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Old 14th July 2015, 08:50 AM   #1
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Arrow Revisiting some "old" ideas from 1970's - IPS, OPS

Hi All,

Came across an interesting article in AES library, published in 1978 by Sansui engineers, describing the vertical differential cascade as a good way of driving VAS - higher slew rate, lower TIM, good stability (see the small picture attached).

So, I thought - why not giving it a try, using the modern active devices.
The whole thing is called Double Complementary Differential Circuit - I call it DC2.

One more thing - an OPS topology, used in a couple of Nakamichi power amps (420, 620) - I consider it great. I have simplified it a bit, still performing very good. In order to have a thermal feedback, Nakamichi places all transistors on the same main heatsink. I think, having the first pair together with the drivers on a separate one could be a good option as well - needs some testing. I call this arrangement Double Diamond Follower - D2F.

Simulation shows rather lower THD and narrower crossower region, comparing to conventional EF2/EF3 topology. No bias spreader required - the left side of a diamond keeps the bias of the right side regulated properly.

Attached is an experimental full power amp schematic (jpg and pdf), combining both cool topologies, along with stability analysis for the whole thing. I have also simulated the D2F OPS with my CF-FET V2.0 front-end - showing an outstanding performance.

Most likely, I'm going to test the OPS first. The whole thing may be built on a single PCB or as two separate modules (servo should be located on the IPS board). Number of output devices may be increased, although, with 2-3 pairs of MT200 Sankens it's already pretty much ok

I did not have time to come up with any PCBs as yet.

So, what do you think? Comments/suggestions are welcome.
Brother Pete, if you'd like to simulate/check/amend/improve something - just go ahead

Cheers,
Val
Attached Images
File Type: jpg Complimentary Diff Circuit.JPG (25.5 KB, 782 views)
File Type: jpg @DC2-Sch-01.JPG (168.4 KB, 793 views)
File Type: jpg DC2 Pilot - AC Analysis.JPG (88.6 KB, 772 views)
Attached Files
File Type: pdf @DC2-Sch-01.pdf (64.9 KB, 100 views)
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Old 14th July 2015, 10:44 AM   #2
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Looks like an interesting project! Only one hard to find device. Are D3 and D6 going to handle the current of the input stage?
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Old 14th July 2015, 04:24 PM   #3
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Interesting OPS. Much like the plain diamond buffer, it affords you more buffer stages with the voltage drop of merely an EF2. I don't see why XO region would be lower than for an EF2 though - I suppose being able to run the drivers fairly hot would have something to do with that.

But MJE340/350 at only ~2 mA? Won't that be a tad slow?
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Old 14th July 2015, 05:12 PM   #4
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these (and other) designs were popular during the "slew rate" wars some years ago.
some designs were so hung up on speed they forgot about things like overload & rail-sticking, common mode conduction in the output stage, latch-up, etc.

be careful or have a stock of spare devices ...


mlloyd1
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Old 14th July 2015, 05:55 PM   #5
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Hi Val,
very interesting, like all of yours
..but look at the supply voltage of the LF411.
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Old 14th July 2015, 09:40 PM   #6
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Quote:
So, what do you think? Comments/suggestions are welcome.
Brother Pete, if you'd like to simulate/check/amend/improve something - just go ahead Click the image to open in full size.
Yup , I'll individually simulate OPS/IPS. Combine either with various other
IPS/OPS's , see whether both are "universal".

Edit - we won't let em' get "bored".
OS
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Old 15th July 2015, 09:11 AM   #7
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Quote:
Originally Posted by jwilhelm View Post
Looks like an interesting project! Only one hard to find device. Are D3 and D6 going to handle the current of the input stage?
Which one is hard to find?

D3, D6 - I hope so. It's close to 30mA, but 1n4148 are rated for 150mA.
In the worst case we will just use something more powerful.
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Old 15th July 2015, 09:40 AM   #8
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2SK170 is getting harder to find in North America. The DIYAudio store is cleaned out of them too. I have a couple left though.
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Old 15th July 2015, 09:56 PM   #9
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Quote:
Originally Posted by sgrossklass View Post
Interesting OPS. Much like the plain diamond buffer, it affords you more buffer stages with the voltage drop of merely an EF2. I don't see why XO region would be lower than for an EF2 though - I suppose being able to run the drivers fairly hot would have something to do with that.

But MJE340/350 at only ~2 mA? Won't that be a tad slow?
Actually, after closer digging into EF2 and D2F, comparing performance with different loads / frequencies at the same quiescent currents - the crossover region difference is barely noticeable, but THD is very slightly lower with the latter one. Well, certain advantage is expected in terms of the bias regulation accuracy, I also like low voltage drop and ability to have the bias fully independent from the VAS current.

Simulation does not show any slow-down, comparing to EF2/3...
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Old 15th July 2015, 10:01 PM   #10
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Quote:
Originally Posted by mlloyd1 View Post
these (and other) designs were popular during the "slew rate" wars some years ago.
some designs were so hung up on speed they forgot about things like overload & rail-sticking, common mode conduction in the output stage, latch-up, etc.

be careful or have a stock of spare devices ...


mlloyd1
Well, CF-FET V2.0 front-end is even faster, but you-re right, this is already close to the OPS speed capabilities. Clipping looks good though - controlled by IPS. D11, D12 prevent sticking in overload conditions.

Anyway, thanks for "heads up"
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