It has long been considered common wisdom that OTL amps are great with the original QUAD ESL. When I had just gotten my first pair of the speaker, I asked Roger Modjeski at an instore seminar for his view on amps well-suited for the QUAD. He had worked on many Futterman amps, and in the 1980's designed his own OTL, which he ended up selling to Counterpoint. He also worked on the direct-drive tube amp used in the Beveridge ESL, fixing some problems he found in the amp when he went to work for Bev. Roger said that though the OTL/QUAD combination was indeed considered by many to be a good synergistic match, he considered it to be the opposite---an OTL being the absolute worst amp design for use with that speaker. The reasons included the matter of the impedance swings in the speaker, of course. Roger has designed and currently offers an ESL with a direct-drive (no speaker transformer) tube amp, which I am dying to hear. |
I was told by out 80+ year old Sydney Quad speaker restorer Otto Major http://www.stereo.net.au/forums/uploads/monthly_12_2014/post-106213-0-29462600-1419899180.jpg who was good friends with Peter Walker, that yes original Quad 57’s were voiced with early Quad prototype tube amps and also maybe Williamson’s?, with not much current ability or wattage and high’ish output impedance’s (low damping factor) From what Otto told me the Quad 63’s was eventually released for sale in 1981 and were voiced around transistor amp/s of the time, and that the Quad 303 and later 405 amp was developed around it, much lower output impedance’s and higher damping factor and current drive than the tube amps were for the 57’s, I think they may have been Williamson’s. http://www.oestex.com/tubes/WW12-1943-BPT.jpg Cheers George |
Not in that sentence, surely! I don't think I jumped the gun; the statement is a common myth and I showed why in my post. You don't need a lot of current to 'control' a speaker, what you need is an output impedance that is sufficiently low, and you don't need 'current' to get that.Ralph you jumped the gun mate, read again, I never tied output impedance and current output together in the same sentence.Then to just complicate what I said above, the current output of an amp comes into as well, to keep the said control over the speaker.This is a common myth. Also to add, an amplifier "with low output impedance", but with not much "current ability" like otl's, have no chance driving a pair of Wilson Alexia's and like, which drop to an EPDR of .9ohm around 100hz (the power region) without serious problems. As an OTL manufacturer I might be recognized as having a bias, but here is the simple truth of the matter: The output section of an OTL has to do the same sorts of things that a solid state amp might do- and that is drive a speaker. It has to be able to make current to do that. Our amps don't have a particularly low output impedance owing to little or no feedback, but if they did, driving this particular speaker really isn't that hard. All of the Wilsons have been fairly easy loads for tube amps in general despite their impedance! We've had very good results with the Sophias, which are a similar load. I've heard that combo many times. I've not heard the Alexias yet. FWIW, the sales manager of Wilson had our amps for some years until an outside organization tried to take over Atma-Sphere about 13 years ago. At that point he sold them to be out of the possible politics. To give you an example of how much current is available in one of our amps, while tampering :) with a set of MA-1s many years ago I noticed that if the output section was deprived of bias, all by itself (IOW not including the filament circuit) it could blow a 15Amp fuse without damage to the power tubes! Certainly, with that sort of current, the power tubes would overheat fairly quickly, but for short (no pun intended) periods of time they can survive much larger amounts of current than one would expect. Our driver section has the ability to drive the output tube grids to about 15Volts positive with respect to the cathodes (which means there can be quite a lot of grid current; this is 15V more than the output section would have on the girds if deprived of bias!) and the output section can be linear in this region. IOW we operate the amplifier class A2. IOW, we have the current, but not the low output impedance. Weird, huh? The fact that the amp does not act as a voltage source is simply because to do so requires about 20 db of feedback. We've certainly built amps that way but never got them to sound as good. Proper application of feedback is a lot trickier than most engineers think! You can't just apply it according to the formulas and expect it to work, because the formulas don't cover everything. This article (be sure to read part 2 as well) does a good job of covering the problems and also proper application, which as far as I can tell is not executed by about 95% of **all** amps employing feedback: http://www.normankoren.com/Audio/FeedbackFidelity.html If the principles laid out in this article were applied in audio today, it would advance the art. So far, its been simpler for us to achieve our design goals by being pragmatic and recognizing that our amps won't drive all loads. But if we can get the amp and speaker to work together, the combination has the ability to cross the line between music and hifi. Roger said that though the OTL/QUAD combination was indeed considered by many to be a good synergistic match, he considered it to be the opposite---an OTL being the absolute worst amp design for use with that speaker.Roger and I don't agree on that one- we have a lot of customers with Quads. I've heard his RM9 (an excellent amplifier BTW) against our amps on ESL63s a lot as a friend used (may he rest in peace) to have them. |
