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The diyAudio First Watt M2x
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Old 17th July 2018, 11:54 PM   #431
Blk Dynamite is offline Blk Dynamite  United States
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Join Date: Dec 2016
Originally Posted by Mark Johnson View Post
Remember that miserable math class in school where they talked about negative numbers? Here's a chance to finally use them in your everyday life.

If VCC=+18 and VEE=-18 then (VCC) - (VEE) = (+18) - (-18) = +36.

Since 36 > 30, it ought to work fine in a Tucson board. As you expected.
Lol my degree is in economics and finance so Iíve had enough math to last me a lifetime. Since Iím new to all this wasnt sure if there was some other aspect I was overlooking
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Old 18th July 2018, 02:22 PM   #432
Mark Johnson is offline Mark Johnson  United States
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Join Date: May 2011
Location: Silicon Valley
The diyAudio First Watt M2x
For those who wish to experiment with gold plating the Mountain View input stage, let me gently suggest "Moffett Field" (an airport inside the Mountain View city limits, where Sergey Brin of Google keeps his personal Boeing 767 airplane).
  1. Start with the Mountain View schematic.
  2. Discard Q2 and Q3 and R4 (node "EMIT" disappears)
  3. Connect C1 to node "SOURCE" instead
  4. Replace J1 with three Fairchild J113 (not J112) JFET devices, connected in parallel. Call them "J1a, J1b, J1c".
  5. Hand test and match J1a, J1b, J1c so their IDSS values are within 10%. Any IDSS greater than 7mA will be fine, but make sure all three JFETs have the same IDSS.
  6. Modify the bias network (Q1 + R3 + crud on the left) of constant current source Q3. Change it in such a way that the current in Q3 is a constant 20 milliamps, pulled from the parallel connection of J1a-J1c. I.e. each of the three parallel JFETs carries 6.67mA of bias current. Make this 20mA independent of temperature and independent of supply voltage, to the extent possible.
Interested builders can fill in the details as they see fit. This may be an opportunity to get rid of the much-maligned MV5075C LED and its horrible "do not substitute" requirement in the BOM.

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