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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: LM317 for PP cathode biasing? posted by DAK on February 19, 2015 at 21:11:18:
The LM317 generates a significant level of broadband "white" noise. In the April 2009 AudioXpress letters section Walt Jung Reports that the LM317 at 10mA had 50 times more noise than a DN2540. Since the common cathode node of a PP amp is not an input that might not be too much of a problem.
How much bias current will pass though the current source? If it is less than 90mA you could use a pair of cascoded MOSFETs. W. Jung writes in the above reference: "...the (cascoded MOSFET) configurations are the most versatile of any discussed. I recommend them without any major reservations." K&K Audio sells a nice kit for little cost. My phono stage uses 17 of these, 4 are used in the line stage and four in each mono-block. They work very well.
My mono-blocks have 300ma of output stage bias. I use a TL431 based current sink. The circuit is shown in the Fairchild data sheet. In place of the transistor I used a power N-channel MOSFET. Instead of the bias resistor connected to the cathode I used a LM334 /DN2530 cascode set to 2mA to bias the TL431. Fairchild makes a low tempco version the KA431. My bias comes up on startup and sits at 300mA and doesn't budge. Cold or hot.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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Follow Ups
- RE: LM317 for PP cathode biasing? - JKT 02/19/1523:06:01 02/19/15 (0)
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