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Lenin 16th February 2002 01:31 AM

HELP! High gain, low noise fet advice needed!
I desperately need some recommendations for a high gain, low noise fet for a phono stage I'm pulling together.

Its made up of 2 linear gain stages, with passive eq. The gain stage topology is essentially a very simple triode type circuit. (Yep, you read that right). With just 1 device in each stage.

I'm trying to achieve extremely high MC gain (good enough for low o/p Ortofons, etc) and I'm just terrified about ensuring that in particular, the first stage is treated with the care it requires. (Hence my search for a high gain, low noise device).

I understand Toshiba fets have a decent sonic reputation . Any recommendations anyone? (Toshiba or anything else)


PH104 18th February 2002 03:30 AM

The 2SK170's are good for this application and should be readily available from MCM or Consolidated. Use 4 in parallel for lowest noise. Borbely might also have 2SK147 which are even better but $$$$.

It would probably be a good idea to cascode either of these FETs, especially if you use them in parallel.

Good luck


arnach 18th February 2002 04:07 AM

The Aleph Ono uses four matched 2SK170s in the first part of MC input stage. You can get these from Borbeley for $2/ea, and matching will add 50% to each or total value, you'll have to check.

-- Aaron

Werner 18th February 2002 07:35 AM

Also look at the DIY Pass Pearl, which is a
two-stage cascoded 2SK170 phonostage, and
the Pacific from William Walther of Maison de ''Audiophile.

I myself started out with a Pacific-like circuit on the breadboard, and then added cascodes in both stages. These are FETs now, but I will try BJTs too. Once I also had paralleled input transistors, but I think the added complexity is not really worth it. YMMV as you desire larger gains. I only
manage about 44dB with 19V rails.

I have another breadboard with a single-stage
phonopre, comprising of an SRPP-like gain stage feeding a passive RIAA through a capacitor. The RIAA includes the 100k volume pot of the linestage.

Promising, but with given FETs it is not easy to get the SRPP thingy right.

HarryHaller 18th February 2002 09:49 AM

Much cheaper at MCM. The 2SK147 is not made any more. I think it wold only have a real noise advantage for MC phono stages. The 2SK170 is very quite and great for everything else. Cascodes are a great idea. Use the 2SK170 at between 8 and 15 vots drain to source voltage for low device capacitance and low noise.


mlloyd1 18th February 2002 01:08 PM

what IDSS does MCM give you these days?
Harry (and others who may have bought from MCM recently):

What IDSS grade (G, BL, V, ??) do you get from MCM?
I'm thinking about ordering some if they have the higher grades.


HarryHaller 18th February 2002 01:28 PM

I don't know if you can specify in your order. I think I usually wind
up with blues or greens which work fine. Violets are real hard to find.


PH104 18th February 2002 01:53 PM

Hi Michael

Harry's right on both about cost from MCM and not being able to specify grade. I asked MCM about this once and they said they were lucky to get the transistors let alone a specific grade. I get ten or so and if they are the grade I want, order more. Then I match and select them.

If you buy them, please post what grade you get. I need a bunch myself, plus the J74 complement.


HarryHaller 18th February 2002 02:08 PM

JFETs from Borbely
The prices for the jfets are in euro dollars. The bias groups for
the K170 and J74 are green and blue. He has the K147s listed but they are EXPENSIVE!


mlloyd1 18th February 2002 07:52 PM

re: MCM's JFETs ...
Phil, Harry:

Thanks. I'll order some and report on what I get. Who knows, maybe I''ll get some V ;)


who is still upset because he can't find his stash of 2SK146 and 2SJ73 duals that he got from Borbely Audio years ago :(

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