Nevod
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In the "Measurements" thread, everyone could see that top-tier electrostatic and isodynamic headphones are more similar than different, having similar FR, CSD, square wave and impulse responces, isodynamics sometimes having better measured parameters. However, their sound is not the same.
I have thought of 3 moments that seem to make difference between them. I must warn everyone from the start that I'm no specialist in these areas and everything below is wild speculation.
1) Air resistance and open area.
ES drivers use very thin stator plates with high open area - more than 50%, it seems. There is minimum amount of trapped air and minimum stored energy. Isodynamics have thicker stators due to thick magnets, and open area varies heavily based on design - Fostex T50RP have very little open area, while Hifiman HE-6 drivers seem to have at least 30% open area. The gaps between magnets also seem to be about equal to magnets' thickness, so the effect of stator thickness should be reduced to a minimum. Electrostatics have edge, but not by much.
2) Motive force and displacement.
There are some opinions that motive force produced in isodynamic designs is much higher than motive force produced in electrostatic designs. Higher motive force means higher acceleration - better treble extension, and higher attainable pressure - more power in bass region. Both in electrostatics and isodynamics, specific motive force is reversely proportional to the membrane - stator gap. Electrostatics actually have inverse square relationship - Fsig = e0*Vpol*Vsig/d^2, where e0 is permittivity, d is stator to diaphragm distance. Actually, Normal bias Stax models seem to have higher specific motive force compared to Pro bias, though not by much. But they do have less excursion, and..
And it's the excursion that allows deep bass and high dynamic range - and hence, deep bass dynamic range. Of two drivers with same motive force, the driver with more excursion would have more dynamic range and deeper bass. Of two drivers with same excursion, one with higher motive force would be able to provide deeper bass(provided that both wouldn't reach excursion limits) and more treble extension.
Isodynamic headphones have much higher excursion in general over electrostatics, so their bass and dynamic range capabilities are understandable. Isodynamics have edge, but not by much.
However, both above points regard large-signal characteristics, and maximum attainable parameters. Motive force directly relates to SPL, and we would anyway listen at about the same SPL, at about the same motive force value be it isodynamic or electrostatic.
And, real music is not 0dbFS sine wave, and contains very low level signals, which have to be considered.
3) Nonlinear effects on percieved detail.
This is even wilder speculation than the above. Isodynamics have much heavier diaphragm than electrostatics, mainly due to voice coil. While in electrostatics, membrane is many times lighter than coupled air, in an isodynamic driver membrane could weight more than coupled air. Heavy diaphragm eats up availible motive force, but its influence is linear and can be remedied by just more power. (Have to set a point to myself there, I'm not sure I'm right).
So, heavy diaphragm shouldn't influence detail level by itself. There must be some nonlinearity which is proportional to diaphragm weight/thickness that is not proportional to signal. Something like static friction force, which is constant, and completely counters any force not strong enough.
Such an effect would be more similar to crossover distortion though, and shouldn't influence small signals superimposed on peaks of larger signals. But that is already something.
Electrostatics seem to have edge, and it's not something isodynamics would be able to counter anytime soon, before some room-temperature superconductors appear.
Feel free to critic any way, as I don't pretend to be correct on any of these points.
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Different side to side diaphragm loading (or damping) should not create asymmetrical diaphragm movement and thus 2nd order harmonic distortion. The diaphragm 'sees' the load on both sides as the total load, and will respond to the total load ( again from both sides) independent of the direction of travel. Now if the behaviour of the damping material was asymmetrical, that is, there was more resistance to the air moving in one direction than the other, then it would produce asymmetrical movement of the diaphragm and thus even order harmonic distortion.
Ah, yes, forgot that the driver has not only to push air on one side, but pull it on the other. Well, that makes everything much simpler.
Yes the LCD-2s do seem to show the best measured performance, certainly the distortion plots are very good with an outstanding CSD. Indeed the CSD shows far less stored energy in the midrange than any others I have seen shown here, the bass is astonishing, okay there are a few small ridges but....
The slope on the square waves is mostly a function of the now airtight the ear cup is, the flat part of a square wave is after all DC - consider a square wave of 1hz, the air will leak out.
Considering the above moment about damping, this doesn't create a need for compromise. Thick, soft, well-sealing earpads - well, LCD's have it. Although I've always thought, why does no manufacturer line the insides of leather earpads with thick felt? That should reduce some reflections at least.
The stored energy in the mids moment is quite interesting. I've read that Sony once used an irregular pentagonal diaphragm in their electrostatic (or electret?) headphones, to reduce resonances on the diaphragm. Maybe, Audeze has used something similar.
It's been said there that no overshoot on the square wave may be due to either very good high frequency responce, or due to some overdamping. And looking on similar HF rollofs on all the high-end phones, I tend to think that this is indeed damping. Plus the fact that, while having not so different responces, the electrostats are still said to be much more detailed than magnetic planars arises some questions that I'll probably start a thread for.
When you say dip are you referring to the absolute 'phase' of the headphones, that is, a positive voltage spike creates a positive pressure spike? I will not get into the debate over absolute 'phase' except to ask: are you sure that Tyll's test rig has correct absolute 'phase' and doesn't invert the signal, perhaps it is LCD-2s that are inverted?
I've got this soon after I've written the post. Inversion doesn't bother me, it's simple to invert once more.
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arnaud,
The normalised FR plots make me think how far headphones still are from real flat responce..
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While we are on the subject, does anybody want to play "spot the difference" between the earliest and the latest SR-007 drivers?
Well, driver on the left has a large port around contacts. IIRC, this port is covered by cable attachment, I've seen some pics.. Driver on the right has the area covered with two intentional ports left.
Stator material is different, but it's hard to guess how just from the picture.
Hi Purrin,
I had a look at your distortion plots of the ESP950s. They are most interesting.
First, a look at the harmonic distortion measurements. Fortunately you show the spectral distribution of the distortion rather than just the THD. I find it interesting that the second harmonic distortion is higher than the third. The theory of push-pull ES consistently shows that the even harmonics should be very low, much lower than the third (actually, theoretically zero). There is a caveat, this cancellation of the even harmonics requires a high diaphragm resistance (the usual quoted value is 10exp9 per square or above, preferably 10exp11 per square). So either there is a problem with the testing <sorry>, Koss has got the diaphragm resistance wrong, or there is some other factor in play that I don't understand.
I believe (as in, I don't have anything to back up) that this could also be due to damping. To cancel even harmonics, you need symmetry, and usually, there is no symmetry in headphones - outer side radiates freely into air, inner is loaded by a sealed cavity. Granted, backside is usually damped, but it is still nowhere near symmetric. To obtain symmetry, one would need ports on the inside, and the ports should provide resistance equal to backside resistance. To make this controlled you'd also need well sealing earcups..
And, I don't want to steal any discussion, but..What really bugs me is LCD-2s square wave and impulse responces - they look near perfect. Minimum overshoot and flat zones are really flat on square wave (though I guess that en even more mighty amplifier for Staxes would correct that). But impulse responce seems to be much better than any other headphone, regardless of technology - most of them dip instead of peaking on the impulse, while LCD-2s plot looks like "it's just as it should be". Makes wonder about the construction of the driver and damping.. Or, reciprocally, why all other phones, Stax included, dip on the impulse.
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And the final mad thought..
What crossed my mind is an such remake of SRX circuit:
Input stage is modified by using a mu-follower/CCS/modulated CCS.
Output stage modified same way as well.
Interstage coupling capacitors replaced by voltage divider connected to highly negative voltage (say, 1.5 kV). Input stage CCSes handle the DC current through that divider.
It is simple and straightforward, compared to active batteries etc.. Drawbacks I see is really poor PSRR and loss of gain - though not very high.
Or should I banish that thought in deep night?
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If you do SRPP you end up in the same troubles as the ray samuels A10.
Same sort of thing as mikhail's original disaster.
Ehm, no, I wasn't talking about using tubes as the plate current source. I was about using mu-follower wired solid state plate current source, output connected to drain of the source's transistor, as luvdunhill said.
Ah, it is only called SRPP is the current source is made using same tube?
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And one more somewhat silly question.
What about plate current source wired as mu-follower/SRPP? (they are pretty much the same, aren't they?). Of course, biased so that other side doesn't get close to zero current through CCS.
Or better leave tube current variable?
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So then anode current sources seem to be even more fit? Some I've seen are rated for up to 1kV, enough for even somewhat boosted SRX..
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I've read that SRX design with EL34 as output (same as Singlepower ES-1) is good.
But how about some more power? Would KT88s be adequate, or it would be better to use 2 EL34s in parallel? Or anyway that would be an overkill and one EL34 is enough, considering load capacitance should be comparable with SR-Omega?
Also, what if the anode resistors would be replaced by some availible kit CCSes?
Primary reason I'm considering instead of KGSSHV all that is because I don't want to mess with finding rare parts, hard to source stuff here, while tubes are reaadily available. Plus the price reason, SRX seems to be gradually upgradeable.
Also, I don't really well understand the first stage of SRX. By looking at it, I'd call it a cross-coupled cascode. What is it's role, to compensate out tube nonlinearities?
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ok, so, by "said to be", are you suggesting these mods without having heard them?
No, of course I'm not suggesting any one else do them. I'm asking the reverse, if anyone did something like that, or generally has insight into what mod would do what, I'd like to hear are the mods worth doing.
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What are you trying to achieve after the mods?
Wachara C.
I'm mainly approaching that from a technical point of view, getting rid of some more or less obvious drawbacks.
They are said to be not as clear, revealing and detailed as top-level Staxes and Sennes, and I'm thinking that these mods may improve on that.
One more mod: 'deresonating' the diaphragm by making spacer opening's shape an irregular polygon. Theoretically, this should remove diaphragm standing wave resonances at high-mids and low-highs.
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I've posted this already on another well-known headphone forum, here however I see DIY is more prominent.
I'm thinking about bying an used pair of ESP 950s and seriously modding them. I've considered making a totally DIY ESL headphone, but making the headband and enclosures for them is too much for my limited skills. Kosses though seem to be a fine ESL headphone as they are, and from pictures of their insides on Wikiphonia it seems that there are some mods easily possible.
1) Removing damping and maybe adding 'phase corrector' like on old Fostex T50 - a felt ring which suppresses high frequencies coming from outer parts of their drivers to even out phase and frequency responce.
2) Regluing drivers to remove possible squealing.
3) Maybe drilling some vent holes around driver to play with air loading on both directions - if needed. As Kosses are regarded as very tonally level, I suppose it's already alright.
4) Using laser CNC to cut a more open and more stiff stators to replace stock ones. Stocks seem to be not very stiff and having too little open area. Also I don't get the purpose of 'tubes' on their stators' holes. These can be replicated by thicked PC board, I believe.
5) Recabling them using DIY 'Verumecce' cable: http://www.diyaudio.com/forums/analog-line-level/194548-new-interconnect-design.html Additional opposite bias would be required though, but that is not a problem.
So, I'm asking more experienced people here, would these mods provide improvements? Maybe someone already tried something like that? Thanks!
Sonic differences Isodynamics vs Electrostatics - theorethical thoughts.
in Headphones
Posted
Well, estats do have greater degree of freedom with regards to stator design, while others have more constraints. Other than that, I agree.