Hi all
Anyone considered using a Class D for driving electrostatic speakers directly??javascript:smilie('
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smash
As far as I can see, the concept would be very efficient and the capacity of the speaker elements could be part of the output capacitance..
Of course the high voltages involved could be a problem , but not unsolvable?
Best
Koldby
Anyone considered using a Class D for driving electrostatic speakers directly??javascript:smilie('
')smash
As far as I can see, the concept would be very efficient and the capacity of the speaker elements could be part of the output capacitance..
Of course the high voltages involved could be a problem , but not unsolvable?
Best
Koldby
This would indeed be tempting since the ESL by itself has quite a reasonable efficiency. This has already been proposed by John Watkinson BTW.
It is not easy to build high-voltage class-d amps for several reasons though.
Since many conventional amps are having problems with ESLs anyway, a good start would be to use any of the known class-d amps available and design its output filter accordingly. I think such an ESL specialist amp would sell well (or why not design an amp with an output-filter than can be easily exchanged by the user in order to match the desired load best ?).
Regards
Charles
It is not easy to build high-voltage class-d amps for several reasons though.
Since many conventional amps are having problems with ESLs anyway, a good start would be to use any of the known class-d amps available and design its output filter accordingly. I think such an ESL specialist amp would sell well (or why not design an amp with an output-filter than can be easily exchanged by the user in order to match the desired load best ?).
Regards
Charles
I was once involved in studies for a switching amp that swings between 0 and 1000 Volts (for a piezoelecric actuator). It is amazing what amounts of power would have been wasted on the snubbers for instance (proportional to V^2 !!!).
But the idea of an amp that is optimised to drive ESLs (on the low-voltage side however) is still tempting.
Regards
Charles
But the idea of an amp that is optimised to drive ESLs (on the low-voltage side however) is still tempting.
Regards
Charles
JohnW
A couple of PL509 in a faraday cage ? Sounds good to me..3Kv is on the high side...as I remember Quads only needed about 750V. I Know the static charge is far higher, but you cannot get close to that without argching.
BTW isen't it more easy to control EMC when the currents involved is small even though the voltage shift is large??
Konrad
No need for a cascode. Just totempole arrangement as in normal Class D..
Best
Koldby
A couple of PL509 in a faraday cage ? Sounds good to me..3Kv is on the high side...as I remember Quads only needed about 750V. I Know the static charge is far higher, but you cannot get close to that without argching.
BTW isen't it more easy to control EMC when the currents involved is small even though the voltage shift is large??
Konrad
No need for a cascode. Just totempole arrangement as in normal Class D..
Best
Koldby
I've the big ESL from Stax and I drive them with a Stax amplifier. Only a few amplifier can drive easely a so difficult load.
I've the project to build an amplifier and connect it directly to the stator. I have thought about a PP 845 but 1250v is not enought.
I've to find tube supporting 3000v or more with 80 - 120W power capability.
brice
I've the project to build an amplifier and connect it directly to the stator. I have thought about a PP 845 but 1250v is not enought.
I've to find tube supporting 3000v or more with 80 - 120W power capability.
brice
The Accustat Servo amps use 6HB5's on about 3Kv rails, circuits are on the web.
Could design a class D 150V OPS and filtering in place of the Accustat transistor driver stage, and then drive the 6HB5's directly, but where to return the FB from the output. Due to the very high impendance of any FB circuit, even very small capacitances will cause signifcant HF rolloff.
John
Could design a class D 150V OPS and filtering in place of the Accustat transistor driver stage, and then drive the 6HB5's directly, but where to return the FB from the output. Due to the very high impendance of any FB circuit, even very small capacitances will cause signifcant HF rolloff.
John
This has been done to a degree with valves.
The T+A Criterion TCI 1RE (what a name) uses standard moving coils for midrange and bass duties but uses a dedicated electrostat panel for the tweeter. The speaker contains a small valve amp that drives the tweeter panel directly without a step up transformer.
The T+A Criterion TCI 1RE (what a name) uses standard moving coils for midrange and bass duties but uses a dedicated electrostat panel for the tweeter. The speaker contains a small valve amp that drives the tweeter panel directly without a step up transformer.
This is also used full-range by Beveridge BTW. The idea of a high-voltage tube amp driving an ESL is rather old hats.
New is only the idea of doing a switching amp that way.
You would have to deal with the freewheeling current of the filter inductor in case of a switching amp. But tubes alone cant cope with this so keep this in mind. anbd before someone mentions an inductorless switching amp: This is one of the worst ideas one could come up to if the load is an ESL. While it can be done with dynamic drivers (I don't even like it in this case) this would be a no-no for ESLs.
I would still vote for a conventional (i.e. non high-voltage) class-d amp whose output filter is optimised for driving ESLs via step-up transformer.
One thing might be interesting: The inclusion of the step-up transformer into the NFB-loop.
Regards
Charles
New is only the idea of doing a switching amp that way.
You would have to deal with the freewheeling current of the filter inductor in case of a switching amp. But tubes alone cant cope with this so keep this in mind. anbd before someone mentions an inductorless switching amp: This is one of the worst ideas one could come up to if the load is an ESL. While it can be done with dynamic drivers (I don't even like it in this case) this would be a no-no for ESLs.
I would still vote for a conventional (i.e. non high-voltage) class-d amp whose output filter is optimised for driving ESLs via step-up transformer.
One thing might be interesting: The inclusion of the step-up transformer into the NFB-loop.
Regards
Charles
ESL switching amp
There is one good reason to try a switching amp for an ESL.
Coil driven speakers are unbelievably inefficient (1% of energy converted to sound or so?) mainly because, to create a magnetic field, they first have to waste energy in the coil, in the form of heat.
ESLs do not have this heat issue. But their "efficiency" is no better. How is that possible? Because they suffer from "quadrature"...what is that? In a capacitor voltage and current are 90 degrees out of phase, meaning in parts of the sign wave large amounts of current run through the capacitor with no voltage between the legs, producing no power since P=V x I. Unfortunately that I is still real power for the amp, so while the speaker wastes no energy, it does get wasted in the amp.
For years I have been thinking whether a special ESL switching amp can be built, which stops current flow during that part of quadrature? If successful we could have a 2W amp driving an ESL to realistic sound levels! Finally a truly portable speaker.
There is one good reason to try a switching amp for an ESL.
Coil driven speakers are unbelievably inefficient (1% of energy converted to sound or so?) mainly because, to create a magnetic field, they first have to waste energy in the coil, in the form of heat.
ESLs do not have this heat issue. But their "efficiency" is no better. How is that possible? Because they suffer from "quadrature"...what is that? In a capacitor voltage and current are 90 degrees out of phase, meaning in parts of the sign wave large amounts of current run through the capacitor with no voltage between the legs, producing no power since P=V x I. Unfortunately that I is still real power for the amp, so while the speaker wastes no energy, it does get wasted in the amp.
For years I have been thinking whether a special ESL switching amp can be built, which stops current flow during that part of quadrature? If successful we could have a 2W amp driving an ESL to realistic sound levels! Finally a truly portable speaker.
Can't be done while remaining linear. You're trying to reproduce a voltage on the plates (actually a charge, but charge is directly linked to voltage and capacitance), a capacitor has a certain impedance and will draw a certain (90 degrees out of phase) current when you apply a voltage, there's nothing you can do about that.
Luckily buck converters (the standard class D amplifier topology) can be full four-quadrant amplifiers. They can return the reactive current to the supplies instead of dissipating it. How do you think supply pumping works, this is no different, just capacitive instead of inductive.
Edit: I was typing while Eva posted, my post refers to toptip's post and panomaniac's relpy.
Luckily buck converters (the standard class D amplifier topology) can be full four-quadrant amplifiers. They can return the reactive current to the supplies instead of dissipating it. How do you think supply pumping works, this is no different, just capacitive instead of inductive.
Edit: I was typing while Eva posted, my post refers to toptip's post and panomaniac's relpy.
Switching ESL amp
Many ESLs will probably work fine under 500V, just not the bass panels.
Another idea is, why not use a small ferrite core transformer to up the voltage at 200-400kHz, avoiding the large transformer lower frequencies require? This may not work though as we still need to get the lower frequencies through and modulating them with a higher frequency may not quite get them through a HF transformer.
Many ESLs will probably work fine under 500V, just not the bass panels.
Another idea is, why not use a small ferrite core transformer to up the voltage at 200-400kHz, avoiding the large transformer lower frequencies require? This may not work though as we still need to get the lower frequencies through and modulating them with a higher frequency may not quite get them through a HF transformer.
How about a self oscillating class D amplifier with post-filter feedback using the leakage inductance of the transformer as output filter L and the capacitance of the ESL as output filter C? Transformer has to be designed to resonate in the 10Khz to 30Khz range together with ESL capacitance, but frequency response is flat up to Fsw/2 because the filter is inside the feedback loop and it's linearized.
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