John Curl's Blowtorch preamplifier part II

[Steve Dunlap]

Thank you for looking Steve,

No problem, Anatech. I didn't even have to move, just glance up from where I am sitting. I have almost full sets of the technical books from Toshiba, Motorola, Texas Instrument and several other companies. It is not easy for me to read a book anymore, so I guess I should consider letting them go. I keep putting off selling off my equipment and supplies. I guess I don't want to admit it"s over for me.

[anatech]

Hi Steve,
You make me a jealous man! A great collection there.

I should remind you that it's not over until you can't think. This is not your problem and you'll need those references until that time comes. I'm thinking that's a long time off.

So, in no way is "it" over for you sir!

My collection has suffered three complete set backs now. Meaning, I have rebuilt my "library" three times from close to scratch. The things I really can not replace are the eight filing cabinets full of service manuals I collected over a 16 year period. That really hurts. So does the loss of many vacuum tube related material.

So hang on there Steve. Your mind is alive and you think clearly. Keep your printed material. I have to say I'm still a bit envious of your material though!

Best always Steve, Chris

[anatech]

Hi John,

No, Anatech, I am a Barbarian, I am known as Conan the Librarian!

Now, what can anyone say to that!!!

Ahhh, just curious John, what do you use for place holders??

-Chris

[anatech]

Hi Salas,
I'm sorry, I forgot to answer you.

Adding a series resistance is perfectly reasonable - it works. It will also increase the separation between the two channels. Don't forget to stagger the capacitor values to maintain a low AC impedance across the band (up to 1 MHz at least). You just have to adjust the voltage before the resistor drop to end up where you want to be.

The question of how much hiss you'll end up with is more complicated, and probably something that others with more experience than I have with very low level signals. Once you have eliminated the supply noise, you still have to contend with air currents and thermal noise. When you are down that far in signal, even air moving over the transistor legs will generate noise. Your solution will therefore be possibly more mechanical than it is electronic. Same goes for damping mechanical vibrations out.

Bet you didn't want to know about the rest of that stuff.

-Chris

[Salas]

Got you. The answer is blowing in the wind (between JFET legs that is).

[syn08]

Originally Posted by ikoflexer

Why are we even considering Vgs = 0? I thought that was not recommended to build a decent ccs.

What happens if Vgs is not zero? In particular, if we use a resistor value between gate and source such that Id is about 1/5 Idss.

That would essentially significantly increase the noise. The Johnson noise of that resistor should be added to the JFET noise.

[anatech]

Hi Salas,
Everything in design is a compromise of some sort.

When is the power supply clean enough? When do I have enough shielding, damping, hair gel? These are all things that only you can decide. In fact, you may not get any more improvements once you pass some point. You have to recognize that point. In fact, this gives you valuable information for the next time. Of course, how much money you can spend for materials is another factor, but not as great as someone going into production commercially.

Why not prototype and experiment? Do your final build once you're happy with things. The stuff I design and build normally works first time. It's the improvements that is never ending.

-Chris

[syn08]

Here are the results for the Norton reference. All measurements are at 24V from a HP6624 low noise power supply. For each device, I measured the Idss and chosed a resistor R so that the output voltage is around 19V. Noise was measured with an HP89410A VSA, in 10Hz-1KHz, 1601 points. The first picture in this message is the analyzer noise floor (input shorted). It can be eventually extracted from the measured noise data (I have all the traces) but this won't dramatically impact the conclusions. No extra filtering (electrolytic or other) was employed in these measurements.

Here's the devices I had at hand (datasheet specs and my measurements). Measurement results are also attached, each picture is identified by the device name.

2N5458
------
Datasheet:
Idss=6mA
Vt=2V
Vn@100Hz=2nV/rtHz

Measured:
Idss=7mA
R=2.7k
Vo=18.9V
Noise=224.8nV/rtHz

PN4392
------
Datasheet:
Idss=25...75mA
Vt=2...5V
Vn@100Hz=Unspecified

Measured:
Idss=47mA
R=390ohm
Vo=18.33V
Noise=109.4nV/rtHz


2SK246Y
-------
Datasheet:
Idss=1.2...3mA
Vt=0.7...6V
Vn@100Hz=Unspecified

Measured:
Idss=1.3mA
R=15k
Vo=19.5V
Noise=124.7nV/rtHz


2SK170BL
--------
Datasheet:
Idss=6...12mA
Vt=0.2...1.5V
Vn@100Hz=1nV/rtHz

Measured:
Idss=9.7mA
R=2k
Vo=19.4V
Noise=302.4nV/rtHz


As you see, my previous comment regarding the JFET Gm and the output noise is totally confirmed. Gues who's the worse contender? The lowest noise device, having an average transconductance, the 2SK170BL. Why? because it has by far the lowest pinch voltage!

And the best contender? The monster Gm device, with unspecified noise, but high pinch voltage, the PN4392 from Fairchild. This exercise is a good example on how intuition can trick us...

One to another, my results show that it is possible to get around 100nV/rtHz with a Norton reference. We'll see in the next days how's with LM329 and LEDs.

P.S. I see the file names are not displayed. The pictures are in the same order as the devices above.

[Salas]

Originally Posted by anatech

Why not prototype and experiment? Do your final build once you're happy with things. The stuff I design and build normally works first time. It's the improvements that is never ending.

-Chris

I will plug it in my existing 56dB. Have to get the DL103R first, and set the TT right for it.

[Salas]

Originally Posted by syn08

Why? because it has by far the lowest pinch voltage!

And the best contender? The monster Gm device, with unspecified noise, but high pinch voltage, the PN4392 from Fairchild.

One to another, my results show that it is possible to get around 100nV/rtHz with a Norton reference. We'll see in the next days how's with LM329 and LEDs.

Yes lets see. I have some 5459s and yummy 1.6V leds in 3 colors. I hope I can spare some 2SKs if it works good with the 5459s. Not when the Jfet is residing under just Vbe margin in some power supplies unfortunately, where I need the low Vt.