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My nephew wants the Magnepan MG10/QR speakers system with the dspNexus and two 2x12 servo subs, so was looking for another pair of planars to replace them.

Found a pair of original Magnepan 1.7 speakers on FB Marketplace in the area for $100 for repair. This was the first Magnepan speaker to use all quasi-ribbons for planar drivers when it came out. One speaker has an open super tweeter quasi-ribbon. Magnepan sells a repair kit for the aluminum foil quasi-ribbon speakers, so ordered a kit. You purchase Acetone separately to remove the old glue to detach the quasi-ribbon material to install the new material for repair. Will pick it up in MN since I am here to visit Danville Signal this week.

Measured the correctly working speaker setup for measurement with the original passive crossover and here is the frequency response plot on center of the vertical tweeter axis. Once I have the second speaker repaired, will do a comparison frequency response plot.

This is a 3-way design with a woofer/mid planar, a tweeter planar and super tweeter planar. The tweeter and super tweeter crossovers are connected in series. My understanding is that the super tweeter is a 1 Ohm load. This is perfect to try a current output power amp for that planar, instead of a voltage output amp.

Amps like the Acoustat TNT200 and modern Hafler Transnova amps both designed by Jim Strickland are transconductance amps designed to drive electrostatic speakers. These are highly capacitive loads that go to very low impedance at high frequencies. Sounds like a good fit for this planar super tweeter.

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Walt

Walt,
I understand your concern, but here are my ideas.

Planar Dynamic speakers like the Magnepan's are basically giant resistors, unlike an electrostat that is basically a giant capacitor with a step-up transformer that lets them be driven by stanadard volatge output amps and then bias the stators. Planars dynamics like current drive as they are in the 84-86dB SPL/1w/1m sensitivity range.

Jim Strickland designed the Acoustat electrostatic speakers along with the TNT200 power amp to be a match for the load impedance at high frequencies that the speaker impedance really represents.

There are very good papers on the use of current output amps instead of voltage output amps for low impedance loads like ribbons. Even Nelson Pass has an F1 amp that is current output. It replaces the output transformer with the current output stage. This is a current feedback amp, so it monitors the current through the load. The transconductance part is the Vo=Io/Vin, in seimens. The load is in the feedback loop and works extremely well for things like servo subs. This is just a HF application of the design.

The 1.7 HF crossover load is the 4.5 Ohm tweeter in series with the 1 Ohm super tweeter, not something a standard voltage amp wants to see, just like the original Apogee speakers with their full range ribbon technology. To do this there is another capacitor in series with the super tweeter to limit the HF range of the driver. Low Frequency excursion is not the forte of planar drivers, so rolling them off faster than one pole gives improved power handling over the drivers usage range.

Besides, I already have a balanced input current output amp design that works, so why not use it with the super tweeter with the fuse for over current protection. I have the fuses you recommended, so will put the second pair to work. Another interesting thing is that current output amps really do not care about the speaker cables and fuses, since they are also in the feedback loop.

[quote="HAL"]Walt, Jim Strickland designed the Acoustat electrostatic speakers along with the TNT200 power amp to be a match for the load impedance at high frequencies that the speaker impedance really represents.

And so it was marketed. I had a pair of Acoustat Model 3s that when driven with a 400-watts/channel SS amp. (Threshold 400) could easily be pushed to clipping. Why? The Threshold could deliver bucket-loads of current without breaking a sweat -- yet the voltage levels delivered to the screens wasn't enough. I then moved on to Jadis Defy 7 that was rated at only 100-watts/channel. With the screens connected to the 16-ohm taps the Jadis drove the piss out of those screens. Later -- I had to run a little 50-watts/channel KT-88-based amp temporarily because of a Jadis power transformer failure. To make a long story short -- the 50-watt/channel amp with auto-transformers (8-ohms in/32-ohms out) provided very powerful playback that seemed to have no limit. WOW!

The Magnepans have the opposite requirement.

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Walt

Maggy's are not electrostatic. There is no high voltage power supply for this speaker. They should be considered as a conventional driver just a low impedance one.

WHAT?! You're kidding! WOW!

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Walt

No, he's not kidding. From what I understand, Magnepans are not electrostatic speakers, but are usually referred to as planar-dynamic/ribbons.

An amp like the Thresholds are a voltage output amp with high current output capacity. They do not like capacitive loads like electrostatic speakers as they disrupt the feedback circuit. They do not like low impedance loads typically with large phase angles. Possibly Krell that use to drive Apogee ribbons directly that were 0.75-1.0 Ohm loads were ok since they do not have large phase angles for the nearly resistive impedance. This speaker does not need that much power for the planars.

A voltage output amp tries to have a 0 Ohm output impedance for the load so the back EMF looks into as close to a short circuit as possible. A current output amp tries to have an infinite output impedance for a load. They do not care about the load impedance and can be dead shorted, since the current through the load is monitored by the feedback network. Totally different than a voltage output amp presented with a short on the output.

The autoformer is isolating the load impedance from the SS amp voltage output stage and has a high output impedance, that is more like a current output than voltage output amp. Makes sense the electrostatic load would work better. McIntosh has used them for decades with SS amps outputs.

Since I like Nelson Pass amp designs and he has designed 2 current output amps for First Watt, might as well give it a shot, since I already designed one with a Chip Amp. Pizza Box Amp 3.0!

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Walt

Being a retired engineer and an audio hobbyist now for over 60 years do any of you really think I would be that ignorant?

Careful.....Asking a question like that might not get you the answer you expect. (I know you're not ignorant, but...)