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In Reply to: RE: Series feed SE OPT with no air gap and low flux?? posted by Tube747 on March 16, 2012 at 23:12:48
Tube747
It is a variation on an old idea called a current mirror.
DC current flowing equal to the regular current in the
opposite direction that cancels the needs for an air gap.
The cancelling currents prevent saturation of the core.
The bypass cap shorts the signal across one winding so
that the iron retains half the inductance of the PP OPT.
Which is still much more than any SE OPT has.
DanL
Follow Ups:
Dan, using that circuit, can we do the same thing with any PP output transformer?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre'
No, you need one that has seperate windings
instead of just a center tap because
the tertiary winding is wired backwards.
Remember if you want a 5K OPT for your circuit
then you need a 10K because you are using
only half of the windings for the signal.
DanL
My pair of Stancor A-8072 should work. If I remember right, they have two separate primary windings.
I also have a pair of Freed output transformers with 4 separate primary windings.
What value should the shunt cap be?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
nt
The cap will effectively short out a winding. This will reflect a very low impedance to the other windings through the core severely loading down one of the tubes.
I don't see the concept working very well in practice.
dave
"The cap will effectively short out a winding. This will reflect a very low impedance to the other windings through the core severely loading down [the output tube]."
I'm sure glad you have my back. Sometimes I leave my brain in the other room.
Thanks
So how about a CCS at one end, set to the idle current of the output tube on the other end of the primary winding of a PP transformer?
That should work, no?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yes, that will work. The currents need to be reasonably matched and make sure you have a load connected since the CCS will fight with the inductance and possibly oscillate.
dave
Hi Dave,
"... since the CCS will fight with the inductance and possibly oscillate."
Would you please elaborate on this?
Thanks
Dave
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
I should also point out that this is a very different design concept to using "loosely" coupled windings to offset the effect of a shorting cap.
dave
"...using "loosely" coupled windings to offset the effect of a shorting cap."
Is that what ABB is doing?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
ABB???
dave
The company that the OP referred to who are making these non-airgapped SE transformers."...Audio Output Transformers for Single Ended Amplifiers
without gap using an Ari Polisois license of Self-offsetting allowing a very low flux (near 0) also for high d.c. idle currents....."Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/19/12
Yes. they seem to be using a design that uses a loosely coupled winding is series with a cap across is.
I tried in spice to get this to work but was unable to get anything near flat results. I still do not see how this idea would work.
dave
Tre'
Depends on the inductance of the transformer.
The Lundahl LL9202/PP worked out to 22uFd.
DanL
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