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Old 1st June 2002, 12:22 PM   #11
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Post 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.
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Old 2nd June 2002, 03:14 PM   #12
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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.
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Old 2nd June 2002, 06:37 PM   #13
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Default 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
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Old 2nd June 2002, 06:53 PM   #14
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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.
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Old 2nd June 2002, 06:59 PM   #15
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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.
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Old 2nd June 2002, 09:51 PM   #16
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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.
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Old 4th June 2002, 03:24 AM   #17
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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
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Old 4th June 2002, 04:10 AM   #18
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...just as long as it's ozone and not smoke...
Good luck.

Grey
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Old 10th June 2002, 02:49 PM   #19
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Quote:
Originally posted by mrothacher
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.
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Old 11th June 2002, 05:39 AM   #20
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Actually, is this a super-symmetry cct as done with opamps?

GP.
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