John Curl's Blowtorch preamplifier

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#17,582
scott wurcer said:
It operates by subtractive neutralization not feedback. The displacement current in Cc is the compensation signal 90 degrees out of phase and increasing with frequency plus the current which is the derivative of the crossover distortion. An easy way to see how crossover distortion goes up with frequency. The displacement current in Cn is just the derivative of the crossover part. So by subtracting them in the current mirror you still have the same compensation but "no" crossover distortion. There is a vector error leaving a small residual.
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Is this why you used the n label, as in Cn, n=neutralizing?

As I understand it, in a tube neutralizing circuit the idea is to cancel the tube's parasitic cap. current by subtracting an equivalent current from the appropriate node. Like placing a negative capacitor in parallel with the parasitic one. This technique creates a gain through the neutralizing C loop of the form -ks/(1+sp). Neutralization is when k=p and in this case the magnitude is always <1.

In the AD797 circuit, my analysis shows the gain through the Cn loop in Fig 32 is -A.Cn/(Cc-Cn). The real circuit will have at least one pole in it due to the finite resistance of the current mirror and this results in a loop gain of the form -k/(1+sp). In this case, k and p tend towards infinity as Cn tends towards Cc. The loop gain looks like an integrator whose knee frequency tends towards dc as Cn tends towards Cc.

These aren't the same at all and that is why I am struggling to understand your statement that the AD797 scheme is "subtractive neutralization not feedback".

Brian
 
#17,583
john curl said:
What's next?
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That’s surely not up to a pitch fork villager to decide, so it’s best that you or one of your dedicated associates decide the next theme.

BTW both syn08 and Edmond is binned for a week, but they are both PFV.

Just one question regarding the class D amp that you liked at the show, do you know if it was based on UCD or B&O or was it another design?
 
#17,589
I made something similar and it is not that bad.
 

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#17,590
mlloyd1 said:
probably something in this family from his op amp book with analog devices:

http://books.google.com/books?id=dunqt1rt4sAC&pg=RA5-PA444&lpg=RA5-PA444&dq=ad745+phono+preamp+walt+jung&source=bl&ots=oUuKit9JV9&sig=GzoHDH8iZ4UJedovEj2fz5rubJY&hl=en&ei=NzDqSdunKZPMMv-U3cwF&sa=X&oi=book_result&ct=result&resnum=1

mlloyd1
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Try Figure 6-15. That's similar to the GFP-565 phono section.

The original Adcom phono design is shown here on the bottom of page 1:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P1293,D1612

The Adcom version doesn't have switchable gain, however.

There was at least one more comprehensive article on this by Gary Galo in AudioXPress spread over four issues starting in 2003.

Part of the appeal of using an AD745 is that it eliminates the need for a blocking cap either at the stage output or within the feedback system. Of course, you could use a servo instead...
 
#17,596
scott wurcer said:
Brian, I will try to draw a little figure tomorrow (we might be dwelling on a semantic issue too). I have been building my second shed. Billy two sheds.... 🙂
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Awesome, Bill.
I have spent my day up to my knees in plumbing challenges. A friend of mine's friend, who is a landlady, was enjoying an unexpected water feature from her kitchen ceiling. I offered to check it out - the source was a leaking tap connector under the upstairs tenants kitchen sink. The floor cavity was soaked. It was a push-fit connector designed for Cu pipe that had been pushed onto a feed of plastic pipe. Not only that, but the plastic pipe was lacking its essential insert. As a precaution, I checked the cold supply too and found it was the same. The maintenance valves were metal compression on to plastic pipe - again the inserts were missing. In the end, I had to trace the pipe feeds under the kitchen, hall and bathroom floors to find another 18 connectors with missing inserts, any one of which could have worked loose and cause a serious flood. 😱 Took me 5 hours.
 
#17,598
I would like to point out that Walt Jung produces app notes for a living, or at least he did previously. I make a limited living from producing my designs and most here could not actually produce them anyway, due to the fact that they don't invest enough in parts and test equipment, and they usually burden ME with their problems. Therefore, I don't publish exact schematics, except for some early designs that are still made from this website, today. Scott Wurcer won't divulge anything that I can use about his AD797, even after repeated enquiry. He is either out of touch with the answers, or he never knew in the first place. I concentrate on OPEN LOOP LINEARITY, as I consider it more important than CLOSED LOOP LINEARITY, and while I consider the latest stuff on the AD797 interesting, I don't find it very useful to me.
 
#17,600
I regret, in passing, jumping on Scott Wurcer, to make an example. I am somewhat tired of 'cheap shots' being fired at me, without due regard to what I hope for from this thread. I like to learn more about other people's circuits, as well as my own. The 'implied insults' and hazing, I find amusing, but tedious, at times. Scott Wurcer and I have known and worked (over the phone) with each other for more than 25 years. I use his IC's, when they meet my needs, gladly, and have done so for decades.
The difference that we have over the AD797 and similar circuits, is related to the overall question of feedback and its relatives: feed-forward and neutralization, as to what they do to audio circuitry and whether they really help the way they are thought to, OR do they just change the distortion into something LESS measurable with normal test equipment.
 
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