What an OTL is, and why you should care

sbo6

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May 19, 2014
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#21
Sorry to interject but Ralph is a titan in the high-end audio industry, and we are grateful for his participation on WBF.

Please consider it verified. I promise you that every OTL amplifier I have ever heard manifests a crystalline transparency which I have not heard from any other amplifier topology.
No problem Ron. I think it's great you have the designer for Atmasphere on the forum providing input. However both of your opinions don't verify as fact. For example, I could say Pass believes his simple circuit Class A topology is the most transparent; that's an opinion not fact hence my original question. I could also argue OTLs are known for horrible bass due to impedance limitations (no output xformer). Does transparency include bass quality? Then there's OTL reliability, heat, power consumption etc. but that's another branch off this tree. Not trying to diminish expert input just trying to keep facts as facts and opinions as opinions.
 

Ron Resnick

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#22
No problem Ron. I think it's great you have the designer for Atmasphere on the forum providing input. However both of your opinions don't verify as fact. For example, I could say Pass believes his simple circuit Class A topology is the most transparent; that's an opinion not fact hence my original question. I could also argue OTLs are known for horrible bass due to impedance limitations (no output xformer). Does transparency include bass quality? Then there's OTL reliability, heat, power consumption etc. but that's another branch off this tree. Not trying to diminish expert input just trying to keep facts as facts and opinions as opinions.
That is all totally fair enough. :) Since our audio observations are substantially subjective only you could verify for yourself to your satisfaction. I agree with you that if we are being technical it is Ralph's opinion and my opinion (to the extent of my amplifier experience thus far) and not fact.
 

Atmasphere

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#23
Hi Ralph

What is the difference between an OPT and having an Autoformer in the path? Were the Circlotron Electro Voice amps "OTLs" with Autoformers built in? I am currently test driving a Circlotron amp that has OPTs. The first thing it reminded me of was your amps driving Tonians at a friend's place which had the somewhat unique ability of delivering sound events "fully formed" at very, very low sound levels. In other words that "comes to life" thing comes in much earlier than usual. You could listen at much lower SPLs without sacrificing intelligibility. I suppose it is because the distortion is lower but also because power delivery is very, very linear. Very little haze.

Thank you in advance.

Jack
The EV Circlotron used an output transformer with a secondary although they could have used an autoformer. Since it was using 6V6s or the like it could not operate as an OTL, but did have the advantage of being able to operate class B without crossover distortion since the magnetic field in the transformer could not collapse. So it was high power and low distortion for using a pair of 6V6s!

No problem Ron. I think it's great you have the designer for Atmasphere on the forum providing input. However both of your opinions don't verify as fact. For example, I could say Pass believes his simple circuit Class A topology is the most transparent; that's an opinion not fact hence my original question. I could also argue OTLs are known for horrible bass due to impedance limitations (no output xformer). Does transparency include bass quality? Then there's OTL reliability, heat, power consumption etc. but that's another branch off this tree. Not trying to diminish expert input just trying to keep facts as facts and opinions as opinions.
One thing that OTLs can do is have a distortion vs. power curve wherein the distortion linearly decreases to unmeasurable as power is decreased. The only other amps that can do this are single-ended, which tend to have high distortion at high power. Now the ear has a masking principle, wherein the presence of louder sounds masks the presence of quieter sounds. Distortion can thus mask detail, in particular at lower power levels. This is one reason why SETs have that 'magical inner detail', but its not there at higher power levels. Most transistor amps have a distortion character where the distortion increases below a certain percentage of power, which means that at low power levels they are going to mask detail compared to an OTL as their distortion will be considerably higher.

Unlike an SET, an OTL can make power without high distortion. OTLs traditionally over the decades have had distortion figures that rival solid state- much lower than conventional tube amplifiers. The output transformer, or the lack of it, is the main reason why- you don't have the phase margin issues if the transformer is absent (and you are also lacking the distortion contribution from the transformer), since the tube circuit otherwise can have some very impressive bandwidth (we easily get full power to over 100KHz for example- that's pretty hard in a high powered tube amp; we have to limit the bandwidth in the driver circuit).

Lower distortion results in greater transparency on the part of the OTL (note: our amps tend to lack the famous 2nd ordered harmonic common with a lot of tube amps since they are fully differential and balanced; even ordered harmonics are canceled throughout the amp, not just at the output). IOW what I am talking about is both measurable and audible.

Regarding your other comments- OTLs can be as reliable as any other amp. However that was not the case before Atma-Sphere came along- if anything we engineered the world's first truly reliable OTLs. I often pull power tubes at audio shows while the amp is playing to demonstrate just how stable the amp is- the amp plays on as if nothing happened. I short them out with a quarter across the speaker terminals- again the same thing when the quarter is removed. Tubes can fail but the amp does not. We achieved this by the use of the Circlotron output circuit, but configured in a different way that allowed for a very simple circuit- only one stage of gain exists in the entire amp (which is one of the features that allows it to rule the roost on transparency- the more stages of gain, the more places for things to get screwed up).

Our amps are full power to 1 or 2 Hz depending on the model. They play bass wonderfully (I played bass in the orchestra from 7th grade till well past college; I expect the bass to be convincing). You are actually drawing from much of the prior reputation of OTLs, which I've come to call the 'Futterman Legacy', although it was really Harvey Rosenburg, more than anyone else, that convinced the world that OTLs were unreliable and couldn't play bass. We've done a lot to reverse that reputation in the last 30 years; most newer OTLs coming on the market in the last 30 years have been Circlotron designs, not Futterman designs. Despite that, we seem to be the only one that has sorted out out to really control the power tubes so that don't have bias runaway or blocking distortion issues. Clipping recovery is instantaneous, something I can't say about a number of competing designs.

The heat of the amp is a function of its class of operation- not the number of tubes (our amps are Class A2; another way we reduce distortion). If one of our amps is in Standby all day long, at the end of the day you can grab a power tube without getting burned. But if the amp is stone cold, after a one minute warmup the tubes are too hot to touch. In practice, a class A solid state amp of the same power makes about 90% of the same BTUs, although due to its being distributed across a heatsink, at a lower perceived temperature.

We are careful to vet each customer's loudspeaker plans, as due to the fact that the smaller amps we make do have a fairly high output impedance, the results will be less than spectacular :) if the amp is put at a disadvantage on the wrong load. Generally speaking, if the speaker is 8 ohms or more in the bass, it will do just fine and easily competes with the best solid state amps on such loads.

Over the years though in looking at amplifier specs in general, one thing that has become abundantly clear is that in high end audio there are only weak arguments for speakers of 4 ohms and less. All amplifiers, tube or solid state, make more audible distortion into lower impedances. If your goal is high quality reproduction, why force your amp to make distortion you can hear (and BTW, the distortion I'm talking about is higher ordered harmonics, to which the ear is keenly sensitive because they are used to sense sound pressure, and so rivaling the best test equipment)?
 
Last edited:

LL21

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Dec 26, 2010
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#24
Fantastic post as usual, Atmasphere. Thanks very much. I 'think' i can actually understand some of what you are saying which was fascinating on both a macro and micro level.
 

the sound of Tao

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Jul 18, 2014
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#25
+1 Great post thanks Ralph, as usual it’s both illuminating and easy to read.
 

sbo6

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May 19, 2014
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#26
The EV Circlotron used an output transformer with a secondary although they could have used an autoformer. Since it was using 6V6s or the like it could not operate as an OTL, but did have the advantage of being able to operate class B without crossover distortion since the magnetic field in the transformer could not collapse. So it was high power and low distortion for using a pair of 6V6s!



One thing that OTLs can do is have a distortion vs. power curve wherein the distortion linearly decreases to unmeasurable as power is decreased. The only other amps that can do this are single-ended, which tend to have high distortion at high power. Now the ear has a masking principle, wherein the presence of louder sounds masks the presence of quieter sounds. Distortion can thus mask detail, in particular at lower power levels. This is one reason why SETs have that 'magical inner detail', but its not there at higher power levels. Most transistor amps have a distortion character where the distortion increases below a certain percentage of power, which means that at low power levels they are going to mask detail compared to an OTL as their distortion will be considerably higher.

Unlike an SET, an OTL can make power without high distortion. OTLs traditionally over the decades have had distortion figures that rival solid state- much lower than conventional tube amplifiers. The output transformer, or the lack of it, is the main reason why- you don't have the phase margin issues if the transformer is absent (and you are also lacking the distortion contribution from the transformer), since the tube circuit otherwise can have some very impressive bandwidth (we easily get full power to over 100KHz for example- that's pretty hard in a high powered tube amp; we have to limit the bandwidth in the driver circuit).

Lower distortion results in greater transparency on the part of the OTL (note: our amps tend to lack the famous 2nd ordered harmonic common with a lot of tube amps since they are fully differential and balanced; even ordered harmonics are canceled throughout the amp, not just at the output). IOW what I am talking about is both measurable and audible.

Regarding your other comments- OTLs can be as reliable as any other amp. However that was not the case before Atma-Sphere came along- if anything we engineered the world's first truly reliable OTLs. I often pull power tubes at audio shows while the amp is playing to demonstrate just how stable the amp is- the amp plays on as if nothing happened. I short them out with a quarter across the speaker terminals- again the same thing when the quarter is removed. Tubes can fail but the amp does not. We achieved this by the use of the Circlotron output circuit, but configured in a different way that allowed for a very simple circuit- only one stage of gain exists in the entire amp (which is one of the features that allows it to rule the roost on transparency- the more stages of gain, the more places for things to get screwed up).

Our amps are full power to 1 or 2 Hz depending on the model. They play bass wonderfully (I played bass in the orchestra from 7th grade till well past college; I expect the bass to be convincing). You are actually drawing from much of the prior reputation of OTLs, which I've come to call the 'Futterman Legacy', although it was really Harvey Rosenburg, more than anyone else, that convinced the world that OTLs were unreliable and couldn't play bass. We've done a lot to reverse that reputation in the last 30 years; most newer OTLs coming on the market in the last 30 years have been Circlotron designs, not Futterman designs. Despite that, we seem to be the only one that has sorted out out to really control the power tubes so that don't have bias runaway or blocking distortion issues. Clipping recovery is instantaneous, something I can't say about a number of competing designs.

The heat of the amp is a function of its class of operation- not the number of tubes (our amps are Class A2; another way we reduce distortion). If one of our amps is in Standby all day long, at the end of the day you can grab a power tube without getting burned. But if the amp is stone cold, after a one minute warmup the tubes are too hot to touch. In practice, a class A solid state amp of the same power makes about 90% of the same BTUs, although due to its being distributed across a heatsink, at a lower perceived temperature.

We are careful to vet each customer's loudspeaker plans, as due to the fact that the smaller amps we make do have a fairly high output impedance, the results will be less than spectacular :) if the amp is put at a disadvantage on the wrong load. Generally speaking, if the speaker is 8 ohms or more in the bass, it will do just fine and easily competes with the best solid state amps on such loads.

Over the years though in looking at amplifier specs in general, one thing that has become abundantly clear is that in high end audio there are only weak arguments for speakers of 4 ohms and less. All amplifiers, tube or solid state, make more audible distortion into lower impedances. If your goal is high quality reproduction, why force your amp to make distortion you can hear (and BTW, the distortion I'm talking about is higher ordered harmonics, to which the ear is keenly sensitive because they are used to sense sound pressure, and so rivaling the best test equipment)?
Thanks, appreciate all the details. WRT your comment about speaker impedance, unfortunately there are many speaker designs that dip below 8 and even 4 ohms, which as you said can limit amplifier choices.
 

Atmasphere

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May 4, 2010
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#27
Thanks, appreciate all the details. WRT your comment about speaker impedance, unfortunately there are many speaker designs that dip below 8 and even 4 ohms, which as you said can limit amplifier choices.
That is certainly true but a lot depends on the speaker and the intentions of the designer. An example might be an ESL. They don't have a box and their impedance curve is based on a capacitor rather than a driver in a box with a box resonance. ESLs typically have a difference in impedance that varies about 9:1 to 10:1 over their range- the highest impedance at low frequencies and the low impedance at high frequencies (which is often less than 4 ohms, some are as low as 0.5 ohms). This type of speaker can be a terrible match for solid state amps as the amp doubles power as impedance is cut in half, which also means its power is halved as impedance is doubled. As a result solid state amps tend to sound bright and bass-shy on many ESLs, because the impedance curve of any ESL is not also a map of its efficiency (with many box speakers, the impedance curve **is** a map of its efficiency at various frequencies). Add to that the brightness that is common with many solid state amps (the result of higher ordered harmonic distortion) and you have a pretty severe coloration.

ESLs and OTLs have been a match made in heaven since the 1950s (back then it was Quads and Futtermans, now days the best combo is Sound Lab with our gear...); this is old news.

Other speakers that don't seem to subscribe to the voltage source concept are full range speakers such as Lowthers and PHY and also most horn speakers. In addition, there are box speakers which are intended for higher impedance amplifiers (interestingly, the first acoustic suspension loudspeaker, the Acoustic Research AR-1, was designed for an amplifier with a 7 ohm output impedance; this is a very famous and respected speaker..) and there are a good number of these made today. In practice, we find a wider range of compatible speakers than one would expect at first blush, if one is assuming that a voltage source is required for neutral uncolored reproduction.
 

sbo6

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May 19, 2014
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#28
That is certainly true but a lot depends on the speaker and the intentions of the designer. An example might be an ESL. They don't have a box and their impedance curve is based on a capacitor rather than a driver in a box with a box resonance. ESLs typically have a difference in impedance that varies about 9:1 to 10:1 over their range- the highest impedance at low frequencies and the low impedance at high frequencies (which is often less than 4 ohms, some are as low as 0.5 ohms). This type of speaker can be a terrible match for solid state amps as the amp doubles power as impedance is cut in half, which also means its power is halved as impedance is doubled. As a result solid state amps tend to sound bright and bass-shy on many ESLs, because the impedance curve of any ESL is not also a map of its efficiency (with many box speakers, the impedance curve **is** a map of its efficiency at various frequencies). Add to that the brightness that is common with many solid state amps (the result of higher ordered harmonic distortion) and you have a pretty severe coloration.
Acknowledged which is one of several reasons why Martin Logan, the largest mfr. of ESLs has implemented hybrid designs for decades (+ the challenges wrt surface area requirements and ESL bass). Also, many models include powered woofers for added compatibility with most tube and SS amps. Furthermore while ESL impedance is inversely related to frequency and can be <2 ohms at very high frequencies (e.g.: ML Montis @ 1ohm at ~20KHz), the source content is minimal reducing impedance issues irrespective of the amp type or topology. Net - In an era of increasingly powerful amps and integrated powered bass most modern SS (or other type) amps work well with ESLs or for that matter any speaker design.
 

Atmasphere

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#29
Acknowledged which is one of several reasons why Martin Logan, the largest mfr. of ESLs has implemented hybrid designs for decades (+ the challenges wrt surface area requirements and ESL bass). Also, many models include powered woofers for added compatibility with most tube and SS amps. Furthermore while ESL impedance is inversely related to frequency and can be <2 ohms at very high frequencies (e.g.: ML Montis @ 1ohm at ~20KHz), the source content is minimal reducing impedance issues irrespective of the amp type or topology. Net - In an era of increasingly powerful amps and integrated powered bass most modern SS (or other type) amps work well with ESLs or for that matter any speaker design.
Section in bold: this statement is incorrect. It is correct to say the Martin Logan (and a few other) are making their ESLs very low impedance in an effort to reduce this problem. But it is not correct to say that any high powered amp will 'work well with ESLs or for that matter any speaker design'. I already stated why, if you'd like the math we can go through it. I think what the issue is here is that you have learned something along the lines of 'all speakers need a voltage source to drive them' which is a false statement. There is more to it than that.

First, read this article, which puts the issue in a nutshell:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Then, before you think that I made this stuff up or are part of the woo camp of subjectivist audio, take a look at this
http://www.hifi-studio.de/hifi-klassiker/fisher/Fisher_80AZ/Fisher_80AZ.htm

-in the center of the 'damping control' scale (which is a voltage and current feedback control) you will see the words 'constant power'.

Now you might ask yourself why that would be marked there, and here is why. When this amp was built, the idea of 'voltage driven' loudspeakers was still being proposed by EV and MacIntosh. A lot of speakers were made back then about which the designers had no idea what the voltage response of the amplifier to drive it would be. To deal with this they installed level controls on the speakers (usually midrange and tweeter level controls). Those controls are not there to adjust the speaker to the room, they are there to adjust the speaker to the voltage response of the amp. My speakers at home are about 5 years old and they have these controls for precisely that reason. Sound Labs (ESLs) have these controls too.

IOW there are a whole class of speakers and amps out there that don't obey the voltage rules with intention- and this is not so they act as anything other than to be as neutral as they can possibly be. The reason was stated by Norman Crowhurst (see Pete Millet's site http://www.tubebooks.org/technical_books_online.htm for free downloads of Crowhurst's work) over 55 years ago- loop feedback adds to the higher ordered harmonic content of any amplifier in which it is used. These harmonics are converted by the ear to tonality, so humans hear them as brightness and hardness. If you want to avoid these colorations, you have to avoid feedback. That means that you don't have a low output impedance to get flat frequency response, but in reality no speaker is flat, and in addition to that the brain has tipping points wherein tonality caused by distortion can be favored over actual FR errors.

The power paradigm is a way of getting flat frequency response without a low output impedance. The thing is, if you use a speaker that is not part of the voltage paradigm with an amplifier that is, you will encounter a tonal anomaly caused by FR errors. ESLs and solid state are an example of that- they are too bright together.
 

sbo6

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May 19, 2014
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#30
Section in bold: this statement is incorrect. It is correct to say the Martin Logan (and a few other) are making their ESLs very low impedance in an effort to reduce this problem. But it is not correct to say that any high powered amp will 'work well with ESLs or for that matter any speaker design'. I already stated why, if you'd like the math we can go through it. I think what the issue is here is that you have learned something along the lines of 'all speakers need a voltage source to drive them' which is a false statement. There is more to it than that.
There is no issue here, you misread and / or misinterpreted my statement. I never said "any SS amp" I said "most modern SS amps ".

Also I've owned several and have heard many MLs and they sound fabulous with SS amps, no high end loss. Try them with Pass for example, you'd be surprised.
 

bonzo75

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Feb 26, 2014
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#31
I have heard ML hybrids with Concert Fidelity (UK Paul's), MSB 200, Gamut monos, also class D Bel Canto. There was no brightness. I will be surprised if Luxman, Pass, and Vitus don't work. I have also heard Soundlabs with Spectral and it was excellent. With Krell sometimes I heard brightness sometimes I did not. I have got thinness in tone relative to valve amps on some but density has been achieved keeping valves upstream in dac, phono, or pre and keeping the SS amp. I once got awfully bright sound using Electrocompaniet on MLs, dunno why.
 

Atmasphere

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#32
There is no issue here, you misread and / or misinterpreted my statement. I never said "any SS amp" I said "most modern SS amps ".

Also I've owned several and have heard many MLs and they sound fabulous with SS amps, no high end loss. Try them with Pass for example, you'd be surprised.
I'm OK with 'most modern amps'. The issue is getting too much in the way of highs, rather than too little. Pass Labs are excellent- one of the best SS amps out there IMO. ML speakers, having about 0.5 ohms impedance at 20KHz, represent one solution to getting SS to work with ESLs. Make the impedance super low so most amps won't make power due to a near short, and put a speaker cable in series with that, and you've tamed the top end.

I have heard ML hybrids with Concert Fidelity (UK Paul's), MSB 200, Gamut monos, also class D Bel Canto. There was no brightness. I will be surprised if Luxman, Pass, and Vitus don't work. I have also heard Soundlabs with Spectral and it was excellent. With Krell sometimes I heard brightness sometimes I did not. I have got thinness in tone relative to valve amps on some but density has been achieved keeping valves upstream in dac, phono, or pre and keeping the SS amp. I once got awfully bright sound using Electrocompaniet on MLs, dunno why.
Sound Lab solves the problems of ESL drive by installing controls on their speakers- they have a Brilliance control, midrange jumpers and bass jumpers. When auditioning amps on Sound Labs, care has to be taken to know the correct settings for each amp in advance so you are making valid comparisons.
 
May 30, 2010
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#33
(...) Sound Lab solves the problems of ESL drive by installing controls on their speakers- they have a Brilliance control, midrange jumpers and bass jumpers. When auditioning amps on Sound Labs, care has to be taken to know the correct settings for each amp in advance so you are making valid comparisons.
One nice aspect of the continuous adjustment of brilliance (treble) is that when you decrease treble (slightly needed most of the time) you increase the treble impedance - this typically raises the horribly low treble impedance a few ohms. For maximum quality, after we find the best sounding position the rheostat should be replaced by quality power resistor of equivalent value. There were several versions of backplates, some with low impedance, but it is very simple to modify any of them to be OTL friendly - it can be done in place, Roger West sends the diagram of the mods and parts if needed.
 

Ron Resnick

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#34
I personally agree with Ralph. I have never heard electrostatic speakers driven by solid-state amplifiers and liked the sound.
 
May 30, 2010
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#35
I personally agree with Ralph. I have never heard electrostatic speakers driven by solid-state amplifiers and liked the sound.
You should listen to Quad ESL63 or modern versions of them with SS. Or Marting Logan's with Electrocompaniet - your old Prodigy's sounded divine with AW250s or Nemo's.

Cello electronics sounded fabulous with Quad ESL's. And a little bird told me once Roger West is known to use Soundlabs with SS ...
 

Gregadd

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Apr 20, 2010
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#37
Once you have heard an OTL paired with electrostatic speakers you'll never go back. Because electrostatics present such a difficult load a lot of people, including manufacturers, believe the answer is to match it with a behemoth solid state amplifer. To my ears, reulting in an unfortunate sound.
 

Atmasphere

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#38
One nice aspect of the continuous adjustment of brilliance (treble) is that when you decrease treble (slightly needed most of the time) you increase the treble impedance - this typically raises the horribly low treble impedance a few ohms. For maximum quality, after we find the best sounding position the rheostat should be replaced by quality power resistor of equivalent value. There were several versions of backplates, some with low impedance, but it is very simple to modify any of them to be OTL friendly - it can be done in place, Roger West sends the diagram of the mods and parts if needed.
Actually Sound Lab discovered an error in their crossover ( the driver is full range; the crossover is for the matching transformers in the backplate) and once that error was corrected, the speaker got a lot easier to drive for any amplifier made, and it sounds better to boot. This backplate is called the 'toroidal II'.


Once you have heard an OTL paired with electrostatic speakers you'll never go back. Because electrostatics present such a difficult load a lot of people, including manufacturers, believe the answer is to match it with a behemoth solid state amplifer. To my ears, reulting in an unfortunate sound.
Right here we see the problem of driving a full range ESL. The Sound Lab and also the older Quads are moderate impedance. But this means they have a fairly high impedance peak in the bass. In the case of the Sound Labs, about 30 ohms. Now if you have a solid state amp that makes 600 watts into 8 ohms, how much does it make into 32? The answer is 150 watts. This is why a tube amp of moderate power can keep up with a rather powerful solid state amp on many ESLs; in the case of the example I just stated, our MA-1 can do the job that it takes a 600 watt SS amp to do.
 
May 30, 2010
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#39
Actually Sound Lab discovered an error in their crossover ( the driver is full range; the crossover is for the matching transformers in the backplate) and once that error was corrected, the speaker got a lot easier to drive for any amplifier made, and it sounds better to boot. This backplate is called the 'toroidal II'. (...)
My Soundlabs have the toroidal II - and long before it had the upgrade for high-impedance, that made it easier to drive - it replaced a resistor with a coil, decreasing the power loss in a part of the crossover connected to the toroidal middle/treble transformer. It was two step process.

The torroidal II increased the efficiency about 2 dB - not bad at all, but not the enormous value (6-9dB) that was mistakenly referred in audio forums by some owners. Soundlab's are large panels, efficiency measurements can not be directly compared to typical efficiencies of point like box speakers at 1m!
 

Atmasphere

[Industry Expert]
May 4, 2010
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#40
I could also argue OTLs are known for horrible bass due to impedance limitations (no output xformer). Does transparency include bass quality? Then there's OTL reliability, heat, power consumption etc. but that's another branch off this tree. Not trying to diminish expert input just trying to keep facts as facts and opinions as opinions.
Just facts then:

I think transparency does include bass quality. Our amps are full power down to 2Hz (1 Hz on the bigger amps) and play bass the best of any amps I've heard tube or solid state. But that does depend on the speaker load. I don't seen the point though of trying to make any amplifier do something for which its not intended. That doesn't serve your amplifier investment dollar very well :)

As far as reliability goes, the Atma-Sphere claim to fame is that we created the world's first reliable OTLs. That was a bit of an uphill battle in the 1980s and 90s convincing people that this is so. But I can short out the speaker terminals (and do so at shows) without damaging the amps, they are unconditionally stable so no oscillations (which was a thing that led to may OTL failures before we came along). I often pull power tubes from amps while there are playing music at show to demonstrate this stability.

The amps are safe with speakers too- despite being direct coupled. If they weren't we'd not have been in business over 42 years at this point.

Heat is a function of class of operation. As a result Nelson Pass makes amps that make nearly as much heat as our amps with similar output power ratings. The filaments of the power tubes don't contribute that much- you can leave the amps on standby all day and grab hold of the power tubes without burning yourself.
 

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