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Zen -> Cen -> Sen, evolution of a minimalistic IV Converter
Zen -> Cen -> Sen, evolution of a minimalistic IV Converter
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Old 28th March 2018, 08:03 PM   #2081
pidesd is offline pidesd  Canada
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hi all,

i remember having asked Patrick (EUVL) some time ago about substituting jfet with mosfet in his I/V circuit but apart from remembering hew did not recommended it, i don,t know exactly why the performance would be inferior.

Is it because of gate leakage current and capacitance linearity?

because, apart from added biasing circuit PSU,probably higher voltage rquirements, and maybe higher capacitance (with lower linearity depending) on the mosfet pair, i don't see what else could deter anyone from trying it.....but then again i'm not a pro

PS: i have a pcm1794 with high swing current and i thought about using 2sk2013/j313
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Old 2nd April 2018, 06:11 AM   #2082
Alex_twn is offline Alex_twn  Taiwan
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Hello,

I quickly implemented my first version of the SEN "to try" on a prototype PCB directly (PCB are very cheap nowadays) and using SMD parts I had in my drawers.
My DAC is the PCM1794 as well and I am struggling with the cancellation of the 0dBFS current of the DAC (6.2mA).
I implemented a 2N4391 in current source mode but I am having issues with the thermal stability, it can quite easily drift over time and not very stable.
I am using 3S LiPo battery.

So I would appreciate if someone had a good recommendation about a low tempco stable current source?

Beside I was able to stabilize the DAC DC offset to a value not too far from 0 and did some listening on one channel only as I haven't assembled the second one since I want to complete the debug on the first one.
I would like to say that at a first listening the SEN seems to sound very good.
I appreciated very quickly the dynamics and low response impact. Was very good. I need to debug & complete the two channels for a more complete listening.

So I am looking for some help on this current source as I am quite excited now to enjoy this SEN completely

Here is how it looks like:

Click the image to open in full size.
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Old 2nd April 2018, 06:44 AM   #2083
pidesd is offline pidesd  Canada
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Does your voltage varies across the source resistor of the jfet, thus indicating the drift?

If yes try this one attached with small signal BJTs.

should perform better as well (maybe a bit more noisier depending on bjt).

Vref is ground,
0V is a negative (say -10V)
Iout is the dac current output



(maybe someone more experimented then me has a better alternative/advice?)
Attached Images
File Type: jpg bjt_ccs.jpg (10.7 KB, 316 views)
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Last edited by pidesd; 2nd April 2018 at 06:50 AM.
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Old 2nd April 2018, 07:36 AM   #2084
EUVL is offline EUVL  Europe
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Use 3x 2SK246BL in parallel, each individually degenerated to 2.06mA using a separate large-ish source resistor.


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Old 2nd April 2018, 09:32 AM   #2085
MagicBus is offline MagicBus  Greece
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Hi,
Coincidentally, I've built a prototype based on this very good idea to null the dc offset on a pcm1792a-same as pcm1794 and I'm testing it these days. I'm using it without the rest of the I/V circuit for now. BF861C seems to work fine.
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File Type: jpg ItoV conversion schematic.JPG (20.0 KB, 307 views)
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Old 2nd April 2018, 10:52 AM   #2086
EUVL is offline EUVL  Europe
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You will find the solution I proposed to be more temperature stable and lower noise than a single BF861.


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Old 2nd April 2018, 12:30 PM   #2087
Alex_twn is offline Alex_twn  Taiwan
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Thanks for all your answers.

Patrick, one question since it is apparently a bit difficult to source the 2SK246. As a replacement should I understand that we preferably look for a high Vp / low Yfs JFET here?
Would the 2SK208 be a possible replacement? (it has a lower current but I coud add more devices in parallel).
I could try as well to couple them thermally.
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Old 2nd April 2018, 12:58 PM   #2088
EUVL is offline EUVL  Europe
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2SK208 is the SMD version of 2SK246.
But you still want as high as Idss as possible.
Minimum GR grade.


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Old 2nd April 2018, 01:18 PM   #2089
Alex_twn is offline Alex_twn  Taiwan
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Unfortunately the 2SK208-GR grade seems as well difficult to order.
On the other side, the datasheet of the 2SK879 seems strictly identical, not sure why they have a different part number but this one is available in GR.
Is it another SMD version of the 2SK246? Should be fine to use as well right?
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Old 2nd April 2018, 01:28 PM   #2090
EUVL is offline EUVL  Europe
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Even smaller package than 2SK208.


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