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Old 16th October 2012, 09:31 AM   #1741
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by fas42 View Post
-- once corrected, the 2nd and 3rd harmonic were both at -89dB. Meaning, very close to identical distortion with badly mismatched JFETs ...

But, and a mighty big one, the offset is at the mercy of the BF862s. If perfectly matched, LTspice predicts an excellent DC null up to 75degC; above that temp the offset goes a-wandering; on the other hand, if using a severely mismatched pair, with gain of 100, the offset varies between -2.4V and 2.4V in the range 7deg to 47deg (default is 27deg)!

One thing, this has been an excellent learning exercise for me - thanks to all ...

Frank
So, you learned that matching the input diff pair isnt all that important to thd but for the lowest/best DC parameters matching is important. For this topology and using high neg fb. Thats great. Your sim shows you should be able to get away without using a dc servo on low gain apps. Just as Scott said it should :-) -RNM

Last edited by RNMarsh; 16th October 2012 at 09:37 AM.
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Old 16th October 2012, 02:25 PM   #1742
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Originally Posted by EUVL View Post
> The BF862 in my experience can not go beyond 4 paralleled devices with even good DIY assembly.

Why is that ? Oscillations ?
How about using a 0402 gate stopper next to the gate pin ??
I know there would be a noise penalty.

SMD inductors are not very linear ??


Patrick
I was speaking of true paralleling where you try to preserve the bandwidth, they are prone to oscillate at multi-Mz frequencies (probably parasitic Collpits mode). An MRF91 would oscillate at 2Ghz with just an inch of wire on the emitter and collector.

There is no gate current to speak of at audio frequencies, and certainly no DC current so I don't see the problem.
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Old 16th October 2012, 02:56 PM   #1743
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Originally Posted by kgrlee View Post
None at all. Just my demo of what can be done if you have a clear idea of what's important for good sound and ignore all Golden Pinnae considerations.

It's a specialist application, now only of interest to vinyl fanatics with low output moving coil cartridges. Probably still the lowest noise such device in the known universe. It's taken more than 30 yrs for someone to come close. What Mr. synaesthesia tells me is that certain devices that I thought were Unobtainium may actually be available.
.06nV preamps exist, even more exotic and specialized. Transformers are your friend.
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Last edited by scott wurcer; 16th October 2012 at 02:59 PM.
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Old 16th October 2012, 03:19 PM   #1744
KBK is offline KBK  Canada
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Originally Posted by scott wurcer View Post
We might be getting carried away with these thermal issues. A simple potting in the right stuff might be enough.

Diamond filled epoxy?

My direct experience is that potting sounds horrible for complex dynamic signals, like audio, where one requirement for expression/condition is never the same as the next. this, for micro signals and for macro signals. (microphone and speaker)

Clocking circuits, which require repeatability in environment, or tracking circuits of almost any kind (depends on the requirements of the tracking, obviously), they can and do benefit from potting. They can be micro adjusted back to a norm, after the given potting has settled to final cure. (months) If time is a concern, then a low level oven cure for a day or two can take the cure to +90% cure rate and then the micro norming can take place. Low level oven is critical, due to lattice structure linkage deviation under high speed cure rates. Deviations should be quite minor after that. This can get product out the door sooner, to cut the cycle shorter, for all the right reasons. Makes for expensive gear, but also good gear. Secondary concern is epoxy adhesion to substrate, in situ. Breakage or shift, on the job. Consider micro etching or even micro scarring (blast, sanding, whatever), for better adhesion.

Last edited by KBK; 16th October 2012 at 03:30 PM.
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Old 16th October 2012, 03:25 PM   #1745
qusp is offline qusp  Australia
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well I guess youre screwed then… are you removing all the cases on every device, resistor, cap, dac with some sort of hardcore acetone, so you are left only with lead-frame?

Last edited by qusp; 16th October 2012 at 03:27 PM.
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Old 16th October 2012, 03:41 PM   #1746
KBK is offline KBK  Canada
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Originally Posted by scott wurcer View Post
Did some quick sims of the split/bootstrapped compensation (jcx). I found with an even split (2-200pF in series) with the center bootstrapped was an improvement, however the bootstrap had to be derived at the output of the emitter followers just before the output pair or the thirds were worse. The distortion numbers start getting silly.

Indeed the settling time is very compromised by this.

Hi Ken, I'm honored. There is very little signal current in any resistors but the feedback network.
Sorry Scott. Did I insult via default silence? I'm not aware of your 'full' background, but I'm aware of 'some' of jneutron's.
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Old 16th October 2012, 03:56 PM   #1747
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Originally Posted by KBK View Post
My direct experience is that potting sounds horrible for complex dynamic signals, like audio, where one requirement for expression/condition is never the same as the next. this, for micro signals and for macro signals. (microphone and speaker)

Clocking circuits, which require repeatability in environment, or tracking circuits of almost any kind (depends on the requirements of the tracking, obviously), they can and do benefit from potting.

The clocking and potting is tricky. Transient mass loading (mechanical) can cause deviation. Besides the given load response of the potting under optimal conditions. The nightmare of repeated brute force and ignorance as a testing/design regimen becomes the projected norm of that doorway of invention.

Edit: There is a simple way past the potting issue for tracking and clocking. But that is the part that is relatively unknown. Otherwise it would have been published as product by now. Perhaps it is, as I don't get out much. (read the trade rags/papers, and so on)

Last edited by KBK; 16th October 2012 at 04:18 PM.
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Old 16th October 2012, 04:25 PM   #1748
KBK is offline KBK  Canada
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If you purposely distort some of the emergent methodologies of making carbon nano-tubes, you will have your nano-diamond based epoxy filler. And, it will come in at a few dollars per pound.
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Old 16th October 2012, 04:33 PM   #1749
KBK is offline KBK  Canada
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Originally Posted by qusp View Post
well I guess youre screwed then… are you removing all the cases on every device, resistor, cap, dac with some sort of hardcore acetone, so you are left only with lead-frame?
One should do this, at least once, so they can get a good handle on what the cases do the sonic presentation. This, of course, in a circuit that does not require the given cases for thermal reasons. To try one against the other, in a controlled environment. This, properly handled... constitutes a fairly decent single cause analysis.

I should go away again, for a while, before I'm seen as saying something stupid. To leave while the getting is good. . Comments on nano diamonds at a few dollars per pound is bad enough.

Last edited by KBK; 16th October 2012 at 04:35 PM.
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Old 16th October 2012, 05:01 PM   #1750
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Originally Posted by rsavas View Post
Other possible candidates to try out, for jfet's ,2SK2394,2SK3557, in SOT23 pkg
Both seem to be available as SMD duals also - MCH5908 and another. Vds is low at 15V max, but Idss goes up to 30 mA and Yfs above 30 mS. No idea about matching.
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