Yes, I’ve seen this suggested somewhere before, but I’m not aware of anyone building one (someone probably has though). I figure that if you had an amp with a 5K primary, that’s a step down to 8 ohms by the ratio of 25:1. Say you then wanted to step up for the ESL by a ratio of 1:75, as a reasonable example. What you really need then is a single transformer that has a 1:3 ratio, from plates to ESL stators. This will have high impedances on both ends, but I’ll bet a better transformer can be made at 1:3 than the two others combined.
This (1:3 or similar xformer) could also be done using hv transistors or mosfets - might be better than what is usually done. May be we need to take page out of Susan's book Zero feedback Impedance thread and make our own.
Bill
Bill
Ouch, why use MOSFETS when you can use still use tubes? 
You don't need to switch to MOSFETs just because the voltage requirement dropped to within solid state range. Granted, it may work. In my experience (Zen et al), even these simple MOSFET amps suffer from rather high levels of phase intermodulation distortion (think FM) due to the highly non-linear internal FET capacitances which vary wildly with signal swings. That's why we love tubes! I'll bet that starts some responses, like poking a stick into a hornet's nest! Have at me...
You don't need to switch to MOSFETs just because the voltage requirement dropped to within solid state range. Granted, it may work. In my experience (Zen et al), even these simple MOSFET amps suffer from rather high levels of phase intermodulation distortion (think FM) due to the highly non-linear internal FET capacitances which vary wildly with signal swings. That's why we love tubes! I'll bet that starts some responses, like poking a stick into a hornet's nest! Have at me...
High Voltage and watches
Yep, I did that; picked up a 1KW ham transmitter we had built 1,400 Volts on the plate cap if memory serves. Touched my metal watch to that cap while holding on nice and tight to the metal chassis.
Wow, best described as having your arm hit with a sledge hammer. Left some little black burn holes on my wrist. Did a good job of making sure I never did it again though
.
There are some spectacular new transistors, IGBT, and MOS-FETs that reach 1200V and well past, not cheap of course and maybe too slow but a lot more drive current than those 833's and you probably only dissipate 500watts in the room instead of a KW
.
Powerex is the first high volatge solid state co. that comes to mind, there are others.
Didn't the old Beveridge ESL have direct drive?
Yep, I did that; picked up a 1KW ham transmitter we had built 1,400 Volts on the plate cap if memory serves. Touched my metal watch to that cap while holding on nice and tight to the metal chassis
.Wow, best described as having your arm hit with a sledge hammer. Left some little black burn holes on my wrist. Did a good job of making sure I never did it again though
. There are some spectacular new transistors, IGBT, and MOS-FETs that reach 1200V and well past, not cheap of course and maybe too slow but a lot more drive current than those 833's and you probably only dissipate 500watts in the room instead of a KW
.Powerex is the first high volatge solid state co. that comes to mind, there are others.
Didn't the old Beveridge ESL have direct drive?
When I said might be better ... I meant better than using bjt or mosfets into a standard xformer - I was NOT saying better than tubes. But it does not hurt to consider all the possibilities!
It seems that for most of us that direct drive electrostsats are dream and somewhat impractical, this leads to a transformer. Just because most amps out there are designed for 8 ohm loads the commercial electrostats are designed to interface with them (with a few exceptions - I once repaired a Accoustat, it used questionable (even for the time) op amps to drive tubes seemed a strange choice of circuitry). We DIY folks should consider what might work better, tubes to transformer to transformer to panels never made any sense at all, an extra lump of iron in there. And silicon (as opposed to silicon dioxide) to iron to panel we should consider what the optimum configuration looks like. Our amps can be special purpose and be all the better for it.
Well I've explained my thoughts a liitle better, but I expect pulling the stick out of the hornet's nest might not help to much!
Of course then there is problem of sourcing the iron - so now you might have to build the panels, wind the xformer, and build an unusual amp ... triple the fun!
It seems that for most of us that direct drive electrostsats are dream and somewhat impractical, this leads to a transformer. Just because most amps out there are designed for 8 ohm loads the commercial electrostats are designed to interface with them (with a few exceptions - I once repaired a Accoustat, it used questionable (even for the time) op amps to drive tubes seemed a strange choice of circuitry). We DIY folks should consider what might work better, tubes to transformer to transformer to panels never made any sense at all, an extra lump of iron in there. And silicon (as opposed to silicon dioxide) to iron to panel we should consider what the optimum configuration looks like. Our amps can be special purpose and be all the better for it.
Well I've explained my thoughts a liitle better, but I expect pulling the stick out of the hornet's nest might not help to much!
Of course then there is problem of sourcing the iron - so now you might have to build the panels, wind the xformer, and build an unusual amp ... triple the fun!
The 1:3 turns ratio (or so) transformer idea for tubes-to-ESLs really does make a lot of practical sense. I never claimed that a DD ESL amp was very practical; I did it to learn and just for the fun of making something like an ultimate statement. I would seriously like to work with a transformer winding expert, which I am not, to make a decent low-turns ratio HV design. It ought to be possible to achieve very high coupling, high primary inductance and low leakage inductance. In other words high bandwidth, although the capacitive nature of the load will be difficult. But in any case better than the two transformer case, I would think.
I didn't mean to make any value judgments, there is a good reason tube amps are still popular.
On the other hand it is perfectly possible to make first class amplifiers out of sand it just may be harder.
Either solution with a step up seems possible and makes more sense than first stepping down and then up again. i.e. If I wanted to built a transistor model I'd start with a couple of hundred volts instead of 50V for my main supply.
One big problem is that the panels have anything but flat reponse and the commercial speakers most likely were tuned with transformers in place.
Of course if it was all easy we wouldn't need the forums.
On the other hand it is perfectly possible to make first class amplifiers out of sand it just may be harder.
Either solution with a step up seems possible and makes more sense than first stepping down and then up again. i.e. If I wanted to built a transistor model I'd start with a couple of hundred volts instead of 50V for my main supply.
One big problem is that the panels have anything but flat reponse and the commercial speakers most likely were tuned with transformers in place.
Of course if it was all easy we wouldn't need the forums
.One of the main advantages of tube amplification for direct drive of ESLs must be that the primary voltage swing on the output transformer is of a similar order of magnitude to that required to drive the ESL. Attached is the schematic of the Quad ESL63 from which it can be seen that the input is connected to 2 step up transformers with primaries in parallel and secondarys in series effectively providing twice the voltage swing. If each transformer was replaced by tube amplifier and the 2 amplifiers fed in antiphase then each amplifier would only have to swing by half of the required voltage which for conventional ESLs may only be in the order of 1.5-2 times the primary voltage and even for the ESL63s may still only be of the order of 3 times. This then has the advantage that each of the amplifiers only has to deliver half the total output power. The other advantage of tube drive with the ESL63s is that the protection circuit could be retained though you would need 2 of them. I hope that all makes sense
http://www.euronet.nl/users/temagm/audio/esl63/esl63_sch2.jpg
Stuart
http://www.euronet.nl/users/temagm/audio/esl63/esl63_sch2.jpg
Stuart
ESL schematic
Typical British product, obscure and possibly clever all at once.
After a quick look I'm guessing the purpose of the 555 is to protect the power amp from ES panel arcing by momentarily placing a short accross the amplifiers outputs? I didn't see where the trigger for TR3 comes from.
I thought I'd read in an earlier post that the required swing was still beyond most typical tubes, something over a kilovolt? So a step up might still be required for any conventional power amp design. i.e you couldn't very well adapt an existing 6550 or KT88 design for this job.
My point being that once you take the step of getting a custom transformer designed and built, other design options become available.
Again I make no value judgements for or against tubes it's more to say that either tube or transistor design may well be equally difficult. The cliche, "no free lunch" seems to the point.
Really old TV sets (1950 to 60's) had very high voltage windings on their huge power transformers, we got that 1400V I mentioned from a vacuum tube voltage doubler so that transformer probably ran 800 VRMS unloaded. Wonder if any of those are still gathering dust in someones attic?
Typical British product, obscure and possibly clever all at once.
After a quick look I'm guessing the purpose of the 555 is to protect the power amp from ES panel arcing by momentarily placing a short accross the amplifiers outputs? I didn't see where the trigger for TR3 comes from.
I thought I'd read in an earlier post that the required swing was still beyond most typical tubes, something over a kilovolt? So a step up might still be required for any conventional power amp design. i.e you couldn't very well adapt an existing 6550 or KT88 design for this job.
My point being that once you take the step of getting a custom transformer designed and built, other design options become available.
Again I make no value judgements for or against tubes it's more to say that either tube or transistor design may well be equally difficult. The cliche, "no free lunch" seems to the point.
Really old TV sets (1950 to 60's) had very high voltage windings on their huge power transformers, we got that 1400V I mentioned from a vacuum tube voltage doubler so that transformer probably ran 800 VRMS unloaded. Wonder if any of those are still gathering dust in someones attic?
So, if you wanted to design a 1:3 transformer, how much power would it have to supply to the esl? Just for discussion, let's say you wanted 2kv p-p audio to the esl and had a 20 meg resistor in series with the high voltage. I believe we can assume a low value for the impedance of the high voltage supply.
P=V squared/R
=(2000)squared/20,000,000
=0.2 watts
That would mean quite a physically small transformer and a low wattage amplifier.
This seems too good to be true, can anyone check the math and my assumptions?
P=V squared/R
=(2000)squared/20,000,000
=0.2 watts
That would mean quite a physically small transformer and a low wattage amplifier.
This seems too good to be true, can anyone check the math and my assumptions?
20 meg resistor in series
?? there is a high value resistor in series with the membrane for constant charge but not in series with the plates - it would form a low pass filter. Don't know what the capacitance of a typical (what ever that is) esl panel is but you need enough current to charge/discharge it (obviously) to follow the signal voltage and I do not think that going through a 20M resistor is going to get you there.
Anyone know off the top of their head what a typical capictance of an esl panel is? Guess I could get out the text and look up the formula and constants and...
Bill
?? there is a high value resistor in series with the membrane for constant charge but not in series with the plates - it would form a low pass filter. Don't know what the capacitance of a typical (what ever that is) esl panel is but you need enough current to charge/discharge it (obviously) to follow the signal voltage and I do not think that going through a 20M resistor is going to get you there.
Anyone know off the top of their head what a typical capictance of an esl panel is? Guess I could get out the text and look up the formula and constants and...
Bill
ESL power requirements
We can make a rough guess;
My ML ReQuest's are spec'd at 1 Ohm at 20KHz. Also at 88dB SPL (from memory I could be off a dB or so)
So 2.83 VRMS (@8 Ohms) to get 88dB SPL will then require 2.83 Amps at 20KHz or 8 watts.
Allowing for a little more volume and some head room seems like 100 Watts would be about the minimum for decent levels, probably 200W would be better. (All this is at 20KHz usually little music power up there, I think the power whould drop linearly with frequency)
I doubt the transformer absorbs much so its not unreasonable to assume almost all the power is to charge and discharge the capacitor.
If I knew the tranformer ratio I could work backwards from the current to get the capacitance.
We can make a rough guess;
My ML ReQuest's are spec'd at 1 Ohm at 20KHz. Also at 88dB SPL (from memory I could be off a dB or so)
So 2.83 VRMS (@8 Ohms) to get 88dB SPL will then require 2.83 Amps at 20KHz or 8 watts.
Allowing for a little more volume and some head room seems like 100 Watts would be about the minimum for decent levels, probably 200W would be better. (All this is at 20KHz usually little music power up there, I think the power whould drop linearly with frequency)
I doubt the transformer absorbs much so its not unreasonable to assume almost all the power is to charge and discharge the capacitor.
If I knew the tranformer ratio I could work backwards from the current to get the capacitance.
A reply from flemming to an older thread at diyAudio:
My els is 40cm X 200cm and the capacitance is 1200pF.
My els is 40cm X 200cm and the capacitance is 1200pF.
ESL capacitances can vary all over the place due to stator spacing and element size, but 1000pF is often used as a design bogey for a mid-sized panel. Imagine the difference in capacitance between a big Sound Labs speaker and a little tweeter cell! The transformer must drive reactive currents into the ESL due to this capacitance, resulting in lots of VA rather than dissipated power (watts). Therefore the transformer will be larger than expected. Plus it will need a lot of primary turns to build up enough primary inductance to maintain bass, and with that big step up ratio, that means a huge number of secondary turns. So it will be much bigger than expected for just that component of power needed for acoustic energy. It will also need hi-pot HV windings on the secondary. If we were to go for a 1:3 being discussed here, the trannie can probably be made smaller. By the way, cone speakers waste even more power in losses having nothing to do with sound, so the ESL just has its own unique challenge.
You are right that the large resistance (20 Meg or so) applies only to the diaphragm bias supply and has no bearing on the audio drive to the stators. BTW, there is a resistive component to the ESL impedance corresponding to the radiation resistance which is the part that dissipates power, in this case dissipated as sound leaving the speaker. Surprisingly, ESLs will appear inductive near the low-end resonance due to mechanical EMF. Professor Hunt’s famous work analyzed these impedance components in addition to the large capacitance.
You are right that the large resistance (20 Meg or so) applies only to the diaphragm bias supply and has no bearing on the audio drive to the stators. BTW, there is a resistive component to the ESL impedance corresponding to the radiation resistance which is the part that dissipates power, in this case dissipated as sound leaving the speaker. Surprisingly, ESLs will appear inductive near the low-end resonance due to mechanical EMF. Professor Hunt’s famous work analyzed these impedance components in addition to the large capacitance.
Thanks, Brian. I'm beginning to understand now. The capacitive reactance is a real problem at high frequencies. The 1000pf esl has a reactance of 16 ohms at 10 khz. 4kv into 16 ohms is a lot of current drive needed.
The use of a step up transformer seems to make the amplifier work even harder as it needs to supply an output current of speaker current x turns ratio. Aaah, you just don't get something for nothing.
I've been trying an output transformer calculator made by one of the diyAudio members. It's a .zip file containing a windows installer. Plugging in different values of transformer wattage and low frquency cutoff really shows the tradeoffs that have to be made in transformer design. You're right about the large number of windings. It shows anywhere from about 800-2000 primary turns, depending on cutoff frequency, core size, etc.
The use of a step up transformer seems to make the amplifier work even harder as it needs to supply an output current of speaker current x turns ratio. Aaah, you just don't get something for nothing.
I've been trying an output transformer calculator made by one of the diyAudio members. It's a .zip file containing a windows installer. Plugging in different values of transformer wattage and low frquency cutoff really shows the tradeoffs that have to be made in transformer design. You're right about the large number of windings. It shows anywhere from about 800-2000 primary turns, depending on cutoff frequency, core size, etc.
VA vs. watts
I'm sure this has been worked out in some detail by others but as I think about the drive requirements for ESL panels I arrive at some assumptions and conclusions that the finshed ESL speaker panels probably can not appear purely capacitive.
Let me attempt to explain my thinking: If you hook a voltage source (an amplifier) to a capacitor the current and therefore power delivered into the load doubles for each octave increase so unless an ESL panel becomes less and less efficient at coupling to the air, the resultant SPL would rise with frequency.
Enter the crossover or compensation network, this network would most likely add inductive reactance to reduce or prevent the rise in accoustic output with frequency. By its very nature this would make the capacitor look more resistive. So the phase angle between drive voltage and ESL panel current is most likely not a simple 90 degrees.
Of course if the compensating network is ahead of the power amp this isn't true, with the exception that the amp would deliver decreasing voltage into the panel with increasing frequency resulting in a more or less constant total wattage being delivered into the ESL panel.
It almost seems that ESL panels should be driven by current sources not voltage sources.
Did I miss something obvious?
I'm sure this has been worked out in some detail by others but as I think about the drive requirements for ESL panels I arrive at some assumptions and conclusions that the finshed ESL speaker panels probably can not appear purely capacitive.
Let me attempt to explain my thinking: If you hook a voltage source (an amplifier) to a capacitor the current and therefore power delivered into the load doubles for each octave increase so unless an ESL panel becomes less and less efficient at coupling to the air, the resultant SPL would rise with frequency.
Enter the crossover or compensation network, this network would most likely add inductive reactance to reduce or prevent the rise in accoustic output with frequency. By its very nature this would make the capacitor look more resistive. So the phase angle between drive voltage and ESL panel current is most likely not a simple 90 degrees.
Of course if the compensating network is ahead of the power amp this isn't true, with the exception that the amp would deliver decreasing voltage into the panel with increasing frequency resulting in a more or less constant total wattage being delivered into the ESL panel.
It almost seems that ESL panels should be driven by current sources not voltage sources.
Did I miss something obvious?
yep its does not behave just like a cap - as Brian posted earlier here. Add to that other stuff in the speaker - I'm most familiar with the quad 67 which is different from most esls but is very instructional to read about no matter what esl you are considering. My quick comments are explained further and much better at the link.
If you look at the Quad 67 - the cap and resistor in the input is to tailor the low responce (the R alters the Q of the bass response - cap shorts for highs) and the delay line not only delays the signal to the rings which makes the diaphragm move as if the source is behind the speaker - as opposed to a piston - but rolls off the higher fequencies - helping with reflections in the mylar controlling the directivity, and I believe therefore reducing the capacitance load at higher frequencies, that is adding some series inductance (sort of).
All very clever - it is explained fairly well at quadesl.org
Bill
If you look at the Quad 67 - the cap and resistor in the input is to tailor the low responce (the R alters the Q of the bass response - cap shorts for highs) and the delay line not only delays the signal to the rings which makes the diaphragm move as if the source is behind the speaker - as opposed to a piston - but rolls off the higher fequencies - helping with reflections in the mylar controlling the directivity, and I believe therefore reducing the capacitance load at higher frequencies, that is adding some series inductance (sort of).
All very clever - it is explained fairly well at quadesl.org
Bill
I'm still trying to figure out what output tube could be used push-pull to drive a step up transformer and esl.
Since my math skills are failing me, I need to simplify the process. Here goes...
If I'm happy with a 10 watt amplifier with an 8 ohm output transformer driving an 8 ohm step up transformer, then I should be just as happy with that 10 watt amp driving a single custom step up transformer instead of the 2 transformers. Assuming no losses in the transformers for the time being.
So the question for anyone that's used a tube amp to drive esl's: How much power do you feel is enough?
Thanks again to all for helping this tube noob.
Since my math skills are failing me, I need to simplify the process. Here goes...
If I'm happy with a 10 watt amplifier with an 8 ohm output transformer driving an 8 ohm step up transformer, then I should be just as happy with that 10 watt amp driving a single custom step up transformer instead of the 2 transformers. Assuming no losses in the transformers for the time being.
So the question for anyone that's used a tube amp to drive esl's: How much power do you feel is enough?
Thanks again to all for helping this tube noob.
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