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Old 18th May 2022, 4:45 pm   #2
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 6,340
Default Re: SE transformer question

Originally Posted by retailer View Post
I just happened to have a stack of laminations that fitted his bobbins so I assembled the first TX with 2 sheets of photo copy paper (around 0.15mm) as a core gap, my homebuilt inductance bridge came up with a primary inductance of 67Hy, with 70mA of DC bias the inductance fell to 28Hy - this seems like a huge drop that I wasn't expecting, is anyone able to comment is this what one should expect ?
Can't comment exactly as there's not enough data - but it doesn't sound unreasonable. What is the core cross-section area, and how many turns are on the primary?

A single-ended output transformer has to not saturate with the standing DC current, and have sufficient margin from saturation to cope with the AC flux swing when the output stage is driven with maximum signal at the lowest frequency of interest. Assuming normal transformer iron (approaching saturation at 1.6T) good practice would have it at a quiescent level of about 0.9T so that an upwards and downwards swing of 0.7T can be catered for without hitting saturation, which would cause distortion.
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