At 09:50 PM 8/27/01 -0500, Ford Peterson wrote:
>Has anyone had experience with this amp design and did they also see the
>gain roll-off below 4mHz?
The caps were indeed chosen to provide some small measure of roll-off at
the low end. The first values used in the original version were 0.1 uF.
That original low-bias version IMD performance was not quite as good as
this higher current version is. The cap values were reduced a bit at a time
until there was some roll off noted at 1.8 as evidenced by lowered IMD
products. The high bias version could probably stand the 0.1 uF caps. In
any case, here should not be 6 dB roll-off. Each pre-amp I have built has
been swept with good test equipment. The gain was constant from 1.8 through
30 MHz. The 0.01 uF ceramic caps I used were very high quality types, not
your typical disc bypass types.
For a different application, I actually used 1000 pF caps and had the gain
roll off below 14 MHz. I would look at the transformers, especially the
core material. I have had mis-marked or defective cores in the past which
caused me no end of grief. If I have time today, I'll try to measure a
couple preamps again and report back on my results.
Thanks for the input. If there is a potential issue with the design I sure
want to know about it. Also, thanks for working with the push-pull design.
I always meant to do that, but since this version worked so well, I never
got around to the push-pull development!
Larry - W7IUV