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Line out audio transformer capacitor question

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diydidi

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Hi
I copied this circuit for the DI output section for another Fender amp.
The DI transformer is on the top right corner of the page of page 6.
My question is how do I determine the right capacitor value on the input prim of the transformer. In the attached schematic they used a 0,1uF.
When I connect the cap with my chosen transformer, an OEP A262A3E, I get a slight boost of the higher frequencies.
 

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A capacitor across the signal would normally cause a reduction in high frequencies because of the time it would take to charge and discharge it through the 15K series resistor but I suspect it is there to resonate with the inductance of the transformer primary and cause a peak in response somewhere at the top of the audio range. The 1K resistor across it would be to dampen the resonance so it didn't sound like telephone audio. There is no right value, you would have to decide a frequency for it to peak at and use the formula "f=1/(2*pi*sqrt(L*C))" where L is the transformer inductance in Henries (check the data sheet) and C is the capacitor value in Farads.

Brian.
 

A262A3E isn't 1:1 transformer, how did you actually connect it?

Instead of the dubious capacitor, I would probably use a voltage divider with lower output impedance. With 1k source, transformer primary inductance can matter. Do you know the transformer specification of the original Mesa Boogie line out design?
 

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