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Old 25th July 2013, 02:59 AM   #1461
EUVL is offline EUVL  Europe
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> Can replacement the 220uF capacitors to 2uF?

Good question.

The 220uF caps are there to provide a low impedance path for the signal current.
If you replace 220uF by 2uF, you will increase the resistance of that path by a factor of 110.
That in turn will cause the floating supply to swing a factor of 110 more relative to signal ground.
Until it runs out of headroom.

Let's assume your R_iv to be 680ohm. And signal current is 4mA. And you use a 24V supply.
So a signal of 1Hz 4mA will cause the top rail to swing between 15V and 9V rel to signal Gnd.
i.e. about the same amplitude as the output signal, as expected.

If you change the cap to 47uF, the top rail will now swing between 19V and 5V.
i.e. 8V pk-pk more than the signal itself.
Anything less than 47uF will cause the JFETs to run out of Vds headroom, and signal will clip.

The situation will improve if the signal current is less.
e.g. with 1.2mA and R_iv 2.4k, and 47uF caps, the signal will cause the top rail to swing between 15V and 9V rel to signal Gnd.
Changing the cap to 10uF will cause the rail to swing between 22V and 2V.

So to be on the save side, I would not use anything less than 220uF for signal current up to 2.8mA rms (or +/-4mA).
If you want to use higher signal current, you should increase the cap value proportionately.


Patrick
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Last edited by EUVL; 25th July 2013 at 03:02 AM.
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Old 27th July 2013, 10:11 AM   #1462
suburra is offline suburra  Italy
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Hi All

I tried to breaboardthe CEN, for my CDplayer PCM63 based
But, the first board have 120mV Input offset
Fet are from Spencer, so well matched
The termic coupled it's "rudimental"

I have not a scope
There is something wrong in my P2P?
Or i have to try gate resistors?

Thank in advanceClick the image to open in full size.[/IMG]

MarcoClick the image to open in full size.[/IMG]
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Old 27th July 2013, 10:14 AM   #1463
suburra is offline suburra  Italy
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Something wrong with image upload

[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]
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Old 27th July 2013, 11:05 AM   #1464
EUVL is offline EUVL  Europe
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Is power supply floating or Ground referenced ?


Patrick
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Old 27th July 2013, 11:18 AM   #1465
suburra is offline suburra  Italy
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Hi Patrick

Battery supply, 2x9V Alcaline (just for try, later i will buy Nimh)
I misured Input Offset 120mV
Output offset belov 1mV
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Old 27th July 2013, 11:24 AM   #1466
suburra is offline suburra  Italy
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Maybe i was not clear in my last post

Battery floating supply
and offset it's -120mV (negative)

Last edited by suburra; 27th July 2013 at 11:53 AM. Reason: other info
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Old 27th July 2013, 12:06 PM   #1467
EUVL is offline EUVL  Europe
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Input offset is mainly influenced by matching between 2SK170 & 2SJ74.
Not much you can do, other than try to get better matched devices.
Suggest you measure Idss of each device individually first.
You can also try to add gate stopper resistors to see if it alleviates the problem.

Output offset is near perfect, as it should be.
The rest looks OK.


Patrick
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Old 27th July 2013, 03:28 PM   #1468
suburra is offline suburra  Italy
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Hi Patrick

I misured the Fets and the matching it's near 0,5%
so i put gate resistance and problem solved!
Input offset below 1 mV for left and right channel
So, THANK YOU

Sound:
I think need a buffer, because i go into a 10K Shunt attenuator (my resistor I/V is 1,4K)
Sound it's clear, trasparent but suffer a bit into bass frequences and dinamics

what you think, Patrick? Some suggestion from you end others members will be welcome

Marco
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Old 27th July 2013, 03:33 PM   #1469
suburra is offline suburra  Italy
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I have a Hotrodded DCB1

May i put it between I/V and my 10K shunt attenuator?

Marco
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Old 27th July 2013, 06:25 PM   #1470
EUVL is offline EUVL  Europe
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Yes, something like DCB1 will do.
Better still our Joachim Gerhard Filter Buffer.
And use a 680p polystyrene cap in parallel with the 1.4k R_iv.


Patrick
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