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Old 12th March 2011, 02:02 AM   #21
artu is offline artu  Chile
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Default clipping the CFA

Here is the clipping behaviour of the CFA. (150% nominal input)

Pix 1: Note that the input stage (VAS input) never cut off (pink and green traces), and the cascodes saturates (dark red and yellow traces) limiting the excursion of OPS voltage to rail (red and blue traces) - 2.6V, so OPS never cut off, and leaves a Vce of 2.6V guard to avoid OPS instability. The output is the violet trace.

Pix 2: Show the symmetry 0.002 DC OFFSET and all distortion are due odd harmonics, leaving the clipping state is free from transients, clipping at 400KHZ or above shows a little (not oscillating) transient when re entering the linear region.

Cheers
Arturo

stable_clipping_20KHZ.png

clipping_symetry.png

Last edited by artu; 12th March 2011 at 02:05 AM. Reason: corrections
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Old 16th March 2011, 07:36 PM   #22
artu is offline artu  Chile
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Default CFA variations

I show 4 alternatives ranging from 10 to 16 signal BJT's and the best FFT (16 BJT's) at 2KHZ for the CFA amp.

Cheers
Arturo
Attached Images
File Type: png ampcc1.png (57.2 KB, 474 views)
File Type: png ampcc.png (61.3 KB, 464 views)
File Type: png FFT_2KHZ.png (23.9 KB, 458 views)
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Old 16th March 2011, 08:23 PM   #23
jcx is online now jcx  United States
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Default how about "the Original" FeedForward?

collected a couple of posts, the sim files are available in the respective threads

Quote:
Originally Posted by jcx View Post
I worked up this "current dumping" Black's Feedforward sim from the jaes paper - sized for a headphone load since I'm using a op amp model with ~30mA output current limit

playing with the signal level it appears that it has the annoying property of increasing distortion as output level is reduced - the max % distortion occurs when the output level just starts to use the "dumper" Q for appreciable load current

feedback is necessary to implement the accurate correction amplifier in Black's Feedforward - I don't believe you can easily combine the sim's op amp and unbiased Q to do better in a "pure negative feedback" topology - so the Feedforward idea is valuable if you insist on "Class C+A"

the shown 2 V sine has ~0.01% output distortion, with the distortion growing to ~0.05% at around 200mV drive
...
Quote:
Originally Posted by jcx View Post
I grabbed Bob's example sim from the TPC/TMC debate in the Cordell Book thread Bob Cordell's Power amplifier book

my poking at it seems to indicate that the distortion is limited by the slow output Q and simply can't be made much better by feedback - especially the THD 20KHz that Bob likes as a simple quality metric (at least without going deep into conditional stability territory with 3-rd order or higher loop gain schemes)

if you can't fix up a amp with more feedback then the other tool is Black's Error Feedforward - rare in audio because of the difficulties power output combiner but famous none the less due to Quad’s "Current Dumping" amplifier

a error feedforward amp is defined by having 2 independent power amp paths to control the output - and the consequent difficulty of combining the outputs without their mutual loading causing unwanted additional (destabilizing) feedback

The VanderKooy Lipshitz Error Feedforward paper http://quad405.com/jaes.pdf shows several schemes - "Current Dumping" actually wraps negative feedback around the "dumper" amp stage - but 2 other schemes are shown - I chose the Parallel Feedforward from their fig 4b - which doesn't intentionally add loop feedback to the main amp and the coupling through the output impedances of the amps is minimal when used with a frequency selective power combiner

so the following is a Composite Amp using a large, slow discrete amp and a fast error feedforward op amp path
the instrumentation op amp measures the error at the diff pair input of the main amp with a gain of 10x, and then the CFA DSL driver op amp integrator provides the required error correction V drive for the Resistive leg of the L,R power output combiner network

Bob's THD 20KHz number is improved by ~20x in this sim

you will need Bob's zip file and to put my asc in the same folder with Bob's Cordell Models.txt file or you can download Bob's transistor models from his
book's site
Attached Images
File Type: png dumper.png (42.5 KB, 427 views)
File Type: png Parallel_Feedforwrd.png (56.2 KB, 443 views)
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Old 17th March 2011, 12:16 AM   #24
artu is offline artu  Chile
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Hi jcx,
VanderKooy Lipshitz Error Feedforward paper completely unknown for me, after a brief reading I understand that the basic principle of this schema is adding a correction (from the auxiliary amp) 'feed forward' signal to the output of the main amp. to cancel the high order products that main amp would never be able to cancel, is this correct?. A question: the purpose of the CFA integrator is to correct the phase of the 'feed forward' signal to be in phase with main amp phase?

Cheers
Arturo
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Old 17th March 2011, 01:22 AM   #25
jcx is online now jcx  United States
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the "non-interacting" power output combiner is a difficulty

the L,R output combiner could be seen as a XO, the low frequency branch thru the output series L is driven by the main power amp

the fast correction amp path to the output through the resistor would be a 1st order highpass

but since the correction needs to add up to unity at the output you need a 1st order roll off for the correction amp's signal - hence the integrator

people like to draw the Quad Dumper circuit as a AC 4-arm bridge, the parallel circuit shown has a similar correction amp signal frequency shaping


I'm not yet really sure of the utility Black's Feedforward Error Correction scheme for audio power amps - except as a "band-aid" for being stuck with a poor performing main power amp
since you still have to accurately measure the error it seems like negative feedback around a better power amp would just be better
In the example of the BobAmp circuit I think doubling up ouput Q to avoid Beta droop and using 5x faster RET would let you get similar distortion improvement with 2-pole compensation

the Vanderkooy paper did suggest Black's Feedforward Error Correction as a way to help out Class D main power stages

Last edited by jcx; 17th March 2011 at 01:28 AM.
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Old 17th March 2011, 03:06 AM   #26
artu is offline artu  Chile
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Quote:
jcx :

the "non-interacting" power output combiner is a difficulty

the L,R output combiner could be seen as a XO, the low frequency branch thru the output series L is driven by the main power amp

the fast correction amp path to the output through the resistor would be a 1st order highpass

but since the correction needs to add up to unity at the output you need a 1st order roll off for the correction amp's signal - hence the integrator

people like to draw the Quad Dumper circuit as a AC 4-arm bridge, the parallel circuit shown has a similar correction amp signal frequency shaping


I'm not yet really sure of the utility Black's Feedforward Error Correction scheme for audio power amps - except as a "band-aid" for being stuck with a poor performing main power amp
since you still have to accurately measure the error it seems like negative feedback around a better power amp would just be better
In the example of the BobAmp circuit I think doubling up ouput Q to avoid Beta droop and using 5x faster RET would let you get similar distortion improvement with 2-pole compensation

the Vanderkooy paper did suggest Black's Feedforward Error Correction as a way to help out Class D main power stages
Thank's for the explanation, not gone yet in deep, at first sight I guess Class D is the logic target for this kind of error correction, given that linear amps can achieve decent error correction by more simple means, unless "being stuck with a poor performing main power amp", and worth a 'minimal intervention' parallel aiding circuit. Anyway it's interesting for me as didn't see it before.

Cheers
Arturo
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Old 23rd March 2011, 12:28 AM   #27
ontoaba is offline ontoaba  Indonesia
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Sorry, long time not showing up, Just finishing my project. CFA is the most stable and I couldn't share my new until I have another.
Your output is true a good one. Give it more bias and it will work very good.
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Old 23rd March 2011, 02:09 AM   #28
artu is offline artu  Chile
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Hi Ontoaba,
Nice to see you again, the CFA besides rock solid stability, it is amusing the high dynamic current capability it has, I didn't see in VFA this unique characteristic (maybe I am wrong), but in ie: at 20KHZ for 31W output at 8 OHM load (16 BJT schema) THD=0.00040%, then for 4 OHM load is 0.00043% and for 2 OHM load 0.00051, distortion is almost independent of high current load changes !!!, I really like it. About the bias, yes it can be set a little bit higher, anyway lot's of room for 'practical' experimentation.

Cheers
Arturo
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Old 23rd March 2011, 07:26 AM   #29
MiiB is offline MiiB  Denmark
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Kenpeter....

I relally like your non switching diode idea....

what if the output stage was made with 2 lateral fets running some class A (like 200mA) and then followed by some bjt's wired up in your non switching manner as current dumpers...
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Old 23rd March 2011, 11:49 PM   #30
ontoaba is offline ontoaba  Indonesia
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Quote:
Originally Posted by artu View Post
Hi Ontoaba,
distortion is almost independent of high current load changes !!!,

Arturo
Hi,
That load characteristics is very good to handle dynamic loads.
It may because it has linear drop voltage. (see picture for detail, it also show how stable they are when rejecting square supply ripple on HEXFETs output stage).

When entering unstable region, VFA input transistor one turned off and another one is saturated. This condition slightly degrade its performance and make VFA to have different speed and condition from classA to switching region.
While CFA still working in its safe conductivity, that is why it stay stable.
Also it has more linear drop voltages that make it able to have classA characteristics in beautiful 6kHz if used with current feedback without global voltage feedback.
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File Type: png error voltage and supply ripple.PNG (14.3 KB, 222 views)

Last edited by ontoaba; 23rd March 2011 at 11:58 PM.
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