struggle between technology and taste...and the fact that those two things will never have a single point of convergence.

lithium's picture

..has every right to voice their product as they see fit. If in their listening they felt that 5 ohm gives a good subjective listening experience with their reference headphones then they are absolutely right to voice it that way. Continuing in the same way, its every consumer's right to reject the meridian in the knowledge of their higher output impedance. As much as we would like our hobby to be of the highest technical competance, the fact still stays that listening preferences and subjectivity will always be a part( as pointed out by tyll). 

Nwavguy did a credible service to the community by challenging dogma but in the process he has given rise to the O2/ODAC dogma. Saying that its the best that is (because of its price performance quotient) without comparisons and logic is frankly in poor taste. The fact is that higher priced amps could be subjectively/objectively better and if someone doesn't feel that way, they are free to use O2/ODAC. 

Impulse's picture

Meridian absolutely has the right to voice their products how they want... But there's something to be said for transparency (as a policy, not as technical merit), which is one of the things NWAVguy was calling for from many manufacturers but that message seems to have gotten lost in all the O2/ODAC hype...

I guess it was to be expected, kinda hard to criticize others objectively while pitching your own ideas (I'm not accusing him of anything mind you, just the way the whole situation got twisted).

Meridian released the Explorer and never disclosed the output impedance, something many other manufacturers now do. More than one person on Head Fi emailed them to inquire with no response, until eventually purrin measured it and the cat was out of the bag. It's funny how the whole thread turned after that.

Had Meridian stated it from the start or when asked it would've managed expectations and anyone that knows they have headphones that wouldn't be a good match could steer clear.

Transparency is something this hobby sorely lacks, it's hard enough to find the opportunity to audition high end gear without having to deal with stuff like this on top of it.

Seth195208's picture

...if every(relatively) low impedance headphone is going to sound different with it based on it's own impedance curve profile. I think it probably had more to do with the best "patch job" they could come up with under the circumstances, without having to radically alter any other parts of the design(expensive). Still though, even that is a perfectly "sound"(blagh) engineering decision.

lithium's picture

I hate the fact that we are expected to buy and then audition stuff. At the same time when we send stuff back we are supposed to feel guilty for taking advantage.

tomasz's picture

I'm not an expert. I build some headphone amps, but only using schematics prepared by people smarter than me (especially my father). Please be tolerant if my thoughts are not really the case in the topic we are talking about. 

Always when I read about the problem of output impedance of a headphone amp I miss adding that low output impedance is good in some aspects but it never come without a cost. It's not that you can just decide about output impedance value without making some sacrifices. The most severe is adding or intensification of global negative feedback, which in my opinion has a most damaging influence on the quality of an amp. 

I would be happy to read some thoughts about it. Sorry for my English. 

Type35's picture

You have to love the hyperbolic language of a marketing pro:

"it has come to our attention that the performance of Explorer could be even better with a small number more esoteric headphones which have a sensitivity or impedance that is low or variable"

I guess when Meridian refers to the more esoteric headphones they mean something like apple earbuds and Monster Beats both in the 20 ohms impedance range and commending the lion's share of the headphone market.

Besides this, what almost knocked my out of my chair when reading Meridian's letter is the use of an internal DSP in the Explorer to "optimise playback sound". Whatever happened to bit-perfect playback? There is no mention of this DSP implementation on the Explorer web pages. Is this something that can be disabled? If so, how does the Explorer sound without DSP?

Finally, Meridian does not mention Windows Vista in the OS requirement specs which leads me to believe that the analog volume control accessed through the computer does not work with Vista similar to the issue on the Audioquest Dragonfly. Granted Vista's market share is small but for the sake of thoroughness and honesty Meridian as well as other manufacturers using this volume control solution should clearly disclose the incompatibility.

The Explorer looked promising as it is extremely difficult to find 192kHz USB powered DACs that produce realistic tone and timbre while being clean and transparent but the above issues are more than troubling (to me at least).

John Grandberg's picture

He is likely referring to the XMOS L1 chip which can also be used for DSP purposes due to its high level of processing power. HERE is the datasheet which discusses it. The example they give is for active monitors with a built in USB DAC - DSP could compensate for low frequency shortcomings or things like that.

I don't see anything else in the picture on page three that would likely be a standalone DSP, and it wouldn't really make sense to add a dedicated chip when the XMOS is already capable of handling that. 

But I confess to being a bit confused as to what he meant by "optimise playback sound according to input" since there isn't any other input aside from USB. And I don't know what the DSP would actually be doing in the Explorer. 

Aerocraft67's picture

"DSP" caught my eye, too. If by "DSP" they mean the likes of "jazz club" or "concert hall" settings on mass market AVRs, then I've quite misjudged this product. 

Twinster's picture

Very good information. thank you Tyll. Would you happen to have any experience with the Centrance DACport with is standard 10 ohms output impedance? 

Impulse's picture

I think Dinny FitzPatrick reviewed it as part of a portable amp roundup he did for InnerFidelity, and I just noticed IF also has an older review by John Atkinson posted (from Stereophile).

Aerocraft67's picture

 

This is an excellent case study indeed, both in terms of the substance of Meridian’s rough entry into a new market segment, and Tyll’s excellent chronicle of it. The thorough, forthright, authoritative, well-written and even-handed piece is the stuff that distinguishes the valuable role of professional publishing from casual blogs and discussion forums. Of course blogs and forums are valuable and fun, too, but with much lower accountability and signal to noise ratio.

I made an uncharacteristic early adoption and impulse purchase of Explorer as it arrived late to the thriving <$300 DAC segment and received highly favorable reviews in that capacity. A contrarian backlash ensued that culminated in the discovery of the output impedance issue by purrin. Although I purchased Explorer primarily with the intention of using the DAC feeding into an outboard amp, I remained interested in the full range of its capability, and the kerfuffle injected nagging doubt into the merit of my decision and almost certainly devalued my investment considerably. Although it continued to sell briskly, my well-regarded $300 DAC turned into an epic failure and laughing stock among certain elements of the headphone intelligentsia in the course of a week or two. You can’t please everyone, but no one likes to feel like a sucker.

Acknowledging the benefit of hindsight, and short of just doing the portable headamp right the first time, Meridian could have saved everyone a lot of trouble had it simply offered a non-portable device and been more forthright about suitable applications (i.e., not low-impedance headphones), or simply revealed the output impedance specification (never mind that many spec sheets are incomplete). Even after admirable actions to resolve a misguided entry into the portable headphone amplifier segment, doubts remain about Explorer’s suitability, illustrated by the revised output impedance that remains a bit generous at 5 Ω, and lack of a full acknowledgement that the portable headphone amp might actually be used with portable headphones, which quite commonly have low impedance and do not respond well to high output impedance, as aptly chronicled here and elsewhere. Dragonfly and iDAC do not have these problems, so it’s not like they’re insurmountable.

For whatever reason, DACs in Explorer’s price bracket come in portable form factor with headamps, and Meridian’s portable headamp was ill-conceived, which sullies what otherwise appears to be a perfectly good DAC in its price range. I suspect this will pose a really good stand-alone DAC value on the used market for those who can ignore the headamp. Maybe about the same price as Schiit’s Modi, a uniquely non-portable stand-alone budget DAC, although I suppose the 192 kHz capability in the Explorer would fetch some premium. Or maybe Meridian will buy them all back and retrofit them with the more suitable redesign, or at least get them off the streets so they don’t devalue the product line that’s barely six weeks old. That’s the last bit of this saga that remains to play out.

deckeda's picture

- head-fi.org community for exposing the problem

- Tyll, for digging deep, clearly reporting and openly inviting others to do the same

- readers here, for noting the "esoteric headphones" bullshit, "CYA" marketingspeak and lack of immediate customer responsiveness. So many companies are keen to have people "like" them on FB or join Twitter but have nothing in place to directly respond with a question privately nor publicly.

HiFiGuy528's picture

I'm thinking of keeping the old one and add a revised one to my collection.  Then I will have best of both since I do like how the old one sounds with the Mad Dog headphones.

miceblue's picture

I firstly just wanted to say thank you for making such comprehensive articles for the "common man". I think you're doing viewers a great favour by providing such articles.

One question I have for you regarding the damping factor effects though. You mention that the difference in the bass response is really evident to you with your tests between the new and old Meridian units. Would someone need to have trained ears to detect the difference, or could the "common man" hear this difference? I often times come across the problem where "audiophiles" know exactly what to listen for when comparing something that a "common man" wouldn't even notice.

Seth195208's picture

...aren't common men (in respect to headphones of course). Laymen? maybe.. If you have low impedance headphones, and you wan't them to sound the way they were intended to sound, you need an amplifier with relatively low output impedance. Does this mean that they will sound better to you? Now that's a different story.

Tyll Hertsens's picture

I'd say an ordinary person, if motivated to try with care, would pretty easily hear the differences between 5 and 50Ohm ouput impedance on the Momentums. With the 300 Ohm HD 600 maybe not. 

Seth195208's picture

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yuriv's picture

Nice write-up, Tyll. But the article makes it sound as if the damping characteristics and the frequency response are totally independent of each other. They’re very closely related. You defined the word damping in terms of amplifiers and their control of the load, but maybe it’s also good to look at an earlier definition. The obvious sources online have it like this:

In physics, damping is an effect that reduces the amplitude of oscillations in an oscillatory system

That particular one is from Wikipedia. They have a reasonable intro even if you remove the math:http://en.wikipedia.org/wiki/Damping

The animation on that page applies to headphones and speakers too. Give the driver an initial velocity or displacement from an impulse or a step, and it will settle down to some final position after some time because of losses like friction and air resistance (mechanical damping), and also, as you noted, electrical damping. How it gets there depends on whether the system is overdamped, underdamped, or critically damped, and also if there are other resonances. (The article in the link talks about those too.) These are all related to the frequency response, and you have some good examples of this from your measurements:

http://www.innerfidelity.com/images/BeyerdynamicT1.pdf
http://www.innerfidelity.com/images/GradoPS1000.pdf

http://www.innerfidelity.com/images/SennheiserHD600.pdf

The T1's frequency response has a huge peak at 9 kHz and the impulse response has an underdamped oscillation (ringing) at about the same frequency, although it's not necessarily from uncontrolled diaphragm motion. The PS1000 has a peak near 90 Hz, and you can see the beginning of a 90 Hz cycle after each rising or falling edge on the 30 Hz square wave. It's almost like an interrupted step response. A more well behaved headphone like the FOTM HD600 has an impulse response that  looks like it has a well damped 3 kHz oscillation to go along with the broad peak of a diffuse field target curve.

You could say that I'm cherry-picking among your graphs to illustrate this point, and that it's really not that simple. Fair enough; you can compute the inverse Fourier transform if you want. The point is this: if the damping has been impaired to the point where the increased decay times are audible, then something will be more underdamped than before, and it should be observable with some kind of time-domain measurement. And the frequency response should also change. Run that PS1000 with a high impedance amp and you’ll see a bigger hump in its frequency response to go along with the resulting one-note bass.

On the flip side, if the magnitude and phase responses aren’t changed by the increased output impedance (like on some planar magnetic headphones and dynamic driver IEMs), then impulse responses and square waves shouldn’t change their shapes. You even wrote an article about this back in May: http://www.innerfidelity.com/content/presto-change-o-measurement-transformations

If that’s the case, then nothing is more underdamped than before. So if you’re hearing something different, then maybe we shouldn’t be so quick to blame it on poorer damping. It might be something else, maybe distortion.

dalethorn's picture

Meridian responded because they needed to. Good luck getting a response like that from Sennheiser or Beyer. And there are worse - Apple, Bose ....

x838nwy's picture

How do I know which version is which? I might have a listen to an Explorer later this week, but I'm not sure which one they'll have. Is there a way to know for sure (in case the guys at the shop doesn't know).

Thanks

C

spacejazz's picture

Hey Tyll, great article. I learned a thing or two about DACs and amplification just by reading this article. Very well written, you do a great job man!

I was curious if there was a way to tell which version of Meridian explorer one has. Maybe by a marker on the serial number? Couldn't find anyone with an answer to that question so I figured I'd post it here on this review.

I have a feeling that my Explorer is the older revision because the bass from my P7 is muddled out of the headphone jack. So I whipped out my Total Airhead and hooked it up via the line out, sounds great. Can really tell a difference between the two amplifiers, the Airhead really brings out the bass and livens up the can.

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