Class AA Opamp Schematic - diyAudio
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Old 16th November 2009, 04:12 PM   #1
Dave S is offline Dave S  United Kingdom
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Default Class AA Opamp Schematic

I believe this concept is similar to the Technics class AA amps, however I cannot see what benefits is brings with op-amps:

NE5534 + NE5532 CLASS AA OUTPUT LINE LEVEL PREAMPLIFIER on eBay (end time 26-Nov-09 06:03:18 GMT)

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Old 16th November 2009, 04:21 PM   #2
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My only comment is that maybe the mods start an Ebay forum so that trying to speculate about the technical content of Ebay auctions doesn't add noise to the core forums.
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Old 16th November 2009, 05:03 PM   #3
Dave S is offline Dave S  United Kingdom
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I was not asking anyone to speculate about the technical content, the schematic is there for anyone to see.
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Old 16th November 2009, 06:53 PM   #4
Mooly is offline Mooly  United Kingdom
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I am intrigued... and unable to "analyse" it in my head.
It is like the Technics idea and also Aubrey Sandmans Class "S" the idea being to use a second amp to make the main voltage amplifier look like it is working into a higher impedance load.

All my instincts tell me it will not be good for sonics... particularly when you consider the typical loads it will work into anyway.

One of the more intriguing "posts" of the day
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Old 16th November 2009, 09:37 PM   #5
Dave S is offline Dave S  United Kingdom
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Well, I was intrigued enough to buy the board.
I am only using the 5532 part with an Alps blue before it at the moment, but even with a rudimentary 78/7915 it sounds pretty good.

My analysis is similar to yours - the upper opamp seems to be correcting the drop in voltage across the 10R output resistor of the lower opamp i.e. the output impedance of the pair is ~0.

I will try a proper PSU at 18V with it soon.

Last edited by Dave S; 16th November 2009 at 09:45 PM.
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Old 16th November 2009, 10:50 PM   #6
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Quote:
Originally Posted by Dave S View Post
I was not asking anyone to speculate about the technical content, the schematic is there for anyone to see.
Sorry about that, I can't see any schematic because of the firewall at my work, so I just assumed it was another "irrationally exhuberant" Ebay listing.
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Old 20th November 2009, 06:06 PM   #7
jcx is offline jcx  United States
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If forced to use the topology there are a few obvious optimizations

U2 buffer should be a different chip to eliminate thermal feedback, faster than U1 to avoid destabilizing the loop and most obviously - higher output current - a cfa op amp as shown meets these criteria but requires good layout, bypassing to work well

it also helps to check that U2 input common mode range exceeds the required output V swing

I trimmed the gain of the current buffer for mostly quadrature error current in U1's output

I also show Class A bias of U1 with bootstrap ccs R8,9, C1


Click the image to open in full size.


I believe better perfromance can be had with faster, higher current buffer chip U2 fully nested inside the feedback loop of U1 in a Jung style multiloop
Attached Files
File Type: txt Sandman_AA.asc.txt (2.5 KB, 99 views)

Last edited by jcx; 20th November 2009 at 06:23 PM.
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Old 21st November 2009, 12:54 PM   #8
Bonsai is offline Bonsai  Taiwan
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I absolutely fail to see what added value the 2nd (i.e. right most) amp has in this circuit. Can someone explain it in plain English?
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Old 21st November 2009, 05:21 PM   #9
jcx is offline jcx  United States
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U2 supplies almost all of the load current - compare the U1:OUT current to U2:OUT in the plot (red, blue trace: right axis for current)

(U2 supplies ~40 mA into the 100 Ohm load plus some more for feedback networks and ccs)

unloading the output of U1 should reduce thermal and gain errors

some people like to claim the U2 amp isn't "in the signal path" so the improvement supposedly doesn't add as much "feedback artifact"


Jung's multiloop amps can give much better performance with the same components but input amp and buffer are in series and you have very large loop gain in the signal path

Last edited by jcx; 21st November 2009 at 05:25 PM.
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Old 21st November 2009, 06:08 PM   #10
jcx is offline jcx  United States
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Default oops, out of edit time...

U2, R1,3,4 form a vccs which senses the drop across R2 and supplies a near equal amount of current into the U1:OUT node

(the op amp subcircuit current direction is positive into the op amp so the displayed current traces are inverted relative to R5 load current)

if the cancelation is accurate enough the current demand on U1:OUT can be low enough for Class A bias of U1 output to be practical as shown - further reducing error in the U1 amp



for a demonstration of U2 function you can remove R1 (or edit its value to 10g, g=GigaOhm) and you should see the output clip as U1 current limits trying to drive 100 Ohm load R5 without the aid of U2

Last edited by jcx; 21st November 2009 at 06:33 PM.
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