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Old 24th July 2012, 12:22 AM   #11
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Quote:
Originally Posted by janneman View Post
These types of circuits that depend on exact matching on device parameters are always very sensitive to small deviations.

I remember that Audio Precision uses a similar circuit for their analog generator output transformer (it's actually a Bruce Hofer patent). They have the resistor representing the DC wiring resistance actually inside the winding so it tracks the temperature. I am not 100% sure and too lazy to check it but I seem to remember that it is the primary DC resistance they were cancelling.

Edit: decided to check it anyway ;-)

This is what Bruce sayz:


jan didden
Hey Jan! Do you think it's anything like the "mixed feedback" system used for output transformers?
http://www.lundahl.se/pdf/ovrigt/feedbck.pdf
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Old 24th July 2012, 01:26 AM   #12
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Hey Jan! Do you think it's anything like the "mixed feedback" system used for output transformers?
http://www.lundahl.se/pdf/ovrigt/feedbck.pdf
Its the same of course. Hofer's patent has expired. In your circuit it seems that you try to cancel the secondary DC resistance though, and thats not the one causing the distortion.

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Old 24th July 2012, 11:30 AM   #13
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Originally Posted by janneman View Post
Its the same of course. Hofer's patent has expired. In your circuit it seems that you try to cancel the secondary DC resistance though, and thats not the one causing the distortion.

jan
Hey Jan, I didn't invent the input circuit. I just tried to work with it from the Lundahl data sheet:
http://www.lundahl.se/pdf/7101.pdf

The advantages of it appear to be a frequency response to almost DC without using a huge transformer.

Commercially, it's used by at least one company that I know of.

I had an idea for using the circuit for something else but I dropped the idea.
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Old 24th July 2012, 12:30 PM   #14
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Well maybe it is not the same as Hofer's. That (Hofer's) essentially tries to cancel the distortion of thye xformer by cancelling the primary DC resistance.
I think Lundahl's is someting else because it does operate on the secondary only.
No idea what that does; maybe as you say flatten the response, but it can do nothing for the linearity.

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Old 24th July 2012, 11:04 PM   #15
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I think we're mixing up input and output circuits. Hofer's as I understand it was an output circuit. I started this thread about an input circuit.
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Old 24th July 2012, 11:14 PM   #16
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Here's an explanation of the zero field input circuit:
"Just a guess but, it appears that the Lundahl circuit is attempting to drive the negative terminal of the transformer through the negative feedback path. Ordinarily, the positive input feedback path would attempt to keep the negative terminal of the opamp near zero volts (+- the DC offset). In this case, the negative terminal of the transformer and negative terminal of the opamp appear to be acting in a "current mode" relationship so that when the input voltage rises, the opamp attempts to move the negative terminal of the transformer toward the positive voltage direction. This would result in a net instantaneous voltage across the secondary of the transformer of zero volts, nulling the field within the transformer secondary. From EE class, I vaguely recall that the inductor represents a high impedance to the changing voltage (as opposed to a capacitor, that represents a low impedance to changes in voltage).

The capacitor C does two functions, apparently. The first is to prevent DC from flowing in the secondary. The second is that it serves as a high pass filter with respect to R (some kind of impedance mirror across the opamp is allowing them to interact, perhaps someone can elucidate). A one pole filter of this type has a rolloff of 6db per octave, I think. The reason why the value of R needs to match the DC resistance of the transformer winding is probably because the two resistances form two legs of a wheatstone bridge. When all currents and voltages are in balance, the lowest THD (center of the hysterisis curve) results. The other circuit that was referenced in a later post does not appear to have a similar function to this circuit, in that there are no impedance elements in positive feedback path."

Zero Field Transformer With DC Servo
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Old 24th July 2012, 11:25 PM   #17
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Studer used it in some of their equipment. This is from a Revox PR99. I don't pretend to fully understand this circuit. I think this is an interesting subject anyway. Things I don't understand are interesting to me.
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