I always wanted to ask...why Bias voltage on ESL? - Page 2 - diyAudio
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Old 10th August 2013, 02:54 PM   #11
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Quote:
Originally Posted by john65b View Post
Now since the Constant Voltage is 6db greater SPL than Constant Charge, what if I just delete one stator voltage on Constant Voltage? So have my 70:1 step up from amp on the high conductivity diaphragm (I have tons of aluminized 12micron Mylar) and have back stator at -3kv, front stator grounded / floating. From what I am reading, should have same SPL as constant charge ESL, and operate much like a Magnepan Single ended (not push pull).
CV(constant voltage) will give the same SPL as the CQ(constant charge) configuration if operated with the same step-up transformer and bias voltage. It is the inverted CV configuration(with its equal but opposite bias voltages on the stators) that is 6dB louder than either the CV or CQ standard configuration. I'll put together some notes and a few pics on the inverted ESL later today.

If you removed one of the stators of a push-pull CV esl, you would be removing one of the large balancing forces in Attachment #1 and the diaphragm would be pulled toward the remaining stator and most likely collapse to it unless the bias voltage was significantly reduced to a point where the diaphragm tension could over ride the force. SPL output would reduce linearly as bias voltage is reduced.

The other issue is that with only one stator, the force on the diaphragm would be non-linear no matter what it's position.

As a precautionary note, if you experiment with metalized mylar (and imperfectly coated stators) be prepared for arcs, melting plastic, and sometimes even a little flame. In my experience, a large value resistor between the HV supply and the diaphragm goes a long way in reducing the severity of the arcing, but does not necessarily eliminate it.
Attached Images
File Type: gif CV_single.gif (39.5 KB, 147 views)

Last edited by bolserst; 10th August 2013 at 03:08 PM.
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Old 10th August 2013, 03:47 PM   #12
john65b is offline john65b  United States
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Yes, I realize this now. Maybe best to think before I type.

I actually ran aluminized mylar on CLS stators with bias dialed down to around 2000v. It sounded different, did not do any critical listening for very long. I did play loudly for a while with no issues with arcs...

Got the aluminized mylar idea from a DIY effort from Curt...

Building an Electrostatic Speaker
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Old 12th August 2013, 02:25 AM   #13
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Quote:
Originally Posted by john65b View Post
Got the aluminized mylar idea from a DIY effort from Curt...
I believe the owner of those ESLs is here on DIYaudio, user name Legis.
He eventually changed out the diaphragms and switched to high resistance coating to lower the distortion.
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Old 12th August 2013, 03:06 AM   #14
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Default Inverted Push-Pull ESL

Attachment #1 provides a comparison of the basic setup for an Inverted Push-Pull ESL and the Standard Push-Pull ESL. As AcoustatAnswerMan pointed out, the Inverted ESL must be operated in CV mode to work properly over the whole audio band, so we will stick with CV mode for the comparison.

Using the same method as in Post#4 for plotting up relative levels of Force on the diaphragm, we can get an understanding of why the Inverted ESL produces 6dB more output for the same input. Sticking with a bias voltage of 5kV, lets suppose the step-up ratio of the transformer is 1:200. So, if we had a 200W amplifier, it would swing maximum peak voltages of about +/-50V resulting in the +/-5000V on the stators. Attachment #2 provides tabulated values for the diaphragm and stator voltages along with the relative Force vs Input voltage plot for the Standard CV push-pull ESL.

Now, if we switched out the transformer for one with a 1:100 step-up ratio and used it to power an Inverted ESL with +/-5kV bias voltages on the stators, you would get the voltages and relative Forces shown in Attachment #3. Notice that at each input voltage level the voltage difference between the diaphragm and each stator match that of the Standard arrangement from Attachment #1. If you kept the 1:200 ratio transformer you would double the voltage swing on the diaphragm, which would double the total Force resulting in the 6dB higher output as shown in Attachment #4

The "extra" output is coming from the fact that having the 2nd equal but opposite biased stator allows us to leverage the entire output voltage from the step-up transformer against that bias voltage value twice rather than having it split in half and fed to the stators before leveraging against the diaphragm voltage. Compare the +/-50V input lines in the table from Attachment #2 with the +/-25V input lines from the table in Attachment #4

One other thing to notice in Attachment #4 is that even though the diaphragm voltage swings higher than the bias voltage on the stators the total force on the diaphragm from the push-pull configuration is still linear as long as the diaphragm is centered in the gap. This is the case for the standard configuration as well. However, with a single ended ESL your signal voltage needs to be limited to something less than the bias voltage to avoid gross distortion.
Attached Images
File Type: gif PP_Inv_vs_Std.gif (13.1 KB, 135 views)
File Type: gif CV_PPstd_200.gif (51.3 KB, 134 views)
File Type: gif CV_PPinv_100.gif (51.6 KB, 130 views)
File Type: gif CV_PPinv_200.gif (53.3 KB, 130 views)
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Old 25th August 2013, 01:30 PM   #15
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Bolserst, I thank you for taking the time to reply in the manner you have (great attachments!!). I liken these linearity differences between inverted and conventional ESL to common arguments with Planar speakers - the further / closer the planar diaphragm moves to / away from the magnets, the bigger they stray from linearity.

But yet they sound very good....at least to me they do...big Maggie Fan here.

I remember testing some new coating on Martin Logan CLS panel with just the back stator connected. It made music, clean sounding, and was noticeably less SPL than with front stator (for obvious reasons) didn't think much of it then, but after I put it all back together, I thought isn't a single stator CV inverted ESL operating like a planar Magnepan / Apogee (what do you call it - Single Ended Push More - Push Less?)

Now the Magnepan 20.X series has the diaphragm between to sets of magnet poles, and approximates a Inverted CV ESL in its operation, correct?

Quote:
The other issue is that with only one stator, the force on the diaphragm would be non-linear no matter what it's position.
Is this true with single pole planars too?

If you ride the signal over the bias voltage (attachment)?


Maybe all this is obvious to everyone reading, and I am a little slow?
Attached Images
File Type: png Untitled.png (104.9 KB, 32 views)
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Last edited by john65b; 25th August 2013 at 01:54 PM.
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Old 25th August 2013, 02:03 PM   #16
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edit - just realized that last blurb and attachment is not possible...and borderline ridiculous....sorry.
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Old 27th August 2013, 04:17 AM   #17
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I am the builder of the metalized stats you mentioned earlier, and legis is the end user. There is a large resistor in series with the HV-supply and the membrane is heavily segmented and curved thereby controlling the resonance and beaming behavior. Distortion may be a little higher, although it most likely also has to do with the interaction between the transformer and the loading capacitance.

The largest advantage is the complete absence of coating and charging issues, it just always works.
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Old 27th August 2013, 11:40 AM   #18
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And another benefit was lower bias voltage? I had my Martin Logan CLS at around 2kv with metalitized mylar. Never had any arcing issues, but never drove then very hard.
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Old 27th August 2013, 07:53 PM   #19
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Quote:
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Is this true with single pole planars too?
No this is not true for single sided planars.
The force exerted on a current carrying wire in a magnetic field is proportional to the magnitude of the current and the strength of the magnetic field it is surrounded by. There is no limit on the current magnitude relative to the magentic field strength. This is more comparable to CQ ESLs where the force is proportional to charge and strength of electrostatic field. Unlike electrostatic forces, the force on a current carrying wire is orthogonal to both the direction of the current and the direction of the magnetic field. Attachement #1 depicts this relationship where magnetic field direction is from N to S, and current direction is from (+) to (-).

Attachement #2 shows a cross section of a single sided planar magnetic showing the forces generated based on the direction of current flow and magnetic field. Note that the magnetic field strength is not uniform, getting weaker with distance away from the magnets. This results in non-linear force on the wires attached to the diaphragm as it move alternately closer and further from the magnets. However, the force is more linear than a single sided ESL which has squared terms in its force equation.

Attachement #3 shows a cross section of a “push-pull” planar magnetic. I prefer to call it a “symmetric” planar since it is a better description of what the second set of magnets does. They provide a substantially uniform and symmetric magnetic field for the wires to move back and forth in. The main drawback to this configuration(besides added cost) is the potential for a cavity resonance in the mid-to-high frequency range.
Attached Images
File Type: jpg Fmag_rule2.jpg (19.6 KB, 37 views)
File Type: jpg Planar_01.jpg (38.2 KB, 42 views)
File Type: jpg Planar_03.jpg (51.5 KB, 39 views)
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Old 28th August 2013, 12:10 PM   #20
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Once again, thanks!!
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