Comparison of JFETs for mic applications

[ricardo]

And forgive my asking but does TINA simulate resistor noise properly?

Yes. But... It has a funky way displaying it.

Can we show this funky display of the noise spectrum for just a couple of different resistors? 2k4, 4k7, 9k1, 18k & 36k would be nice. Ideal resistors are fine.

I didn't claim SimpleP48 would cure Global Warming and bring World Peace. Only that it would be quieter than your SupaDupa circuit in certain frequency bands.

So a good noise spectrum display is obviously essential for this shootout. Constant Bandwidth instead of the Constant Relative Bandwidth on page 10 of Zephyr.doc is OK But log frequency scale please. Linear Frequency will scrunch up the important parts of the display so we won't be able to see much.

Can TINA produce a noise spectrum curve like the one on page 10 of Zephyr.doc ?

If the answer is 'No', then we have to find a different way to investigate my claim that SimpleP48 is quieter than your SupaDupa circuit in certain frequency bands. Any suggestions for alternative methods?

---

I'll talk about the requirements for the 'THD with level' side of the shootout in separate posts. I'm still interested in this though I'm sure your SupaDupa circuit will have as great an advantage over SimpleP48 as it has in number of parts.

As a start, are you happy to use SimpleP48RCA for this (and the noise test if you wish) instead of the original SimpleP48? This equally complex circuit was in direct response to people who wanted lower THD & higher max. spl handling over the original. Should not be a problem as this is a sim. shootout.

 

[ricardo]

Much of today's unserious software runs rings around what was possible under DOS.

Moderators, I beg your indulgence for some wi**y wan*ing, an opportunity not often available to beach bums like me in such august company

When I emerged after nearly a decade in the bush circa 2008, I found that though bells, whistles and fancy pics/animations bla bla were now commonplace, the ability to do useful stuff in software was still pretty rare.

In the meantime, I found the power of even cheapo computers had increased so much such that stuff I'd only dreamed off was now, not only possible but easy even with my primitive DOS compilers.

I present 2 examples.

Circa 1990, I dreamt up, in theory, Professore Farina's Log sweep to measure Response & THD in the theoretically shortest possible time. At that time, the computing power, and especially the A/Ds were too $$$ as I wanted lots for Factory Test.

More than 1.5 decades later, Angelo visited dis beach bum in Cooktown and was surprised to be shown Jurassic code that replicated his method. I use my own versions of Angelo's method to measure speakers, microphones bla bla

The other example is How to measure, align and dream up Digital Filters for a Tetrahedral Ambisonic mike. I know this is something among Thor's many Supa powers. I met up with Peter Craven, the co-inventor of the Soundfield Mike, at AES, San Francisco 2006?, and presented the results and also the commercial manifestation of these musings.

Stan Lipshitz made useful comments on my digital juju there too

You may like to ask Prof. Farina, Peter Craven (whom Michael Gerzon said was the smartest person he knew) and Stan about their opinion of the ravings of the "Mad Chinaman"; one of my nicknames in my previous life.

Alas, Michael Gerzon has passed on so I can't ask him for a reference. I met him and Peter circa 1980 when I was doing the Mk4 Soundfield; IM not so HO, the best microphone of the 20th century. Certainly the most $$$ and complex which should surely gladden Thor's heart

PERHAPS you should consider to be dragged kicking and screaming into the windows age. Or Linux for that.

Alas. I've tried to do this on 3 separate occassions this Millenium but was thwarted by computers dying on me. This is my 4th attempt but I need to have something Jurassic working to check what my software should do.

---

Now back to software mike shootouts ...

 

[thor.zmt]

When I emerged after nearly a decade in the bush circa 2008, I found that though bells, whistles and fancy pics/animations bla bla was now commonplace, the ability to do useful stuff in software was still pretty rare.

Well, you are 100% wrong. Just saying. And all software managed to run on Windows 7, no problems.

I guess anyone looking for useful computer software to do useful work will find it. And anyone looking to have his prejudice confirmed that only programs under DOS can do useful work will find confirmation.

Alas. I've tried to do this on 3 separate occassions this Millenium but was thwarted by computers dying on me. This is my 4th attempt but I need to have something Jurassic working to check what my software should do.

I just packed up my little workshop and moved it.

My "Labrat" Komputermaschiene is an antique Lenovo Thinkpad T440. For lab use, recording and DJing I always buy an ex lease 3 year old minimum spec T series Thinkpad.

Extremely reliable. Extremely Serviceable, even a 10 year old T series Thinkpad will have most any FRU (Field Replaceable Unit) available.

The keyboard will survive a coffee spill and the whole machine is easily folded up and can be used to beat latter day luddites to death, then opened again and continue emails.

As T series Thinkpads (at least up the one I got) have replaceable CPU, Memory and HDD's, pay 100 Bux for the machine, pay another 100 Bux for a totally obsolete top of the line Intel CPU fitting it, 16-32G in memory sticks, a 256G SSD and you are off to the races.

Mine runs of course all the sim software and EDA (though real work is done on the big workstation). Attached to this 15" Screen laptop are:

Quantasylum Audio Analyser QA404

RIGOL 4 Channel 100 MHz DSO

RIGOL Function Generator

MINIDSP EARS calibrated against G.R.A.S H.A.T.S with Calibration file

MINIDSP UMIK USB measurement microphone Calibrated against Earthworks with calibration file

Electronic Load for testing powersupplies and batteries

What can be done with small setup that fits a small work desk is mind-blowing to a guy who started out in the Comecon.

Packed up it easily fitted a 100 liter box on wheels including my old-fashioned 3 Channel 5A Lab supply.

The abilities modern computers and computerized, computer controlled test gear are off scale compared to what we made do in the 80's, extremely affordable.

And modern simulation software goes as far as simulating passives with S-parameters if you are thusly inclined.

We absolutely live in a golden age when it comes to what computers do for us to support audio design work. My simple desk can do stuff almost instantly that a complete engineering department would take month in the 80's.

Thor

 

[thor.zmt]

Can we show this funky display of the noise spectrum for just a couple of different resistors? 2k4, 4k7, 9k1, 18k & 36k would be nice. Ideal resistors are fine.

TINA displays either noise density (spectrum) or SNR with reference to an arbitrary level across bandwidth.

I didn't claim SimpleP48 would cure Global Warming and bring World Peace. Only that it would be quieter than your SupaDupa circuit in certain frequency bands.

Which frequency bands would that be?

The numbers for 20-20k SNR re 94dB did not favour SP48.

If the answer is 'No', then we have to find a different way to investigate my claim that SimpleP48 is quieter than your SupaDupa circuit in certain frequency bands. Any suggestions for alternative methods?

Simple inductive circuit analysis?

SP48 operates the J-FET as transconductance element with no degeneration into in effect 6.8kOhm load, giving with a 1mA/V transconductance J-Fet a gain of ~ 7.

With a transconductance of 1.2mA/V we expect an equivalent noise resistance of our J-Fet of 1V/1.2mA or around 833 Ohm, giving ~ 0.5uV noise in the flat band, or around 4nV|/Hz.

We have the full influence of the remaining input capacitance and miller capacitance, attenuating the microphones signal for SP48.

With 1.2pF Crrs and a gain of 7 we get 8.4pF plus 4.5pF Ciss, for ~ around 13pf load capacitance on a 44pF microphone, causing ~ 2.3dB attenuation.

So SNR of THD raw J-Fet is degraded by 2.3dB. The 6.8k load(s) contribute ~ 1.5uV each, or ~ 2.1uV combined.

The 0.5uV noise of the J-Fet sees an input referred load resistor contribution of around 0.4uV, for a total noise of 0.65uV

The cascoded split load inverter in my circuit in effect eliminates any capacitive effects. The gain to each load resistor is 1, but each resistors contributes it's noise.

As these resistors need to be scaled for ~ 5V according to the actual Idss, I used the highest Idss class with 10k loads.

The load noise will be ~ 1.8uV per resistor.

So purely Ein at the gate will be around the 1.8uV, theoretically an 8dB noise disadvantage, IF NO OTHER SOURCES CAUSE EXCESS NOISE.

---

I'm still interested in this though I'm sure your SupaDupa circuit will have as great an advantage over SimpleP48 as it has in number of parts.

It does. And my design was made for studio recordings, not live in a concert hall.

Of course, adding a single capacitor (say 33pF S-G for a 44pF capsule) could improve that a lot.

As a start, are you happy to use SimpleP48RCA for this (and the noise test if you wish) instead of the original SimpleP48?

Moving goalposts already Richard?

How about NO.

I am ok to ADD this.

Please post the exact circuits you want me to use, assuming a J-Fet with ~ 370uA IDSS and a Yfs of ~ 1.6mA/V (this is the model I have to work with).

And a capsule with 44pF capacitance and -35dBV Sensitivity and 26mm Diameter (11mm Radius for Brownian motion noise).

Thor

 

[gyraf]

I have several pieces of essential software that requires me to keep hardware alive for both DOS/Win3.11 and winXP.

Some programs simply won't run on modern hardware (or rather modern abstraction layers). Try to real-time a parallel port or mount a paranoid protection dongle, and you'll know what I mean..

@ricardo - are you just looking for a xp install cd, or for the whole computer?

/Jakob E.

 

[thor.zmt]

Some programs simply won't run on modern hardware (or rather modern abstraction layers). Try to real-time a parallel port or mount a paranoid protection dongle, and you'll know what I mean..

I had this problem with an AP2 that needed an ISA Card and an antique version of windows. Eventually the last PC with an ISA slot in the part of Scheena where I was died. So now I had a 10k USD boat anchor.

Solution, USB to AP2 interface, use USB 2.0 isolator and also kill the ground loop that plagues the the AP2. Run a laptop. Could have done years earlier, burlt wasn't on the old inertia thing if it ain't broke.

As was, the AP was down for Several weeks.

At this point in time I have invariably found alternative (and usually better) more modern software, hardware or suitable bridge hardware, to move forward and get off unsupportable legacy hardware.

It seems easier to keep old hardware/software alive, beyond a certain point I find it counterproductive and borderline latter day ludditism.

And I'm really conservative on hardware / software replacement to start with.

Example, using an old pentium to get hardware control of a parallel port, vs running an I5 10 gen with a USB 2.0 480M/sec to parallel dongle and emulation.

Which REALLY responds faster? Virtual machines and multiboot allow configurations that remove componts that drag down performance.

For example, I find having a second boot configuration on my laptop that is maximally slimmed down for DAW / DJ Software use. That is literally a second install of the OS, double boot, configuration for best performance, low latency etc.

Anyway, I'm preaching to those who probably don't want to listen. But if you rely on a legacy system enough to keep it alive well past it's "Recycle by" date, you probably should start looking hard enough for a replacement or way out. And if not, let it die, move on.

Otherwise we'd all still 50MHz tube scopes.

Thor