Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?
@ramtubes --- Roger, thanks for responding. Yes, ... there is only one bias pot to control the bias of a pair of KT-150s. The bias pot is used to adjust the bias in mVs of the set tube. ARC recommends 65mVs. The other tube in the pair is a slave. If the tubes are well matched, the slave will measure 65 mVs, plus or minus 3 or 4 mVs.
No, … I do not have a schematic and I do not know if the drivers are direct coupled to the output tubes. As you probably gleaned from the ARCDB website, each pair of KT-150s are driven by one 6H30 driver tubes; 4 in total.
Does the following ARCDB excerpt give you a clue about your direct coupling question? The ARCDB website states that the " power-supply energy storage has been doubled to some 1040
joules. All-new interstage coupling capacitors using technology and
materials first incorporated in our Anniversary Edition
preamplifier effortlessly link input stage to output stage, which
is powered by two matched quads of Russian KT120 [KT-150s in the SE version] output tubes
driven by 6H30 twin triodes. There is more than ample multistage
solid-state regulation. Output stage coupling is a combination of
“ultralinear” and Audio Research’s patented 'partially cathode-coupled' topology, which is superior
to conventional pentode or triode operation."
I seem to recall reading that ARC runs the KT-150 at half that max amount. The New Sensor website says that the KT-150 "CARRIES A 70 WATT PLATE DISSIPATION RATING WHICH PROVIDES FOR PUSH-PULL AMPLIFIER DESIGNS UP TO 200 WATTS OUTPUT." I believe that a quad of KT-150s in the Ref 150SE is configured to produce only 150 watts. So I surmise that ARC chose the tube because it sounds good and is pretty robust. And ARC is not pushing the KT-150s to the max Q limit.
As far as sound is concerned, I ran my amp with KT-120s and KT-150s. IMO, there is a world of difference in terms of sonic saturation and a sense of bass extension and control. In plain terms, the KT-150 is a great sounding tube, but expensive.
Your comment about tap selection is interesting. In a post directed to Ralph Karsten I mentioned that my speaker are nominally rated at 8 ohms, but that is a misnomer. The impedance and phase angle curves are rock and roll.
In particular, impedance drops below 8 ohms between 65 and 700 Hz and below 4 ohms between 70 and 150 Hz. There is a big impedance peak between 600 and 6000 Hz. The good news is my speakers are rated at 92 db sensitivity.
Based on a Stereophile article done on the Ref 150 some years back, Atkinson suggested that one try the 4 ohm taps for the reasons you suggested above. Weird thing is that I have gone back and forth between the 4 and 8 ohm taps any times and I keep settling on the 8 ohm taps. Distortion and damping factors considerations aside, the 8 ohms taps sound better, even though I sense the amp is adding a little not too unpleasant flavor to the presentation.
And yes, …. please keep me in mind that if you decide to expand your tube repertoire to include KT-150s. I sense it is the power tube de jure for many manufacturers and as the KT-150s age, the need for replacements will make a market for replacement tubes.
Care to guess what the useful life of the KT-150 is in my amp given what I posted above? Is there any way to measure tube life and quality with a tester?
Amen to this Tube Amp post-we need more engineers on here...
HI hemigreg,
Your have brought up some good points here and I hope you take my expansion on them as a compliment. Many of the readers of this thread have been pulled into impedance and power thinking by some new paradigms that I do not particularly agree with. So here goes.
We do need more engineers on here, for one thing to keep the known laws of electricity correct in our discussions. Some manufacturers like to make up stories to defend their equipment's performance, or lack there of. I am always entertained when John Atkinson, a very strait shooter, measures an amplifier and finds horrible results. It is unwise to send JA and amplifier that measures poorly and then have to defend it in the Manufacturers Comments.... which are even more amusing.
QUOTE :;"’6-18-2018 2:05amThis is an old topic...literally. In the times when tubes were dominant amplification devises, the output transformer was a necessity (one might say a necessary evil. The tubes are amplifying voltage, so that they work with high impedance loads. The loudspeakers normally have quite low impedance. Thus a transformer is required to match the output of tube amplifier and the loudspeaker. When first transistors have emerged, the schematic design did not evolve immediately. Thus, early transistor amplifiers were very similar to the matured at that time tube amplifiers.
I think its safe to say that when engineers first got transistors they didn't know what to do with them and the evolution of design topologies took awhile to develop into what we have now. I was learning transistors at the same time and had the same early RCA and Motorola books. I was 14 years old and it really wasn't all that difficult to get something that worked. We were all making the circuits out of those manuals which were the most advanced at the time which was 1965. However those schematics did not look like tube schematics at all.
However, over time it was realized that transistors work better and amplifiers of current as opposite for tubes which work better as amplifiers of voltage. Thus, transistor amplifiers can and do work well with low impedance load such as loadspeaker directly.
While we consider that transistors are indeed current amplifiers they do not respond linearly to voltage inputs as tubes do. Pentodes act like transdconductance devices which means their current is proportional to their input votage which is a very handy thing to drive a speaker since we start with voltage (from the preamp) and want current for the speaker. Speakers are current driven devices by nature of their physics.
What makes tubes need a transformer and transistors not is the fact that high current transistors come easily but high current tubes do not The most popular transistor in the world is the 2N3055, which Dynaco, NAD and many others used. For $1.50 it can do 15 amps of current. The best Horizontal Output tubes (mistakenly called Video Tubes because they were in TV sets) can only do one amp of current but they can do this at a very high voltage, thus the need for the transformer. We are really not matching impedances here, we are using the transformer to exchange voltage for current, as all transformers do. Indeed output transformers are difficult to make, more on that if people want to know, and expensive because they have to get the best workers in the factory to make them. I had one guy I trusted and he made all the RM-9 outputs with exceedingly consistant results.
Now, still transformer (autoformer) can assist with the loudspearkers with various impedance...i.e. 4 Ohm vs. 6 Ohm vs. 8 Ohm providing more stable load to a SS amplifier. However, transformer alone is not a perfect transfer devise and making quality transformer for audio output is difficult and costs a lot of money. The typical issues with output transformers are: reduced damping factor and difficulties with driving complex loads, slew rate reduction, additional distortions, etc.
This is the biggest myth being promoted in this threat. An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier. A good transformer does not reduce damping factor appreciably, or slew rate or change distortion except at the frequency extremes. M6 iron is extremely linear, low hystersis, and low eddy current loss. Those fancy amophous irons don't make transformers appreciably better.
What Transformers do is present to the amplifier a particular voltage and current that the output devices like. The taps on tube amps are there for that purpose. You might as well call them A, B and C and choose the one you like best. At least you have a choice. What an Audoformer does is give you that choice with ampifiers that do not have taps.
The RM-10 produces an unbelievable 40 watts with one pair of EL84/6BQ5 tubes (typically 17 watts) at less than 1% distortion with an 8 ohm load on the 8 ohm tap. Many people do not need 40 watts and they can get 25 by putting their 8 ohm speaker on the 4 ohm tap resulting in twice the damping and 1/10 the distortion. Most people do this, I encourage them in the manual to try it and it works on all well designed tube amps. Try it!
Sorry this is long, but there is a lot of work to be done on this topic.
I looked up your amp on the ARC database but no schematic for the 150. Great site for those who are curious about ARC gear. From what you tell me it appears there is only one bias pot per channel. Do you have a schematic for me to look at? I need to know if the driver tube is direct coupled, I do that in the RM-200 and they did in some amps.
I was the first to computer test tube and match them to very tight specs in 1982. I discoverd the Two Point match where once we find the grid volage for a particular current we then find the transconductance. If one matches both of those numbers the tubes will track over a wide range or voltages and currents. Others, as far as I can tell, still set a grid voltage and get a current but they aren't doing what I am doing.
I also test for grid leakage, which is one aspect of what ARC used to call "Low Gas". What they wanted was tubes that didn't run away when they got hot as ARC tubes do..But tubes usually arent Gassy when new, they get gassy from running hot.
On life all I can say is that I consulted with Sylvania Engineers, spend a full day at the plant in Altoona, PA and learned a lot. I was advised that their power tubes can last 10,000 hours if run at 50% of rated dissipation. Its dissipation that kills tubes, makes them gassy and run away. ARC likes to run tubes at high idle currents such as 60-70 MA and at 500 volts thats can be 35 watts which is the max rating for 6550s. Tube life is not linear with dissipation and max rating can reduce life by a factor of 10, so 1000-2000 hours is typical for many amps, though not for my amps which run them at 15 watts in the RM-9.
So it was good for them to go to the KT120 which is rated higher and the KT 150 higher still. Im not sure I agree with those ratings but they certainly are bigger tubes and have a hight dissipation. I would say the KT150 is a bit of overkill but thats what ARC likes to do. While I keep a large stock of KT-120s on hand I have not yet bought a large number of KT150 as they are much more expensive than the 120s. However I think i can beat their price and know I can do better matching if I can get some demand for those tubes. So lets see if that will occurr..
Yes I am indeed Roger A. Modjeski last time I checked :)
@georgehifi - Yes let's go with the MkIII. I'm sure while ruffling some feathers on the active preamp side, you could ruffle a few more as there have been a few vendors venturing into the passive LDR space at prices quite a bit above what I paid for the Lightspeed. I still consider it one of the best bangs for the buck in this business.
@bifwynne - I work with Roger and handle the online tube store for him (tubeaudiostore.com). I know we have a quad of KT-150s lying around, but I imagine we can get more if necessary. You may want to read some of Roger's articles on tubes and tube matching. You can find them here:
Also, Ken Stevens of CAT has trusted Roger to match tubes for his customers CAT amps and preamps who he sends to us on a regular basis. Roger does the matching to Ken's stringent specifications. The cost is not cheap as there is a lot of work involved. As you have experienced, you may be able to find tubes at a lower cost, but you subtract quality from that as well as it relates to proper matching. I have purchased tubes from some very reputable vendors, but IMO no one tests and matches them as well as Roger. In fact, Roger has been working on developing a tube tracer that can be used by the layman to test tubes. It's been a slow process, but it hopefully it will see the light of day soon.
Amen to this Tube Amp post-we need more engineers on here...
QUOTE :;"’6-18-2018 2:05amThis is an old topic...literally. In the times when tubes were dominant amplification devises, the output transformer was a necessity (one might say a necessary evil. The tubes are amplifying voltage, so that they work with high impedance loads. The loudspeakers normally have quite low impedance. Thus a transformer is required to match the output of tube amplifier and the loudspeaker. When first transistors have emerged, the schematic design did not evolve immediately. Thus, early transistor amplifiers were very similar to the matured at that time tube amplifiers.
However, over time it was realized that transistors work better and amplifiers of current as opposite for tubes which work better as amplifiers of voltage. Thus, transistor amplifiers can and do work well with low impedance load such as loadspeaker directly.
Now, still transformer (autoformer) can assist with the loudspearkers with various impedance...i.e. 4 Ohm vs. 6 Ohm vs. 8 Ohm providing more stable load to a SS amplifier. However, transformer alone is not a perfect transfer devise and making quality transformer for audio output is difficult and costs a lot of money. The typical issues with output transformers are: reduced damping factor and difficulties with driving complex loads, slew rate reduction, additional distortions, etc.
From my perspective, McIntosh polished out the technology and because of the quality, manages to make very good sounding amps. The majority of other brands simply moved on ( which at this point seem to be the right thing to do) and still makes good amps. At the end of the day it is not the technology but the end result that matters "’
@ramtubes. Sorry for not being current with the handles of some of our members, but if you are Roger M, I have a few questions.
Matching KT-150s in my ARC Ref 150 SE amp is a bitch. ARC sourced tubes match much better. When I refer to matching, I am speaking to matched pairs that can be biased within close tolerance, say 3 to 5 mVs. ARC suggested optimal bias is 65 mVs on the set tube, but the bias on the slave tube in the "matched pair" can be between 57 and 73 mVs (+/- 8 mVs).
Do you know any 3rd party tube venders who can match KT-150s for ARC amps at ARC standard? ARC charges $200 per tube; $1600 for a complete retube is outrageous. Most 3rd party tube venders charge half that amount. However, a batch of KT-150s that I bought several years ago from a well known tube vender have bias numbers that range from 5 to10 mVs per matched pair, … and in most cases 8 to10mVs.
Btw, do you really think that ARC driven KT-150s have a useful life of only 2000 hours, …. the recommended time for replacement? How can one tell if a tube has *really* reached the end of its useful life? If you say, one should replace the tubes when the amp doesn't sound good reminds me of the story of the frog and the pot of boiling water.
The story goes like this. If you drop a frog into a pot of boiling water, he will jump out. However, if you put a frog into a pot of cool water and slowly bring the pot to full boil, the frog will just boil to death. The later variant is like listening to tubes. The sound degradation occurs so slowly, you don't realize that the tube is spent until the you get the urge to replace.
Thanks guys, I appreciate the compliments. Making amplifiers has been my life's work as I find them forever fascintating in all their many forms. Though I have put into producion only three power amps and two preamps, there are lots of designs hanging around waiting for the right person to come along.
I know BIF, Roger doesn’t look any different than a regular guy, but damn! It’s humbling to listen to him talk about amplifier design; I wonder how anyone can have so much knowledge in a normal-sized head. I talked to Bill Johnson a few times, and to Tim de Paravicini (EAR-Yoshino), David Manley, Keith Herron, Ralph Karsten, Frank Van Alstine, Max Townshend, and numerous other really good Hi-Fi engineers. They are all very interesting of course, but I have the distinct impression that Roger Modjeski understands amplifier design as well as any human in history. That he and his products aren’t more well known and widely owned actually doesn’t surprise me any more than the fact that most of my favorite music is made by artists known to only the hardest-core music lovers, like we here on AudiogoN.
The late Brooks Berdan was my Hi-Fi dealer for many years. He sold me Hi-Fi, I sold him vintage drums. He was a Music Reference dealer, and loved Roger and his amps. He told me his wealthier clients would pass up MR products because they didn’t cost enough (no bragging rights), or weren’t being reviewed or talked about enough in TAS and Stereophile, didn’t have the High End cashe’ of ARC, VTL, Jadis (Brooks sold a lot of Jadis), etc. Brooks had a LOT of used ARC amps in his pre-owned racks, their owners tired of the breakdowns, repairs, frequent tube replacement, updates, etc. His tech Tom Carione showed me all the burned circuit boards in the ARC amps he had repaired every time a tube blew. He had never seen a Music Reference amp come back for repairs---never. That’s my kinda amp!---Eric.
I checked out the Roger Modjeski YouTube video. Awesome. I've often observed that there are 3 types of smart people in the world, namely: regular smart; very smart; and scary smart. Roger is scary smart and down to earth at the same time.
I think he might have tossed a tongue in cheek zinger at ARC. Roger snickered that expensive ARC power tubes need to be replaced every 2000 hours. In contrast, Roger said his early tube amps (RM-9 ??) could run their power tubes for years.
Not being a EE, I can't say whether tube life in ARC amps is good, bad or neutral. But I can tell you that ARC charges $200 for a single KT-150 tube. My ARC Ref 150 SE takes two quads (8 tubes) -- for a total cost of $1,600. Ouch! I should mention that while I can buy the KT-150s from well know tube vendors for half that price, I have had problems getting closely matched pairs from one well know vendor.
If Ralph (Atmasphere) picks this post up, ... what is the tube life is in your amps. Btw Ralph, …. Roger mentioned a couple of times that negative feedback, *if used properly*, is not all that bad a design feature. Not taking a position, just passing along what RM said.
I can't be certain but I have owned my Lightspeed for around 9 or 10 years.
Wow I think your right, the MkII, was released around 2006 when all MkI's were recalled and converted. I think it's time for a MkIII, what do you think Anthony? Upset the active preamp brigade some more?
Paul was awesome. I had the pleasure at hanging out with him at RMAF several years ago. We met in Bobby P.’s room as Paul was a Merlin owner. It was there that I mentioned to him the Lightspeed and he bought it soon thereafter. In addition to the 9SE, Paul also loved his RM-10 and used the Lightspeed with that amp as well. I can't be certain but I have owned my Lightspeed for around 9 or 10 years.
I posted this on the thread Roger (ramtubes) started to discuss transformers, but thought I would do it here as well.
Roger gave a 90 minute talk on tube amp design at the 2015 Burning Amp Festival. If you own a tube amp, or are thinking of getting one, you will be glad you watched the video. The video is on You Tube, but also on the Berkeley Hi Fi School website.
Go to "berkeleyhifischool.com" and put your browser over "RESOURCES". A drop-down screen will appear---click on "VIDEOS". On the right hand side of the page is a row of "ARCHIVES". Click on October 2015, and the video will appear. Prepare to learn a lot!
ramtubes I also would like to note that the OP of this thread, Paul R owned a RM-9 Special Edition. We miss Paul, a really good guy.
Totally, "top bloke" had many late night discussions with him. He really loved his RM-9 Special Edition which he said was a perfect match with my "Lightspeed Attenuator" which he also started thread on, which exploded to over 18 million hits, made my life hell and interesting at the same time, rip Paul miss you heaps.
I did read your white paper before entering this discussion. Lets just
agree to disagree. Peter Walker designed the 57 to be used with an
amplifier with a damping of 20 and even specified the series inductance.
To say that these speakers will play with a damping factor of 1 is not
fair to the speaker, no matter that some people like a widely altered
frequency response. How can we discuss little differences in distortion
when the frequency response has been so altered to make the speaker
unreconizable?
You may not remember one of your customers, Bill Toberman (RIP, lived a few miles from me), but he had a set of RM-9s which beat out any ARC introduced to his system. He had Quad ESL63s. At the time I didn't think our amps would work with ESLs, but when Bill convinced me to bring them to his house, I was proven wrong- the MA-1s and Quads proved to be a match made in heaven. No loss of highs either, and this in direct comparison to the RM-9. The ESL57 proved even easier to drive and more accessible, since one of my employees had a set. In a nutshell, the FR is not as unrecognizable as you suggest!
We've also gotten Best Sound at Show using our amps with ESLs... all I'm saying here is there is more to this than meets the eye at first blush.
As we all know, there is theory, but there is also practice and we have many happy Quad customers. The thing is, OTLs can provide far more current than most people realize; I've seen one of our 12-tube output sections take out a 15 amp fuse...
As an antique radio collector I have given a lot of thought to how early
SE amps (the 45 in particular) got relatively flat response without
feedback. In that case the driver was a single cone in an open baffle
cabinet so the impedance rise was not so severe and relatively flat.
These early radios sound pretty good. However that does not represent
modern popular speakers.
If you recall the old days, speakers like Altec, Klipsch, JBL and others had level controls for the midrange and tweeter. These controls were not for room adjustment, they were there because the amplifier to be used had an unknown voltage response. **That** was how frequency response was handled in the old days prior to the introduction of the Voltage Paradigm. This practice persisted into the early 1970s as the industry switched over; KLH had level controls for their tweeters as did the AR-1, the world's first acoustic suspension loudspeaker. Of course, now open baffle speakers are back- one of them is in that list I posted above and its not the only one by any means. Google: https://www.google.com/search?q=open+baffle+speakers&client=ubuntu&hs=UEV&channel=fs&...
Nowadays there are entire generations of audiophiles that grew up thinking that the voltage rules are the only game in town. In mid fi, that's certainly true but in high end the power rules have been in play for decades. That is why SETs are around. We've been making zero feedback OTLs now for over 42 years. The reason all this is going on is due to objectionable distortions- and what kinds the ear doesn't care about. That latter statement is why the tubes/transistor thing has been raging on and on, and is why tubes are still in production (horns too), 60 years on after being declared 'obsolete'...
Just to be clear, we've been working on our own take on a class D amp for the last 2 years; not using anyone's modules, since we realized we had something to bring to the table. That amplifier acts as a voltage source; please don't think that I don't understand what's going on here. The paper was written because many people ask why we don't use feedback, and an explanation was needed that went beyond 'we don't like it'.
The reason 'paradigm' is used is because someone inside the paradigm (which is a platform of thought) will regard that which is outside the paradigm as blasphemy or heresy- it can't possibly be right. But this is history: the Power Paradigm used to be how everyone did it. These days our understanding of physiology has greatly expanded so we can better see why there can be a benefit to this approach.
ramtubes Ralph, I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?
Hear hear, good to see someone else thinks the same, about amps becoming tone controls with wild speaker impedance curves. Low output impedance (damping factor) and current are the only fix to make the amp stay reasonably flat. Or I suppose you could have an "inverse tone control" to counter it, but the amp in question will start to gag itself very early in volume level.
Ralph, I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?
Thats a nice little list of expensive speakers that represent a vanishingly small part of the market. I agree a single driver speaker like the Lowther will work just fine with your amplifier.
As an antique radio collector I have given a lot of thought to how early SE amps (the 45 in particular) got relatively flat response without feedback. In that case the driver was a single cone in an open baffle cabinet so the impedance rise was not so severe and relatively flat. These early radios sound pretty good. However that does not represent modern popular speakers.
The Eminent Technology LFT-8b, while employing magnetic-planar drivers as do Magneplanars, is rated as 8 ohms by it’s designer, Bruce Thigpen. While Maggies’ impedances are solidly in the 3-4 ohm range, the ET LFT driver is an almost purely resistive 11 ohm load (the speaker’s 8 ohm rating a consequence of it’s dynamic woofer, for frequencies 180Hz down).
Maggies require a LOT of power, very expensive in a tube amp. The matter is exasperated by their 3-4 ohm load, almost all tube amps putting out half as much power there as at 8 ohms (the notable exception being Music Reference/Ram Tube’s Roger Modeski’s RM-200---100w/ch @ both 8 and 4 ohms!). If you bi-amp the ET LFT-8b, you can use a modestly-powered tube amp (the RM-200 works splendidly, as I have heard does the Atma-Sphere M60) on the m-p drivers, and a ss amp on the woofer. The panels and the woofers each have their own binding posts.
@ramtubes Roger, you might want to read this: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php If the Quads were close to the wall behind them, they will often have one-note bass. This often happens when a solid state amp is employed; the customer will note that there is no bass on account of the higher impedance of the speaker into which the amp can't make power, and so will move the speaker around, eventually finding out that if they move it closer to the wall behind it, they can finally get some bass. But this is not how *any* planar is supposed to be set up.
ESLs don't respond so well to voltage rules, depending on the speaker- the Quad ESL57 and 63 being pretty good examples- also the Sound Lab ESLs, AudioStatic and even Martin Logan (if you use a set of ZEROs to allow for their low impedance). The reason is, unlike box speakers, the impedance curve is not also a map of its efficiency, which is pretty much the same across its bandwidth (on account of its impedance being based on a capacitor rather than a driver in a box with resonance). This is typically a 9 or 10:1 change in impedance! If the amplifier output power varies with this curve, the result will be too much highs and not enough bass.
The solution we've found with our customers using Quads is to have them pull the speakers further out into the room, so there is at least 5 feet behind the speaker to the wall. In this way the bass normalizes and customers report that the Quad is playing bass quite well.
I know of no speaker maker who designs for high output impedance amplifiers.
I can name a few- Coincident, Classic Audio Loudspeakers, DeVore Fidelity, Audiokinesis, Lowther, PHY, PureToneAudio and many more. Essentially, any speaker that works with an SET is working with an amplifier of high output impedance.
We are quite a ways off-topic; if you would like to discuss this further it would be a good topic for another thread.
A friend brought over a circulotron amplifier and we connected it to my Quad 63s I was immediately overhelmed with boomy one note bass. Knowing the Quads and most ESLs have high impedance in the bass I hooked up a generator to the amp and voltmeter to the speaker and found a very sharp 8dB peak at 50 Hz, the cause of the boomy one note bass. The treble was down several dB for the same reason; the non flat impedance curve.
Cone speakers have similar impedance peaks at resonance in addition to peaks and dips in the entire curve due to multiple drivers and their crossovers. This cannot be ignored unless you really like the modification to the frequency response. Very few speakers have flat impedance curves as this is hard to achieve and still get good frequency response. I know of no speaker maker who designs for high output impedance amplifiers.
One other point. The proper marriage of the ESL and OTL was the KLH 9 speaker and the Futterman amplifier. It was a good marriage for two reasons. The KLH 9 was a 16 ohm speaker that did not go below 12 ohms and the Futterman was a high damping amplifier. The Futterman, like all OTLs love a high impedance load and the KLH was. Now we have ESLs that dip to an ohm which no OTL likes.
I know of no other combination of ESL and high output impedance amplfier that is good one.
excellent explination on the higher order harmonics.... Musicians know this stuff in and out and can appreciate harmonics big time since you tune string instruments for intonations,harmonics etc.... the music we hear or think about is usually more the many order harmonics instead of just a plain note. think Foriear transformans math and wave theory. same for light the eye likes vs the actual Frequency, back to the amps, they should not enhance and particular range or require feedback to quell a too bright range-Just increase current/voltage flatly from 0-30K. design your differential signal work in the pre and equilizer if you want. this can be seen on a RTA live. think when you tune your guitar on how super high the lead sounds when its only a few 1000hz,
@bifwynne , Well, since you asked, the answer is ’distortion vs actual frequency response’.
All amps make distortion. Some distortions (IMD and higher ordered harmonic distortion) are more audible than others. Of the two mentioned, the latter is what makes solid state amps bright and harsh, and does that to tube amps as well, although they are not as harsh as solid state because there is simply less of this form of distortion. In a zero feedback amp, there is respectively (solid state or tube) even less. That’s why they sound smoother. The problem is, solid state amps have non-linear capacitances built into the junctions of the output devices. This capacitance is responsible for higher ordered distortions (brightness and harshness) that still affects a zero feedback solid state amp.
Since the ear uses the higher ordered harmonics to sense sound pressure, it is more sensitive to these harmonics than most test equipment, since it has to cover a 120-140db range! This is an insidious, inconvenient truth that the audio industry does not like to face. This is further complicated by the Fletcher-Munson loudness curves, which place many of these upper harmonics in the most sensitive region of the ear’s response.
That makes things particularly tricky.
The reason zero feedback tube amplifiers exist is that a good number of designers have come to recognize the problem of higher ordered harmonic distortion. One fairly easy way to sort that out is to avoid using feedback, since it is known to add such distortions of its own in the process of otherwise suppressing distortion. So this means that the designer has to use other means to suppress distortion.
This results in an amplifier with a fairly high output impedance.
The thing is, if you chose the speaker carefully, the result is **far** more neutral than is possible with the conventional approach (its not subtle- you can hear it right away, whether an audiophile or not). The reason this is possible has to do with two salient facts: first, no speaker made anywhere at any price is flat. Second, the ear converts distortion into tonality, and has tipping points wherein that tonality *is favored* over actual frequency response.
The latter fact is why two amps on the bench can have identical frequency response but one will sound bright and the other will not!
I’ve found over the years and decades that if the speaker requires the amplifier to employ feedback to get flat frequency response out of the speaker, that the result has no chance whatsoever of sounding like real music! At best it will sound like a very good stereo (this bit is tricky; many people are happy with the status quo of a good stereo, because they think getting it to sound like real music isn’t possible!). Its this difference where I have drawn the line, and why over 42 years we’ve avoided feedback.
**That** is why bother with a zero feedback tube amp. Sorry for the derailment.
I use I am using a Mac C2500 tube preamp mated to a Mac MC 152 amp with the transformers driving GE Triton Reference speakers and this combination has gotten me off the merry go round as it is the best sound I have ever had in my listening room.
My other system uses. Mac MA6600 Integrated with transformers. I was considering a MA6500 but Audio Classics who know more about Mac than anyone else with the exception of McIntosh told me I would lose the lush romantic Mac house sound without the transformers. Needless to say I am thrilled with all my Mac purchases and am done.
@bifwynne I run into that too- we make OTLs as you know, but there are speakers out there that on paper would seem to be too difficult a load, but in practice work out just fine with our amps. So your experience does not surprise me in the least.
Feedback is helping you out here, and my surmise is that most of the speaker impedance curve is actually higher than the dips in the bass (the bass region also has peaks BTW). That's a pretty simple explanation for why this is working for you.
Quite often the 4 ohm tap on an output transformer sacrifices some performance- in some cases you can lose an octave of bandwidth off the bottom just by going from the 8 ohm tap to the 4 ohm tap. So it can be a mixed bag in the real world (as opposed to the math world of theory).
So here is my question. What is happening electrically if there is an
impedance mismatch? Is the amp producing distortion if there is an
impedance deviation from 3000 ohms? If so, does having larger wattage
and power supply capabilities ameliorate distortion effects?
How
about negative feedback? In my case, I understand that the Ref 150
uses 14db of negative feedback and as a result, performs like a voltage
source amp. Does that feature help to reduce distortion effects?
I
have not touched on phase angle or sonic coloration issues caused by
impedance mismatches between the amp's output taps (nominally, the 4, 8
or 16 ohm taps) and the speaker's actual impedance at a particular
frequency.
As to sonic coloration, I recall that the output
impedance of the ARC Ref 150 off the 8 ohm taps is around 1 ohm or
less. This results in some sonic coloration, ... but not a lot. In a
Stereophile review of the ARC Ref 150, John Atkinson measured the Ref
150's voltage output variation off the 8 ohm taps to be (+) or (-) .8 db
when presented with a synthetic speaker load.
Btw, are similar issues presented with OTL tube amps?
@bifwynne
All amps always make distortion. If the amp's tubes are loaded below 3000 ohms, distortion will rise. If loaded above that, its power will decrease as will its distortion.
The feedback allows the amp to behave as a voltage source. George has it wrong; the amp can make double the power into 4 ohms as it does into 8 because voltage sources do that (this is by definition). The deal is, the 4 ohm power is the maximum power for which the amp is rated and you have to be on the 4 ohm tap. If the impedance then rises to 8 ohms, the amp will make 1/2 of its output power. Its a bit confusing, because tube amps accomplish the task of being a voltage source in a way that is opposite of how solid state amps do it.
The issues are the same with an OTL, but of course no transformer is (usually) involved. Instead the tubes see the load directly.
was quite pleasantly surprised to see the amp double down from it’s 8 Ohm tap down to 4 Ohms
If you don’t change the tap, it doesn’t double at all when the load impedance halves from 8 to 4ohms And you can’t change the trany tap from 8 to 4 half way through a musical note, when it decides to dip from 8ohm to 4ohms then back up again. Autoformers good interim fix for amp/speakers if the amp can’t drive the speaker without it, till you get the right amp
I have to admit that I was quite pleasantly surprised to see the amp double down from it's 8 Ohm tap down to 4 Ohms, but from there down to 2 Ohms, it was even worse than I expected.
In my case I have a McIntosh MC452 (solid state amp with autoformers) driving a pair of Focal Sopra No2 speakers. Focal lists the "nominal" impedance at 8 ohms, but the "minimum" impedance at 3.1 ohms, so my dealer recommended that I connect them to the 4 ohm taps on the amplifier.
Ralph, could you clarify a point regarding impedance matching between tube amps and speakers with roller coaster impedance and phase angles numbers.
I think you mentioned that the function of an output transformer is to step down the high voltage from the output tubes to the amps output taps. In addition, the output tranny will step up a speaker's impedance presented to the output power tubes at the primary windings of the tranny.
Maybe a simple example will help further clarify my understanding. My amp is an ARC Ref 150SE which uses KT-150 tubes in push-pull fashion. I recall that the output impedance of KT-150s is 3000 ohms. In a theoretically perfect world, if I had a speaker with a perfectly flat 4 ohm impedance function, plugging the speakers into the amps' 4 ohm taps would result in a stepped up impedance of 3000 ohms presented off the primary windings of the output tranny. That would be a perfect match with the power tube's output impedance.
We all know that in the real world, most speakers have roller coater impedance functions. As a result, the impedance off the primary windings of the output transformer will vary from the optimal 3000 ohm target. ARC says, just use the taps that sound the best.
So here is my question. What is happening electrically if there is an impedance mismatch? Is the amp producing distortion if there is an impedance deviation from 3000 ohms? If so, does having larger wattage and power supply capabilities ameliorate distortion effects?
How about negative feedback? In my case, I understand that the Ref 150 uses 14db of negative feedback and as a result, performs like a voltage source amp. Does that feature help to reduce distortion effects?
I have not touched on phase angle or sonic coloration issues caused by impedance mismatches between the amp's output taps (nominally, the 4, 8 or 16 ohm taps) and the speaker's actual impedance at a particular frequency.
As to sonic coloration, I recall that the output impedance of the ARC Ref 150 off the 8 ohm taps is around 1 ohm or less. This results in some sonic coloration, ... but not a lot. In a Stereophile review of the ARC Ref 150, John Atkinson measured the Ref 150's voltage output variation off the 8 ohm taps to be (+) or (-) .8 db when presented with a synthetic speaker load.
Btw, are similar issues presented with OTL tube amps?
But what about speakers with impedance swings all over the place, from 3
ohms to off the charts well at well over 20 ohms like 802D3s (nominally
8ohms). It’s swings like this that tax the best tube amps with their
transformer outputs. How, exactly, do these impedance swings effect
autoformers???
Are the electrical properties of autoformers
similar enough to those of tube output transformers to cause similar
issues when driving speakers with crazy impedance and phase curves?
I clicked on a link within the post that attempts to tout the benefits of an Autoformer; essentially better matching to speakers with different impedance values. Ie 4 ohm taps to 4 ohm speakers, etc. All well and good for “nominal impedance”.
But what about speakers with impedance swings all over the place, from 3 ohms to off the charts well at well over 20 ohms like 802D3s (nominally 8ohms). It’s swings like this that tax the best tube amps with their transformer outputs. How, exactly, do these impedance swings effect autoformers???
Are the electrical properties of autoformers similar enough to those of tube output transformers to cause similar issues when driving speakers with crazy impedance and phase curves?
But, let us be real. Many fine speakers are not easy to drive, and autoformers are there to keep the amplifier safe and sound ( pun intended ) and better match the speaker. Enjoy ! MrD.
These speakers then should have different amps that can handle their loads.
With easily driven speakers ( impedance ), I much prefer the Mac amps without the autoformers. Same with OTL tube amps. But, let us be real. Many fine speakers are not easy to drive, and autoformers are there to keep the amplifier safe and sound ( pun intended ) and better match the speaker. Enjoy ! MrD.
You shouldn't question what they do, they have their job moderating and are good at it. Like I said I doubt very much the post was deleted because of a reference to another audio website, it was probably whatever else was said, in the way it was said.
Well, my last post was removed because I mentioned another audio website. I was saying that there is a recent review of the new McIntosh 611 monoblocks if anyone cares to read it and see how "poorly designed" the reviewer found them to be.
Thank you Ralph for keeping it honest and cutting through the misinformation. Certain individuals keep polluting the threads with personal opinions based on complete ignorance. I now simply scroll over their posts.
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