Quote:
Originally posted by jbuntz
Geoff, I was wondering something about the neve preamp design. Normally transformers generate primarily 3rd harmonics but the DC across the primary winding on the output tranny will add some 2nd too since normally the bilinear transfer curve is symmetrical (looking at the integral of the signal) but the DC shifts the symmetry so that negative values distort sooner...are my assumptions wrong? |
Hi
I don't claim to be a transformer expert but the onset of distortion from the biased transformer is adjusted to ensure that the point of clipping/saturation is equal on both ends of the wave.
I think that you can compare transformer bias, in basic operation, to tape bias. You can't efficiently magnetise the particles on the tape unless you apply a bias... usually an ac tone but you'll find on really cheap cassette recorders they even used dc bias.
The 70mA or so dc passing through the primary of the LO1166 biases it to a nice point on its magnetising curve and the audio is modulated on top of this. Whenever I look at the 283 circuit it always reminds me of the sort of amplifiers that they fitted in old record players. The ECF triode/pentode combination where the triode is the pre in front of the output and the pentode drives the transformer.
If you look at really early Neve circuit diagrams like the B100 and B104 you'll see that Rupert drew his transistors so that they looked like valves with the base a horizantal line through a circle (like a grid)and the emitter, an arrow rising at 90 degrees through the bottom of the circle to meet the base in the centre... rather like a cathode.
I think that valves were an influence in the early days.
