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Old 8th September 2003, 10:31 PM   #1
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Unhappy Douglas Self op. amp. problem..

Folks, take a look at figure 3 here:

http://www.dself.dsl.pipex.com/ampin...te/discrop.htm

Douglas Self claims 6ppm THD+N at 1KHz , (fig. 4), for his discrete op. amp., but simple AC analysis in spice shows there is no way he could have swept stimulus across the audio band into this design, as it would have certainly broken into violent oscillation along the way...
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Old 9th September 2003, 02:15 AM   #2
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Default Re: Douglas Self op. amp. problem..

Konnichiwa,

Quote:
Originally posted by mikek

Douglas Self claims 6ppm THD+N at 1KHz , (fig. 4), for his discrete op. amp., but simple AC analysis in spice shows there is no way he could have swept stimulus across the audio band into this design, as it would have certainly broken into violent oscillation along the way...
What did you expect? Did you ever get a chance to "fix" on of his mixer designs and I am talking about basic groud routing on the channels and the bus here, nothing fancy....

Sayonara
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Old 9th September 2003, 04:58 AM   #3
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The point here is, if its completely impossible to run this design without oscillation, then the THd+N graph caould not possibly have been obtained legitithemately........the whole thing stinks.....makes one wonder....are those THD curves in his book worth the paper they are printed on????

moderator....i have attached image!
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Old 9th September 2003, 07:59 AM   #4
sonnya is offline sonnya  Denmark
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hi Mike.

I did a sim this morning at home, with 2sa970,2sc2240..

Yes it is unstable, but i ended up with a peak at around 7MHz, and a phaseshift much more normal than yours..... I only got a posetive phaseshift not the one you have shown with two shifts!?

Are you sure...... I am not shure that your phaseshift is ok.... sorry. I double checked my circuit twice..

By the way... The highest harmonics was 123dB down. So i would suspect that he could have measured it... I could easily sim it. It only shows unstability when subjected to a squarewave, ringing but it dies out...

Like to hear you answer...

A fix could be rasing the noise gain at high frequencies.... 33pF between the noninverting and the inverting input!?!?

Sonny
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Old 9th September 2003, 08:15 AM   #5
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I say don't draw conclusions too fast. As Walter Jung points out you must do a follow up with real life circuits. Be positive Mike K
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Old 9th September 2003, 10:00 AM   #6
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Mikek,

you better sometimes build than only simulate. Simulation often does not give good results at high frequencies (my experience). The real life is not the simulated one ...
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Old 9th September 2003, 10:05 AM   #7
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Since Douglas Self doesn't seem to have specified the transistor types he used, it is unclear to me how you can check the stability or the distortion of his design, either by simulation or by measurement. Both stability and distortion may be strongly dependent on the actual transistor types used.
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Old 9th September 2003, 11:09 AM   #8
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Default Re: Douglas Self op. amp. problem..

[Douglas Self claims 6ppm THD+N at 1KHz , (fig. 4), for his discrete op. amp., but simple AC analysis in spice shows there is no way he could have swept stimulus across the audio band into this design, as it would have certainly broken into violent oscillation along the way... [/B][/QUOTE]
----------------------------------------------

Any sonic advantage of ppm distortion seems to me to be negated by the two coupling caps he uses
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Old 9th September 2003, 01:32 PM   #9
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In case that capacitor coupling has cut-off frequency (-3dB) somewhere between 0.1Hz and 1Hz, speaking about foil capacitor distortion is only a superstition.
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Old 9th September 2003, 01:48 PM   #10
sonnya is offline sonnya  Denmark
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i have forgotten to say. My sim is with perfect caps and a zero generator impedance. with a impedance of source is 1k the distortion will most likely go up to around the 6ppm.....
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