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Old 29th November 2011, 08:21 PM   #21
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Originally Posted by Mooly View Post
Many many thanks for your insight Scott.

Sorry if these are daft questions just trying to expand my knowledge now.
You say "the gate voltage equilibrates with the back to back diodes biased by the gate leakage current. Diodes ? plural, two of. Do you mean the intrinsic "diodes" found in a normal junction FET and if so, that the leakage is both sufficient and presumably equally matched between the two ? Sufficient on their own to be used for biasing. (I am thinking that as the FET is symetrical there must be two diodes.)

Is there any reason why just one diode is shown in the picture and also in the device in Jaccos link.


I appreciate the electret in isolation has no DC path of its own... I have just never heard of FET's being left essentially with the gate OC to self bias if that understanding is correct.

So legarem is going to need a fairly specific FET for this to work.
Yes, the other diode is the gate junction. To be sure you can just use back to back low leakage diodes. At zero volts a diode has a resistance of around 2VT/Is where Is is the saturation current and VT is 25.9mV (kT/q). This can be 10^15 Ohms in some cases. Say you have a pico amp of leakage, the diode looks like .058/10^-12 Ohms or so. Computer diodes have high Is and don't work well.

Sorry, it's not here but another site where every few months this comes up and sorry also I don't have an easy to read reference that explains this.
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Old 29th November 2011, 08:30 PM   #22
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This is what I strongly suspect
I have it working on my bench right now with a 2SK170 and 2-2N4418 with the drains cut off used as back to back diodes. The 2SK170's used as diodes contribute too much capacitance. Do you know how much C the electret cartridge has? We shoud be able to get you up and running with something. 2SK222's are hard to find but they would be good but as followers BF862's work also and you could use extras as the diodes.
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Old 30th November 2011, 12:33 AM   #23
legarem is offline legarem  Canada
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Originally Posted by scott wurcer View Post
I have it working on my bench right now with a 2SK170 and 2-2N4418 with the drains cut off used as back to back diodes. The 2SK170's used as diodes contribute too much capacitance. Do you know how much C the electret cartridge has? We shoud be able to get you up and running with something. 2SK222's are hard to find but they would be good but as followers BF862's work also and you could use extras as the diodes.

I don't have any idea of the electret input capacitance of the cartridge. I really have to find a fet alone because there's not too much place in the cartridge.
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Old 30th November 2011, 03:26 AM   #24
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Originally Posted by legarem View Post
pinout
It's SGD, viewed from fet azz, trust me. (cross my heart and hope to die)
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Old 30th November 2011, 08:16 AM   #25
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Originally Posted by scott wurcer View Post
Yes, the other diode is the gate junction.......................
Many thanks again Scott for the explanation. Learnt something new today.
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Old 30th November 2011, 03:44 PM   #26
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I don't have any idea of the electret input capacitance of the cartridge. I really have to find a fet alone because there's not too much place in the cartridge.
In that case it can't hurt to just try either a 2SK30 or 2SK118 which should be easily available and just tack them in and see (gate floating which usually works). They will run at Idss so the 30k might be too big, you could try it on the bench first.
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Old 2nd December 2011, 01:48 AM   #27
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Originally Posted by jacco vermeulen View Post
It's SGD, viewed from fet azz, trust me. (cross my heart and hope to die)
Hi Jacco

I zoomed your cartridge internal picture to see how the wires were connected.

I can clearly see that the fet are connected as source followers.

The pinout is GSD. Left pins of the fet goes to the electret needle
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Old 2nd December 2011, 09:28 AM   #28
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That would the first, even the oldest Toshiba JFETs in a metal can (e.g. SK11/2SK12) have the Gate inbetween the Drain and Source.
Other Japanese N-channels, K68, K150, K163, even uPA68h : DGS or SGD.
GSD is a common lead sequence for Japanese MOSFETs, e.g. K213-K216, K2013.
(an artist impression doesn't make it right)

Toshiba Gate diode for Mooly =>
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Old 2nd December 2011, 12:03 PM   #29
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Thanks Jacco...
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Old 2nd December 2011, 12:58 PM   #30
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Here is the real beast
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