This is a somewhat complicated field. The short answer is about 960 volts peak to peak - about 340 volts RMS. If you would like to know what this actually means in this application, I have put this page together to give a feel for the requirements of driving electrostatic headphones. Stax of Japan is the premier company in this field. Read on if you are curious.Quote:
Originally Posted by 1audiohack
Firstly, check out this two page thread. That site if full of misinformation and BS, but the contributors "Kevin Gilmore" and "Spritzer" are the gold standard for information about electrostatic headphones and their ancillary equipment. They are the world leading experts outside of the Stax inner sanctum, which seems to closely follow what they say in any case.
http://www.head-fi.org/t/582518/elec...oltage-ratings
Here are two posts from it.
From Kevin Gilmore:
Electrostatic Amplifiers Voltage ratings
I get asked about this fairly often, there are a number of different ways of measuring the voltages
so here in one place is all the amps i know about, and all are rated the same. The voltage is
peak to peak voltage measured from one stator to the other stator.
For solid state amplifiers, the output voltage is usually very close to the power supply rails.
For OTL tube amplifiers, the Rp of the output tube(s) usually takes about 60 volts off of the
maximum voltage swing, for Stax made amplifiers the negative power supply is increased to
compensate for this. These are theoretical maximums, in practice the voltage swing will be less.
There is of course more to these amplifiers than just the voltage ratings, Slew rate into a
capactive load is also very important, i will be measuring some of these amplifiers and will
add slew rate at a later time.
2400 volts peak to peak (+/-600 power supplies)
koss esp950
2300 volts peak to peak (+/-575 power supplies)
KG’s new Silicon Carbide jfet based space heater.
2000 volts peak to peak (+/-500 power supplies)
Stax T2, DIY T2, KGSSHV (ixys parts)
1800 volts peak to peak (+/-450 power supplies)
KGSSHV (sanyo parts)
craig has said his new electra amp does 1800 volts, power supply unknown
Ergo Jecklin
1600 volts peak to peak (+/-400 power supplies)
BH, BHSE, singlepower ES1,ESX (after being modified to remove the 100V offset)
1400 volts peak to peak (+/-350 power supplies)
KGSS,Stax T1,T1s,T1W,313,323,717,727,sra-14s,srm-1 mk2,srm-monitor
woo audio wes
1300 volts peak to peak
sra-10s/12s
1200 volts peak to peak (+/-300 power supplies, and +600 capacitively coupled)
KG tube #1,aristaeus,HEV70,HEV90,RSA A10, Stax srm212,srmXh,srm300,Exstata
Egmont,Cirolan,SRM-1,srm-3,woo audio ges,srm310
1100 volts peak to peak
sra-3s
srm-001 (when the batterys are at full power)
From Spritzer:
Well peak-to-peak has become a bit of an industry standard in the DIY world with only Stax using RMS. Still it is easy enough to convert, RMS*1.41*2 to get the P-P value. The SRM-300(310) is listed at 350VRMS so that would be roughly 1000V P-P. Stax always post conservative specs plus these are measured, not simply derived from the rail voltages (in this case +600V regulated).
My amp is not directly listed there, but Stax publishes these specs:
SRM-007tII SPECIFICATIONS:
Type :Vacuum tube output stage Low noise dual FET input Class A operation, Pure balance DC amplifier configuration Earspeaker driver unit Vacuum Tube :4 (6FQ7 / 6CG7) 6S4 in my modified example Frequency Response DC to 100,000Hz +0,-1.5dB w/ SR-007 or SR-404 High Harmonic Distortion :Max. 0.02% (with 1kHz, 100V r.m.s. ) Gain :54dB (x500) Input Terminal :3 ( XLR x 1, RCA x 2) with 1 RCA parallel out Rated Input Level :200mV / 100V Outputs Max. Input Level :30V r.m.s. at min. volume Input Impedance :50Kohm, 50kohm x 2 during balance Max. Output Voltage :340V r.m.s. (1kHz) Standard Bias Voltage :PRO 580V x 2 Power Voltage Frequency :117V (50Hz / 60Hz) Power Consumption :55W Temperature & Humidity :0 to 35ēC / 90% max. without condensation Dimensions :7.7 (W) x 4.1 (H) x 16.5 (D) in Weight :10.4lbs Color :Silver or Black
I am not claiming that my example is higher voltage, I don't know, but rather that it yields the stated voltages without drama or just plain crapping out. Calculated (RMS X 1.41 X 340) it comes out to 959 volts p-p. Spritzer said it sounds like a KGSS, which has a 1400V peak to peak, so I don't wonder why I am happy with the sound. Bear in mind that electrostatic drivers don't need much current, but the amps have to be beasts because of how the capacitive load varies with frequency. Again, from Dr Gilmore:
Virtually 100% capactive which means that the reactance varies linearly with frequency. Starts at literally meghoms at DC, and goes all the way down to under 18k ohms at 20khz. Moisture in the air has some impact on this.
To put this all in perspective, Stax headphones can be put into three groups, in decreasing difficulty to drive: the SR-007 series, the current top of the line SR-009, both still in production, and everything else. The first two are large, round and expensive and may be the World's most comfortable headphones to wear. My SR-007Mk2 is exceedingly difficult to drive in a high fidelity manner. The modifications to my amp put it into the "not the best but works adequately" category. The best amp currently for sale is the BHSE - the Blue Hawaii Special Edition - by Justin at Headamp. I have heard it and it is superb. I cannot afford one.
Reading the entire thread will impart more information about the demands placed on these amps. FYI, Kevin Gilmore is an engineer who works for the math department at Northwestern University, so he is in the Chicago area, where I met him in 2010 at a huge international headphone event. My audio buddy and I brought some gear and participated. I listened to my phones on his DIY T2 amp and I have seen the light. And the heat. He told me it would literally fry an egg. The original T2 bankrupted the Stax corporation. Spritzer is the online name of a gentleman in Iceland, who I finally met there too. We were already friends from online exchanges and some equipment going back and forth. He modified my Stax amp. Talking to these men about Stax is like talking to Nelson Pass about solid state power amps. Take anything you hear to the bank.
Hope this answers your question.
PS The bias voltage for Stax headphones was changed some years ago. If you apply the high bias to the older units it will destroy them. Be careful.
Perhaps a little glossary is in order. The PRO 580V X 2 reference indicates two 580V sockets. The 580V (high) bias refers to the voltage biasing the headphones, not the amp circuits. The legacy non pro phones used about half that. KGSSHV is a Kevin Gilmore Solid State High Voltage (amplifier).
If you think the voltages are high in these amps, there is a fellow that built some using mercury vapor rectifier tubes. It gives you a lovely blue glow and something like 5000 volts. Goes to show what it can take to go all the way with this quest. Images posted below, pictured with a Stax SR-007. He obviously knows what he is doing. Here is a quote from him about two top amps:
" I'm usually perplexed when people talk about "power" in relation to electrostatic headphones and amplifiers.
The headphone is a very high impedance load consisting almost exclusively of capacitive reactance. It requires a large voltage swing and enough current in the amplifier output stage to maintain slew rate and frequency response, but it consumes next to no true power no matter how much you "throw" at it.
Electrostatic amplifiers are voltage, not power, amps. Conceptually they are more like an ultra high gain preamp rather than a traditional speaker power amp, which unlike a voltage amp, is designed to transfer both current and voltage into a low impedance load. The only signifigant "power" involved in an electrostatic amp is the dissapation of the output devices and their respective loads.
Rather than an undefined or mislabeled discussion of an amps output "power", it might be better to discuss the amp in terms of it's ability to drive a load. Driving ability, reduced to basics, is a product of gain(usually about 60dB), output voltage swing (usually about 600Vrms), and output stage current (usually 15-20mA to ensure slew rate at frequencies of interest.) I'm passingly familiar with the designs of both amps, and as far as I know, both easily meet these requirements.
Attachment 63589Attachment 63590Attachment 63591Attachment 63592
