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In Reply to: RE: Cool posted by dave slagle on March 18, 2012 at 08:46:15
Hi Dave,
"... since the CCS will fight with the inductance and possibly oscillate."
Would you please elaborate on this?
Thanks
Dave
Follow Ups:
I have used a simple LM350 as a CCS on a tertiary winding of an output for a GM70. The goal was to run 2A of current through a low turn winding to offset 100ma of primary current.
In testing I found out the hard way that there was about 700V at 60Khz appearing across the primary with the CCS operating. This was without the Rp of the tube or a speaker load both of which damped out the oscillation. The OT also had an amorphous core which may have played a role.
In any event the basic idea of not putting two current sources in series is the basis for my warning so they will both fight to be in control which can lead to oscillation.
dave
Thanks for reporting this incident. I find it very curious - do you have a thought of what the oscillation mechanism is? And what is important about an amorphous core?
Dave
This was a dozen years ago for the GM70 amp I took to NY Noize. A choke approximates a current source and when you put two current sources in series they fight each other. I suspect the CCS was winning but the choke has that "inductive kick" that the regulator then corrected for.
I think the amorphous core was key because it has lower losses at high frequency and since this was happening at 70Khz the core may still be in the picture.
dave
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