AudioCircle
Other Stuff => Archived Manufacturer Circles => Music Reference => Topic started by: Roger A. Modjeski on 23 Oct 2011, 04:09 am
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I am happy to see the interest in OTL amps. As most of you know I designed the Counterpoint SA-4 which some say is the best amp they have ever heard. To my knowledge they made 50-100 pair, all on PC boards. Mine will be hard wired, simpler and more user friendly.
The big question is size. The power output of any OTL is limited by the maximum current one can safely draw from the output tubes. This is about 1.5 amps for the tubes I will be using. Peak power is that current squared times the load impedance. Your RMS power is half that. OTLs do not like RMS testing so that the RMS power should be rated rather low and one should be more concerned with the peak power.
Good speakers have become more efficient since I designed the SA-4 and many listeners are now happy with more definition and less SPL. Therefore we must re-evaluate out power needs.
Please vote.
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With my listening levels and sensitive speakers I rarely use more than 1 watt.
You asked on another string if anyone would want a pre/main separate switch and jacks?
Absolutely! With that feature and a low enough price I could get two and finally realize my dream of bi-amping with OTLs. Could have done it with two of Bruce Rozenblit's low power amps, but I'm not a big fan of his designs for several reasons.
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OTL's:
sound open and uncomplicated with the best HF. No punch and extension in the bass and not as much layering & depth.
so no transformer no bass extension , no depth and layering to the music.
roger can these issues be addressed in a OTL?
rob.
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OTL's:
sound open and uncomplicated with the best HF. No punch and extension in the bass and not as much layering & depth.
so no transformer no bass extension , no depth and layering to the music.
roger can these issues be addressed in a OTL?
rob.
My OTL (zero feedback, 30 watts) has plenty of bass extension, depth, and layering. In fact I have compared it to my EM-7 12v and RM-10 MkII and it gives up nothing to these two amps in those areas, while providing some additional benefits, one of which is a bigger sound stage, most notable in depth.
My previous OTL (feedback, 15 watts) also exhibited similar qualities, although it didn't make the cut against the EM-7 12v or RM-10 MkII for other reasons. If I had to knock OTL's I'd say they could be quieter, but with my current OTL I'm really splitting hairs here because any noise I hear is with my ear to the driver, and it's just some 120Hz noise. I'm positive Roger will ensure his design is extremely quiet, but not sure why he would have to address the other issues.
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Yeah, bass extension has never been a problem with my OTL. In fact I went from an Pass Aleph 30 to my Atma-sphere S-30, in large part because of the better bass performance with my speakers. I have full range drivers, and they were overdamped by the Aleph 30, a problem I didn't have with most of the Nelson Pass First Watt designs (F1, F2, F3 were all great in terms of bass, especially the F1).
Roger, when you give these power numbers, I assume you are referring about OTLs into 8 ohms? (since power output goes up into higher ohm speakers, for those less familiar with OTL design).
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Hi,
I need a monoblock passive/integrated(just one input for CD) OTL with two 6C33 for about 20W.
No PushPull, preference to SE Parallel.
The speaker is a FR crossoverless, OTL with popular tubes are useless to me, as I stock some 6C33.
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Also occurs to me that if folks are going to discuss their experiences with OTLs here, we should specify exactly what OTL you had experience with. Not all OTL amps are created equal. Some are more reliable than others, some have greater negative feedback, etc.
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Mine is a Transcendent Sound. I bought the single ended OTL that puts out 1.5 wpc. Needs efficient speakers, but very full ranged and detailed sound. I have also tried their 40 wpc monoblocks which is a pp class ab amp design.
Steve
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My OTL's are 140 watt Class A mono blocks. Given the relatively low sensitivity of my speakers (Vandy 5A's), lower power amps aren't the best choice.
George
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I am also using an Atma-Sphere S-30 and for a while simultaneously had the Transcendent Sound T-16. The S-30 stayed in part because my Audiokinesis Jazz Modules were voiced with this amp and they prefer a higher output impedance from the amp. The S-30 (zero feedback configuration) is in the 7 ohm range while the T-16 was 1 ohm. Just an overall better match for my speakers which are 12 ohm maximum, 8 ohm minimum, and pretty much 9 - 10 ohm throughout most of the frequency range. In fact, I use the 12 ohm configuration on my EM-7 12v which sounded better than the the 3 ohm configuration with these speakers.
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George,
How can your amp run in class A? The power line requirements would be industrial.
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Clio09,
I have many times wondered about the Transcendent OTLs. Can you say more about the differences that you heard between the 2 in your system?
BTW, for those of you with speakers that don't require much gain, I highly recommend the jumpers available for the 2nd pair of 6SN7s with the Atma-sphere S-30 (and I assume the Atma monos too, but not certain).
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George,
How can your amp run in class A? The power line requirements would be industrial.
Not quite at industrial levels, but they do use a lot of juice.
I run my whole system on a dedicated 20 amp line. Each amps pulls around 500 watts from the wall when turned on. So figure around half of the 20 amp line goes toward powering the amps.
George
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George,
How can your amp run in class A? The power line requirements would be industrial.
http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713
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Also occurs to me that if folks are going to discuss their experiences with OTLs here, we should specify exactly what OTL you had experience with. Not all OTL amps are created equal. Some are more reliable than others, some have greater negative feedback, etc.
To compare amps these are the relevant things:
1. power into what impedance
2. number and type of output tubes per channel
3. output impedance or damping
4. amount of feedback
5. retail price
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Roger,
There seem to be three popular methods of OTL. Futterman, Circlotron, and Rozenblit. Could you please comment on them ( for educating us) and the popular tubes used? There probably are only about a dozen people in the country that understand OTL amps. Thank you.
steve
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http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713
I spent some time on this thread a few days ago. I couldn't make much sense out of it. Please help us. Has Ralph spoken to this topic?
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http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713
Seems like a pretty worthless thread to link to. There is no substance, just some troll making a comment.
George
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To compare amps these are the relevant things:
1. power into what impedance
2. number and type of output tubes per channel
3. output impedance or damping
4. amount of feedback
5. retail price
Since you asked Roger...
Most of what you are asking for with my amps in on the Atma-Sphere website.
http://www.atma-sphere.com/Products/#MA-1
George
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Seems like a pretty worthless thread to link to. There is no substance, just some troll making a comment.
George
Really? I find Ralphs technical comments interesting from what I remember.
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Really? I find Ralphs technical comments interesting from what I remember.
Ralph only posted once in that thread (at least that's all I see) and it wasn't an overly technical post, it was about being compared to Futterman designs.
Am I missing something? :scratch:
George
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Ralph only posted once in that thread (at least that's all I see) and it wasn't an overly technical post, it was about being compared to Futterman designs.
Am I missing something? :scratch:
George
I guess he removed a BUNCH of his posts then.
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roscoeiii, the T-16 for the money, especially used which is how I acquired it, is quite a bargain. It certainly has a number of the qualities an OTL is known for. If you have speakers that can work well with a 1 ohm output impedance (think solid state amps) then I have no doubt this amplifier can provide quite high levels of satisfaction. With my speakers it sounded slightly anemic versus the Atma-Sphere. Yes it is half the power but even when I pulled a couple tubes from each channel of the S-30, I found the same results (also, I use the jumpers in my S-30 so the gain of the amp is very low and sensitivity is close to 6V). Biggest difference was the bass. Before comparing the T-16 to the S-30 I was always a bit unsatisfied with the bass of the T-16. After comparing it to the S-30 it was evident that the bass of the S-30 was not only more extended, but fuller and more natural sounding as well. Again, I run my S-30 without feedback and I also have the beefed up power supply.
FWIW I use the S-30 with a Resolution Audio Cantata and Otari MX-5050 BII tape deck. Both are connected directly to the balanced inputs (I swap) cables when I want to switch sources) and drive the S-30 via their internal volume controls.
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Hi Clio09,
Thanks for the update. Yes, I really like the bass on the S-30. Is the conversion to zero feedback a mod that Ralph does? There's a pretty low amount of negative feedback as is, IIRC. And one of these days I will scrape up enough cash to get the upgrade to 3.1 status (I have a 2). And then maybe it will be time for the power supply upgrade, or the caps, or...
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Roger,
There seem to be three popular methods of OTL. Futterman, Circlotron, and Rozenblit. Could you please comment on them ( for educating us) and the popular tubes used? There probably are only about a dozen people in the country that understand OTL amps. Thank you.
steve
Other than a few very low power single ended designs, OTLs are push-pull and usually not class A though they do, as all push-pull amps, have a class A region. The class A region may be just a few watts on a 100 watt OTL. More about that later. A single ended OTL is hampered in that the output current will be only twice the standing current. As the standing current is likely to be limited to 0.2 amps per tube the peak output power is that squared times the 8 ohm load resulting in 0.32 watts per tube. More tubes, more power. As you double the number of tubes you quadruple the power, hence it is tempting to have a lot of tubes.
In all push-pull amps there is a tube or bank of tubes for the positive half cycle and the same for the negative. The Futterman amps employ a single power supply around 300 Volts with the negative end on ground. He also employs a large output coupling capacitor (1000 uF) to couple the 150 VDC at the output point to the speaker.
In the Futterman the output tubes are arranged as a "totem pole" one above the other with 150 V at their junction. The top tube can pull up to 300 volts and the bottom can pull down to ground. The amount of current is about 1.5 amps per tube. The tubes idle at about 100-200 mA per tube resulting in 15-30 watts dissipation per tube. To get more power just put more tubes in parallel. I used 4 in parallel per bank in the SA-4.
In the SA-4 and the designs I am working on now I use a split supply of plus 150 and minus 150 V (referenced to ground) and direct couple to the speaker thus eliminating the output capacitor making it a DC amplifier. To keep it centered (zero offset) I use a servo and a protection circuit should a tube short or large DC appear at the output.
The Atmasphere amps are a Circlotron configuration which is essentially a bridge with two floating power supplies of about 135 Vdc each neither referenced to ground. The output has a virtual ground at center of the speaker (if it had one). The original Electrovoice amp had this ground on the center tap of it's output transformer. The EV amps had an output transformer which Ralph eliminated by using a great number of tubes in parallel. I have worked on a MA-1 which has 12 output tubes per channel. Although Ralph feels tube matching is not important, I did match the output tubes so that they shared current equally. Otherwise some ran hot and others cool, shortening the life of the hot ones.
I note that the Atmasphere S-30 has an odd number of output tubes per channel. I have not seen a schematic of this amp and would be interested in looking at one. I suspect the odd tube may be the driver for the other parallel output tubes and Ralph may be going into positive grid voltages to get more power. Any comments?
As to Rozenblit's cicuits, the early ones I saw in Audio Amateur and Glass audio were of totem pole (Futterman) design but he was trying to improve on the driver. There were several mistakes in that article. I don't know what he is doing now. Again, a schematic or perhaps the patent number would help.
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Roger thank you for your contributions here as well as starting this quite informative thread.
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And Roger, I notice you added an autoformer question to the poll. Maybe you don't want to bias these results, but eventually, I'd like to hear your thoughts on autoformers with OTLs.
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The Atmasphere amps are a Circlotron configuration which is essentially a bridge with two floating power supplies of about 135 Vdc each neither referenced to ground. The output has a virtual ground at center of the speaker (if it had one). The original Electrovoice amp had this ground on the center tap of it's output transformer. The EV amps had an output transformer which Ralph eliminated by using a great number of tubes in parallel. I have worked on a MA-1 which has 12 output tubes per channel. Although Ralph feels tube matching is not important, I did match the output tubes so that they shared current equally. Otherwise some ran hot and others cool, shortening the life of the hot ones.
Roger,
Matching output tubes can definitely make a noticeable difference. A friend did this for me and it was easy to hear the improvement.
BTW, the newer MA-1 amps have 14 output tubes per amp and 5 6SN7 tubes.
Thanks for allowing us to talk about competing products on your Circle. That is a classy thing to do and shows your level of maturity, as well as confidence in your products.
George
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No bass? You should have been to the CES show back in the 80's when crazy Harvey Rosenberg, rip, brought in The Quad ESL-63's with the NYAL OTL 1's. Every person that walked into his room looked for the subwoofers that he had none of. It was the highlight of the show. No one knew Quads could sound like that.
Roger, do you remember when Harvey pulled that off?
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And Roger, I notice you added an autoformer question to the poll. Maybe you don't want to bias these results, but eventually, I'd like to hear your thoughts on autoformers with OTLs.
I use Zeros with my MA-1's and Vandy 5A's and the system sounds better when they are in the chain. Depending on your speakers, you might think otherwise.
When I tried Zeros with M-60's and single driver, higher impedance speakers, I felt that they not only didn't make things better, they made things worse. By worse, I mean that you could clearly hear that the Zeros had been added to the equipment chain and were negatively impacting the sound.
George
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Thanks for allowing us to talk about competing products on your Circle. That is a classy thing to do and shows your level of maturity, as well as confidence in your products.
George
+1
Look forward to seeing what OTL products you decide to produce.
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As to Rozenblit's cicuits, the early ones I saw in Audio Amateur and Glass audio were of totem pole (Futterman) design but he was trying to improve on the driver. There were several mistakes in that article. I don't know what he is doing now. Again, a schematic or perhaps the patent number would help.
A simple google search for me showed his patent clearly. The patent number is: 5604461. The paper he presented to receive his patent is right here (http://www.freepatentsonline.com/5604461.pdf).
What he is doing now, is a significant departure from his older circuits in Glass Audio. The patent eventually became what was in his T8 amplifier. His Beast is his best application of the aforementioned patent, with some improvements, and more powerful as well.
Best,
Anand.
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Thanks Roger, this topic is both informative and unique because most designers have an agenda of their own which does not include input from the circle.
At one time, I would have been against using an auto-former, but considering your past clever transformer applications, and the potential to reduce the number of output tubes, I can only say go for it.
Steve
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No bass? You should have been to the CES show back in the 80's when crazy Harvey Rosenberg, rip, brought in The Quad ESL-63's with the NYAL OTL 1's. Every person that walked into his room looked for the subwoofers that he had none of. It was the highlight of the show. No one knew Quads could sound like that.
Roger, do you remember when Harvey pulled that off?
This thread might be of interest:
http://www.whatsbestforum.com/showthread.php?2025-Constant-power-tubes-vs.-solid-state-amps-and-wild-impedance-speakers&highlight=quad
Lin
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Roger
I think the S30 gets away with odd number of tubes because the tubes are 6AS7, which are dual triodes, so 5 tubes per channel equates to 10 triodes per channel.
Regards
Gnnett
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Roger
I think the S30 gets away with odd number of tubes because the tubes are 6AS7, which are dual triodes, so 5 tubes per channel equates to 10 triodes per channel.
Regards
Gnnett
I considered that which means the fifth tube would have to be split, one triode pull up and one pull down in the same envelope. Seems like a lot of trouble for one extra triode. I am wondering if the fifth tube is a driver, such as a cathode follower to the outputs. Anyone care to check into this?
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Thanks Roger, this topic is both informative and unique because most designers have an agenda of their own which does not include input from the circle.
At one time, I would have been against using an auto-former, but considering your past clever transformer applications, and the potential to reduce the number of output tubes, I can only say go for it.
Steve
The greatest problem in making transformers for conventional push-pull amplifiers is the coupling of the primary sections to each other and to the secondary. I have spent years developing transformers that do this well and the ones in the RM-200 MK II are the best so far.
With an OTL topology, the transformer can become an autoformer and there is but one primary winding verses two in a conventional amplifier. The coupling problem goes away. Now the transformer can be outside the feedback loop making the internal feedback far more stable.
I have been experimenting with a totem-pole output stage with only one pair of tubes. The driver is also very simple, DC coupled and balanced drive with only two stages. This is far fewer than any existing OTL amp that I have seen. With a single pair of output tubes I can get 60 watts into 600 ohms with a good class A range of about 20 watts. Without the autoformer this same amp is a measly 6 watts.
Here's the Transcendent patent. It is far from simple. http://www.audiodesignguide.com/otl/5604461_tif_1_s0_35_r0.gif
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roscoeiii, the S-30 has 2 dB of negative feedback in stock form. Ralph, being very familiar with my speakers suggested I disable the NFB circuit and see if I liked the results. So he walked me through the process on the phone which was quite simple. I did like the results so I left the NFB circuit disabled. I also have the 3.1 upgrade.
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I hope I'm not opening up a can of worms, but no one has mentioned the Berning designs when it comes to OTL. I know Ralph and David have debated whether Berning designs are really OTL, but I think very highly of them nonetheless.
Any thoughts?
As for NFB in an OTL circuit. Is it really necessary?
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I hope I'm not opening up a can of worms, but no one has mentioned the Berning designs when it comes to OTL. I know Ralph and David have debated whether Berning designs are really OTL, but I think very highly of them nonetheless.
Any thoughts?
As for NFB in an OTL circuit. Is it really necessary
First I would like readers to temporarily drop all the notions they might have about feedback.
Here are the problems of amplifier design as I see them.
How does one achieve low output impedance which is essential for most speakers to sound good because they were very likely voiced on an amplifier with low output impedance?
The output impedance of the S-30 with 2 dB of feedback is 7 ohms. Without the feedback it would be about 20% higher making it about 8.4 ohms. I would venture to guess that any sonic difference you heard by disconnecting the feedback was due to the higher output impedance.
I do not care to make amplifiers with such a high output impedance and I generally shoot for an output impedance that is 1/10 of the load impedance. This means that the output voltage will increase only 1 dB if the load goes to infinity. The 7-8 ohm output impedance of the Atmasphere allows the output to rise 6 dB when the load goes high. This is a very audible difference as some loads go very high, usually in the bottom end.
Since the NYAL OTL/Quad 63 combination has been mentioned I have observed the following. A circlotron OTL was brought to my home and connected to my Quad 63's and there was incredibly large but not so good bass. At 50 Hz the Quads have a very high impedance and at high frequencies a very low impedance. I connected an AC voltmeter directly to the speaker terminals and observed an 8 dB rise at 50 Hz. This of course is very noticeable. This is not what Quad had in mind when they voiced the speaker with their very low impedance transistor amps. The top end was also rolled off about 6 dB due to the low impedance there. Yes, feedback is necessary and not a bad thing in a loop that does not include the output transformer.
Feedback also reduces distortion but since that comes down with output impedance I am more interested in setting the output impedance. Generally when I get this to my bogey value the distortion is less than 1% at full power and 0.1% at average listening levels.
As to the Berning design it is mostly solid state and looks more like a switching amp. It is not the kind of OTL we are discussing here.
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I guess he removed a BUNCH of his posts then.
Not that I remember, anyway.
Its a simple fact that we could have sold a lot more amplifiers if we only used negative feedback. The reason that we don't is that its use causes the amp to violate a fundamental rule of human hearing: how we perceive the volume of a sound. Negative feedback can increase trace amounts of odd-ordered harmonic distortion (while overall otherwise decreasing THD). Its the trace amounts (specifically the 5th, 7th and 9th harmonics) that are used by the ear/brain system to figure out how loud a sound is.
So when these harmonics are altered even slightly, the amp will sound brighter and louder than it really is. This is one reason why two amps on a bench can measure flat but one is bright and the other is not.
Many speakers (not all, by any means) are designed to expect the amp to have some sort of voltage response at its output that is or approaches a constant voltage characteristic. With tubes this pretty well means that you will have to run feedback. The result will be unnatural brightness and unnatural loudness cues. IOW, with such speakers **it will never sound real**.
If the result will never sound real, why bother?
more on this topic: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php (http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php)
from Roger:
I note that the Atmasphere S-30 has an odd number of output tubes per channel. I have not seen a schematic of this amp and would be interested in looking at one. I suspect the odd tube may be the driver for the other parallel output tubes and Ralph may be going into positive grid voltages to get more power. Any comments?
We built the driver circuit in our amps to be able to push the power tubes into the class A2 (grid current) region. The 6AS7G remains quite linear in this portion of the curve and so allows us to do so. Conversely, the tube does not go into cutoff very easily either, so (as long as the amp is driving the right load) they are class A2 designs. There are those that claim that this is not 'true' or 'pure' class A, and such statements would be true if we were claiming that the amp was A1. But its not, and does trade off some linearity for more power. However, we also gain instantaneous overload recovery, with no issues of bias stability, both of which are problems in most OTL designs.
The S-30 has an odd number of tubes only because it worked out that way- as many of you know, OTLs have an economy of scale, the smaller you make them the less efficient they become. So the additional tube sections offered by the 5th 6AS7G were helpful.
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Not that I remember, anyway.
Its a simple fact that we could have sold a lot more amplifiers if we only used negative feedback. The reason that we don't is that its use causes the amp to violate a fundamental rule of human hearing: how we perceive the volume of a sound. Negative feedback can increase trace amounts of odd-ordered harmonic distortion (while overall otherwise decreasing THD). Its the trace amounts (specifically the 5th, 7th and 9th harmonics) that are used by the ear/brain system to figure out how loud a sound is.
So when these harmonics are altered even slightly, the amp will sound brighter and louder than it really is. This is one reason why two amps on a bench can measure flat but one is bright and the other is not.
Many speakers (not all, by any means) are designed to expect the amp to have some sort of voltage response at its output that is or approaches a constant voltage characteristic. With tubes this pretty well means that you will have to run feedback. The result will be unnatural brightness and unnatural loudness cues. IOW, with such speakers **it will never sound real**.
If the result will never sound real, why bother?
more on this topic: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php (http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php)
from Roger:
We built the driver circuit in our amps to be able to push the power tubes into the class A2 (grid current) region. The 6AS7G remains quite linear in this portion of the curve and so allows us to do so. Conversely, the tube does not go into cutoff very easily either, so (as long as the amp is driving the right load) they are class A2 designs. There are those that claim that this is not 'true' or 'pure' class A, and such statements would be true if we were claiming that the amp was A1. But its not, and does trade off some linearity for more power. However, we also gain instantaneous overload recovery, with no issues of bias stability, both of which are problems in most OTL designs.
The S-30 has an odd number of tubes only because it worked out that way- as many of you know, OTLs have an economy of scale, the smaller you make them the less efficient they become. So the additional tube sections offered by the 5th 6AS7G were helpful.
Ralph,
Thanks for your information. I have great respect for your work. Going into the positive grid area is a good idea and I have done it in some of my designs. Have you measured the peak output current using a tone burst? I am curious as to what you can get.
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Hi Roger,
Thanks! Not to get too much into a mutual admiration society thing but that's the same with me :)
I've not messed with tone bursts especially. We *have* done things with DC voltage settings on the grids of the tubes though. The tubes can support so much current (for short periods, pardon the pun, but we are talking seconds, not milliseconds) that half of the power tubes in one of our amps can blow a power fuse that is a multiple of the correct rating quite easily without themselves being damaged. This is why having the correct fuse in place is important in our amps- having one that is over-rated can have the same results as a shorted output transistor in a solid state amplifier.
This should not be surprising though- an OTL has to drive the speakers directly and so can have some transistor-like abilities in that regard. Most people are quite surprised at how much current the output section can handle without damage- to put some numbers to that one section of the 6AS7 will handle 500ma peaks, far beyond the 120ma limit commonly seen in many spec sheets! Eric Barbour, who used to be with the Svetlana company's US office, observed this years ago and published the resulting curves on their 6AS7G datasheet.
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Roger was asking about the schematic for an Atmosphere amp. This thread discusses an Atmosphere design.
http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp.html
Retsel