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Old 16th March 2006, 04:10 AM   #1
rlaury is offline rlaury  United States
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Default Class Double A

Hi Group:
I seen a few references made to amplifiers as being "Class Double A" Is this another name for "Class A Push Pull" or is it another topology. I haven't seen any schematics.
Any ideas??

Thanks
RonL
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Old 16th March 2006, 06:16 AM   #2
lndm is offline lndm  Australia
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http://www.cc.jyu.fi/~svhiippa/Technics.pdf#search='class%20aa%20amp'

Looks like they used mosfets and bjt's concurrently.
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Old 16th March 2006, 01:20 PM   #3
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rlaury,

Goto:

http://www.diyaudio.com/forums/showt...hp?postid=6192

Alain.
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Old 16th March 2006, 05:34 PM   #4
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"Any ideas??"

How about this? (Would have to be class A and class A of course to qualify for class AA)

Don
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Old 16th March 2006, 05:47 PM   #5
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Don
why do you want such a complex feedback topology if you can easily use feedforward, probably with better results?
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Old 16th March 2006, 05:59 PM   #6
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you beat me to the punch line, was drawing the schematic.

Wait till you see the NEXT one, Hawksford EC of a SS amp using a tube amp for EC.

edit:
OOPs, had to fix the gain of the Tube Amp
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Old 16th March 2006, 06:11 PM   #7
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How about this:
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Old 16th March 2006, 06:27 PM   #8
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Hmmm, for some reason the edit function wouldn't take a new version of my schematic in the last post. Here is a corrected gain version:

Hi Darkfenriz,
Yes, except I think you need something to define the tube stage gain.
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Old 16th March 2006, 06:53 PM   #9
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Here is a version with FeedForward ErrorCorrection (FFEC) and Hawksford feedback style error correction ( HEC).

The R6/R5 HEC feedback from the Tube Amp tells the SS amp it is not sounding enough like a tube amp, so fix it man!

But the "fix it" signal to the SS amp is adjusted to less than unity fixup so as to avoid loop instability (around R6, SS Amp, R2/R1, Tube Amp to R6 again). The feedforward path thru the xfmr then fixes the residual error by FFEC. Some gain tweeking will no doubt be required to get this all to work correctly.

Don
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Old 16th March 2006, 07:57 PM   #10
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My schematic in post #8 needs another fixup. The R2/R1 ratio needs to be the SAME as the R4/R3 ratio, so might as well use the same resistors (as Darkfenriz has shown in his diagram). Then the tube amp stage needs to be specified as unity gain when the xfmr. ratio is included (rather than a high op amp like gain) to compute the feedforward error correction. I was too eager to get some power from the tube amp stage also.

Similarly, the feedforward EC combined with feedback EC schematic in the previous post needs to have R2/R1 = R4/R3, so can combine them here too. Here, the xfmr ratio needs to be lowered some further so as to put less feedforward correction into the output since the SS amp is already correcting some of the problem via R6/R5 feedback.

Don
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