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Old 4th September 2012, 10:13 PM   #701
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I like augmented emitter followers.









Edit: like'em more than "Cascoded output stages"
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Last edited by Wavebourn; 4th September 2012 at 10:15 PM.
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Old 4th September 2012, 10:35 PM   #702
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Quote:
Originally Posted by bcarso View Post
…I'll get a noise measurement of it soon, but according to Dimitri they manage about 1nV/sqrtHz.
Good enough! Looking forward to confirmation as well.


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…Until NXP is persuaded to make a BF862 dual (and the latter was developed for AM radios, so the prospect may be unlikely) they are very attractive.
Your right. Easy to overlook the obvious connection shortcoming with the other apparent virtues of the 2sk2145.


I was thinking vertically in that circuit. Delete the degen resistors and save the space by using two K2145s instead of four with the BF862.


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Now you are getting practical! These other aspects are often viewed as an afterthought, some of which that can be brushed aside by invoking a better power supply etc. Considering them at or near the outset is wise.
Absolutely. How well can a simple circuit work as a functioning op amp. Judging each on these secondary effects is just as important as they may overshadow the primary performance metrics if not measured and understood for a particular use or implementation.


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Originally Posted by bcarso View Post
Another consideration is to look throughout, especially near the input stage, at how much signal swing changes power dissipation, i.e., thermal distortions, and also how the design responds to more-than-zero source impedance. And --- how does it handle overloads?
For JFET diff pairs in small footprint circuits I have been looking at simple folded cascodes using MMBFJ111 JFETs with Vgs thresholds in the 6 volt range. Any suggestions?


Overlaod recovery, clipping behavior, assymetrical clipping - all good points to consider. So are possible shorted outputs during cable changes... pop then smoke???

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Dave, I found your proposal very interesting.
Thank you. Been looking at variations of it for a while and like it for its simplicity. This and several of the other examples posted can be made pretty small.


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But now, what circuit are you talking about?
This one from Scott was specifically what I was suggesting parts for.

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Generally, are we talking about several circuits, every one heading for another direction?
Several circuits recently. It really has been all over the place since starting.


Just my opinion but I think we should build a number of them then test to see how well things really work. Patrick (EUVL) is working on a version of this Kaneda/Constellation like circuit. I have a layout in progress of my SE circuit. Scott's and others deserve some solder at some point as well.

I hope that others will build more of the proposed circuits. Some of these have little chance of fitting on a dip8 footprint like Patrick’s and mine so the API 2520 footprint could be used instead for ease of comparison...

Dave
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Old 4th September 2012, 11:22 PM   #703
coluke is offline coluke  Italy
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Got a bunch of J113s from Mouser, did a (not so) quick selection and tested again the design in #451 (with some major changes - no cascoded input fets: I'm a lazy guy ). This, besides reminding me why I really don't like designs requiring selected devices, makes me wonder wonder if the results are worth the time spent (well, not that long - 1 hour or so, but it was pretty boring, and I'm so lazy...): THD is now 0.0013% @ 0 dBV into 30 ohm in the 100 Hz - 10 kHz range, low order only (2nd & 3rd), BW is ample and DC drift just absent or so (output offset changed by less than 1 mV over 2 hours) - same operating conditions I described in my previous post. Quite a remarkable result for a 10 devices design with a far-from-optimum layout (still breadboarded, I mean); but I'm pretty sure I can't hear THD in the 0.005% range, so my somewhat philosophical question is: I'm quite happy to lower further an already pretty low THD figure by selecting devices, but maybe trying to design a circuit which doesn't rely on selection to perform well is a better approach - isn't it? To me, at least: all in all I'm not an audio professional, but an experimental physicist - have to throw in the circuit a bunch of randomly selected fets and see what happens, and if I can manage to hear any difference at all...

Ciao,

L.
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Last edited by coluke; 4th September 2012 at 11:47 PM.
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Old 5th September 2012, 12:55 AM   #704
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Not sure, but I thought I had read somewhere that paralleling input devices reduces noise?

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Old 5th September 2012, 12:57 AM   #705
bcarso is offline bcarso  United States
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Quote:
Originally Posted by fitzfish View Post
I hope that others will build more of the proposed circuits. Some of these have little chance of fitting on a dip8 footprint like Patrick’s and mine so the API 2520 footprint could be used instead for ease of comparison.
Dave
Perhaps not DIP8 footprint on a single board, but one can make a layer cake and get a lot more parts down if needed.
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Old 5th September 2012, 12:59 AM   #706
bcarso is offline bcarso  United States
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Not sure, but I thought I had read somewhere that paralleling input devices reduces noise?

_-_-bear
It will reduce "series noise" (aka "voltage noise") and increase "parallel noise" (aka "current noise"). With FETs there is very little of the latter at audio frequencies, so it can help IF you can tolerate the increased capacitances.
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Old 5th September 2012, 01:19 AM   #707
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Perhaps not DIP8 footprint on a single board, but one can make a layer cake and get a lot more parts down if needed.
Absolutely right. I have some layouts like that. Truth be told, a few were fixes once in production (not audio)... Some discrete amps use right angle boards for most of the components with the dip8 footprint simply for the transition to the standard pin locations.

The AP 2520 footprint was an easy (read more space) option without 3D construction so I brought it up. Dip8 interests me most so I foresee some vertical construction in my future.
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Old 5th September 2012, 01:38 AM   #708
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> With FETs there is very little of the latter at audio frequencies,
> so it can help IF you can tolerate the increased capacitances.

Ergo the quietude of power mosfets with their many parallel cells.
(mentioned before, but I think a mostly unexplored avenue)

> IF you can tolerate the increased capacitances
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Old 5th September 2012, 01:39 AM   #709
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by coluke View Post
Got a bunch of J113s from Mouser, did a (not so) quick selection and tested again the design in #451 . . . hear any difference at all...

Ciao,

L.
Nice result Coluke!
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Old 5th September 2012, 01:59 AM   #710
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Ciao,

L.
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