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Old 5th January 2011, 05:35 AM   #11
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If a portable battery-powered solution were my goal I'd go for the direct drive amp with a smps providing the rail voltages. The smps transformer would be much smaller and lighter than two audio transformers for sure. Use SMD wherever possible (it's really not that hard to solder).

If you get to that point you might think of commercialising it (I know I would buy one )
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Old 5th January 2011, 08:50 AM   #12
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Quote:
Originally Posted by kavermei View Post
If a portable battery-powered solution were my goal I'd go for the direct drive amp with a smps providing the rail voltages. The smps transformer would be much smaller and lighter than two audio transformers for sure. Use SMD wherever possible (it's really not that hard to solder).

If you get to that point you might think of commercialising it (I know I would buy one )
I would of checked Geiger-Mueller power supply...
and
Surely enough some people have already taken care of the task:
Bias Supply Powers Low-Power Geiger-Mueller Tube - Maxim
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Old 5th January 2011, 08:58 AM   #13
markusA is offline markusA  Sweden
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Does anyone have irl experrience with this?
Does it actually work?
Is the sound on par with traditional soultions?
Is it scalable?
What are the practical limits?
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Old 5th January 2011, 09:49 AM   #14
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Quote:
Originally Posted by markusA View Post
Does anyone have irl experrience with this?
Does it actually work?
Is the sound on par with traditional soultions?
Is it scalable?
What are the practical limits?
What I would do:
1) make use of L and DS capacitor of MOSFET to get into ZCS mode
2) check Unitrode/TI for appropriate CMOS controller i.e. constant off time
or
use CMOS 555 and VCO (can't recall PN)
In regard to limits - current draw is zero to none - so check the low leakage general switching diodes like bay135 - to save some juice.
Solution by itself is rather starightforward from the times of vibrators or shall I say vibroconverters.
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Old 5th January 2011, 10:06 AM   #15
markusA is offline markusA  Sweden
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Before diving into anything detailed...
- Scalability!
What bias is needed?
Do we need a fixed bias or is it ok for the bias to follow the input?
Is it ok with a "dirty" bias or do we want a pure smooth dc? What are the tolerances?
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Old 5th January 2011, 10:34 AM   #16
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My design so far adjusts from 0 - 600V. It's a PIC micro sending 5V PWM into a small transformer then a voltage multiplier. Though this might be changed to 3.3V (easier to run on a single cell)

I suppose the diaphragm would loose its charge very slowly, so a dirty bias would smooth out on the capacitors of your voltage ladder. You want your bias to stay more or less round say the 500V mark. If it where to change dramatically I think the SPL of the headphones would begin to vary?

My entire design would be surface mount. Solder paste + a cheap gas soldering iron with a heat blower tip = professional looking boards =D.
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Old 5th January 2011, 11:13 AM   #17
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Quote:
Originally Posted by markusA View Post
Before diving into anything detailed...
- Scalability!
What bias is needed?
Do we need a fixed bias or is it ok for the bias to follow the input?
Is it ok with a "dirty" bias or do we want a pure smooth dc? What are the tolerances?
One would just have to make sure that the time constant formed by the HV supply + series resistor and the Cds (diaphragm-to-stators capacitance) is larger than the worst-case "quiet passage" length, otherwise the bias HV would sag during prolonged quiet passages in the music. The bias HV would be modulated by the signal which is not good.

It's easy to "clean up" the bias since the current draw is so tiny. A few nanofarads before the diaphragm series resistor is probably enough.

Careful construction will help since any leakage paths between diaphragm and stators would discharge Cds. In other words, if one can avoid these leakage paths, the diaphragm series resistor can be made quite large (think giga-ohms) helping the diaphragm keep its charge over prolonged quiet passages. These values are realistic -- I made 0.5m^2 panels which work fine with 400Mohms bias series resistance.
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Old 5th January 2011, 11:32 AM   #18
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Multiplier is a peak type rectifier
so in a general sense PWM would not help.
Surely @ small duty ratio it would "work".
You need either boost or inverting topology.
Preferably resonant or quasiresonant.
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Old 5th January 2011, 03:00 PM   #19
markusA is offline markusA  Sweden
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Interesting ideas here.
My interest lies in full size esl's and the thread is about headphones.
You got me thinking but I'm not going to derail the thread.
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Old 8th January 2011, 09:58 AM   #20
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Since efficiency of ESL driver varies with bias voltage, you sure don't want any system that varies bias (like signal-based bias unless with voltage control).

That holds true at start-up as the bias goes from zero to 100%, of course. But it depends on your set-up. For me, I keep my tweeters de-energized because they charge-up in just a minute or two and their absence isn't too distracting during the charge-up period. But for the typical big-range ESL, it would sound awful unless you are playing music that only needs the ESL like Haydn quartets during warm-up.

Ladder bias supplies are very simple but some of these others are complex - esp. the Geiger counter lab-like supply. But that supply has AC filtering at the output.

That's not found in hifi ESLs much (enormous time-constant with the super-high-resistance membranes) but might make sense since the ladders produce a very peaky output.
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Last edited by bentoronto; 8th January 2011 at 10:02 AM.
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