Martin Logan frequency correction???

Hi experts,

I have read some different opinions regarding the way the curved ESLs of Martin Logan work.
Some claim that the crossover of for example the Sequel2, has frequency corrective elements. Others claim that this is not the case and it is all about impedance correction.
Because beaming of high frequences is largly reduced by the curved design, one should expect that there is only little correction needed in the frequency response.
Who knows about this???
If you post the schematics it would be easier to see what is what.

The curve reduces slightly the HF beaming, but not entirely.

In general the ML's interface - at least the ones I've seen - are somewhat inferior to some of the patented schemes that have been used for matching the xfmr(s) to the cells... for this reason they ended up with a hyper low imput Z with their original CLS design...

Generally speaking, you can't "match" a step up device to a given "full range" cell and still have good sensitivity - the tradeoff is sensitivity vs. bandwidth. This was shown nicely by Peter Walker in his 1950s articles on ESLs in Wireless World. Similarly, if you match for good HF extension, you'll probably have weaker bass... which will "work" if you knock the HF down so that you end up with a nominally "flat" response... this may be what ML was doing with the CLS. Not sure.

_-_-bear :Pawprint:


Martin You guess right. The Sequel 2 needed quite heavy equalizing. First of all the frequency response of the panel itself wasn´t that linear. Second the quality of the audio trannies was quite cheap, with a too great leakage inductance, that lead to a resonance peak around 7kHz with a -12dB slope above that resonance frequency. See the attached pic.
C1 in togetherness with L1, R1 and the ESL gives a very steep highpass response with a lot of overshoot (R1 is used to damp the overshoot) to correct the phase cancellation of the panel itself. R5 is used to get the efficiency down to woofer level and C2 corrects the falling response above 7Khz. The notch (C3, R3, L2) corrects the midrange around 3kHz.

black: principle frequency response of panel and trannies
green: electrical frequency response of ESL with crossover but without C2 and notch
blue: electrical frequency response of ESL with crossover but without notch
brown: electrical frequency response of ESL with crossover but C2=5µF
orange: electrical frequency response of ESL with crossover but C2=10µF
red: electrical frequency response of ESL with crossover (original, C2=15µF)



  • sequeliisimu hf-circuit b.jpg
    sequeliisimu hf-circuit b.jpg
    93.1 KB · Views: 1,464
different responses


this pic shows the frequency responses of the Sequel2 panel with and without crossover and using the original audio transformer or the excellent Amplimo toroids.

black: Panel with original trannie without crossover
blue: Panel with original trannie and crossover
green: Panel with Amplimo trannie without crossover
red: Panel with Amplimo trannie and crossover



  • sequel2 simu1.pdf
    17.4 KB · Views: 345
different responses


this pic shows the same responses but to compare the quality of the trannies.

black: Panel with original trannie and without crossover
blue: Panel with original trannie and with crossover
green: Panel with Amplimo trannie and without crossover
red: Panel with Amplimo trannie and without crossover

Without crossover the superior high frequency response of the Amplimo (greater bandwidth) is obvious. The linearity of the panel itself is not the best.
With crossover the differences are much smaller. Like in a clockwork the corrective features of the crossover help to minimize the linearity probs of the panel and the low quality of the trannie. The overall result is very accaptable. Nice work indeed ;)



  • sequel2 simu2.pdf
    17.5 KB · Views: 254


here´s a comparative measurement of my own DIY panel versus a Prodigy panel (which is roughly half the size of the Prodigy)

I use a curved panel with smaller holes and greater opening factor and an even smaller spacing than ML. Therefore the efficiency of the panels is nearly the same.
My film is thinner than ML´s which could be the reason for the extended bandwidth and higher efficiency above 8kHz. The fundamental resonance is higher with the ML because the thicker film can be stretched harder. It is quite clever of ML to ´use´ the frequency response of the panel down to the fundamental resonance. With a damped 12dB HP filter You can get a very linear amplitude response together with very high efficiency without the need of a lot of corrective crossover components. Dynamic headroom is exceptional because of the hard stretching of the membrane and the low diaphragm travel above 200Hz.
Such panels can play easily soundlevels above 110dB and can even reach very high efficiencies of around 100dB/2.83V/m



  • vergleich ml prodigy-esel.pdf
    19.3 KB · Views: 374

The measurements are very interesting. But I always thought that elektrostatic panels itself have a linear frequency response automatically (at least to a very large extent). Only limited by the mass and tension of the film. How is such a non flat response possible (like the Sequel). Where is the difference between the Sequel and the Prodigy? Mechanical difference?

Thanks for each explanation.

Flat response


ESL´s show good linearity measured under nearfield conditions, but not necessarily under farfield conditions. Apart from room interactions there´s phase cancellation that spoils linearity below a certain frequency thats determined by the physical properties of the ESL. Furthermore standing waves occur on the membrane depending on the size, diaphragm tension, diaphragm properties and the surrounding of the membrane (e.g. a 17cm wide diaphragm will nearly always show unlinearities around 1kHz and 2kHz).


It´s interesting, that elektrostatic speakers are not automatically nearly perfect.
So I probably have to repeated optimize and measure my future first DIY-ESL. :(
Does it also makes a difference from which material the spacers are? Maybe the damping from a double foam tape? Has the prodigy also a foam tape at the edges and a hard material (plexi?) for the inner (horizontal) spacers?

Best Regards


I guess the spacer material does make a difference in the cost-department. The foam-tapes are quite expensive though easy and cheap to work with. But the ML Designs are very optimized with regard to manufacturing ease and costs. So the hard plexi will probably save some cents. Another reason may be that the tapes come in just a few thicknesses and it might be that the right thickness is not in production. A third reason might be that the thickness of the spacer depends on the mechanical forces impressed by the diaphragm tension. The Prodigy´s fundamental resonance is at app. the membrane is stretched very hard. This strong force may compress the ´inner´ spacer.
To my knowledge the outer spacers are made from hard plexi too (but app 50% less thick), the same as it is for the outer frame.
Don´t know if the hardness of the material plays any importnat role with regard to damping resonances. Guess the effect is negligible in praxi.



2001-10-23 8:44 pm
Hi Calvin,
do you have any experience with MLs like the Aerius, SL3 or the newer Aeon? I am interested to buy one of these used an ask myself how is the linearity of the panel, the quality of the transformer and the senitivity in these models. Any tuning hints?

Do you think it´s useful to do the frequency correction active with an parametric EQ? An active XO with a (steeper) slope seems interesting to me as well.


Hi Calvin,

Great measurements. I didn't expect to read about such experiments with the amplimo-transformers in the ML's! Very interesting.
I have 4 amplimo's ST105PP each with 1:50 step-up. So I can't wait to fit a pair of them into my sequels.
Did you use the ST105PP or the other type with 1:75 step-up?
What kind of audible effects should I expect? (subjective, I know)

martin logan crossover

Hi all,

It took some time, but I managed to draw the crossover of my sequel 2. It turned out to be two crossover sections, one for the bass and one for the esl, parallel.

The bass section is quite simple. A large coil in series with the woofer. Parallel there is a large capacitor and also parallel a resistor in series with a switch (bass control).

The x-over for the esl is much more complicated:
Sorry no picture but I try to explain:
- thirst the audio signal passes a capacitor. After the capacitor (and parallel to the primary taps of the transformer there is:
1) capacitor, 2 Ohm resistor and coil in series
2) 5 Ohm resistor, 1 Ohm resistor and coil in series;
between the 5 and 1 ohm resistor, a capacitor is connected with the input.

The secundairy part of the transformer is STRANGE to me.
The center tap is connected with the zero of the high voltage unit , no surprise.
Only one of the secundairy taps is connected with a resistor before being connected to one of the stators. Why not a resistor on both the secundairy taps??????????Amplimo transformers must have a resistor on both secundairy taps as is advised by amplimo. Has this something to do with the curved design (asymetrical spacers?)
Also there is a resistor between the two secundairy taps.

I know that the value of any secundairy resistor is matched to the type of audio-transformer, so what to do when installing the amplimo's?

By the way, there is no way to get the amplimo within the original enclosure without doinig some surgery. To little space there!

Greetings, Martinjan


2008-02-24 9:47 am
Hi all,

Is it possible to deconnect or bypass the crossover, so that the ESL can be used full range?

I would like to use the Sequel2 (without their woofers) with a Sub-woofer (REL Strata2).

It would be:
From 10Hz to 100Hz: Rel Strata
From 100Hz to 22kHz: Sequel2

sounds feasible or ...