Experimental Design - Comments, Flames Welcome

[Michael Rothacher]

First, to the uninitiated: please don't build this circuit. It is only a conceptual schematic. I'd really like to hear the thoughts of your group and, if he would do me the honor, the "Big Guy" (NP)! Particularly, I'd like to figure out if there's a simple way to employ mosfets in the OP.

Thanks

Mike

 

[Michael Rothacher]

MOSFET version

Here's the mosfet version almost ready for alligator clips and duct tape!

 

[Nelson Pass]

OK. but I have to think about it for a while.

 

[Michael Rothacher]

Excellent! Thanks Mr. Pass!

 

[GRollins]

I glanced at if briefly. Need to get back to painting walls, etc. My first impression is: Circlotron.
If so, bear in mind that that output stage is patented by Ralph Karsten.
I'll try to look at it in more detail later.

Grey

 

[reinhard]

Circlotron

Yes Circlotron,
I don´t kwow the content of the Ralph Karsten patent, but there are many Circlotron amplifiers out there: Einstein OTL, Audio Valve and every power amplifier from Balanced Audio Technology,
tube and transistor.

Reinhard from Germany

 

[Michael Rothacher]

Grey, Reinhard:

Thanks for your replies. Yes, I would say the OP qualifies as a circlotron. It does remind me of an old OTL tube schematic I saw in TAA. I had a look at the Karsten patent and I don't believe this circuit is is directly comparable. To my unqualified eye it is just a balanced bridge with batteries in two of the arms. In fact, the inspiration for the OP came from a 1960's U.S. Navy transistor book. I was originally trying to work-up something along the lines of a two stage "Son of Zen" and this where I ended up. I'm working on a currnet flow diagram to help clarify the circuit action in my mind. It is problematic at the moment, and I haven't figured out a good way to apply overall feedback. I wonder if th "X" feedback topology would work? Also I think I see an easy way to dynamically bias the OP, but it'll have to wait for the weekend. I really appreciate your comments. Please keep them coming.

Mike

 

[Michael Rothacher]

If I may go off topic for just a moment, I've been wanting to thank Nelson Pass for a decade or so. I built the A40 when I was in the Army, stationed in Germany, and I've been having fun with this hobby ever since, due mainly to the generous contributions of Mr. Pass. It is a terrific hobby.

Have a great vacation.

Mike

 

[GRollins]

I still haven't had time to go back over the circuit but, to the extent that it is (maybe) a Circlotron, yes, X will work. Keep an eye on the phase and all will be well.

Grey

 

[djk]

http://www.diyaudio.com/forums/showthread.php?threadid=99&perpage=15&pagenumber=5
Each half of the bridge acts like a current source for the other.Only one polarity output device needed.A 50W amp could be made from one pair of IRFP22N50A ,a 277W device available from DigiKey for $2.45 .Since no devices are paralleled no matching should be needed.If you have trouble visualizing this just draw it out on a sheet of paper.Prior art shows both tube (Atamasphere) and BJT (Sumo) versions of this idea.
http://www.diyaudio.com/forums/showthread.php?threadid=1796&highlight=circlotron

 

[Joe Berry]

circlotron resource

For anyone interested, there is a good source of early information on this topology at http://circlotron.tripod.com. The site focuses mainly on the erstwhile Electrovoice Circlotron product line, but also describes other implementations of the floating-bridge output stage concept that emerged under other names (e.g. cross-shunt push-pull, parallel-oppposed) in products and patents from the US, Finland, Japan, and Russia.

For more on the later implementations, you can do a search at http://www.uspto.gov on the Bongiorno (Sumo BJT) patent (US 4,229,706) and the Karsten (Atmasphere triode OTL) patent (US 4,719,431).

And just in case anyone reads the thread djk links to above, don't believe my statement that the Lamm amplifiers are of this type -- they're not.

 

[djk]

Its a question of the feedback loop and the bias scheme. If we refer to Pass 5,376,899 figure 7 we see the Bongiorno 4,229,706 (Sumo Model 9), figure 8 is the Sandman design from Wireless World 1971, and figure 9 is the Pass X design. It would be interesting to compare the three different feedback schemes on the same amp. My main concern is the bias. The Bongiorno bias scheme would work with the Hitachi type lateral FETs, but not the IR type. I guess a bias servo will be needed.

 

[reinhard]

Joe Berry / Lamm

Hello Joe Berry,

of what type are the Lamms. When NP spokes about nine gain stages in the big L, does he mean Lamm?

Reinhard

 

[Joe Berry]

djk, I think the bias scheme in the Bongiorno patent is designed to respond to changes in base current, so it is really only meant for BJT output devices. Maybe you could work up a variation on the idea suitable for MOSFETs.

The experimental circuit described in this thread is so heavily biased into class A that it may not need any thermal compensation beyond source degeneration, even with IR types.

 

[Joe Berry]

reinhard, the review of the Lamm ML2.1 by Bascom King (available from http://www.lammindustries.com) has a circuit description. I think it has five stages overall, including a vacuum tube Vas stage. The output stage is compmentary MOSFET push-pull. I don't know which "L" NP was referring to, but my guess would be Levinson.

 

[P.Lacombe]

Mike,

I think that it's a problem with the non-linearity of the driver stage M1 and M2. I suggest current-feedback whith some resistors in the source of the devices, and/or global feedback from outputs to inputs.

Regards, P.Lacombe.

 

[Michael Rothacher]

Thanks for your replies! This is excellent information. I've worked up a version with overall feedback. However, the source resitors on the input differential, as suggested by P. Lancombe, might be just as effective. The bridge bias is determined by the Vgs and transconductance of the output mosfets, so heat may be a problem. I hope Joe is right about the high bias, so complex thermal compensation isn't necessary. An adjustable current source for the input pair could trim the bridge bias as well. DJK, I really liked the idea about comparing the different feedback topologies. It would be easy enough in this circuit to try no overall feedback, normal global feedback, instrumentation amplifier feedback, even super-symmetry. I've had a look at all of the patents mentioned. It is amazing how many variations of bridge amplifiers are out there. The prototype is almost ready so, we'll see if this one actually works. I can smell the ozone already.

Thanks

Mike 😀

 

[GRollins]

...just as long as it's ozone and not smoke...
Good luck.

Grey

 

[Circlotron]

It would be easy enough in this circuit to try ... instrumentation amplifier feedback, even super-symmetry.
The prototype is almost ready so, we'll see if this one actually works. I can smell the ozone already.

Thanks

Mike 😀 [/B]

Any results yet? I know what an instrumentation amplifier is but what is i/a feedback? and super symmetry?

GP.

 

[Circlotron]

Actually, is this a super-symmetry cct as done with opamps?

GP.