John Curl's Blowtorch preamplifier part II

[Steve Dunlap]

Quote:
Toshiba 'Small Signal Transistor' manual 1992
Something everyone around here (Toronto, Canada or even the service sector) has been searching for, but never finding. That's any year! Even a PDF (from anywhere) would be great!

Looking at the shelf behind my computer, I see I have the 1993 version. It's a big book. PDF would take forever.

[john curl]

Sorry Dimitri, but the actual value is 1.57 times the -3dB bandwidth.

[anatech]

Hi Salas,
Why four diodes in series? Or is this the voltage you wanted to end up with?

1.8Vf Red or Yellow or Green color Leds well bypassed only with a high value low ESR cap, fed by low noise Jfet CCS

Well, to filter the supply, you need to use a resistance in series with the reference before your circuit. If you don't, then all this will do is create a time delay as the capacitor slowly charges up. It will not really filter that much out when you compare it to the impedance of your Vref.

Hi Steve,

Looking at the shelf behind my computer, I see I have the 1993 version. It's a big book. PDF would take forever.

My gosh Steve! I wouldn't actually ask anyone to do that. Especially not you! That's asking way too much of anyone.

I was merely hoping that Toshiba had scanned the book in. The easiest way would be to cut the spine off the book and use an auto-feeder on the scanner. No, I expected to pay something for a book, back in time. They simply did not seem to be available to us. One day, maybe.

Just imagine all the technical information that is sitting around, unused and possibly being thrown out.

Thank you for looking Steve,

-Chris

[john curl]

My business associate accidently spilled a full cup of coffee on my 1992 edition. I was livid, but he didn't mean to do it. These handbooks are meant to last a lifetime.

[anatech]

Hi John,

My business associate accidently spilled a full cup of coffee on my 1992 edition.

That only proves you use it. The signature of a technician ...

These handbooks are meant to last a lifetime.

Yes, they really should. I have a library where I collect technical books (and read them). There is nothing like sitting down with a book, you can't get the same appreciation with a computer and PDF.

I had a business partner that stupidly placed all my early books on a freshly painted bookshelf! He was the idiot that painted it! I think your imagination is filling in the blanks. What do you say to someone that dim? They are all gone, I got angry just seeing them like that. Cost me a lot to replace most.

-Chris

[Salas]

Originally Posted by anatech

Hi Salas,
Why four diodes in series? Or is this the voltage you wanted to end up with?


Well, to filter the supply, you need to use a resistance in series with the reference before your circuit. If you don't, then all this will do is create a time delay as the capacitor slowly charges up. It will not really filter that much out when you compare it to the impedance of your Vref.

-Chris

In the previous pages I was cooking for a DC stable practical to make with available in my severely limited common DIYer's drawer components, very low noise about 7V Vref for biasing the base of the cascode BJT for 4 parallel 2SK170BL low MC input stage, since I am looking to get a 0.25mV MC. So I want to boost my gain avoiding as much hiss as possible.

You suggest I feed from the voltage reference node through a 100R to a big shunt cap if I get you right?

[john curl]

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

[dimitri]

Sorry Dimitri, but the actual value is 1.57 times the -3dB bandwidth.

You caught me , yes, actual noise bandwidth is pi/2=1.57 times larger, 1.26=sqrt1.57 correction factor to calculate correct noise density

[syn08]

Ok John, now that you realized that it's not at all easy to bring a JFET at the zero tempco point (BTW, you didn't consider the large Idss variations and the large noise impact of a source resistor) perhaps it's a good time to address a few more things you didn't seem to fully understand.

First, your statement here: http://www.diyaudio.com/forums/solid...ml#post1961619

Let's see how this goes. Let's consider the simplified case of a JFET having the gate connected to the source, feeding a resistor to the ground. Obviously, the current through the resistor is Idss. Now, asume you need an output voltage of Vo volts. Therefore, the resistor has to be R=Vo/Idss.

So far so good. Now, the JFET is at Vgs=0 therefore the transconductance is Gm=2*Idss/Vt where Vt is the threshold/pinch voltage. How much is Au, the current source gain? Obviously Gm*R

And guess what's the final result, after a simple substitution above? The gain is Au=2*Vo/Vt. It does NOT depend on Idss or otherwise directly on the JFET Gm. The only chance to lower the current source gain is to use a JFET with large pinch voltage.

Let's plug in some numbers:

Assume Vo=19.5V (as in your Vendetta phono preamp) and a few JFET options:

K170BL with Vt=0.7V and Vn=1nV/rtHz
J201 with Vt=1V and Vn=6nV/rtHz
J203 vith Vt=2.5V and Vn=4nV/rtHz

Doing the math, from an output noise perspective, the values are:
K170: 38nV/rtHz
J201: 162nV/rtHz
J203: 43nV/rtHz

As you see, there is absolutely no direct relationship between the Gm and the output noise. J203 has the largest transconductance and still it's at par with the K170, noise wise. OTOH, J201 has the lowest transconductance and at the same time by far the worst noise.

Therefore, here's the bottom line: Given an required output voltage, use the JFET with the lowest noise and the largest pinch voltage. Determine the required resistor, based on the device Idss. The output noise will be: 2*Vn*Vo/Vt + the resistor noise, where Vn is the device noise.

Of course, a large Vt JFET will have a second order effect on the noise: the voltage across the JFET will be Vt and the power dissipation Vt*Idss. This may heat a little the JFET (in particular if it's a SOT323 SMD part), hence it's noise will increase. Of course, small Vt JFETs won't experience the same degree of heating.

To be continued...

[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.