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The Stax Thread III - Page 125

post #1861 of 4038
Quote:
Originally Posted by milosz View Post
 

I think people are somewhat off-base talking about the reason that some of us feel that the 007 (esp. mk I) really come alive with an amp capable of higher voltage swings -  and it is NOT just volume.

 

The SLEW RATE is important. Most electrostatic amps just can't swing ENOUGH voltage FAST ENOUGH (with low distortion) into the reactive load of an electrostatic driver.

 

If everything else was equal - slew rate, distortion, etc - then higher voltage swing would only mean a louder output.  But when it comes to electrostatic amplifiers, things are NOT equal, and so the QUALITY of sound that a given electrostatic earphone is capable of has limits imposed on it by the amplifier's capabilities.  An amplifier that is capable of delivering the best in terms of high slew rates and low distortion is also capable of greater-than-average voltage swings.

 

Listen to an 007 on a DIY T2 then listen to it on an SRM-T1 - or even the  SRM-727 etc-  you'll hear the difference.  And that difference is NOT just "moar volume."

 

Quote:
Originally Posted by dude_500 View Post
 

Unlike in conventional headphones where the cable does not measurably affect the output of the headphones, in electrostatic headphones it actually can definitely play a major role. It depends entirely on the amplifier whether or not it will matter. Electrostatic drivers are capacitors, and the cables are capacitors. As it turns out, the cable is quite a fraction of the total capacity of the headphones. Increasing this capacitance may or may not change the sound quality. Electrostatic drivers run at such high voltages, that slewing the voltage in these capacitors actually takes milliamps or even tens of milliamps of current (might not sound like much, but we're at hundreds of volts!). If the amplifier has sufficient drive power and is well designed, it really shouldn't change anything. If it's a lower power amp or lower design quality, it might be at least measurable.

 

In terms of conventional headphones, imagine a case with a 300 ohm driver, where the cable is 100 ohms. Changing this cable to 300 ohms by extending it would change the impedance of the system and would change how it is loading the amplifier. Fortunately for conventional dynamic drivers, cables are <<1 ohm.

 

Given the bias currents and topology of the Blue Hawaii, it is my opinion that there might be a very slight measurable difference using a 15 foot cable, but I would be shocked if the difference could be perceived by any human (of course assuming you're unaware of the difference in cable length). On the other hand, if one were using a Stax transformer box, I would be quite surprised if the average listener couldn't tell a difference.

 

 

These quotes deserve to be united since they are both related. I have found both beautifully explained. Thank you.

post #1862 of 4038
Quote:
Originally Posted by milosz View Post
 

I think people are somewhat off-base talking about the reason that some of us feel that the 007 (esp. mk I) really come alive with an amp capable of higher voltage swings -  and it is NOT just volume.

 

The SLEW RATE is important.

The SLEW RATE is meaningless.

 

Quote:

Originally Posted by milosz View Post
  Most electrostatic amps just can't swing ENOUGH voltage FAST ENOUGH (with low distortion) into the reactive load of an electrostatic driver.

 

 

Classic unsubstantiated claim.


Edited by dripf - 3/26/14 at 8:15am
post #1863 of 4038
Quote:
Originally Posted by dripf View Post
 

The SLEW RATE is meaningless.

 

 

Could you please elaborate ?

post #1864 of 4038
Quote:
Originally Posted by kothganesh View Post

Could you please elaborate ?
This should be good... popcorn.gif
Looking forward to dripf's fourth post.
post #1865 of 4038

Assuming a linear audible frequency response (which is reasonable) from the amplifier, we can use the output Vrms to calculate the slew rate of the amplifier. This is meaningless as it describes nothing more than we already know.

 

The context of SR in the case of ES headphones is the calculation of current required to output 20 kHz at a required loudness. This is simple SR * capacitance. Where there is a current insufficiency SR is effectively limited.


Edited by dripf - 3/26/14 at 7:25am
post #1866 of 4038

Although, if you do transpose the formula into SR = current / capacitance, things aren't looking so great for my hot air.

 

By this I mean that I've proven milosz's point. :rolleyes:


Edited by dripf - 3/26/14 at 12:06pm
post #1867 of 4038

I'll be the first to admit that I have a fairly limited understanding of what such factors mean in practical applications (i.e. outside of circuit topology theory and conversation topics for electrical engineers).

 

In particular I'm unsure of the implications of damping factor and slew rate in the electrostatic realm.

 

That's why I'm genuinely curious as to how and when the theoretical limits of electrostatic amplification might be breached, particularly from individuals who have first-hand experience with such audible distortion/clipping. This is not just for something like a SRM-252S or SRM-323S or SRM-T1, but also for justifying the increments from a KGSS to a KGSSHV to a BHSE to a SRM-T2.

 

I have the money for just about anything short of a SRM-T2/DIY-T2, but I just can't justify the outlay. Let me say that my first-hand experience with the KGSSHV was less than revelatory.


Edited by 3X0 - 3/26/14 at 2:14pm
post #1868 of 4038

Slew rates DO matter. Most amplifiers have such high slew rates that they are thought not to matter- and certainly I doubt whether anyone could hear the difference in two amplifiers, one with a slew rate of 200 V/uSec and one with a slew rate of 20 V/uSec - all other things being equal.  But if you had an amplifier with a 1 V/sec slew rate, do you really think it would sound OK?

 

When driving the reactive load presented by the 007, most electrostatic amplifiers tend to start showing power bandwidth and slew rate problems at their max voltage output levels.  

 

I'll post some 'scope pictures if I get around to it. Square waves even as low as 1 kHz don't really look all that great from the output of a T1 driving an 007 at ~90 dB SPL, while the DIY T2 is close to flawless.  

 

You CAN hear the difference.  Be sure you have actually tried listening to a 007 mk I on both a DIY T2 and a T1  (or 727 etc)  before you claim  that these things don't matter.

 

I suspect that IM also increases from the T1 and other amps at high-ish drive levels into an 007.  With a steady-state test signal (sine wave etc) the amp's negative feedback will reduce IM and THD levels rather quickly, but music is NOT a steady state signal, and I think that the lesser amps are introducing IM into short-duration signals.  This is just a guess. I have no way to measure this.  My distortion analyzer relies on sine wave test signals, as they all pretty much do.

 

The DIY T2  is extremely linear even at high output voltage levels, even without negative feedback.  So is the BHSE.  


Edited by milosz - 3/26/14 at 3:35pm
post #1869 of 4038
Quote:
Originally Posted by 3X0 View Post
 

I'll be the first to admit that I have a fairly limited understanding of what such factors mean in practical applications (i.e. outside of circuit topology theory and conversation topics for electrical engineers).

 

In particular I'm unsure of the implications of damping factor and slew rate in the electrostatic realm.

 

That's why I'm genuinely curious as to how and when the theoretical limits of electrostatic amplification might be breached, particularly from individuals who have first-hand experience with such audible distortion/clipping. This is not just for something like a SRM-252S or SRM-323S or SRM-T1, but also for justifying the increments from a KGSS to a KGSSHV to a BHSE to a SRM-T2.

 

I have the money for just about anything short of a SRM-T2/DIY-T2, but I just can't justify the outlay. Let me say that my first-hand experience with the KGSSHV was less than revelatory.

 

The slew rate is how fast the output voltage can change. If you ever hit the slew rate limit, there is substantial distortion because the output can't "keep up" with the input. You shouldn't actually ever be breaching this if the amplifier is properly built for the load it drives. I measured my Blue Hawaii's unloaded slew rate at about 125V/uS (it is entirely possible the BHSE has superior slew rate, I didn't spend a whole lot of time tuning my DIY BH for slew rate, and the published schematic has typos so every DIY BH is going to be a little different). This means that at full 800Vp-p output, there is no slew rate distortion up to 50kHz. I always forget to measure it with headphone load when I have it in the shop, but it is safe to say it's going to be quite a bit lower. 


Basically, since the slew rate limit is caused by the amplifiers inability to provide sufficient current to the load to slew its voltage. Since the current required to slew a capacitor is proportional to the capacitance, adding more capacitance will directly decrease the slew rate of the system.

 

 

I think it was stated that slew rate doesn't matter, because it actually doesn't unless you're hitting the wall. At reasonable volume levels with a reasonably quality amp, you're just not hitting the slew rate limit. But the thing is, if you start adding long cables and multiple headphones, you might. Now, that's not to say there aren't other forms of distortion caused by increasing the capacitive load. But calling any of those slew rate limitations is an incorrect use of terminology.

 

It blows my mind that industry standard is to put two output jacks on electrostatic amplifiers. It is completely absurd to listen to an amplifier with two headphones attached. Sure, this might not matter on a T2, but just about everything else including the Blue Hawaii does not have enough power to recklessly be throwing away slew rate and capacitive loading distortion. I believe it started as amps having two jacks, one pro bias and one standard bias, and now it just kind of stuck because your amplifier would look worse with only one jack. But in fact, the second jack should NEVER be used if you care about sound quality. Those of us who build electrostatic headphones have noticed that etching away even one or two square inches of unnecessary stator area affects the sound produced, especially in the trebles where the slew rate is much higher. Now add a second pair of headphones...

post #1870 of 4038
Quote:
Originally Posted by dripf View Post

Although, if you do transpose the formula into SR = current / capacitance, things aren't looking so great for my hot air.

By this I mean that I've proven milosz's point. rolleyes.gif
Good conclusion 😄
post #1871 of 4038
Quote:
Originally Posted by milosz View Post

Slew rates DO matter. Most amplifiers have such high slew rates that they are thought not to matter- and certainly I doubt whether anyone could hear the difference in two amplifiers, one with a slew rate of 200 V/uSec and one with a slew rate of 20 V/uSec - all other things being equal.  But if you had an amplifier with a 1 V/sec slew rate, do you really think it would sound OK?

When driving the reactive load presented by the 007, most electrostatic amplifiers tend to start showing power bandwidth and slew rate problems at their max voltage output levels.  

I'll post some 'scope pictures if I get around to it. Square waves even as low as 1 kHz don't really look all that great from the output of a T1 driving an 007 at ~90 dB SPL, while the DIY T2 is close to flawless.  

You CAN hear the difference.  Be sure you have actually tried listening to a 007 mk I on both a DIY T2 and a T1  (or 727 etc)  before you claim  that these things don't matter.

I suspect that IM also increases from the T1 and other amps at high-ish drive levels into an 007.  With a steady-state test signal (sine wave etc) the amp's negative feedback will reduce IM and THD levels rather quickly, but music is NOT a steady state signal, and I think that the lesser amps are introducing IM into short-duration signals.  This is just a guess. I have no way to measure this.  My distortion analyzer relies on sine wave test signals, as they all pretty much do.

The DIY T2  is extremely linear even at high output voltage levels, even without negative feedback.  So is the BHSE.  

Milosz, by any chance would you happen to have any square wave measurements at lower dB SPL, like 50dB or less. I have not done any listening comparisons between my DIY-T2 and other electrostatic amps i have had, but am curious, particularly since I tend to listen at very low volume levels.

Dude, regarding the 2 output jacks issue, I had been under the impression from comments made by Kevin Gilmore that there was little to no difference in running two headphones at the same time - but I could be mistaken on this. Maybe a BHSE, Liquid Lightning, Electra or WES owner could ask their respective makers the question.
post #1872 of 4038

I think the KG statement about 2 ES headphones driven from a stat amp concurrently from the two outputs is not a problem since the Z load is very different on electrostats to that of conventional dynamic transducers. My memory is vague here as it was in the old Stax thread and on the other site but I could be wrong.

post #1873 of 4038
Quote:
Originally Posted by georgep View Post


Milosz, by any chance would you happen to have any square wave measurements at lower dB SPL, like 50dB or less. I have not done any listening comparisons between my DIY-T2 and other electrostatic amps i have had, but am curious, particularly since I tend to listen at very low volume levels.
 

 

My DIY T2 is back apart now, boards and so on being fit into the casework.  No measurements possible for a while yet.

post #1874 of 4038
This has me curious to measure various step responses on the 007 and 009 through my 727 amp and bhse when it gets (any day or week or month now smily_headphones1.gif ).
post #1875 of 4038
Quote:
Originally Posted by arnaud View Post

This has me curious to measure various step responses on the 007 and 009 through my 727 amp and bhse when it gets (any day or week or month now smily_headphones1.gif ).

I'd be interested to see what the signal output from the amp itself looks like, the acoustic output of the 007 and 009 are of interest too but I'd be curious to see if you can see any difference in the voltage from the amp.  CAUTION:  use isolated 'scope! High voltages!

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