[Icarium]

Ah right the TV tubes yeah I've seen a few of those they are pretty neat.

 

[MrMajestic2]

Quote:

Originally Posted by krmathis /img/forum/go_quote.gif Care to share how much they are? Guess you have been in personal contact with them, cause I have looked at their several times (in the past though) and never found any prices..

I found a Dutch site that sells them. The LL1630 is 114 Euro and the LL1650 is 154 Euro

http://www.tripleaudio.nl/index.php?...ress=&showart=

 

[spritzer]

Quote:

Originally Posted by Icarium /img/forum/go_quote.gif Ah right the TV tubes yeah I've seen a few of those they are pretty neat.

Many are in abundant supply but the problem is to sort the wheat from the chaff. Then there are the old DHT favorites, 845 and 211 which with a B+ in excess of 1kV can swing plenty of juice.

 

[krmathis]

Quote:

Originally Posted by MrMajestic2 /img/forum/go_quote.gif I found a Dutch site that sells them. The LL1630 is 114 Euro and the LL1650 is 154 Euro http://www.tripleaudio.nl/index.php?...ress=&showart=

Thanks!
I have nothing to compare against. But depending on how they perform the price don't scare me off that much.

 

[jcx]

Quote:

Originally Posted by Icarium /img/forum/go_quote.gif And now I must of course ask what tubes/semiconductors can do 1800 volts peak to peak ;p

what electrostat headphones are rated for 1800 Vpp?

there are 1200V N MOSFET, p-p bridge could swing that much V

 

[spritzer]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif what electrostat headphones are rated for 1800 Vpp? there are 1200V N MOSFET, p-p bridge could swing that much V

Most of them can handle double the bias voltage with ease so around 1200v but who wants to stop at the recommended specs? What speakers are rated at 1kW yet there are quite a few 1000w+ amps out there. That being said it's not really the voltage swing I'm after beyond a certain point but current and a very stiff PSU. All electrostatic drivers are just large capacitors so the impedance fluctuates and drops in the treble and bass. To maintain the same voltage swing into a lower impedance you need current and that's where 95% of the amp designs out there run out of steam causing difficult phones such as the Stax Sigma and Omega lines to sound very midrange centric with flabby bass and rolled off highs. Most of the phones don't "need" all this power but they sure like it.

 

[kevin gilmore]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif what electrostat headphones are rated for 1800 Vpp? there are 1200V N MOSFET, p-p bridge could swing that much V

koss are rated at 2400 vppss

those mosfets have hugh gate capacitance. Very hard to drive.
One of the many reasons a hev70 sounds so bad.

 

[jcx]

I'm in favor of lots of headroom

I just thought electrostatics were limited in bias V by diaphragm instability, would you really want 2x the design bias?


There may not be any semi's designed for the electrostatic headphone load, most power MOSFETs will have more output Cds than the headphone + cable

since these designs are already accepting single digit % efficiencies I don't think gate drive power per se is a real problem

of course my approach would be philosophically the opposite Kevin's - high feedback loop gain with composite op amps, probably with cascode/common gate output devices

high output current cfa like the TPA6120 driving the MOSFET while providing loop gain, OPA637 input, global feedback op amp, maybe even another gain boosting stage in the feedback loop

I would agree that many op amp based discrete output HV amps may not have been well excecuted in the past - but today's cfa op amp are faster than many discrete transistors you might use in pre/gate-drive stages

 

[minivan]

where is the best place to buy the stax socket for the amp or the stax socket they used on the extension cable?

 

[kevin gilmore]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif of course my approach would be philosophically the opposite Kevin's - high feedback loop gain with composite op amps, probably with cascode/common gate output devices

The closed loop gain needs to be 60db. Which means your open loop gain
would be 90db or more. At 20khz, good luck with that. The group delay
is going to kill you and the thing is going to ring like a doorbell when
you try and compensate for a pure reactive load. Its not the output
capacitance of the fet that is the problem, it is the gate capacitance.
No matter how hard you drive it, its still going to move only so fast.

So lets see what you got.

 

[spritzer]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif I'm in favor of lots of headroom I just thought electrostatics were limited in bias V by diaphragm instability, would you really want 2x the design bias?

We aren't talking about bias voltage here but drive voltage. The bias voltage is directly tied to the size of the D/S gap and messing with it isn't wise. Most headphones are under biased to have some headroom built in for extreme humidity so there is some wiggle room but we normally leave it well alone.

 

[jcx]

I don't know esl technology well, but don't you have problems with drive amplitude exceeding bias – at least at low frequency where the constant charge approximation can't hold – or has diaphragm coating tech greatly improved over the years to give << 20Hz charge leakage corner frequency?


I'm sure Kevin Knows a lot more about the limitations with several successful designs, but just looking at the raw numbers I don't immediately see problems

adding raw loop gain in composite amplifiers is no challenge with cfa op amps easily providing 60 dB flat gain to near 1 MHz, with nearly ½ A output drive, I parallel the TPA6120 in my high loop gain amp which would give guaranteed 800 mA min

As a candidate output device the IXYS IXFA 3N120 has:

Ciss 1050 pF
Coss 100 pF
Crss 25 pF

at 20KHz there would be < 10 mA required in any of the device parasitic C, looks like the tpa or other cfa would have no problem driving this 1200 V MOSFET to >200KHz power bandwidth, small signal bandwidth may be internal (polysilicon?) gate resistance or TO-220 package parasitc limited - but in any case in the MHz



as for allowable loop gain at audio frequency I'm a big advocate of “cheating”, most engineers don't know about or ever apply any more than simple dominant pole compensation, I've continued reading beyond the typical undergrad EE curriculum, particularly seeking out broader “Control Theory” books rather than just EE oriented amplifier design

Classical Feedback Control (much more readable than his 1986 "Feedback Maximization", and a revelation to someone serious about practical Control Theory but still a "buggy" text, needs a new edition)

Using non-Nyquist, conditionally stable loop gain profiles I've directly measured 30K ( = 90 dB) loop gain at 20KHz with a 2 op amp composite multiloop, I have a later circuit that I simply don't believe I can measure the 20 KHz gain of without building or buying a lot more capable measurement tools – my ESI Juli@ channel cross-talk currently limits me to a little below 110 dB

Lurie's treatment of nonlinear stability with conditionally stable high rate loop gain roll-off is fairly unique in my experience, using Lurie's ideas I can build stable amps with respect to output clipping and input slew rate limiting despite a region of 3rd order gain roll-off in the global loop gain

I haven't tried these ideas in a esl amp but I see no reason to believe past efforts that don't incorporate them should define the “state of the art”

 

[pabbi1]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif I haven't tried these ideas in a esl amp but I see no reason to believe past efforts that don't incorporate them should define the “state of the art”

Why not? We're all game here... and, besides, Spritzer needs a new project if he would ever finish the long awaited Blue Hawaii Redux.

 

[krmathis]

Quote:

Originally Posted by minivan /img/forum/go_quote.gif where is the best place to buy the stax socket for the amp or the stax socket they used on the extension cable?

Allied Electronics I presume.
Socket: WPI - 78-S6S - Allied Electronics
Plug: WPI - 86-71-6S - Allied Electronics

 

[spritzer]

Quote:

Originally Posted by pabbi1 /img/forum/go_quote.gif Why not? We're all game here... and, besides, Spritzer needs a new project if he would ever finish the long awaited Blue Hawaii Redux.

That would be the new all DHT amp Kevin designed but I doubt that I'm crazy enough to build it...