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-   -   Phono pre, valve or jfet (http://www.diyaudio.com/forums/analogue-source/75368-phono-pre-valve-jfet.html)

lndm 7th March 2006 06:43 AM

Phono pre, valve or jfet
 
I have spent many hours reducing noise in my phono pre. I use a 12au7 for the first stage as it is supposed to be low in microphonics.

My amp is now quiet but I know I could reduce noise much further if I opted for a jfet. Would anyone recommend this?

SY 7th March 2006 11:33 AM

Why not split the difference and use a cascode input stage, jFET on the bottom, tube on the top? I use an NTE458/E88CC combo, but the Toshiba 2SK147 is a popular choice. Very, very quiet.

lndm 7th March 2006 12:22 PM

Very interesting. What sort of current/supply rail could I use?

SY 7th March 2006 12:29 PM

I used a 200V regulated supply. There are a lot of other options. Allen Wright has some good examples at his Vacuum State site and in his Preamp Cookbook. I've done things rather differently, but Allen's approach has the considerable virtue of being documented.

lndm 7th March 2006 12:37 PM

Excellent. I'll go to allens site and do some research. After all, thats part of the fun of DIY :).

Thanks.

Mikewong 4th April 2006 11:58 AM

Quote:

Originally posted by SY
Why not split the difference and use a cascode input stage, jFET on the bottom, tube on the top? I use an NTE458/E88CC combo, but the Toshiba 2SK147 is a popular choice. Very, very quiet.

SY,
Do you have the actual schematic?
I saw one from 'borbelyaudio' published in Costruire Hi-Fi some times
ago.

SY 4th April 2006 12:25 PM

I built that preamp back in the early 1980s, so the schematic doesn't exist in digital form. I'll see if I can sketch something up. Basically, it's single-ended, the tube has a 180V rail, 45k plate resistor, a 6.9V reference sent to the grid, no resistor between the cathode and the FET drain, and a small source resistor (100 ohm). FETs were selected in-circuit (a lot of tedious plugging in and out).

AW's use of the FET/tube in CC differential is a better approach, I think.

pixpop 4th April 2006 03:11 PM

I just finished building a Pearl, which has 4 parallel JFETs on the input. The sound is very beautiful, and very quiet. MM only though. Pass has a different phono stage for MC carts.


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