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Nazo 4th June 2012 06:14 PM

Any idea at all how the AD8656 OPAMP compares to the AD8620?
I asked over at Head-Fi and for the most part I only got criticism about everything rather than any sort of response to the question itself, so I thought maybe it wouldn't hurt to ask over here and see if by any chance anyone here could tell me anything on the matter.

So right now I'm using a Fiio D3 connected to a CMoy with an AD8620 OPAMP and it just couldn't possibly suit me better -- I absolutely love it. I've seen someone describe it as making a CMoy sound like a $100 amp, but for me it sounds even better than the iBasso D3 I got a long time ago for a different headphone. Perhaps it's not the best of the best for some, but for me it is. Still, it is a little inconvenient having the Fiio D3 connected to a soundcard then connected to a separate amp, dealing with the amp (especially as far as portability goes -- I'm powering it via 6xAAs since a 9V seemed to drain too quickly, so the case I have it in had to be a little larger and more inconvenient to carry around,) and etc. I've read that the AD8656 is supposed to be completely compatible with the Fiio D3 and that it works very well with it and even sounds great. Certainly it could be a lot more convenient if the one device could basically just do everything. Still, I wonder if it would be a "downgrade" for me to go from the AD8620 I'm using now to that one? It occurs to me that, given the model number, it might actually be somewhat similar to the AD8620 (maybe even in the same series or something?) If it basically sounds the same or even a little bit better, it would be a great way to go.

I still haven't decided for certain if I'll do this in the first place (I haven't really done much actual soldering of chips in the first place, plus I'll have to rig up a volume control somehow -- though I have an idea or two on that -- since most things don't actually let you adjust the volume of the digital outputs and the D3 itself lacks any sort of volume control, so the whole thing is definitely inconvenient) but obviously there's no point in even going to the troubles to try it if it's not really going to be any better. (As a pre-amp the TILMV358 seems to be fine, so while the AD8656 might technically still even be an upgrade for this purpose, it wouldn't be worth it for me at all if I wasn't going to be able to use it directly as a headphone amp.)

By any chance would anyone have any idea how the two really compare? I don't know if anyone would have had much opportunity to compare the two side-by-side, but perhaps someone might have enough of an idea of how each sounds to be able to more indirectly compare? It absolutely has to be really close to be worth all of the trouble for me. In fact, one person's "better" might actually be worse for me.

BTW, IF I am going to do this, I need a good source to actually get the AD8656. I got the AD8620 easily enough (already in an adapter even) from eBay, but the AD8656 looks to be tougher to track down. It seems that Mouser at least doesn't have it which is where I'd normally think to look for things like chips. I saw one old post here where someone else wanted to get the AD8656 to replace the TI LMV358 in the Fiio D3 with a link to Digikey which didn't look bad, but is that the only source I should look at? (I'm in the US btw.) Also, which one is the right one to directly replace that TI LMV358? I see a ton of different versions of the AD8656, most of which are the same price and everything. If it comes in different chip "packaging" that isn't compatible with the D3 obviously it could be bad to order the wrong one.

sgrossklass 4th June 2012 09:26 PM

If you take a look at their respective datasheets, you'll notice that AD8610/20 and AD8655/56 are rather different beasts indeed.

The AD8656 is a CMOS (MOSFET) opamp with rail-to-rail input and output that'll operate at supply voltages of 2.7 to 5.5 V (including e.g. a LiIon cell). Output current is typically +/- 75 mA. Capacitive drive is up to 500 pF, which is decent but may still require an isolation resistor in the 10s of ohms, with everything that implies in terms of load interaction. (A common 3 m Sennheiser headphone cable weighs in at about 1 nF.)

The AD8620 is a JFET (input?) opamp that'll swing to about 1.5 V (input) / 1.2 V (output) from its rails and operates from about +/- 4 V to +/- 13 V (8 to 26 V). Output current is typically +/- 30 mA. (Low-impedance load driving capability seems about equal to a NE5532, which is decent but by no means exciting.) Capacitive loads pretty much do not impress this one at all.

BTW, a Cmoy is not exactly a great headphone amp. It's a useful circuit for driving mid-high impedance cans with a decent selection of opamps when doing so. When it comes to lower impedances and fussy loads requiring low output impedance (BA IEMs and such), choices are few and far between. The JDS Labs cMoyBB uses an OPA2227 so that seems to be working well for designs with nearly no output impedance, as does the lowly NJM4556 (which proved very well-behaved, maybe not so much of a surprise for a part designed to drive headphones in the first place). Eventually the rail splitter limits current output, which is why the cMoyBB allows doubling 'em up.

Overall, a cMoy will only go so far. A buffered design may give considerably better performance (see e.g. the O2).

BTW, I'm under the impression you're worrying too much about electronics. Not once did you mention the cans used (or to be used).

Nazo 4th June 2012 10:50 PM

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I wish I could say those numbers really meant something to me. Well, for one thing, I do realize I'm sort of comparing apples to oranges. Those two OPAMPs were intended for different applications and, in fact, the Fiio D3 IS a different application from a CMoy amplifier. I'm just wondering if the Fiio D3 outfitted with an AD8656 might come remotely close to sounding like a CMoy with an AD8620 connected to a Fiio D3 with the stock TI LMV358. Otherwise I'm better off sticking with the CMoy being external and it isn't worth the time, effort, and cost to go through all the trouble of tracking down the AD8656 chip and then modifying a Fiio D3 with it.


Originally Posted by sgrossklass (
BTW, a Cmoy is not exactly a great headphone amp.

For its pricerange it's pretty hard to beat though. I doubt you can find anything better at that range that allows me to use this chip (remember, its sound is pretty much ideal for me even compared to some far more expensive amps) or one similar enough in every way. If you can, I'll admit I'd very much like to see it. Especially since I've managed to break my current CMoy and have been forced to buy another (which I haven't done yet because I need one with a socket since I haven't been able to find anyone willing to sell me one with an AD8620 already in it.)


BTW, I'm under the impression you're worrying too much about electronics.
I'm worried about the sound. Right now this setup is right where I want it. Any changes from here I might make would be for greater convenience only. I'm not looking to find an amp that sounds very different but supposedly complements my headphones better, and I'm aware that my headphones aren't the highest quality i


Not once did you mention the cans used (or to be used).
Sorry, I originally posted in Head-Fi since I thought it would make more sense to ask them than anywhere else I know of, but instead I'm getting responses like someone suggesting that I ask iBasso to make an amp with an AD8620 (huh? That wasn't even the question at ALL!) or someone fussing that they think my Fiio D3 isn't the most portable solution in the world (which still doesn't make sense since I was only talking about eliminating having to use an external amplifier -- the Fiio D3 was to be present either way, so its portability can't be any less by eliminating having to carry around an external amplifier in addition to it...) On Head-Fi it has a user profile page where you can list your sound equipment which I sort of try to keep up to date and I'm used to the idea that if anyone wants to know what I'm using they can just click on my name and see it there.

Anyway, I'm using a modified pair of HD555s. I've not only done the "soundstage mode" but I've gone a bit further and done a bit more. (The metal grill is just gone basically. Instead of either the plastic or metal there is only a soft speaker grill cloth meant more to protect it from gathering any dust inside than anything else.) I've also put the Canare L-4E6S cable on it. (I've attached a picture in case you're at all curious.) The end result is something that just simply suits me. It may not be the best of the best and definitely isn't for everyone, but for me it basically is just perfect. I actually also have had a few others in the past and even still have MarkL-modded Denon AH-D2000s that I've been trying to sell for a while now. None of the others have suited my preferences and hearing quite like these modified HD555s have done. (The ultimate irony is I got them on a whim thinking they might be more convenient for my portable devices when they were marked way down on Amazon once and did the mod more just as a fun project, then ended up loving them so much that I've abandoned everything else and will probably have to buy and mod another pair someday so I'll have a backup.) It's worth noting that on the go I sometimes use a pair of modified HD280 Pros instead (due to how well sealed they are -- I don't want other people hearing what I'm listening to and don't really want to hear too much outside noise either) which I've modified with the Bluetak mod and put in a couple of capacitors to act as a first-order high-pass filter to some small extent (it's not perfect, but it's sufficient for its purpose.) I don't really like the HD280s at all, but with the AD8620 they do tend to still sound a bit better. (I should add here that I don't actually use them too often.)

Then, when I combined them with that AD8620, not only is the sound more accurate and detailed, but it just seems to fit with them for me. I love it like this and want to keep it that way. I'm willing to some extent to accept "similar" but I'm a little hesitant even about things others find to be better since not all of the "better" things suit me so well. That iBasso D3 for instance is obviously a far higher-end headphone amp than my CMoy, yet it just doesn't work for me like it does. I don't hear that detail or get the enjoyable sound from it that I get with the AD8620 CMoy. (It seems to me like the AD8620 overall a lot warmer by comparison. I don't really understand why the iBasso D3 seems to not have as much detail and everything though. I guess it just is so much more neutral that I simply don't notice those details as much. Overall to me the iBasso D3 just seemed "boring" for lack of a better term. At this point I'm trying to sell it too, lol. I really only got it to complement the D2000s anyway.)

Anyway, yes these are low impedance headphones, but trust me, the CMoy drives them just beautifully with clarity and detail through the full range (or at least all I can hear of it. Beyond 16KHz or so I don't really hear much due to hearing loss from age and too much music too loud over the decades.) Even the bass is just amazing and I absolutely love it. (At least with the HD555s.)

BTW, if you haven't guessed, I don't call myself an audiophile and I can't give you exact descriptions of anything to do with sound. And yes, this makes identifying exactly what it is that I'm looking for a LOT harder for not just others, but myself as well.

sgrossklass 4th June 2012 11:41 PM

That LMV358 is a funny little circuit... folded cascode, mixed BJT/CMOS. With a GBW of a few MHz and a slew rate of 1 V/s it's not really a speed king, and it's very noisy... but a typical supply current of 210 A for both channels is impressively low. It can source and sink a healthy amount of current, too.

Quite expectedly, specified distortion performance is nothing much to write home about, so assuming it is in fact used to buffer the DAC output, I'd expect significant measurable (though not necessarily audible) improvements when using the AD8656 instead.

What sort of output resistance does your cMoy have? And which HD555 variant is that? The common "bass-light" 50-ohm version actually benefits from a bit of extra output resistance, which fleshes out the bass to a normal level in particular. That could explain why you found the iBasso D3 (presumed to have a low-impedance output) to be "boring", 'cause the "real" sound signature of these HD5x5s is just that. AH-D2000s, by contrast, don't care much one way or the other and should sound pretty much exactly the same - their impedance is nearly constant over frequency.

Nazo 4th June 2012 11:54 PM


Originally Posted by sgrossklass (
Quite expectedly, specified distortion performance is nothing much to write home about, so assuming it is in fact used to buffer the DAC output, I'd expect significant measurable (though not necessarily audible) improvements when using the AD8656 instead.

It's only acting as a buffer for the DAC right now though. How much difference would there really be while it's just doing that? I have to say though, this Fiio D3 sounds better than any USB DAC or soundcard I've ever used before all the same. I feel no need to change out the OPAMP just for it standing between the DAC and a headphone amp. If it could act as a headphone amp on its own (as I've read that the AD8656 supposedly can do in the Fiio D3) and I liked it at least close to as much as the AD8620, that would be worth the trouble, but just as a buffer I don't want to deal with all that and possibly ruining a Fiio D3 in the process of attempting it. At least. Not right now. I may still someday do it -- there's no doubt that it could only be a step up from that TI OPAMP -- but if it's not going to act on its own it isn't worth the trouble of doing it today. (Remember in particular that I'm on a budget here.)


What sort of output resistance does your cMoy have?
I really don't know. Like I said, I seem to have managed to fry this one and will have to get another. Wouldn't they tend to be around the same sort of range though?


And which HD555 variant is that?
The 50 ohm version.


The common "bass-light" 50-ohm version actually benefits from a bit of extra output resistance, which fleshes out the bass to a normal level in particular. That could explain why you found the iBasso D3 (presumed to have a low-impedance output) to be "boring", 'cause the "real" sound signature of these HD5x5s is just that. AH-D2000s, by contrast, don't care much one way or the other - their impedance is nearly constant over frequency.
Well, it's more than just the bass. I would say that the bass on the CMoy sounded just beautiful. As far as bass goes, I like it to be at least somewhat close to neutral and accuracy is probably more important than loudness. The HD555s with that AD8620 CMoy seemed to have just the right level of bass with all the accuracy I could want and more (I was hearing individual notes of synths that I didn't even think WERE individual notes before.) If it is "bass light" as it stands right now, then that's actually right where I like it. I'm aware that frequency response graphs on places like HeadRoom are showing a low frequency bass rolloff, but to me these just sound right, so either the CMoy was just perfect or I don't really need or want more.

I felt like the iBasso D3 was a little too neutral I guess though. I think even in the mid ranges it didn't suit me as well and possibly the highs too (I'm a bit sensitive to certain high frequencies, so I liked that with the D2000s since that actually helped tame them a bit, but by comparison the HD555s seem to be just right for me in those ranges and get along well with normal amps.)

I still can't really tell you what exactly it was "detail-wise" about the AD8620, but with it I was noticing all sorts of little things in the music I'd always missed before with every other amp I've ever used (including CMoys with OPA2132s and 2777s, so this isn't just a benefit of CMoy designs or something, but that specific chip.) Everything was just more accurate and detailed to the point that it sounded like a FAR better amp than a mere CMoy and I liked it better than even other "budget price with quality" amps like the Fiio E6 (which I understand uses an AD8692 and a good power source?) I wish I could quantify things more for you, but all I can really say is the AD8620 just seemed to bring something amazing to that CMoy.

EDIT: Well, given that I no longer have that CMoy I think I'll dust off the old iBasso D3 and use it for a little while. I'll probably ultimately be using the Fiio E6 -- I think it suited me better -- but in the meantime I'll see if I can get a better idea of exactly what it is I don't like about that iBasso D3.

jcx 5th June 2012 01:21 AM

as a line out the D3 may have too small coupling Caps for 50 Ohms - expects >> kOhms loads driving amp inputs/volume pots

driving a low Z through small coupling Caps will lose octaves of bass

may also have too high series R - often included to protect angainst oscillation of the op amp with unknown cable capacitance - 50-100 Ohmms is commonly used but I have a desktop SACD/CD player with kOhms in series with the line out

Nazo 5th June 2012 01:32 AM

I don't know, supposedly it works, but I can't find much by way of verification. The biggest irony is that Google keeps trying to send me to this very thread (I can't believe it's already indexed on Google...)

Let me ask this though: IF I were to consider an amp such as, just for example, the iBasso D-Zero (which is among the cheapest of the higher quality amps from companies like those) which also just happens to use the AD8656, how close would it sound?

jcx 5th June 2012 03:53 AM

the most important determining factor in "sound" is frequency response, electronics can be very flat, mostly inaudible frequency response variations - literally 10,000x less than mics, headphones, speakers over our most sensitive hearing region

but some cirucit design features can cause audible frequency response changes - thats why we want to know what the circuit is
half way compentent manufacutrer's selections of op amps appropriate to the application circuit are seldom the cause of audible frequency response variations

using the D3 to directly drive 50 Ohm headphones is likely beyond the designer's intent - again seeing the circuit would allow informed guesses of how it would perform

Nazo 5th June 2012 06:48 PM

Perhaps this will help? This guy pretty much opened it up as soon as he got it and took excellent quality pictures along the way.

I finally got a straight answer from someone in Head-Fi saying that they think the AD8656 doesn't sound nearly as good in even a full amp like the D-Zero as the AD8620 does in a CMoy though. Barring anyone with the opposite experience I guess I'll have to consider this idea a bust and stick with the AD8620.

jcx 6th June 2012 08:01 PM

nice pics, can't say exactly without schematic but can see a 0805 ceramic directly connected to one op amp output pin - not promising for "audiophile" quality if its a DC blocking output Cap - the only ceramics with enough uF use horribly nonlinear, piezoelectric dielectrics

can't tell what the sot-23 are doing, could be transistors for a active gnd splitter

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