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Old 16th September 2012, 08:54 PM   #981
coluke is offline coluke  Italy
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Quote:
Originally Posted by fitzfish View Post
Have you ran a curve trace of both to see how closely they are complimentary?
Nope - only Id matched, but I wonder if close matching is worth the effort at all: I mean, if throwing in 4 randomly selected devices you get 0.0005% THD at the expense of a slight DC unbalance...

L.
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Old 16th September 2012, 09:48 PM   #982
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Originally Posted by Joachim Gerhard View Post
Choco, i like the extension you made. If it sims well i would like to build a prototype.
Joachim,
I started to sim, but had no models of reasonable JFets.
After seeing silly results as a consequence of using silly types I decided to check for models of the 2SJ74 and 2SK170.
I have to ask for patience and cannot make promises on a time schedule.
Busy week ahead...
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Old 16th September 2012, 10:28 PM   #983
dimitri is offline dimitri  United States
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Quote:
George Wilson presented an all NPN and NJFET op-amp in >>1968<< that had 200V/usec slew rate and 50MHz BW.
Quote:
I never saw any followup
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Old 16th September 2012, 11:10 PM   #984
RNMarsh is offline RNMarsh  United States
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Default Followup -

Quote:
Originally Posted by scott wurcer View Post
I found it interesting that George Wilson presented an all NPN and NJFET op-amp in >>1968<< that had 200V/usec slew rate and 50MHz BW. This WAS potentially good for audio but I never saw any followup.

Yup, the same Wilson as in the current mirror.
Maybe we could do it for him. RIP. Do the followup for audio.
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Old 16th September 2012, 11:56 PM   #985
RNMarsh is offline RNMarsh  United States
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Default How much gnfb does it take to get below .001% ?

I have never, in my life time, had a hard time getting low thd in an amplifier. It just isnt that hard. If you have a symetrical and balanced circuit, it doent take much gnfb to have very low thd. I think i showed that with a measured .0005% thd + n with modest gnfb. Decades ago I had <.002% at 22 v p-p on the very first line stage I ever designed. That one had only 20db gnfb.

So, remind me again -- I am not sure what we are doing now for a discrete opamp design? What is it that we can do discrete better than an IC opamp? -Thx - RNM
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Old 17th September 2012, 12:03 AM   #986
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linuxguru

I think there's something wrong with your sim. The descending slope on the right might be caused by a LF drift in the caps, it's not always there anyway... The skirts round the fundamental don't look right either.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Spice directives for simulation courtesy of keantoken.

2V in, 6.4V out (peak) into 16 ohms, as shown. < 2 PPM THD. It's not the least number of components, but you can get these transistors for $0.03 ea. Surely that's got to count for something.
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Old 17th September 2012, 12:07 AM   #987
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Bases of Q11-Q14 are connected to the wrong end of R6.
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Old 17th September 2012, 02:04 AM   #988
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Damn. Thanks.

You'd think it would make more difference to the sim. I moved it and got 0.00016.
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Old 17th September 2012, 02:15 AM   #989
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Thanks, Dimitri. As shown, Wilson's topology has a minor biasing problem for the 3rd-stage darlington, but that's easily fixed with a dual-diode in the emitter. The CCB can be grafted on for the O/P stage, but the darlington VAS won't be good for sonics, at any rate.
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Old 17th September 2012, 03:54 AM   #990
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Originally Posted by RNMarsh View Post
So, remind me again -- I am not sure what we are doing now for a discrete opamp design? What is it that we can do discrete better than an IC opamp?
As Scott has reminded us, the existing discrete opamps for audio run +/- 24 VDC rails for +24dBu output into 600 Ohms. No sense doing less. Class A output at that level isn't difficult either.

Some things not always clear in monolythic opamp performance are high frequency common mode distortion levels with high (maybe 10K Ohm or so) source impedance, clipping overload recovery behavior, and sensitivity to power supply imperfections. A truly hot dog design would address these things too.

Thanks,
Chris

Last edited by Chris Hornbeck; 17th September 2012 at 04:02 AM.
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