- Posted by Naz (A) on May 08, 2007 at 05:26:03
In Reply to: Low vs High Operating Points posted by danlaudionut on May 8, 2007 at 03:45:27:
Dan, you'll get all sorts of opinions but ultimately you know the drift, you'll have to make your own mind up. FWIW, I personally think that many differences of opinion arise simply because there really is no absolute rule that can be applied to every case.There’s no doubt that in most triodes generally higher current results in more linear operation. Also, higher voltages result in higher linear voltage swing, but this is not always required. Many factors need to be taken into account and a good starting point is always to plot the load line and set the operating points along the most linear portion of the line having regard to the required voltage swing.
However … when it comes to OP stages or any stage where a lower than optimal impedance load must be used then the previous stage becomes way more important and IMHO the pair of stages should be viewed as one and the operating points should be set according to the best overall result. For SETs, that generally means that lower operating current for the driver though less linear can often be used to offset the higher expected distortion of the OP stage due to opposite phase cancellation. This is both measurable and very audible IME. Since distortion in the OP stage is usually dominant higher current normally works better here as does higher load impedance. I also think that this is the reason that so many report such good results with (less linear) Pentode drivers for SETs.
Cheers,
Naz
