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hrnoevil 21st January 2011 09:34 PM

JFET used in ARC PH3 hybrid phono pre amp
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I m first time writer here. I m planning to build a phono pre amp using the audio research PH3 schematic. However, I m having problem identifying what is the JFET Q1 and Q2 used in the first stage. It's called *6537* on the schematic.
This advantage of using JFET is to minimize the noise level.
I tried to search products in Fairchild, Toshiba, Vishay etc. and getting nowhere. The closest one I found from products sold in eBay is using Toshiba 2SK170 and I m not sure if this is suitable for this schematic.
You help is very much appreciated.
Attached schematic for your reference.

Vancouver BC Canada:)

Zen Mod 21st January 2011 10:24 PM

to my eyes - 2SK170 ( GR or BL suffix) will work perfectly there

or just one 2SK369

jackinnj 21st January 2011 10:28 PM

If you can't find 2SK170 you could use PN4393 --

Salas 21st January 2011 10:33 PM

Those *6537* look like in house code numbers. Probably some standard one, hidden. R2 and R3 would contribute enough noise so to undermine any Japanese special fet choice. An uninformed guess would be something like J112 would work. But you need to ask someone who has been actually servicing ARC to really know.

wrenchone 21st January 2011 11:02 PM

I hate to be all sour and stuff, but the circuit looks kinda like a fustercluck to me. That there are two fets being run in parallel with strangulated drain current (they get a little over 1ma apiece when you count in the contribution of R11, R12, and R18 to the current through R3) tends to indicate that the chosen devices didn't have a lot of transconductance. PN4393s wouldn't work here as they would need a lot more negative gate voltage (read lots more voltage across R3) to hold them back at the operating currents chosen here. You could likely get one piece of selected 2SK170 BL to more or less work. Drain current should be somewhere between 2.3-2.7ma, depending on which of the schematic voltages you actually believe. You want the current to be on the low side of that range so you have some drain voltage fro the fet.
This circuit looks like it's set up for MM cartridges, so I'd be tempted to ditch the original input circuit and use a 2SK170 cascoded with a triode to get rid of the Miller capacitance. If you really as me though, I'd try something like SY's phono circuit without the input transformer first.

Zen Mod 21st January 2011 11:05 PM

me , Sir , don't like ARC circs ;

had my share of copying them , when I was young and enthusiastic :clown:

Salas 21st January 2011 11:11 PM

Do you gents agree that something like J112 could fall into place in that circuit as it is? (I do find that CCT non purposeful enough too by the way).

Zen Mod 21st January 2011 11:16 PM

yes ;

according to datasheets , both J112 and J113 will do ;

as I remember from various schematics - at ARC they were used to adjust resistor values to exact Jfet used in situ .

NB that I didn't play with J112/113

wrenchone 21st January 2011 11:25 PM

I took a look at the datasheet, and it looks like you could get the J113 to work. They're cheap enough to easily afford a pile for selection purposes (that is, if you want to bother with this circuit in the first place).

Salas 21st January 2011 11:29 PM

At least we answered the original question. I.e. What could that Jfet be or suggest something that works in that circuit as it is.

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