John Curl's Blowtorch preamplifier

Status
Not open for further replies.
Jump to Latest
#9,402
myhrrhleine said:
How's the sound compare?
Modern IC vs. Modern transformer?
Click to expand...
john curl said:
I would bet on the transformer.
Click to expand...

🙂 Hate to crush peoples (audiophile golden ears) illusions.
But I gladly will try to do it, anyway. In the name of sober reality.
😀 😀

Both the question and the answer above, shows some illusive illogical thinking!
They are not free from all the myths of 'sound' tweaking and listening sessions.

Those tweaking sessions AudioPhile people on all levels are doing.
Most of the time biasing and fooling them selves. To the bone!
And getting a twisted image of reality of sound waves produced.

Misguided by the pressure from the golden ear crowd & their followers
and based on the many, many daily myths presented in forums such as our here at www.diyaudio.com

If you repeat one lie or story enough many times
it becomes the truth of the day.
And you have to have lots of personal integrety,
to see it's is as much a truth as in stories of people
taken away by aliens from outer space.
Or the conspiracy theory that USA did never make it to the moon.
... or any other common myth of yesterday & today..

----------------

I would bet, Mr Curl, if you're using very good gear
... you hear no one!!! Nothing by transparancy.
Neither good signal Transformer nor good IC
will contribute to sound at audioble levels.


It is much truth in the statements and similar opinions we see often
from sensible audio designers:
In our days of very refined audio circuits, 192/24 bits level,
there is much that points to, logically,
that only if one circuit is badly designed, with higher distortions,
there is any audible contribution to the sound we hear & can hear.
Click to expand...

What makes sound in my Lineup book, today, is:
1. The sound source. With its quality or lack of quality ..
2. The sound reproducers.
The sound creator devices: >>>> HeadPhones and LoudSpeakers.
--------------
--------------

Because only there, and nowhere else in our audio chain of hi-tech components, we can find signal screwed up at the LEVEL of 0.2 % - 1.0 %.
Even an ordinary Audio Op-Amp like OPA2134 mentions
THD like 0.00001 %
This is about 100 -120 dB down .. from Speakers

And when I mention speakers/headphones at that level of distortion
0.2 % to 1.0%, it is not some cheap ordinary gear, that most of us have at home.
Even those that dream about good speakers & read them reviews of High-End in fact many times listens to some home built mediocre speakers system.
Like Diy Transmissions Lines 😀 😀

No, what I talk about is absolute High End gear for thousands of money.
Having comparably low dist to Ordinary sound producers.
But vanishing HIGH Distortion of Sound
compared to wires + opamps + power amplifiers.

Lineup articles on Audio things
Lineup - regards without sounds - sound of silence

..
 
#9,403
Lineup;
like sometimes a grape juice may be sour and alter the state of mind, singers may have cellphones in pockets. Transformer balanced gear is more immune than electronically balanced one with a sand.



john curl said:
Ciss in a 2SK170 is 30 pf or more! A 2N4416 has 2.2 pf. Big difference.
Click to expand...

Thank you John! I'll check it. The capacitance itself is not the evil; the evil is it's variation with voltage between gate and source.
 
#9,404
Scott,
when I want to have a specific Idss value for a JFET mode, I change both the beta and VTO values.

Maybe we should continue in the Spice thread.



Sigurd

scott wurcer said:


Your SPICE is definately different. our NJF defaults to the earliest JFET models where your model gives very non-physical characteristic curves. Pinchoff is abrupt and always obviously at VTO (it looks like a bipolar). After asking around I think I can get a better model running. Did you intend for VTO (VP) to be the same across the entire range. The first model was labeled for the 2SK170_BL so I just assumed there would be a mean value for each color code in yours with the beta adjusted over a more limited range. That's what I thought I was missing, the same VP for the entire 4:1 Idss range is not very physically real. It's funny we do almost no design with JFETs where a good fit matters.

With modern lithography the old planar FETs have absolutely cavernous dimensions. Since the parameter spread has not improved much I would think the implant dose and drive which enter into the VP and IDSS equations to essentially keep beta constant are more important.

The short channel FETs add some additional variation but accounting for 4:1 Idss variation by geometry alone I would doubt.
Click to expand...
 
#9,405
All MIC amps I have seen use only one rail PS (Phantom power) but then one needs a cap in series with the signal and that will never be optimal, IMO.

Using + and - rails instead would create some practical / mechanical issues but they can be over come.
That is how I would do it.

Anyone seen any dual rail mic amps?


Sigurd


john curl said:
This is a sad situation. We have all kinds of amateurs designing microphones. Unfortunately, they are not the best possible solutions. Yet, nobody seems to know the difference.
Click to expand...
 
#9,406
Adding a cascode will lower the input capacitance - is that what you are thinking about?

Borbely wrote a bit about capacitive induced distortion in the JFET series of articles.



Sigurd


john curl said:

The first factor of concern to me is: What is the input capacitance of the input fet? This is more important than most here realize. That is why I brought the subject up. Sure the 170 will work, and it does have low voltage noise, BUT what about the distortion added to the capsule transfer function? Isn't that important as well?
I have found several RF fets that do have low input capacitance AND are pretty quiet as well. It is not so hard to find alternative devices that would work somewhat better. Does anybody know what I am talking about, does anybody have any suggestions?
Click to expand...
 
#9,407
It is more complicated than just a cascode. That is the easy part. It also has to do with bootstrapping and inherent input capacitance. You will find that the vast majority of professional mikes use a low cap jfet. I just checked out specific examples from Sony, Schoeps, and B&K. All low cap devices under 10pf.
 
#9,408
Sigurd Ruschkow said:
All MIC amps I have seen use only one rail PS (Phantom power) but then one needs a cap in series with the signal and that will never be optimal, IMO.
Click to expand...

In my new phased mic array made with cheap Pabasonic electret capsules (modified to bootstrap sources!) a phantom power is used only as a signal to switch on a battery power. It is too weak to power cirquitry driving without expensive transformers 200 Ohm load plus capacitances of cables.
 
#9,409
john curl said:
Ciss in a 2SK170 is 30 pf or more! A 2N4416 has 2.2 pf. Big difference.
Click to expand...

In 1950 Walter Kistler invented the charge amplifier, you continue to quote performance issues with the way certain people choose to do things, there are other ways. The truth is going into sub nanoVolt noise is pointless with a microphone in general, so the point is somewhat moot.

Quote from Wiki....

Charge amplifiers are usually constructed using op amps with a feedback capacitor. They thus act in a similar manner to an integrator. Since the transducer acts in a similar manner to a differentiator, the two transfer functions cancel and the output voltage is proportional to the charge produced by the transducer. Stray capacitance at the input to the amplifier is not detrimental to operation because this capacitance is always at a virtual ground.
 
#9,411
Wavebourn said:
Thank you John! I'll check it. The capacitance itself is not the evil; the evil is it's variation with voltage between gate and source.
Click to expand...


john curl said:
It is more complicated than just a cascode. That is the easy part. It also has to do with bootstrapping and inherent input capacitance. You will find that the vast majority of professional mikes use a low cap jfet. I just checked out specific examples from Sony, Schoeps, and B&K. All low cap devices under 10pf.
Click to expand...

I just dug into that for the first time. Not knowing much about circuits and all.

Why do you guys call it capacitance, when it actually isn't? It just looks like capacitance, but is only related to the 'classic' interpretation of capacitance.
 
#9,413
Thanks Dimitri, I was looking last night for a good derivation and you found it in the AES preprint, on pages 5-6. B&K pointed it out to me back in'74, but I could not find any clear derivation in their app notes. There is, however, a GREAT app note on negative capacitance in the 'B&K Technical Review' 1996 Vol 1 by Erling Frederiksen (the guy who first told me about it, at least I think it was him), but it got too esoteric for presentation here. Check it out on their website!
 
#9,414
john curl said:
Thanks Dimitri, I was looking last night for a good derivation and you found it in the AES preprint, on pages 5-6. B&K pointed it out to me back in'74, but I could not find any clear derivation in their app notes. There is, however, a GREAT app note on negative capacitance in the 'B&K Technical Review' 1996 Vol 1 by Erling Frederiksen (the guy who first told me about it, at least I think it was him), but it got too esoteric for presentation here. Check it out on their website!
Click to expand...


Ok. I see. I was thinking of a non-linear LCR circuit added in the right way to the right spot as a linearization/correction for this issue. However, unless carefully implemented, it is still a point of the horse leaving the barn and then chasing it. Chasing the dragon's tail, as Bearden likes to put it. Temporally speaking, it must be implemented in parallel with input. Ground stability and connectivity of such with respects to the fet, signal source, and amplification would also play a great part in the effectiveness of such a thing. Folks in Scott's line of work should be able to implement something workable and repeatable on a decent scale.
 
#9,416
got it. took me a bit. Any corrective LCR applied to the input, must derive from the V+ or supply. What you are doing, is applying a polarized non linear spring to the input to correctively shape it-to deal with the non-linearity created by 'gate capacitance'. Which isn't really capacitance.

Also, it seems as if the inductor should be at the input side and the cap at the supply side, with the resistor in the middle. It'll take some fiddling........
 
#9,417
This is like deja vu, I would like to see evidence of a single popular professional recording microphone that uses any of these negative capacitance techniques. I generally find them esoteric engineering exercises that preclude interchangability and require difficult trimming in assembly (if they ever were outside the laboratory).

We have drifted again, sorry I'll stop now.
 
#9,418
KBK said:


I just dug into that for the first time. Not knowing much about circuits and all.

Why do you guys call it capacitance, when it actually isn't? It just looks like capacitance, but is only related to the 'classic' interpretation of capacitance.
Click to expand...

Because it is a common term.

The definition: varactor diode
a diode designed to have a repeatable and high capacitance vs. reverse voltage characteristic. A two terminal semiconductor device in which the electrical characteristic of primary interest is the voltage dependent capacitance.
 
Status
Not open for further replies.
Top Bottom