[Roger A. Modjeski]

Hello to all and welcome to the brand-spanking-new MR circle. Thanks for the vote of confidence and commentary from so many of you. For starters I would like to clarify some of the technical terms we use in the field of amplifiers and speakers. Please let me know if the level is good for you, or too advanced or too simple. The topic will be "BIAS". I will get that post up by Sunday evening or sooner.

Here's an offer that I would like to hear back on, preferably here so we all get to hear what each has to say.

I have spent many years as a teacher at several colleges. I currently teach a course called "Just Fix It" at Santa Barbara City College, Adult Ed. division. I also taught a guitar amp class where we built 3 watt single ended amplifiers using a 6V6 and a 12AX7. Somewhat like a Fender Champ but with bass, mid and treble controls in place of a single tone control.

I would be happy to offer a weekend seminar on preamps, amplifiers and speakers to those who would like to come to my lovely town of Santa Barbara, CA and spend a weekend learning what goes on behind the front panel along with tips to make your system sound better. Bring the wife and kids. They can enjoy our lovely beaches while we get to nerd out to our heart's content. The school has offered me use of my regular classroom on the weekend to hold such a class. We have room for 40. My lab there also has 6 oscilloscopes, oscillators, Fluke multimeters. Although generous, the school has to make some money and we have tentatively planned a cost of $200 for a Saturday and Sunday class. We would meet at 9 till noon, break for lunch till 1 and go till 5. I promise you will be entertained and enlightened.


Roger

[topround]

Roger,
Kudos to you to try such a thing It is great to have such a person be able to explain how such things work. If I was on the west coast I would definitely sign up for such a class, even if it were over my head   which it is
You will be a welcome addition to this audio circle

Welcome aboard!!

mike

[Whitese]

I am an east coast dweller, but that is a great idea...! 

[Double Ugly]

Welcome (again) to the forum, Roger. 

Unless the past few days prove an anomaly, my concerns about creating a new manufacturer's circle sans manufacturer were unfounded.  I'm genuinely happy to wrong, and I look forward to trying to figure out what you're talking about (meaning I listen, but unfortunately don't build ). 

Best regards,

-Jim

PS - Congratulations on the brilliant (and extremely generous!) idea of offering seminars.  It certainly serves notice of your love for and dedication to audio.  Would that I could attend! 

[Roger A. Modjeski]

Jim,

Thanks for that. Any topics you would like to see covered?

Roger

[Nick B]

A 2 day seminar.... I think my head would be spinning, but that sounds like a great idea  . I used to live in so. California, but still have family there. So I might just be able to sign up someday if you have such an event.
Nick
 

[Srajan Ebaen]

Hello Roger,

Why not jump feet first in the frying pan and cover the contentious subject of Music Reference amplifier power ratings which tube experts elsewhere claim are impossible from the tubes you use? I know you've got the numbers on your side but just how do you do it and why hasn't it been done before?

Or, for a different subject, explain the genesis of your newest spud amp concept and how you selected the output tubes you've chosen?

Just creating some unnecessary work for you on a sunny Monday morning in Cyprus 

[Roger A. Modjeski]

As to power rating I presume you are speaking of the RM-10 and the 245.1; both at twice the normal power? I assure you these power ratings specified are accurate. The nay-sayers are like flat-earthers.

For the RM-10 the answer is high plate voltage, low screen voltage, low plate current. All of which make the output transformer impedance many times higher than previous applications. It's a new application I created for the EL-84. Most designers take one of the published applications as if there were no alternatives. One could write 50 distinctly different applications all with different power output, b+, plate current and output transformer impedance. The 245.1 application is new also with higher plate voltage, lower plate current and runs the tube around 8 watts dissipation, 2 watts lower than max where most designs run them.

I'll have a white paper up soon about why I choose the EM7. The short answer is that it has low output impedance, sufficient gain and low losses which allow the amp to approach 50% efficiency. Typical 300 B amplifiers get about 1/4 their dissipation as output. These are all round numbers give or take a bit, but they illustrate the magnitude of difference.

Roger

[Double Ugly]

Any topics you would like to see covered?

Thanks for asking, but no. 

I have a very limited understanding of even the basics, so virtually any aspect of the topic will be ripe with learning opportunities.   

-Jim

[Roger A. Modjeski]

oh, there must be something aa

[Grover]

Welcome Roger and thanks for starting a Circle.


I'd be extremely interested in taking a weekend course.  I'm only about an hour or so away from Santa Barbara.  One concern I have would be matching the course topics to the experience level of participants.  I've built a few kits, but don't really know the theory and am not any sort of engineer.

As far as white papers and topics for discussion:  It might be interesting to go through some basic tube circuit design.  We've all heard terms such as SRPP, Cascode Follower, Constant Current Source - but what do these really mean in a design sense?  What are the benefits and/or drawbacks of each.

It might also be interesting to talk about capacitors.  How does size affect sound performance in coupling capacitors - is bigger better?  What does a coupling capacitor do  as compared to a power supply capacitor? 

 

[DavidDG]

Hi Roger,

If memory serves me, I recall that you designed an OTL amplifier for another manufacturer, but i can't remember who.

I would be interested to get your thoughts on differant OTL circuit types and what you believe to be the best.

Regards,

Dave

[Roger A. Modjeski]

Welcome Roger and thanks for starting a Circle.


I'd be extremely interested in taking a weekend course.  I'm only about an hour or so away from Santa Barbara.  One concern I have would be matching the course topics to the experience level of participants.  I've built a few kits, but don't really know the theory and am not any sort of engineer.

As far as white papers and topics for discussion:  It might be interesting to go through some basic tube circuit design.  We've all heard terms such as SRPP, Cascode Follower, Constant Current Source - but what do these really mean in a design sense?  What are the benefits and/or drawbacks of each.

It might also be interesting to talk about capacitors.  How does size affect sound performance in coupling capacitors - is bigger better?  What does a coupling capacitor do  as compared to a power supply capacitor? 

 

I am accustomed to hitting things on many levels. I currently speak to  students with a wide range of experience making sure everyone leaves with something.

Has anyone read Art Dudleys article on OTL's in the most recent Stereophile? If so what do you think of it?

[Roger A. Modjeski]

Hi Roger,

If memory serves me, I recall that you designed an OTL amplifier for another manufacturer, but i can't remember who.

I would be interested to get your thoughts on differant OTL circuit types and what you believe to be the best.

Regards,

Dave

The OTL I designed, first for Music Reference, became the Counterpoint SA-4. It won quite a few awards both here and in Japan. They only built 100 pairs I am told. It was not a happy collaboration, but hey, they are gone and I'm still here. I could revive that circuit with some better output tubes if there is interest.

To give credit where credit is due, my OTL design started with the circuit that Julius Futterman developed. I had worked on a pair of the "Harvard Electronics" version around 1974 when I was working at Atlantis Sound. I later had the pleasure of visiting him in his Manhattan workshop. A very sweet and dedicated man  winding his own transformers and hand building amps in BUD chassis. He showed me a log book of every amp he had made.

The improvement I made was to make the whole circuit DC coupled from input to output and eliminate the big electrolytic capacitor that coupled the speaker to the amp. This is similar to what I did in the Beveridge RM-1 preamp. Since then I have read several articles on his circuit, written by people who do not understand how his circuit works. I also had the dubious pleasure of re-working a pair of Sans Pariel amps for a local customer. They attempted to improve Futterman's design and made a real mess of it. There were four adjustment pots and a meter but the adjustments interacted making the amp very difficult to set up. They would have done better to just copy his. When I was done, I had basically re-designed these amps to be as close to Futterman's as possible. This was no easy task as they were built on PC boards. They sounded much better and were stable for the first time in their life.

The other OTL design I know of is the Atma Sphere, which is a version of the Wiggins Cyclotron. Over the past year I have been helping a client get his going. I have made a few modifications to the fusing and measuring routine making the amp much more reliable and easy to adjust. Ralph, the designer, has been very helpful in this process.

The most important thing to know about all OTL's is that they are limited in current. There is no way around it. The best tubes are good for an amp or two. My design had 8 output tubes, 4 to pull up 4 to pull down. They could provide 8 amps to the load at best. On an 8 ohm load that's a little over 100 watts. On a 4 ohm load about half that. But on a 16 ohm I got about 200 watts. Too bad there aren't more power hungry 16 ohm speakers out there.

Anyone care to share their experience with any of the OTL's?

Did anyone catch Art Dudley's OTL article in May issue of Stereophile pg. 33? Does anyone have any experience with the Joule OTL that he is writing about?

[Whitese]

Good reading Roger...

I just sold yesterday my OTL Atma-Sphere...I honestly didnt want to deal with so many tubes and the heat, but it did sound extremely good. 

Have you ever had a chance to listen to David Berning's OTL's? he has a SET (300B or 811) OTL and a ZH270 ?

[richidoo]

Hi Roger,
We have a mutual friend in Henry C from Raleigh. He is VERY excited for his RM9 to arrive. He thinks you are DA MAN! and from what I have been reading about you at his suggestion, I think he is right!

My OTL story: Last summer, I had a few guys over to listen and gab. Some brought gear. One brought his SansPareils. He bought them in NY many years ago and they soon after caught on fire. Nobody could fix them, so he stored them for 20+ years. A talented local tech got them going again without schematics and I heard them on Quad 989s to my delight a few months previous to them coming to my house. So then here they are connected to my new Legacy Focus speakers. We were hearing a slight hum and couldn't figure it out. Lots of gear around, powr supplies, you know the typical gear party mess. Suddenly the inputs were disconnected from the amps, someone trying to isolate the slight buzz. Within seconds the left amp started warbling around 800Hz and just went up from there like a screech to ultrasonic. A couple seconds later I see lightning coming out of the ribbon tweeter. What seemed like an eternity later someone finally shut off the power. Being relatively new to high end audio, I was about to throw up thinking my speakers were now junk. Bill at Legacy inspected everything on their bench and no harm done. New tweeter made it good as new, but I will always keep a guard posted near any OTL that comes in my house again!  But what an incredible sound they make playing music, that is...

I'm looking forward to meeting you, maybe at RMAF, or maybe at a seminar. I would certainly be up for that.  I would like to hear you speak about strange and wonderful tube circuits, like cyclotron, aikido, White follower, and of course your innovative designs, etc.  Also the very basics to help a new tinkerer start experimenting. How to make a cheap and quiet 300V preamp PS, The basics of good power amplifier design, etc. Might not squeeze all that into two days though! hehe   

I vaguely remember Art's article. I heard the Joules at RMAF playing on Analysis Audio ribbons. It was very lovely and almost ballsy, but I thought not such a good match to that speaker. Highs were missing much above ~12k, but LPs of Louis and Ella, Sinatra never sounded so good to my ears.
Rich

[hiradi]

Hello Roger...
I would just like to say "Hello" from Australia    Way back in the late 70's Richard Teuber started  me of with your phono and pre-amp and here I am 30 years later finding myself returning to your very musical gear, running a RM 10 into Horning Alkibiades speakers ,very ,very musical !!!!...it's been a long journey and I am looking forward of reading more of your articles here...keep up the good work...

[Roger A. Modjeski]

richidoo,

Thanks for sharing your experience with the OTLs. One has to be very careful with the design to keep catastrophe at bay. Disconnecting inputs from a working amp is dangerous for sure. One thing I do recommend when hunting down hum problems is to turn the power amp off. Let it discharge for a minute and put shorting plugs in the inputs. Then you will here what I call the "resident hum" of the amplifier. An open input is not valid as most preamps provide a fairly low source impedance which we want to mimic. If you knew the output impedance of the preamp you could put a resistor of that value in the shorting plug. That would be a very accurate way check things out.

The OTL I designed for Counterpoint (SA-4) had a relay and detection circuit to prevent DC damage to the speaker. Preventing a full power 800 Hz tone would be another matter. But I am thinking about it as I write this.

Roger

[Roger A. Modjeski]

Hello Roger...
I would just like to say "Hello" from Australia    Way back in the late 70's Richard Teuber started  me of with your phono and pre-amp and here I am 30 years later finding myself returning to your very musical gear, running a RM 10 into Horning Alkibiades speakers ,very ,very musical !!!!...it's been a long journey and I am looking forward of reading more of your articles here...keep up the good work...

Glad you found your way back. Wasn't it Jeffery Teuber?

[DavidDG]

Hi Roger,

Thanks for the read on OTL's.

I'm curious if you are working on a preamp to replace the RM-5?

Regards,

Dave