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Old 23rd June 2004, 07:46 PM   #41
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I think that sam9 did a good job of summing up the pros and cons of each.

Bode plot:

Measure the phase of the output compared to the input phase. See how much gain the compound has when the two are in phase..........

Does that give you some insight??

Jocko
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Old 23rd June 2004, 09:47 PM   #42
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Hi Jocko.

I think that you mostly speak about the ouput CFP or Sziklai pair. For the moment, i am not working on the output stage.
I must say that my first schematic was not clear on this point.
But i am very interested by all your advises !
I am trying input stage and VAS, in Microcap, in bench, and soon i will listen...

I would like to test a few thoughts, i have in mind.

http://www.diyaudio.com/forums/showt...778#post422778

An idea of the design i am working on...



Click the image to open in full size.
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Old 24th June 2004, 02:21 AM   #43
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Default Go ahead......hate me....

What is the purpose of the "CFP" in the input stage????? Probably the last place that I would use one.

But just the way that I would do it......

Doesn't mean that it won't work.

Jocko
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Old 24th June 2004, 02:59 AM   #44
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"What is the purpose of the "CFP" in the input stage????? Probably the last place that I would use one."

So I would have thought, but I now find both D. Self and Ben Duncan refer to them in differential amps at the input. Where you normally have a differential pair you use to CF pairs (more likely called Sziklai pairs in this context) to provide local fedback.
I moded an existing LT Spice circuit to insert these to see what happens. "THD+N" (mostly the "N", I think) is slightly reduced. We're talking a change of ~.0001%! But in real life the transistors are neither ideal or perfectly matched to the real benefit may be greater. No mention is made of potential foroscillation. In the Duncan diagrams it is indicated that all four should be thermally coupled which sounds like an entertaining endeavor for people who like to fiddle with small objects! I would want to be thoroghly convinced I had a problem before applying this fix.
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Old 25th June 2004, 02:39 PM   #45
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Default Re: Tricky means......

Quote:
Originally posted by Jocko Homo
The silly things are prone to oscillation problems. The solution depends on how you are using them.

For a power amp..........

I used them for years, and I thought that they sounded good (after they were stabilised), but then I tried a diamond buffer. Everyone who heard that version.......including all the customers who forked over $$$ to have their amps retrofitted....... agreed.

In power supplies........

They seem better suited there. Current demand is usually constant, you can put caps everywhere to stop them from singing, and the output impedance is flat up to around the alpha cutoff frequency...usually around 100 kHz or so.

Jocko

Sure OSCILLATIONS problems !!!! But i progress, slowly but i progress.
O.K. for the input stage YEAAAAHHHH !!!!!

Just a question:

What is an audio power amplifier else, than a modulated power supply ? (Voltage source from 0Hz to 100khz or more).
Well I speak about CFP's in class A not in AB or even worse B, not only due to crossover THD, but in class A you have less changes in the transistor characteristics, and i think it's very important for our purpose. Today I think in a CFP output stage, but maybe i will change my mind, I need tests. The diamond buffer looks very interesting too...

For the moment :

JFET + BJT / CFP = marvelous FFT plot (fine spectra), nice looking square wave, no input capacitor, no CCS ! What moreover ! .... LISTEN maybe ! But I have to solve a few problems with VAS, before ...
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Old 26th June 2004, 08:46 PM   #46
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Well, after a few small changes in the design… Lots of problems with small H.F. oscillations, about 4/5Mhz, so Miller cap adjust, base resistors, cascode configuration, and very, very important, good ground, and decoupling power supply rails. (I must say, that my stabilized power supply is not very good).
Now the prototype works fine, haem… in test bench !
I just need to buy... batteries… In old listening tests, for my “active” crossover, I notice that my stabilized power supply sounds very bad ! And this time, I want to get rid of it.

Below, some FFT graphs. I can’t show oscilloscope traces of square wave signal, I have nothing for capture or photograph the screen, but there is quasi no ringing. Band-pass is 2 MHz and the gain is 20 dB. At 0dB input, VAS output is about 20V peak to peak.

T.H.D. graphs.

Input -20dB
Input -10dB
Input 0dB

I.M.D. graphs.

Input -20dB
Input -10dB
Input 0dB
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Old 27th June 2004, 02:00 AM   #47
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Default Re: DON'T SAY THAT!!!!!

Quote:
Originally posted by Jocko Homo
The resident vacuum cleaners will suck that up and come out with the world's greatest extreme power supply next week.

And I'm gonna blame you!

Jocko
Gedday Jocko,

How is sunny Texas,

WRT CFP in PS, I totally agree they have a good balance
between BW, settling and OP Z, and I used them for years.

Funny though, I gave a few customers a "short out" option
for the CFP tranny, ie; in an N channel CFP follower, short the P
channel E - B and turn it into a plain vanilla single tranny EF.

They all preferred the plain vanilla version.

This was in DAC's where very wide BW would be important,
nevertheless it was an interesting experiment. I was actually
biased toward the CFP, probably placaebo because I knew
it had lower OP Z.

I would also like to note you can do a three stage CFP
and get even more "OLG" for lower OP Z and even more
stability problems



Cheers,

Terry from sunny Sydney.
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Old 27th June 2004, 04:39 AM   #48
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Default Ssssshhhh.......!

Don't give them any more ideas.......crikey!

Jocko
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Old 27th June 2004, 08:22 AM   #49
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Lightbulb CFB in Powersupply Regulator

Hi,
Just an example of CFB in a powersupply regulator for a preamp.
Attached Images
File Type: gif cfbpowersupply-sm.gif (94.6 KB, 791 views)
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Old 27th June 2004, 08:41 AM   #50
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What advantage of CFB amp?
CFB amp is used in inverting mode but "the power of love" is non-inverting mode
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