Direct drive Martin Logans

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#2
Having never tried it myself, I can't give you a definite answer. My first concern would be if the MLs can stand the 5kV bias voltage supplied by the amplifier. If ML uses a lower bias voltage, then I would steer clear of this idea. At best the frequency equalization applied by the servo amplifier would likely need some (or a lot) of tailoring to make it sound right. I seriously doubt this would be a plug-n-play situation.
 
#5
Calvin is right, ML's are very high capacitance, being a plate stator design without electrical segmentation. Iirc they are over 1000pF for a panel. That makes them very current hungry.

As a rule of thumb, you can expect short peaks of full amplitude up to a few kHz in music. 1000pF at 4 kHz equals 3.8kOhm. In order to drive such panel with several kV, you need in excess of 1A of peak current availability from your DD amp. No way any DD amp will deliver that, not even close. Most designs struggle to reach 20mA.

Long ago I designed and built a few DD amps and I made a lot of measurements. My conclusion at the time was that you need at least 100mA pp to drive an electrically segmented fullrange panel to realistic levels. As Calvin mentioned, you have to multiply that by 10 for a non-segmented panel. Which is in pretty good agreement with my ballpark calculation above.
 
#9
I have read about someone who did it with tubes, but he made a special purpose build amp for it. As fair as i remember it was not that easy, the trafo in the speaker somehow compensates for some lacks in the panel.
I was considering it my self but having real high voltage in cables together with my kids and dog seem a bit too dangerous. 😉

Now i'm settled with a Music Lab D200 and a pair of vantage 😉
 
#11
Correct, the panel arrays (3 and 4 panels per speaker) driven by the Acoustat amp were full-range and non-segmented. Segmentation for Acoustat did not start until the Spectra series, and by that time the direct-drive amps were but a distant memory. Can't comment on the total capacitance of the Acoustat array vs. ML panels.
 
#14
Hi,

iIrc the Innox tube amp could be ordered in two variants ... a lower voltage one for non-segmented panels and a higher voltage one for segmented panels.
At one of Munich´s HighEnd shows Innox performed with Silverstatic ESLs, which are of the segmented type.
They featured the EL/PL509/519 type Beam Power Tetrodes, matched and screened to withstand the higher voltages(!) of the segmented panel variant.
Those tubes just have a meager 35W of Anode dissipation (the 6HB5 used in the AudioExclusiv amp, known as "Pütz" featured even lower 18W)
I recall them replacing the tube sets at least about 3 to 4 times within three days!
I assume the same would have happened to the lower voltage amp variant feeding into high capacitance non-segmented panels.
In any regard are most amps I know of not capable of an full power bandwidth much higher than 5khz.
Due to the typical power distribution in music they might just get away with it for most of the time .... but there will be musical events where they simply will run out of steam ... either voltage-wise or current-wise.

jauu
Calvin
 
#15
Yes, those are the limits you'd run into. In my high voltage amp I used a similar 6HS5 current-output stage, current-source loaded by another 6HS5 to direct drive ESL 63's. The saving grace here was that the '63 has transmission lines to drive the various segments which makes the speaker look almost resistive at about 300 to 400k.
It works fine, full power to 20kHz at +/-2400V supply but I'd like a bit more power to play it really loud.

Jan
 
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#16
The Acoustat amps use 6HB5. I sit 12' away. I can get 110 db at my listening position with the Monitor 3s. I listen to them every day. Only one tube has failed in the last 6 years. I've replaced the entire set once just to be safe.
Calvin said:
Hi,

iIrc the Innox tube amp could be ordered in two variants ... a lower voltage one for non-segmented panels and a higher voltage one for segmented panels.
At one of Munich´s HighEnd shows Innox performed with Silverstatic ESLs, which are of the segmented type.
They featured the EL/PL509/519 type Beam Power Tetrodes, matched and screened to withstand the higher voltages(!) of the segmented panel variant.
Those tubes just have a meager 35W of Anode dissipation (the 6HB5 used in the AudioExclusiv amp, known as "Pütz" featured even lower 18W)
I recall them replacing the tube sets at least about 3 to 4 times within three days!
I assume the same would have happened to the lower voltage amp variant feeding into high capacitance non-segmented panels.
In any regard are most amps I know of not capable of an full power bandwidth much higher than 5khz.
Due to the typical power distribution in music they might just get away with it for most of the time .... but there will be musical events where they simply will run out of steam ... either voltage-wise or current-wise.

jauu
Calvin
Click to expand...
 
#17
When I was repairing my Beveridge 2SW2 I found that the power supply transformer is a source of error.
this DD amp is working with +/- 3200Volts.
And to achieve that a voltage doubler is used. DD amp is possible to build but you really have to know what you are doing. Resistors, caps, transformer, diodes, tubes, wiring, PCB coating, everything needs attention. Transformer is really hard to find. I tried to get some replacement.... i did not succed. Had to do do a work around.
 
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