[Roger A. Modjeski]

OTL amps have traditionally been monsters of consumption both tubes and electricity. Now that efficient speakers and low power amplifiers have a following, I decided to make a small, efficient and reliable OTL.  Many of you know I designed the Counterpoint SA-4 which was the first direct coupled Futterman type OTL with a servo. My new OTL-1 is a far better design and goes back to develop some of Julius Futterman's earliest work which was never put into production. His OTL-3 made by him and New York Audio Labs was a difficult beast to tame and was unfortunately made in the dark days of inefficient speakers and listeners who needed lots of watts. In developing it for high power he lost some of the simplicity of his earlier designs. The OTL-1 uses only one pair of output tubes per channel, has a servo to keep DC below 20 mV and an Impedance Converter to achieve 10 watts into any load. Output tubes are horizontal sweep types connected as triodes. They are much better suited to the high current peaks that the typicaly used 6AS7, 6080 and 6C33 types that were intended as pass tubes in power supplies. These are tubes are not designed for high peak currents as the horizontal sweep types were.

For those who are new to OTL amps there are several major design topologies, so don't lump all OTLs in one basket. If there is interest I can go into the various ones.

The facilitator has requested we discuss here rather than in the Music Reference Circle. This is a great place to discuss this low wattage OTL where I don't have to preach low wattage to people who still think you need 999 watts reserve power to control a speaker playing at 1 watt.

For additional information you may want to see the original discussion.   http://www.audiocircle.com/index.php?topic=121659.0

[Freo-1]

Roger, thanks for bringing this topic up.  I would like to keep this topic in the Low Wattage circle, since it involves both your low power design approach and high efficiency speakers.

Well done.

[Roger A. Modjeski]

Roger, thanks for bringing this topic up.  I would like to keep this topic in the Low Wattage circle, since it involves both your low power design approach and high efficiency speakers.

Well done. 

Ok, I will modify my post and suggest to keep it here.  I would like to reference the link as there is additional information there.

In addition I just posted this about low cost amps and preamps and the situation with Chinese amps as I see it.  http://www.audiocircle.com/index.php?topic=123179.0

[JLM]

Does anyone have a good OTL primer for those of us without a background in this stuff?

TIA

[Larkston Zinaspic]

http://www.atma-sphere.com/Resources/OTLs_Defined.php

Ralph Karsten (who has visited here previously) seems to have some good info on his website, but it's clear that Ralph and Roger disagree on quite a bit. In the article above, Ralph says,

"For many years the public has associated the weaknesses of the Futterman circuit with OTLs in general. Fortunately modern OTLs have solved the earlier problems of the Futterman by (for the most part) using entirely different circuitry. In fact, every manufacturer who has ever attempted to produce a Futterman amplifier has ultimately gone out of business".

Regardless of this, having heard how good Roger's amps are, I'm interested in the new Music Reference OTL. Would like to learn more, if I could.

 

[Jon L]

Impedance Converter to achieve 10 watts into any load.

I use a small SET into a Stax step-up transformer into electrostats, e.g. Stax SR009 which has 145K impedance. http://www.wooaudio.com/headphones/stax/sr009.html
Would this "impedance converter" work well with a Stax step-up transformer (such as SRD 7 Pro), or is there perhaps chance this impedance converter can be customized for electrostats somehow?

[Roger A. Modjeski]

http://www.atma-sphere.com/Resources/OTLs_Defined.php

Ralph Karsten (who has visited here previously) seems to have some good info on his website, but it's clear that Ralph and Roger disagree on quite a bit. In the article above, Ralph says,

"For many years the public has associated the weaknesses of the Futterman circuit with OTLs in general. Fortunately modern OTLs have solved the earlier problems of the Futterman by (for the most part) using entirely different circuitry. In fact, every manufacturer who has ever attempted to produce a Futterman amplifier has ultimately gone out of business".

Regardless of this, having heard how good Roger's amps are, I'm interested in the new Music Reference OTL. Would like to learn more, if I could.

Thanks for Ralph's quote. I doubt my OTL will sink Music Reference.  I like Ralph and always enjoy his unusual thoughts about the voltage an current paradigm, however he has a real thing about Futterman. I hope he can refrain from such comments about my OTL. Maybe I should stop saying its base on Futterman's work but I like to give credit where credit it due. There is one time I think he said in Stereophile "The Futterman never should have seen the light of day". Understanding and making the Futterman circuit is challenging. I've been on and off the project since 1984.

[Roger A. Modjeski]

Does anyone have a good OTL primer for those of us without a background in this stuff?

TIA

This is your primer...    What specifically do you want to know?

[Roger A. Modjeski]

I use a small SET into a Stax step-up transformer into electrostats, e.g. Stax SR009 which has 145K impedance. http://www.wooaudio.com/headphones/stax/sr009.html
Would this "impedance converter" work well with a Stax step-up transformer (such as SRD 7 Pro), or is there perhaps chance this impedance converter can be customized for electrostats somehow?

I read the link but didn't see the transformer box specs. However I will say that a high voltage direct drive amp would be the ideal thing and I have made one for my STAX. Great phones BTW

Would also need to know what amp you are using. You need to be in the low power range of it and you may not be hearing the STAX at their best.

[Jon L]

However I will say that a high voltage direct drive amp would be the ideal thing and I have made one for my STAX. Great phones BTW

Have you considered making your Stax direct drive amp available commercially?  Most Stax fans feel Stax currently does not make an amp capable of driving their top-line 'stats to their full potential (Omega II, SR009, etc), and capable third-party amps usually command long wait-times and large prices 

[JLM]

This is your primer...    What specifically do you want to know?

How about a reference to an OTL 101 class/book?  Don't know enough to ask many questions.  Gathering in bits and pieces is extremely frustrating/confusing and I'm too old/impatient to spend decades learning.  You don't want us to dumpster dive the internet for scraps of junk information do you?  Isn't there a handbook (hopefully short) or a few easy to understand links that can spell out the basics of OTL?  Or is this an exclusive club (that I thoroughly detest)?

And while I'm at it, can you recommend a "tube amp for dummies" reference?

[JohnR]

Perhaps rozenblits book beginners guide to tube audio design. I think his first otl is described there, good primer.

[JLM]

Thanks John.

[Roger A. Modjeski]

Perhaps rozenblits book beginners guide to tube audio design. I think his first otl is described there, good primer.

I have read rozenblits in Glass Audio and I find some of his explanations to be flawed. This is a nice way to say some of his information and thinking is incorrect and some of it is going about it a difficult way. So sadly I don't recommend his book.

Many of the people who write about OTL amplifiers think they have to match the output impedance of the tube to the load by piling on more tubes. One never reaches the speaker impedance and that is not really the point unless one is trying to design a no feedback amplifier. When they start talking about matching source and load impedance for "maximum power transfer" they are using a concept that Edison used in his first power plant in NYC in 1884. Edison knew, perhaps by experience, that you get the most power to out of the station when the load and source impedance are equal. At that point half of the power is lost in the generator and half goes out to the load.  He was burning up a lot of coal and wanted to deliver as much electricity as he could. In doing this he was running his generators at only 50% efficiency. It was really the best he could do at that time.  All 20th century generators, like those big ones at Hoover Dam, run at over 95% efficiency. This concept does not apply to amplifier design and is bad way of looking at things especially when high impedance pentodes are employed.

I am wary of anyone who says the drive in the Futterman is unequal as he and many others do. Saying this tells me they have not studied the circuit and do not understand it well enough to see that the drives are equal. If they can't see that I am suspicious of the rest of what they have to say. Most designers who do not use the Futterman circuit start their papers by telling the reader how wrong it is and how they have gone about solving the problem, which is not a problem at all.

I invite specific questions that will lead to a better understanding of OTL amps.

[steve f]

I just took down my post. Roger answered a couple of concerns in his post.

My biggest questions involve an overview of how the Futterman and Rozenblit circuitwork. They are both so different from anything else out there. Is a brief overview possible to help us get a group conversation started.

Steve

PS: I look forward to the new small Music Reference amp.

[matt.w]

Have you considered making your Stax direct drive amp available commercially?  Most Stax fans feel Stax currently does not make an amp capable of driving their top-line 'stats to their full potential (Omega II, SR009, etc), and capable third-party amps usually command long wait-times and large prices 

If so, I'd love to see the inevitable forum war between Roger and Kevin Gilmore or Birgir

[Roger A. Modjeski]

If so, I'd love to see the inevitable forum war between Roger and Kevin Gilmore or Birgir

I don't know them. Do you have links to something I can read?

[matt.w]

Kevin Gilmore is an amp designer in his spare time, and is basically the authority when it comes to modern electrostatic headphone amplification.

His Blue Hawaii (http://headwize.com/?page_id=751/) is manufactured by Headamp (http://www.headamp.com/electrostat_amps/bhse/index.htm). There are a bunch more designs out there for DIY consumption, including a reverse-engineering of the Stax T2.

Birgir goes by 'spritzer' on forums, is incredibly knowledgeable about electrostatic headphones and Stax history, and I would say informally heads up the Stax hobbyist community.

To check out activity in this area don a flamesuit, keep your head low, and head over to www.head-case.org

/OTL hijack

[Freo-1]

Kevin Gilmore is an amp designer in his spare time, and is basically the authority when it comes to modern electrostatic headphone amplification.

His Blue Hawaii (http://headwize.com/?page_id=751/) is manufactured by Headamp (http://www.headamp.com/electrostat_amps/bhse/index.htm). There are a bunch more designs out there for DIY consumption, including a reverse-engineering of the Stax T2.

Birgir goes by 'spritzer' on forums, is incredibly knowledgeable about electrostatic headphones and Stax history, and I would say informally heads up the Stax hobbyist community.

To check out activity in this area don a flamesuit, keep your head low, and head over to www.head-case.org

/OTL hijack

I've used a procedure I found from spritzer on that website to adjust my Stax Headphone Amps (it worked like a charm). 

A few years back, I heard a pir of heavily modified K-Horns with a low powered SE amp.  It was stunning, and made me re-think the whole K-Horn and low wattage approach.  I would dearly love to hear Roger's OTL with those type of speakers.

[Roger A. Modjeski]

Kevin Gilmore is an amp designer in his spare time, and is basically the authority when it comes to modern electrostatic headphone amplification.

His Blue Hawaii (http://headwize.com/?page_id=751/) is manufactured by Headamp (http://www.headamp.com/electrostat_amps/bhse/index.htm). There are a bunch more designs out there for DIY consumption, including a reverse-engineering of the Stax T2.

Birgir goes by 'spritzer' on forums, is incredibly knowledgeable about electrostatic headphones and Stax history, and I would say informally heads up the Stax hobbyist community.

To check out activity in this area don a flamesuit, keep your head low, and head over to www.head-case.org

/OTL hijack

The Blue Hawaii looks interesting and the low price makes me wonder where it is made. I presume it's direct sale so that helps. It does have a lot of transistors. The use of EL-34s is a bit of overkill. It appears there is a kit.

Head-case looks a place someone could waste a lot of time. Can you send me a link or two that are interesting?