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Old 22nd June 2004, 08:48 AM   #21
Jocko Homo is offline Jocko Homo  United States
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Default On second thought.......

They won't be able to figure out how to stop it oscillating, because theirs obviously will. No app note to copy to tell them how, either.

"Larry, that giant sucking sound that you hear.........."- Ross Perot, maniacal egomaniac.

Hey........he would fit in well here!

Jocko
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Old 22nd June 2004, 08:50 AM   #22
Elso Kwak is offline Elso Kwak  Philippines
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Red face 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
Yeah, and I we have to deposit 50 box beforehand.......
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Old 22nd June 2004, 09:49 AM   #23
hjelm is offline hjelm  Sweden
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Default Jocko Homo dont do this to us

Do not know if you are willing to share the information with us Jocko but:

1: In the Poweramps did you use them as Emitter followers for the output stage? If so do you think it was the non linear load that was causing the problem, i.e. would it work in a line buffer?

2: If used as emitter follower then the stability seems to improve when a cap is placed across the r1 in the schematic in post #3.
Intuitively it could be seen as making it a pure single transistor emitter follower for the higher frequencies since you are "shorting" the resistor r1?

I do not know how to think with stability and the CFP.
What is the equivalence of open loop gain for a CFP so i could estimate phase margin, is it possible at all?
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Old 22nd June 2004, 01:49 PM   #24
Jocko Homo is offline Jocko Homo  United States
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Yes. I used them for the output. As did Nelson Pass and Jeff Rowland at times.

No, it was not a function of load. They just have a tendency to oscillate in the 2-5 MHz range. All by themselves.........

As for a line buffer......

Never had the need to drive so much current into a very low impedance load. Yes, those would be easier to stabilise. But, I have concluded that open-loop designs, such as the diamond buffer sound better. And they are easier to work with.

Never tried using a cap to reduce the gain at HF. Not sure it would help much, as the goal was to keep the impedance flat to aorund 100 kHz, and that only leaves a little over 1 decade before the problems start.

Open loop gain??? I dunno......never tried to put a number on it. I did enough Bode plots to determine phase margin. Seems that the actual numbers in a real world application were more important. With real world loads.

Jocko
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Old 22nd June 2004, 02:51 PM   #25
sreten is offline sreten  United Kingdom
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D Selfs book (and internet pages) also show that whilst a CFP
has superior large signal linearity when used in an output stage
the EF configuration has much lower distortion around 1W,
whilst the CFP has lower distortion near full power.

When the distortion spectrum is considered the CFP looks less
and less attractive as a superior alternative to the EF as the
output stage configuration, IMO it simply isn't for music.

sreten.
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Old 22nd June 2004, 03:00 PM   #26
JonMarsh is offline JonMarsh
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Default Speaking of Diamond buffers...

Speaking of diamond buffers, I'm surprised these guys haven't come up more around here in the discussions of unusual amplifiers... or even come up at all?


NHB


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This ought to be worth a little Hoovering with the local crowd....






~Jon
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Old 22nd June 2004, 03:50 PM   #27
hjelm is offline hjelm  Sweden
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I must confess my utter and incredible ignorance when it comes to bode plots. I just do not see how i do it for a CFP in emitter follower configuration. Its gain is less than 1 but it can still oscillate? How should i think?
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Old 22nd June 2004, 03:59 PM   #28
Ouroboros is offline Ouroboros  England
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The reason for the CFP having a gain of one is because of the tight feedback loop from the collector of the second transistor to the emitter of the first. It's quite easy by adding a couple of resistors to make a CFP with gain, and this is a useful building block.
Even the 100% feedback unity gain version will have a gain peak at some high frequency because of delays in the NFB loop. Beware, they can and do oscillate!
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Old 22nd June 2004, 04:40 PM   #29
sajti is offline sajti  Hungary
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Hi Eva,

I also found this problem with CFP. If You apply no load, and drive Your amplifier with high level, and high speed square wave (no input filter!), You will find that the current from the PSU will increase, due both output devices draw the current in same time.
The solution is very easy: Connect together the base of the output devices with a small RC network. I used 10ohms+22nF+10ohms. The resistors to avoid oscillation, because of the incductance of the wire on the PCB. I tried some value for the capacitor, with measuring the current. 22nF results no current increasing with 50kHz square wave.

sajti
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Old 22nd June 2004, 08:36 PM   #30
Justcallmedad is offline Justcallmedad  France
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Hi sreten.

In wich D. Self book, do you refer to his CFP works ?
I haven't find very much things on CFP in "D.Self on audio" (the one i got).

It's surprising that for a CFP the thd rises at low levels maybe because its a class AB stage ?

Maybe on the web, i will find more information...

If you have more precise documentation on this, i am interested of course.
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