Jfet/bipolar PCB and Schematic (finaly)

[Elso Kwak]

Hi Sonnya,
Two people communicating in a foreign language can easely give rise to a misunderstanding.
So you do mean lowering the noise of the LED's. But I think the LED's are <B>not</B> noisy at all!. It is a different mechanism than in the zener or avalanche diode that are very noisy especially the above 5V types. In a moving coil cartridge preamplifier I once used a 3.3V zener. It was extremely noisy and uneven. You could hear the avalanche. Later I replaced it with a string of LED's. The noise was gone!.[Circuit is in fig. 7 of the MAT04 datasheet; low noise instrumentation amplifier]

Bases of the transistors of the current sources can by bypassed with a cap to ground, just to be on the sure side.
Please see the book Motchenbacher and Fitchen, Low-Noise Electronic Design, Wiley & Sons, page 180.

[sonnya]

Hi again!

It is still worth trying even if the LED has LOW noiselevel. The LED bias resistor will add some noise to if it is bigger than ~ 1KOhm. Even if the LED has a low impedance, the transistor will generate some currentnoise on its base .... i think??

mmmhhh ... Elso .... figure 7 in the mat04 datasheet!?... try upload a new one ... haha

Sonny

[Elso Kwak]

Hi sonnya,
I am sorry it is fig.<B>6</B> of the MAT04 datasheet. I always look at the bottom of the fgure. I just downloaded the datasheet. Actually in the fig.6 it is a 3.9V zener(1N748A) but in the text the 3.3 V is mentioned because of the "soft knee". I have been using this circuit for a <I><B>balanced</B></I> moving coil cartridge preamplifier. Sounds very good. Hope this helps.

[sonnya]

Mmmhhh .. Elso i have just read the thread through again. I will make a new design where i will make new currentsources. Make a new folded cascode, where i will enhance it with dual cascode.

The output stage - I think i will leave it as it is.

The JFET Q1 and Q2 ... I am going do make the layout so that i can interchange it with a Mat02 (Has a realy low offset .. 100 times better than the JFET's and a "linear" HFE)

I am going to make it as a doublesided PCB (5 boards 100*160mm can be done for ~2500DKR by ELPRINT)

I am going to use it for the MAT04 for the currentmirror .... if i can get them.

Sonny

[sonnya]

Now i understand what you mean ... thanks for the advice ... It is easy to hear any noise with MC-levels ...

Sonny

[Elso Kwak]

Hi sonnya,
I give up. Please have a look at this thread at the AudioAsylum!
http://www.audioasylum.com/audio/twe...ges/50592.html

[sonnya]

I don't want to fight with you!... LED's do generate noise .. take it leave it . It is still a PN design. It is right it does not generate a lot of voltagenoise.

Its series resistance applies to the diodeformula : ~ re = 25mV/A (at room temperature).

At a current of 100uA it will generate : ~2nV/sqroot(Hz)
At a current of 1mA it will generate : ~0.6nV/sqroot(Hz)
At a current of 10mA it will generate : ~0.2nV/sqroot(Hz)

Yes it is VERY small values .. Every time you raise the current your currentnoise will go up and it will multiply it self with the series resistor .. Wich also generate thermal related noise ...
I dont know the value of the currentnoise.. I have not been able to find any noisefigures on LED's at all!! It is not the normal application for LED's

The biggest reason i can think of why use a LED as refence is that it have the same Tempco as a transistor and to keep the circuit simple.

JFET do have noise figure around 3nV (For the J113). If you look at the currentnoise it is a lot lower than a BJT .. maybe 100times!
So you only end up with the 3nV and the thermal noise generated by the resistors and caps! surrounding the JFet.

If you choose the right JFET (in my case J113) to a specifiet current you end up with a ZERO tempco. Even if it would change 3% i would be happy .. The LED - transistor circuit can not do this much better... They say 2% over a complete temp. range

The reason why i was talking about to be able to interchange JFET Q1 and Q2 was to make hearing test on those . I wan't quit using JFET'S as a inputstage.. I was not happy with variation in IDSS!!!
You will have the same problem with DUAL's from unit to unit ...


I have not read the audioasylum link yet! I am going to do this know.

Sonny

[hifidaddy]

Hi folks,

finally, I got the schematic from my friend and scanned the interesting part. It is the line stage, which works together in one part with the power output, so the biasing and the power transistors are missing.

BTW, the light emitting diodes are shown the wrong way round, the JFET is a N-JFET, and all transistors not marked differently are 2SC2320 or 2SA999. The minus voltage is top, the plus voltage bottom.

regards,
Hartmut from Munich

[mikek]

folks...looks like Elso Kwak has got a something up his whatsit....

[Elso Kwak]

Originally posted by mikek
folks...looks like Elso Kwak has got a something up his whatsit....

What do you mean exactly mikek?