Simplistic NJFET RIAA - Page 796 - diyAudio
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Old 4th September 2012, 09:15 PM   #7951
Rush is offline Rush  United States
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Quote:
Originally Posted by vgeorge View Post
Measured lots of fets so far no fuse blown on DMM.
I blew a $30 DMM fuse doing this. I now measure voltage.

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Old 5th September 2012, 08:55 AM   #7952
AndrewT is offline AndrewT  Scotland
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A 1k0 series resistor connected to +ve supply.
Clip the G+S together and connect to -ve supply.
Attach DVM across the 1k0.
now tap a wire from 1k0 to D of DUT.
In worst case short a 20Vdc supply will pass 20mA, That's 400mW through the resistor.
In the no short condition the DVM reading in Vdc is equivalent to the mA of current passing.

if the Vds is equal to the manufacturers specified test voltage and if the Tj is equal to the manufacturers specified test temperature then that mA reading is the Idss.

The best we can achieve is a close approximation to Idss, due to errors in Tj. We can get very close on Vds, but super accuracy here is wasted if Tj is wrong.
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Old 5th September 2012, 08:58 AM   #7953
AndrewT is offline AndrewT  Scotland
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Salas,
7.9Vds is perfectly within range of a 2sk170 Lsk170 for avoiding excessive gate leakage current.
Most designers/builders who make comment on this gate leakage issue are referring to Vds >>10Vds and Id>=Idss.
7.9Vds is ~half the voltage that might cause a problem, if Id were high and Tj was high.
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Old 5th September 2012, 01:15 PM   #7954
RCruz is offline RCruz  Switzerland
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I believe 7.9V is the minimum Vd to use with K170 to avoid non linearities.

In the Simplistic first stage, Salas bias the cascode bjt at around 8v so we get 7.9V for Vd in the lower k170.
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Old 5th September 2012, 01:20 PM   #7955
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Salas View Post
J310 around 40mA Fairchild gives me twice as much Vgs (=Vds across 8mA K170 under it in full IDSS cascode CCS test) compared to 15mA BF245C Fairchild. So take care not to see more than 7.9V across your K170 when you will apply with your Rs. After that point gate leakage jumps. There are linearity and noise differences between 245 & 310 which do not impose them selves in this config enough.
So BF245C is a much better choice.

Fortunately BF245C is easily obtainable here throu Distrelec.... I am now working in the layout to mod my 48db build
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Old 5th September 2012, 02:09 PM   #7956
RCruz is offline RCruz  Switzerland
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Hi Salas... do you knw these EMZ caps: https://www.distrelec.ch/ishop/Datas...B33063B_de.pdf ?

After cascoding the second stage I will need to retrim the high freq cap... can I use those EMZ ?
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Old 5th September 2012, 02:14 PM   #7957
Salas is offline Salas  Greece
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Quote:
Originally Posted by RCruz View Post
So BF245C is a much better choice.

Fortunately BF245C is easily obtainable here throu Distrelec.... I am now working in the layout to mod my 48db build
No its not much better per se in cascode. Its just tamer for Vgs. In the cascode the upper Jfet's noise won't directly correlate with the lower Jfet's that sets that performance. 245 15mA and 170 8mA did a clean swing of 12Vpp in a test I run with less B+,Rs, and RL than the projected 2nd stage values. It will likely be a bit more in the phono. So its enough. You can use J310 anytime too, logically. Although I did not run that exactly in the same way yet. I did in cascode CCS full Idss mode. There I saw its happier for Vgs naturally due to its high Idss characteristic of course. There is a Toshiba chart showing the Igsx VS Vds & Id leakage thing for K170. Interpretation depends on what is considered high regarding the input signal level and how high is Rg (few mV & 1Meg in our case). All Id levels follow the trend non the less (our 2nd stage is at about 4mA Id). Should be more at higher than 25C also.
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Old 5th September 2012, 02:21 PM   #7958
Salas is offline Salas  Greece
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Quote:
Originally Posted by RCruz View Post
Hi Salas... do you knw these EMZ caps: https://www.distrelec.ch/ishop/Datas...B33063B_de.pdf ?

After cascoding the second stage I will need to retrim the high freq cap... can I use those EMZ ?
Haven't used any that I remember of, only a friend had an MZ crap bike when we were 15 and had some silly falls, but EMZ look far more decent in what they do.
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Old 5th September 2012, 02:31 PM   #7959
RCruz is offline RCruz  Switzerland
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Originally Posted by Salas View Post
Haven't used any that I remember of, only a friend had an MZ crap bike when we were 15 and had some silly falls, but EMZ look far more decent in what they do.
Anyway.. the word crap is there so.... what do you think about wima FKP ?
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Old 5th September 2012, 02:37 PM   #7960
Salas is offline Salas  Greece
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If its jinx having two common letters with an old budget bike that gave bone fractures to an old mate after refusing to corner, then FKP have none of those letters.
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