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Old 19th March 2013, 12:29 AM   #111
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by Telstar View Post
Point 3 I think was correct (PSRR).
For the very basic CFA topology that Michael seems to be stuck into, yes.

For a true high performance CFA (intended for audio), no. Look at this: http://www.ti.com/lit/ds/symlink/lme49871.pdf

102dB PSRR
1.2ppm distortions in 600ohm load
Under 1mV offset.

Quoting the SR and bandwidth numbers for this CFA would be, in the context of audio, obscene.
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Old 19th March 2013, 12:31 AM   #112
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Quote:
Originally Posted by Telstar View Post
Point 3 I think was correct (PSRR).
Start using CFA with CCS and PSRR is leveled with noise floor.
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Old 19th March 2013, 12:54 AM   #113
jcx is online now jcx  United States
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Quote:
Originally Posted by Telstar View Post
I have to disagree with the above statement. An amp with 200khz at the canonical -3db does introduce phase shifts in the audio band and is therefore sub-optimal.
gotta love the things people think thay can get away with saying - no matter how easy to refute

200 kHz single pole is -0.043 dB, 8 ns delta group delay at 20 kHz re 1 kHz

its not the 5.6 degrees or the 800 ns total delay - it is the deviation from a pure delay that is the time "distortion", and again that is 8 ns

the reason 200 kHz LP is "cannonical" for audio is because it is technically and by even audio guru listening tests inaudible

Last edited by jcx; 19th March 2013 at 01:04 AM.
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Old 19th March 2013, 07:50 AM   #114
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Lazy Cat View Post
Start using CFA with CCS and PSRR is leveled with noise floor.
Not true , wether you re using resistors , basic current sources
R + zener + R , active sources or even perfect sources (in simulators)
the PSRR will be the same +-0.5db.

Taking Gabor s design as workhorse , it has basicaly 34db worse
PSRR that an equivalently biaised symetrical differential VFA ,
and your VSSA , wich is the same design , has the same mediocre
PSRR , the current source argument being just wishfull thinking
but has not the slightest technical ground.
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File Type: gif PSRR CFA VS VFA.gif (27.0 KB, 176 views)
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Old 19th March 2013, 08:03 AM   #115
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Waly View Post
For a true high performance CFA (intended for audio), no. Look at this: http://www.ti.com/lit/ds/symlink/lme49871.pdf

Quoting the SR and bandwidth numbers for this CFA would be, in the context of audio, obscene.
Perhaps that the most obscene is that this chip is obsolete according
to its datasheet.
One has to wonder why....
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Old 19th March 2013, 08:24 AM   #116
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by wahab View Post
Perhaps that the most obscene is that this chip is obsolete according
to its datasheet.
One has to wonder why....
Simply because this chip, originally designed by National, is superseded, performance wise, by other products designed by TI. I'm sure you'll find your way in the TI catalog.
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Old 19th March 2013, 08:35 AM   #117
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Quote:
Originally Posted by wahab View Post
Not true , wether you re using resistors , basic current sources
Real circuit measurements are the ones to trust. Modulation of DC rails resulting in lower than noise floor suppression of the output voltage modulation in VSSA, in both cases, observing the scope-analyzer screen, as well as listening to speaker noise. Usually VFAs I ever tested had more problems with the same rail modulation signal (DC+AC). Simulations are so approximate that I trust only the real world's measurements results.

BTW CFAs are always better sounding performer in all aspects.
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Last edited by Lazy Cat; 19th March 2013 at 09:04 AM.
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Old 19th March 2013, 08:40 AM   #118
wahab is offline wahab  Algeria
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No doubt that TI has their own equivalent products but overall ,
i think that those low impedance inputs opamps are hardly finding
their ways given improved processes.

Theses topologies are convenient to design high speed op amps
using slow devices but once the components frequency transistion
is increased (GHZs ft transistors) the speed advantage gradualy shrink.
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Old 19th March 2013, 08:51 AM   #119
Bonsai is offline Bonsai  Taiwan
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I am surprised you are so anti CFA Wahab. At least, they are symmetrical!

Ignore that false claims on both VFA and CFA sides and think about the additional scope you have in your design arsenal!
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Old 19th March 2013, 10:00 AM   #120
CBS240 is offline CBS240  United States
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If you choose to use a separate power rail for the VAS than for the driver and output stage, it is of little consequence to include a small signal (highish frequency) voltage regulator to better the PSR figures for a CFA circuit.(and VFA for that matter)

I see no reason not to analyze the shortcomings of a design and build around it to mitigate those shortcomings. The result becomes more ideal, and that should be the goal, no?

The last amp I built is made using the CFA definition and in fact uses a half wave voltage doubler to power the VAS rails. Vripple is ~0.5V @ 60Hz, yet PSR is very good as there is no trace of the Vripple sawtooth signal on the rails or the output. However, if the regulator is disabled then the output is struck with 60Hz BZZZZzzzzz.
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