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Old 25th August 2012, 03:20 PM   #251
bcarso is offline bcarso  United States
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Appeal to all: can we please reference the design we're speaking of when we post? When other references are interspersed one is left scratching the head as to which is meant.

It doesn't always have to be a quote, but something like "the schematic shown in #231" would remove ambiguity.
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Old 25th August 2012, 03:35 PM   #252
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by EUVL View Post
Scott,

A SMD dual JFET in SOT23 is too good for me to give up, so I went back to the Toshiba website.
It says (see attached) internal connection common source.

So it can at least be used for input LTP.

Toshiba never says whether their FETs can be used with D/S swap.
All the ones I tried can, even though some has a slightly different Idss when reversed.
But this is solvable even if the lower FET has lower Idss when reversed.
Just tie a resistor or a CRD from Vout to the negative rail.

So I am still on for this one.

I think Dave posted the wrong link.
Here is the link to John's circuit using N-JFETs as input.

Discrete Opamp Open Design

And I also put it here.
I'll do a PCB layout in SMD. I like this.


Patrick

Ugly. Too many FET's.

For the input stage for high Zin (but they will need cascoding) ok, but for the rest of the circuit I would go bipolar. I guess the 8 trannie limit is off the table. 10ppm at 20 kHz 12 V pk into 600 Ohms easily achievable with 11 or 12 active devices, but to get to 500ppb I am compelled to got to 14 or 15 active devices.
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Old 25th August 2012, 03:57 PM   #253
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Quote:
Originally Posted by bcarso View Post
Appeal to all: can we please reference the design we're speaking of when we post? When other references are interspersed one is left scratching the head as to which is meant.

It doesn't always have to be a quote, but something like "the schematic shown in #231" would remove ambiguity.
He mentioned BJT version of Blowtorch presented here.
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Old 25th August 2012, 04:12 PM   #254
EUVL is offline EUVL  Europe
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I don't like bipolars, so the first one John posted will do for me. But everyone to his own taste.
Also the small Dual FETs allow a single aided layout on a DIP8 footprint.
So it gets my vote.

The circuit in post#240 I also like, and can be made with all FETs.
But with no more P-FETs widely available, it does not lend itself to the general public who do not have private stocks.


Cheers,
Patrick
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Old 25th August 2012, 04:16 PM   #255
AndrewT is offline AndrewT  Scotland
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and three spare PINs for tapping into the circuit for "other" duties/corrections/compensations
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Old 25th August 2012, 04:27 PM   #256
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by dirkwright View Post
Well, that was succinct.
The topology used to be almost a Standard one before complimentary push-pull and current-mode fb topologies replaced it.
The reason for that output stage is same as for the diff stages before it - at the time good compliments were virtually non-existant and better match was found with same type transistors.

I used the same topology for my passive RIAA phono stage around 1980-ish but with jfet front end and bipolar Vas for high gain/gnfb and compl bipolar output. Distortion is very low and good PSRR as well could be attained. At that time, the HP thd meter limit was .0018% -- the circuit was better than that and used 6 active transistors (not counting current sources). Today, I think i would use Scott's circuit if i wanted a modern high gain linear circuit.



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Last edited by RNMarsh; 25th August 2012 at 04:39 PM.
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Old 25th August 2012, 04:39 PM   #257
EUVL is offline EUVL  Europe
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> and three spare PINs for tapping into the circuit for "other" duties/corrections/compensations

0.4mm plate through holes to connect top layer to bottom layer.
I did not quite manage to get it all in one without jumpers.
And I am sure you don't want to solder 0.4mm enamel wire jumpers.




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Old 25th August 2012, 04:40 PM   #258
bcarso is offline bcarso  United States
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Quote:
Originally Posted by Wavebourn View Post
He mentioned BJT version of Blowtorch presented here.
I was talking, among other things, about JC's "rational" design remark, which wasn't clear to me. But in general, we have a tendency to post thinking it is going to appear in immediate succession to what we're referring. And that's often not the case in a busy thread.
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Old 25th August 2012, 05:03 PM   #259
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Let me clarify things for you all: Over the decades, I have had a series of 'requirements' for making an audio gain stage, developed after years of making prototypes. This designs meets MY criteria, but then I designed the whole thing and put it into a power amplfier, 30 years ago. I guess I should mention that, but that is NOT why I said that it was 'rational'. I meant that this design example meets MY criteria, and does not fall down in some way. Now, if I am allowed, I will try to show where many other designs fall away from meeting the criteria that I find useful and important. Not ALL designs are failures in some way, but they have yet not been presented here to any degree, recently.
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Old 25th August 2012, 06:08 PM   #260
EUVL is offline EUVL  Europe
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Of course you are allowed.
We are all eagerly waiting to learn from you, John.


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