Zen -> Cen -> Sen, evolution of a minimalistic IV Converter

[ryanj]

Tham,

I dont know much about the TDA1545A, but I know it has a bias current of a max of 785uA. So if its anything like the TDA1541A with a current bias of 2mA, then you will need to compensate for that.

 

[Dr.H]

Hi Patrick,

I have found a cheap source of BF245C with IDSS 12-25mA. Could these be used in the SEN?

 

[mandalore]

Hi Patrick,

I have found a cheap source of BF245C with IDSS 12-25mA. Could these be used in the SEN?

Well, I'm no expert, but I think that they would not be suitable because of their small transconductance ( 3-6.5 mS ), which means that SEN would have large input impedance. For example 2SK170 has 22 mS, and recommended 2SK369 40 mS - the higher the value, the lower the impedance.

But wait for Patricks answer, he certainly has much more knowledge than me

 

[EUVL]

Your answer is just perfect.




Patrick

 

[Dr.H]

Thanks guys; what about paralleling the input jfets?

 

[EUVL]

According to the old Siemens datasheet, the BF245 has relatively low capacitances.
So you may, if you so wish, parallel 4 pieces of them to make up for 1x 2SK170.

You will be burning >100mA per channel though.
So large batteries required.


Patrick

 

[Dr.H]

Wow, so 16 BF245's for 4 SK170's?
Perhaps I'll use a breadboard for a quick try and see approach.

I'll be using a DC supply so no worries on batteries.

 

[regal]

Wow, so 16 BF245's for 4 SK170's?
Perhaps I'll use a breadboard for a quick try and see approach.

I'll be using a DC supply so no worries on batteries.

What would the input impedance of the paralelled BF245's look like? Feasible to get below 5 ohms ?

 

[EUVL]

If you use 32 BF245 in parallel then you probably can get to 5 ohm, yes.
So in one single ended SEN you would need 64 BF245s, or 128 for one channel balanced.

And I'd love to see that done.




Patrick

 

[Joachim Gerhard]

I need a floating supply for another project. I have posted a floating supply here but i think this new one is more interesting because it is more symmetric. Also the CCS´s.
This is not dimensioned yet and i also think about alternative CCS´s so it shows only the principle at this moment. See one alternative.

 

[Alexandre]

Nice.

But I wonder, is the simmetry in the CCS important?

If not, you could use two DN2540 cascode current sources. I think Walt Jung found it to be the best measuring one.

OK, maybe it´s overkill.

 

[Joachim Gerhard]

The symmetry isolates both inputs from the incoming voltage. If that is important i can not say. If you look at it as a series connection it may not matter. What flows in only one CSS flows in the total circuit.

 

[Alexandre]

This, with good trafos, should be a great alternative to batterie.

I found a cheap source of R-Cores locally and I´ll try some things with them. The R-Cores have two completely separate primaries, one on each spool. So I thought, this can make a perfect 1:1 isolation trafo. And it´s super efficient, it hardly warms up IME.

The other trick, to use it as a normal step down trafo, is to use one primary and one secondary, from different spools. Drastically reduce capacitive coupling between windings.

(I didn´t try this as of yet)

 

[qusp]

ok back on track, SEN is on the bench for the morning, i hope to have some beginnings of sound by sunday to report for the 9018/12 brigade. just one channel for now

 

[Joachim Gerhard]

Alexandre, this use of a transformer is creative. I will draw a schematic with the mosfets too.

 

[Alexandre]

Should be possible to do this with the stock R-Core, here with my 127V mains.

For 220V you have to use the primaries in series, so...

 

[Joachim Gerhard]

Here is the Mosfet version. Sure, your trick only works when you can use only one primary.
You could use one primary on 230V too but then you get the double output voltage. Using a transformer with half output voltage and you are back in business .....

 

[Joachim Gerhard]

Here is a current source design by Mr.Evil. It is good to 50mA. I need around 25mA and i have the parts so i will give it a try.

 

[Alexandre]

The double CCS adds safety, in case you are using the mosfets. They can fail and pass much more current. (I have ruined a few playing on protoboard with low voltages, I´m not even sure how. Static?).

Altough not a big concern for the cascode CCS, if well soldered in place, which it will be of course.

 

[Joachim Gerhard]

It drops double the voltage though.