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Salas Shunt v1.2 notes

Posted 27th May 2010 at 01:00 PM by klewis
Updated 27th May 2010 at 01:11 PM by klewis

The ones I posted as symmetric 1.2 in page# 190 can do for +/-24V. Those are the ones that Bill initially tested for supplying his OP AMPS. There is a Q2 2N5459 JFET which is 25V VDS max that will set a 30V absolute max Vin given that it sits on 1.85V and there is a Vgs and R1 drop above it. Respect that Vin max. That Q2 must have 7mA or more IDSS for the cascode to work efficiently for a 7-9mA 2SK170BL Q9 under it with the given R4 (220R). If you want to feel free from Vin restrictions or choosing JFETS, you can do the BJT cascode left side. See post #2432 for negative example.
Making an additional positive one like those for 5V, only needs R13 1k trimmer to be 5k trimmer so it can go that low.
R1 is changing current setting, by 0.6V/R1=CCS current. Steadily predictable because that 0.6V is Q7's Vbe. Does not vary significantly per part or batch as with Leds Vf.
As for Vref and output caps subjective preferences, as Andrew said.
Good luck!

post #2486 is start of new board layout for v1.2

Anyway, guys remember if you will try to listen for output terminations. 3 steps:

1. 22-47uF solo lytic medium ESR across output. Most immune to reactive loads, most predictable for noises/oscillations if you can't check out the system with scope. Less bandwidth (talking MHz not audio), excellent phase margin (75deg), not so smooth phase undulation though.
2. 10uF+0.5R (1R//1R) Zobel across output. Very good BW, same phase margin, smooth. Film cap of choice.
3. 4.7uF+1R even more BW and 10 deg less margin but in best balance, given the load is not weird. Film cap of choice.
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