OTL amplifier and electrostatic loudspeakers


I would like to connect in an amplifier "Atma-Sphere M50" a loudspeaker Quad 988. Are there experiences whether OTL amplifier and elektostatische loudspeakers well function? Thanks for your cooperation!
3eb809b2 01cd 4551 b407 cf2ef57e29c0graphitteller
Hopefully, Ralph will chime in, but in the meantime...
It looks like Quad says 100wpc/sensitivity at 86db, so your Atma's are a little on the low side of power. You should be able to drive them, but I wouldn't crank it up too high.
That said:
I would suggest buying a pair of Paul Spelt Zero Autoformers to handle any low impedance dips of the Quads-I see Stereophile measure a dip to 4 ohms at lower frequencies-not terrible, but why tax your tubes?
If you could relieve the amps from the lower frequencies with a sub/s, you'd probably be good to go.
Bob
Ralph @atmasphere on this board should be ultimate authority @gdnrbob so rightly said

I would try it with and without a set of ZEROs (www.zeroimpedqance.com).
With that speaker our customers seem to like both with and without. Overall the speaker does not have any severe impedance dips, so if you use a set of ZEROs don't use more than 2x.
The ZEROs would mostly be used to restore the highs - the amps otherwise should be fine in the bass. Make sure the speaker is at least five feet from the wall behind them.
@OP. 
Well, there you go...
B
one reason why I appreciate manufacurer presence and expertise here :-)

@tomic601 , as we both own Vandy's. I concur. A great manufacturer stands by their product. Considering that I bought my 3a sigs used, Mr. V. called me back the next day, and even the following day to see if I still had a problem. Now that is exceptional service.

Do you have Atmasphere's?
B
@gdnrbob - no , not yet.....I have my Dad’s old but hot rod MC240 in my “vintage” system  where the 30 wpc or 60 wpc OTL would shine... and then the Treo in a Condo where monoblocks sans tube cages would be in danger of the 4 and 5 year old nephews.... and the Vandersteen M7 amps in my reference system..... so I am for now an ardent admirer and wannabe kit amp builder...
the war department and my sanity stand as temporary obstacles....
graphitteller
  OTL amplifier and electrostatic loudspeakers
"Atma-Sphere M50" a loudspeaker Quad 988



With this sort of power from the OTL’s and an output impedance of 4.1Ω, on a speaker that’s only 83db!!, I would say no. Get the right amp, and not a expensive interim fix with autoformers.

M50-60
60 watts per channel into 8 Ω load
45 watts per channel into 4 Ω load
80 watts per channel into 16 Ω load

https://www.stereophile.com/images/archivesart/quad989fig1.jpg

Cheers George
We have enough customer running Quads that I can safely say that they are a great combination.

We originally designed the amps to be used optionally with an outboard autoformer. Thus years ago we marketed the autoformer as the Z-Music autoformer.

The design of our amps is based on the idea that feedback adds distortion that is easily heard by the human ear as brightness and additional harshness.
So we designed the amps to have good linearity without feedback. Triode power tubes, single stage of gain, class A2 operation, fully differential from input to output and direct-coupled output.
It was obvious that such an amplifier would have a higher output impedance (we of course built amps with feedback, some of them switchable, so it was easy to hear the detrimental effects it had). So the Z-Music was designed to allow the amps to deal with lower impedances. It was pretty effective; in his review of the MA-1 in The Absolute Sound Steven Stone used the MA-1 with a set of Apogee Full Range loudspeakers, which are 1 ohm impedance- and it worked very nicely! Other than impedance, that speaker was easy to drive.

We don't make the Z-Music anymore as the ZEROs had slightly better performance. Neither product was/is expensive, so this has proven over the decades to be an elegant solution for lower impedance speakers coupled to lower powered OTLs and other amps like SETs that normally have a higher output impedance due to no feedback.

We occasionally get the question- doesn't that eliminate the best qualities of the OTL? And the answer is no because an output transformer like the ZERO or Z-Music isn't possible unless the rest of the amplifier already has a fairly low output impedance and no DC at its output (both devices don't block DC as they only have one winding, and the turns ratio is extremely low so no distributed capacitance to kill the bandwidth). As a result, both products have bandwidth that is well in excess of the amplifier used with them- the ZEROs go for 2Hz to 2MHz! Usually the output transformer is the bandwidth limit of the amp but with this system that isn't the case. In the meantime the amp still has all the qualities that make it work as I described above.
 Meanwhile my OTL final amplifiers (Atma Sphere M-50) are revised or repaired. This was made in Croatia. I think it the best man I is in Europe for these works could find. I did not want to let carry out a dispatch in to St Paul (MN / the USA) the dispatch there and back would already have cost 400 US $ to me. One had advised against a dispatch in a wooden box in the USA me in Germany, one meant the wooden box "might not have entered" and in a carton I did not want to send. Thus it has well gone out for me and the devices. I had not played this combination. Also I say quite clearly: The use from ZEROs  would not have been possible for me. If I use ZEROs, I also need no OTL amplifiers. By the introduction the first tacts were already a pleasure. This clarity, this directness, these timbres!!! I could not simply believe it. This combination OTL (Atma Sphere M-50) and the "QUAD ESL 988" works like a magnifying glass all mistakes in the music arrangement before it are shown pitilessly. Maybe one more sentence like me on OTL amplifier has come. In summer, 2014 erschienin to the German magazine "ANALOG" an article of the German Austrian engineer and scientists Dr. Artur Seibt under the title "Tube - unequalled". In he writes basically "... gives no tube sound, the tube is the most neutral construction element of the electronics". Further he writes basically: "... everything what deviates from this neutrality, does not make the tube, sonder the surroundings of the tube". To me, as a studied electrical engineer, was clear immediately what he means:" The source transformers make the so-called tube sound". Since this time I looked in Germany or Europe for payable used devices. In May, 2018 I bought the M-50 amplifiers in fanzösisch languages Switzerland. I must say the time of the search has been worthwhile. Thanks, for it that you have read my long implementation. Thanks also to Ralph Karsten that he has fortified to me here at this point not to try this combination OTL / QUAD ESL (in Europe commonly) because he it knew functioned. 
Hey, he speaks better English than most of us speak German. ;-)

Durch die Kritik: Der gleiche Aufsatz in Deutsch und nicht mittels PC-Programm übersetztes Englisch.


Inzwischen sind meine OTL-Endverstärker (Atma Shere M 50) revidiert oder repariert.

Das wurde in Kroatien erledigt. Ich denke es der beste Mann (Malden Krcelc) den ich in Europa für diese Arbeiten finden konnte. Nach St. Paul (MN / USA) wollte ich die Endstufen nicht senden, es hätte schon 440 US Portogebühren gekostet. Man hatte gegen eine Absendung in einem Holzkasten in den USA mir in Deutschland empfohlen, dies nicht zu tun. Möglichweise hätte man den Holzkasten nicht in das Land gelassen und in einem Pappe-Karton wollte ich einen Verstärker nicht in die USA senden. Ich hatte diese Kombination (OTL Atma Sphere M-50 und QUAD ESL 988) noch nicht gespielt. Auch ich sage ganz klar: Der Gebrauch von ZERO-TRAFOS kam für nicht und niemals in Frage!!! Wenn ich NZERO-TRAFOS verwende, brauche ich auch keine OTL-Verstärker.

Bei der Inbetriebnahme waren die ersten Takte bereits ein Vergnügen. Diese Klarheit, dieses Direkte, diese Klangfarben: Es war ei Genuss!!! Ich konnte es nicht einfach glauben. Diese Kombination OTL (Atma Sphere M-50) und der „QUAD-ESL 988“ Arbeiten wie ein Vergrößerungsglas, alle Fehler in der Musik-Wiedergabe davor wird gnadenlos aufgezeigt! Vielleicht ist ein mehr Satz, wie ich auf dem OTL-Verstärker gekommen bin. Im Sommer 2014 erschien in zur deutschen Zeitschrift „ANALOG“ ein Artikel des deutsch-österreichischen Ingenieurs und der Wissenschaftler Dr. Artur Seibt unter dem Titel „Röhre - unübertroffen“. Darin schreibt er sinngemäß „... gibt keinen Röhrenklang, die Röhre ist das neutralste Bauelement in der Elektronik“. Weiter schreibt er sinngemäß: „ ... alles, was von dieser Neutralität abweicht, macht das nicht die Röhre, sonder die Umgebungen der Röhre “. Zu mir, als ein studierter Elektrotechniker, war sofort klar, was er indirekt sagte: „Die Ausgangs-Übertrager und die passiven Bauelemente sogenannten Röhrenton“. Seit dieser Zeit habe ich in Deutschland oder Europa nach bezahlbare OTL-Endverstärker gesucht. Im Mai 2018 habe ich die M-50 Verstärker in der französisch-sprachigen Schweiz gekauft. Ich muss sagen, dass die Zeit des Suchens sich gelohnt hat. Dank dafür, dass Sie meine langen Ausführungen gelesen haben! Der Dank gilt auch Ralph Karsten, der mich hier an diesem Stelle gestärkt hat, diese Kombination OTL / QUAD-ESL zu versuchen (in Europa unüblich), weil er es gewusst hat, hat funktioniert.