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. |
@geoffkait wrote: " Is A Stradivarius Violin Easier To Hear? Science Says Nope" Maybe not to the average listener, but to a trained, professional violinist? Yes, absolutely! Many years ago I worked at the Banff Centre, which is where professional artists, musicians, & conductors go to hone their skills. Over the years, the Banff Centre has built up a sizeable collection of Steinway grand pianos (mainly through bequeathments). At last count (~2016), they had approx. 115 Steinways. Every Steinway had a name. Musicians and conductors would be in residency at the Centre for anywhere from 3 weeks to 4 months. Some would specifically request their favourite Steinway by name. In 2004 I asked one of the piano techs (who have a contract with the Smithsonian in Washington to repair their Steinways) if someone could actually hear the difference between the pianos. The tech told me that he did a blind sound test with Oscar Peterson in 1974. Oscar, who was the director of the new Jazz Program that summer, was being very picking in selecting a piano for the summer, so the tech arranged to have 20 pianos brought together onstage for Oscar to test. The tech blindfolded Oscar and then played each piano for him. Oscar was able to correctly identify all 20 pianos by name, blind-folded. |
In fact I would love it if I could make the output impedance of our amps lower. The problem I have with solid state is that many semiconductors have a non-linear aspect about them that causes them to have higher ordered harmonics (at low levels, but as I pointed out earlier, the ear is very sensitive to that sort of thing) and hard clipping. The only devices that I have found that don’t are the static induction transistors made by Sony. IMO They had a chance to really set the audio world forward, but in true Sony fashion (which is to come up with an innovation and then shoot themselves in the foot) failed to make a full complement of driver and voltage amplifier devices to go with their rather amazing output devices. Then when you add in high levels of feedback (there are various forms of feedback and I’m speaking in general terms), you end up with micro changes in transient values of the harmonic structure, all smeared out of time and out of level, shape, expression, and so on. This is sometimes...the new detail found in some recordings. People mistake this false data as real and write the given amplifier up as being revealing. Yet one good listen to the given amp by an aware person who has heard differently (and possibly understands what is going on), will drive said person from the given room with hands clamped on ears. The brain and aural learning, generally moves forward. It is the cognitive speed and lack of aural projection (parallel aural paths in forms of known neural discernment and learning)..which makes the difference in this question and answer pairing (in it’s evolution). Ie, the brain has a masking and pre-load issue that it uses to keep aural experiences from taking too long to discern -it pastes in learned data on new and current aural experiences. Learning to defeat that mechanism takes time and awareness. Or, for the intrepid audio explorer -"what has been heard, cannot be unheard". Complex crossovers with low impedance considerations..exacerbate said issues, and make the given high feedback amplifier even more illiterate with fine transient function - and increase false harmonic structure. Which drives those who hear it, even faster.... from the given room. This falseness is, more importantly -- covering up real detail and real harmonics. Using feedback in audio is tricky, at best. This is cognitive function like learning and intelligence. It takes time to learn to discern these things, aurally. Some people ’get it’ sooner, some people ----never do. |
Good thread. I appreciate this type of discussion because it provokes good technical insight as to speaker reaction due to various impedance variables and damping factors. I have ZU Definition IV speakers driven by Macintosh MC60 amps. The impedance curve on the Defs are lowest at 50Hz to around 225Hz, being around 6 Ohms at the lowest, with a prolonged rise through the mids and treble. Damping factor of the Macs is said to be 12. What, if any, difference in frequency response of my speakers should I hear due to the varying impedance curve? And just to throw something else out there, I recently purchased a quad of NOS TungSol 6550s that completely changed the bass characteristics of the speakers. The bass is softer but eminently more tuneful, with considerably more detail and nuance. |
The complexity of the scenario is go great, that all you can do is try it and see if it works for you. It's not quite random, but it is so complex that these scenarios really are individual in nature. The paring can be measured and listened to but fully fleshed out ahead of time and then take that and compare it to how your personal discernment and learned aural function relates? Not so much. Generalizations like horseshoes and hand grenades (close is enough to get some sort of job done), is just about all that is predictable. |
@geoffkait Yes, I did read the article. I don't understand your question though? Nowhere in it are they talking about differentiating between "live" instruments? Which was the basis of my post as I was responding to your statement: " Why couldn’t they pass the test? The same reason why a panel of experts cannot tell the difference between a Stradavarius and any other well made reasonably good sounding violin." I then provided a real life example disproving this statement. |
@Nordicnorm Not sure what you're referring to when you say they were not live instruments. Here's a link to a discussion of the two Stradavarius tests. http://www.thestrad.com/blind-tested-soloists-unable-to-tell-stradivarius-violins-from-modern-instru... |
nordicnorm @geoffkait Hmmm. Interesting. Perhaps there isn’t as much difference between violins? What do I know? I’m only a trumpet player. ;^) >>>>>>I actually think it doesn’t necessarily mean that at all. I think there may very well be significant differences between/among good violins that trained musicians cannot distinguish, or cannot distinguish in a test, in particular a controlled blind test. Some people don’t test well. 😀 My evidence: exhibit no. 1 is that some of the worst sounding audio systems I ever heard were those of musicians. Now, all you musicians out there, please don't send me a bunch of angry emails, I'm sure some of you can hear OK. 😀 nordicnorm That doesn’t mean that musicians are unable to tell the difference between other instruments (as in my Oscar Peterson example). >>>>>Actually, I suspect it might mean that musicians may not be able to tell differences between other instruments. It’s the process or perhaps the test subjects themselves that’s flawed somehow. Can trained musicians tell the difference between cables? Power cords? Fuses? Can you? |
Notwithstanding my personally-held belief that high end stuff sounds better to me, I am willing to concede that some of the claimed superiority I (we) claim to hear in amplifiers or perhaps cables as well is due to the fact that the products we are attempting to A/B are not level matched to within a very tight tolerance. There are just too many people that seem to fail these comparison tests when the products are level matched to ignore the statistic totally. I think. |
Damping factor of the Macs is said to be 12.@dentdog with that much feedback, none at all. OTOH, the fact that the amp runs feedback means it will *sound* a little brighter on top due to added trace amounts of higher ordered harmonic distortion. The emphasis is added on account of on the bench no change in frequency response would be seen. |
What Ralph said https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt This explains why my amps are designed as Class A to 2/3 power. The switching distortion at high output is insignificant compared to everything else. However at very low levels switching distortion is audible (at least on test signals) - so it is preferable to be in Class A at low levels (of less than 1/10 of a watt) The article confirms to me that only carefully designed listening tests at very low levels will lead to audible differences between well designed high quality power amplifiers. Alternatively, stressful loads or extreme power requirements would audibly differentiate the amplifier with better performance. |
