Zen -> Cen -> Sen, evolution of a minimalistic IV Converter - Page 161 - diyAudio
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Old 23rd November 2013, 05:05 AM   #1601
pidesd is offline pidesd  Canada
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hi guys,

is there any reason why this topology would not work well? for a lower input impedance...


edit: maybe the circuit is not right as the gates are not reference to ground...now is bed time
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Last edited by pidesd; 23rd November 2013 at 05:16 AM.
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Old 23rd November 2013, 07:35 AM   #1602
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Yes, no Gnd (or Vref) reference for the gates.
See Moving Coil Cartridge Head Amps

The minute you have that Gnd reference, you have current flowing through from floating supply to Gnd.
So the two current loops are no longer totally separate.

Not the case with JFET, since no gate current, and no biasing resistor network necessary.


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Old 27th November 2013, 06:03 PM   #1603
pidesd is offline pidesd  Canada
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Default for d1 lovers

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Old 13th December 2013, 01:31 PM   #1604
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Since there are so many pages, can anyone point me to the discussion (if any) to adapt the cen/sen I/V to the BII/BIIISE?

Thanks
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Old 16th December 2013, 10:32 PM   #1605
alazira is offline alazira  United States
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Nicmac discusses his build here:
Zen -> Cen -> Sen, evolution of a minimalistic IV Converter
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File Type: jpg Sen 9018.jpg (212.7 KB, 785 views)
File Type: jpg Sen 9018b.jpg (249.3 KB, 782 views)
File Type: jpg Buff II Dac out to IV.jpg (208.4 KB, 772 views)
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Old 8th January 2014, 01:49 PM   #1606
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if i'm going to use 2sk117 for cen, what would the p channel equivalent to sj74 be?
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Old 8th January 2014, 07:47 PM   #1607
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No p-channel equivalent. You need to use SEN.


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Old 9th January 2014, 01:52 AM   #1608
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Can i still use sk117 for sen?

Id be happy as long as it works

170 is hard to source in asia
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Old 11th January 2014, 01:59 PM   #1609
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Read the article.


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Old 18th January 2014, 12:53 AM   #1610
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The origin of this circuit, as mentioned in the article, was Dr. Leach's MC headamp.
Moving Coil Cartridge Head Amps

The natural question arises as to whether we could use the JFET version for the same purpose.
This is being discussed independently, as I found out by chance, elsewhere :
audio-talk :: View topic - j-FET / Triode Phono front-end
(see post by JamesD on Mon Oct 07, 2013 8:26 pm)

As you would expect, we did consider this application for ourselves.
The biggest problem is DC offset.

If the Idss of the JFETs do not match and thermally track perfectly all time during operation, 2 things will occur :
1) A DC offset will be seen by the MC pickup at the input, causing a small DC current.
2) This current has to pass through R1 and/or R2, thus causing imbalance of the Vds of the two FETs.

This has been verified in Spice (I am not prepared to risk my MC pickup for that).
The second point above can be solved by replacing R2 by a Zener diode, but this does not solve the DC current problem.

To improve thermal tracking, one can :
a) use a monolithic dual JFET such as LSK389 or 2SK2145;
b) degenrate the JFET to the appropriate Zin for the MC, which also improves Id stability.

But we are trying to limit DC current to µA level, when the JFET bias is ~8mA.
In the Leach circuit, he was using 125µA bias and this was also done by a resistor network.
So there is a chance of better thermal stability.


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