Pinout for a 2SK170 JFET

sbeach · Jul 3, 2007

I want to be certain about this.

In the image, is the bottom 3-D pinout pic correct? In looking at the Toshiba spec. sheet (upper two images), in the 2nd pic, are you viewing the JFET from the top or the bottom? It SEEMS to be a bottom view. If so then the source & drain would be reversed compared with the bottom 3-D pic. I'm too confused. Please help me figure this out. Thanks.

evm1024 · Jul 3, 2007

Reverse the Source and Drain in your drawing. The top 2 drawings are front (facing the flat) and bottem.

Rossi · Jul 5, 2007

Isn't the bottom pic taken from a Dorsey schematic?

sbeach · Jul 5, 2007

Yes, the bottom diagram (apparently incorrect) is from a reprint of Scott's "Making Mics Better: a Practical DIY Project" article for Recording Magazine. I found it on the web at ...

http://www.pfarrell.com/music/shanghaimic.html

Since the article was a few years old & no one had noticed this discrepancy, I assumed it must be accurate - hence the confusion when checking it against the Toshiba spec. sheet. The lesson, I guess, is DON'T ASSUME!!!

Skip

Kit · Jul 5, 2007

"I am not a number, I'm a free man!" "Hahahahaaaaaa!!!!!"

Iron Maiden. :thumb:

bcarso · Jul 5, 2007

You can't go too far wrong with a JFET with the gate in the middle, since they are more or less symmetrical, usually. But there can be subtle differences.

tk@halmi · Jul 5, 2007

[quote author="Kit"]

"I am not a number, I'm a free man!" "Hahahahaaaaaa!!!!!"

Iron Maiden. :thumb:[/quote]

Isn't that from the TV series "The Prisoner".

- Who is number one?
- You are number six.

evm1024 · Jul 5, 2007

[quote author="sbeach"]Yes, the bottom diagram (apparently incorrect) is from a reprint of Scott's "Making Mics Better: a Practical DIY Project" article for Recording Magazine. I found it on the web at ...

http://www.pfarrell.com/music/shanghaimic.html

Since the article was a few years old & no one had noticed this discrepancy, I assumed it must be accurate - hence the confusion when checking it against the Toshiba spec. sheet. The lesson, I guess, is DON'T ASSUME!!!

Skip[/quote]

When I modded my Oktava I also was a bit confused. I put my bets in the vendors datasheet.

Kit · Jul 6, 2007

Isn't that from the TV series "The Prisoner".

I dont know, but it would make sence as the the song by Iron Maiden is also called "The Prisoner" if i remember it correctly.

I havent seen the the TV show, but the song is cool.

guavatone · Jul 6, 2007

I can't remember the directions on the Oktava mod and how I followed them, but I may have mine in reverse. The level seemed to drop a bit. WHould a lower level be a symptom brad?

Samuel Groner · Jul 6, 2007

www.groupdiy.com/index.php?topic=21568

You shouldn't have yours in reverse if you followed my advice...

Samuel

guavatone · Jul 6, 2007

Thanks samuel, but that advice wassn't for the Oktava. I did the Oktava 2-3 years ago, it was one of my first diy mods.

Has anyone modded the 319 and had to flip the sk170 to sound better?
-I am lazy, nevermind, I'll find the oktava.

IIRC the actual pictures Dorsey gives in his article should be the guide, right?

Gus · Jul 6, 2007

why are people changing the fet to a 170 in the oktavas?

I have no problems with the stock ones. Plus SD( I am assuming that is the "mod") does not tell you how to readjust the circuit IIRC. Any written numbers on the transformer?

bcarso · Jul 6, 2007

[quote author="guavatone"]I can't remember the directions on the Oktava mod and how I followed them, but I may have mine in reverse. The level seemed to drop a bit. WHould a lower level be a symptom brad?[/quote]

Wouldn't expect it to make much difference. According to my model (which may be faulty) the parasitic drain and source resistances are equal. Maybe I will try reversing a simple amp with one and see if there is any discernible change.

EDIT: Looking again I see there is a small difference between the zero-bias gate-drain C and the gate-source C. So maybe there could be a small gain effect in a low capacitance transducer application, that would favor the correct pinout.

bitman · Jul 6, 2007

Are you kidding!!! SOB! :evil:

I made an ambisonic mice with 4 Dorsey head amps from that mag article.
The mics noise floor wasn't what it should be. A tad hissy. The boards are sqeaky clean so that was'nt it. I just kinda gave up and used it anyway.

Could reversal of that fet be the issue?
Oh someone say yes, say yes, say yes.

Rossi · Jul 7, 2007

[quote author="Kit"]

Isn't that from the TV series "The Prisoner".

I dont know, but it would make sence as the the song by Iron Maiden is also called "The Prisoner" if i remember it correctly.

I havent seen the the TV show, but the song is cool.[/quote]

Yes, it's from The Prisoner (Iron Maiden used a sample). I recently rediscovered that series on Youtube. I faintly remembered it from my childhood days (must have been an early re-run). Absolutely fascinating. I'm about to buy the DVD box. :grin:

About the 2SK170 - I discovered the discrepancy between the datasheet and the Dorsey schematic at some point. I think I tried the FET both ways and didn't notice much of a difference. I decided to follow the datasheets rather than SD, though. :green:

If you have hiss, it's probably because the FET isn't biased correctly (as Gus implied). I don't see a compelling reason to change the FET. I have two MK-219 and two other Oktavas. They're all pretty low noise. The pencil numbers on the transformer cover are the values for the bias resistors, which tells you that they're hand-adjusted for each mic. Pretty nice for such a cheap mic. Changing the FET requires you to re-adjust the bias. The Toshibas are quite consistent, but for optimal performance the resistors should be adjusted individually. Or just retain the original FET. :wink:

bitman · Jul 7, 2007

Well thanks for that Rossi.
I'll give it a go.

Gus · Jul 7, 2007

Rossi

I implied nothing. I left hints to help people find their own answers.

I wanted people to think a bit before just blindy changing parts in their microphones thats why I posted about the writing on the transformer. I like the Oktava 217 circuit I don't like the caps or capsule

Has anyone heard or recorded an improvement with changing just the FET?

ribarr4 · Jul 7, 2007

Hi all. I have been watching this forum and am glad to finally be able to contribute.

The Dorsey diagram is incorrect, but because of that when I was designing DOA several years ago I did a bit of extra testing. I turned all of the sk170s & sJ74 around (and there are 7 all together) and it speced' out exactly the same and passed audio exactly the same. There is supposed to be a difference because the channel is asymetrical and I suspect that will affect certain applications, but made not an ounce in my design.

Richard

Kit · Jul 7, 2007

. I turned all of the sk170s & sJ74 around (and there are 7 all together) and it speced' out exactly the same and passed audio exactly the same.

At small signal levels, the jfet conducts in both directions. IE drain and source are interchangable. Hence the use as a AC gain controlling device. Distortion will rise with signal level.

Pinout for a 2SK170 JFET

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