A couple of points for you to ponder.
A single OPA-695 won't make 5V RMS. However, it's capable of a voltage
gain of 10X, 20dB power, so you only need a single stage to go from 5mW to
500mW. A pair of them connected differentially will output 5V RMS, and
provide some 2nd harmonic cancellation as a bonus, but, they probably shouldn't
drive less than a 200 ohm load for best distortion numbers.
Or... a single '695 could be followed by a +/-15V rail part, a THS3001 (or a
more recent part from that family) for instance, to output an easy 5V RMS... or
10V from a differential pair. Again, this scheme may not be happy driving into
Note: Both of these op-amp approaches only exhibit exquistely low distortion
on the low bands. At higher freqs., their distortion numbers rapidly
As the 4CXwhatever grid is a high impedance point, a tuned matching network
might be better fit than a wideband transformer for single band operation.
Your proposed 20dB negative feedback around the driver-final means that you
will require more grid drive, say somewhere around 50V RMS to the driver, to
maintain the same overall power out. The op-amp pre-driver may not provide
enough oomph in this instance since a 10-1 voltage gain, employing a tuned
matching network sourced from 200 ohms, would imply a grid impedance of 20K
ohms. Where's that free lunch? :-)
73 & Good evening,
-- "Tom Cathey" <K1JJ@comcast.net> wrote:
Thanks for the suggestion.
You make a good point of possibly eliminating the 6146 pre driver. I thought
about that myself, but didn't have enough knowledge to know if I would have
enough gain left to reduce the overall gain with 20 db of negative feedback
afterwards. Figured I'd just try it and see.
Well, I know that the 4CX-XXXX final [they're all similar] will need about
300 volts of swing. And the OPA-695 will do 10db gain. That's 5mW to 50mW. I
figured I'd put two OPA-695's in cascade for 20db = 500mW. Into 50 ohms,
500mW = 5V. So we have 5V to drive the 4CX-250 driver. And, the 9:1
transformer will make it X3 = 15V, right? This is a 9:1 impedance ratio =
3:1 voltage ratio...?
Anyway, with 20 db of negative feedback from the final to the 4CX-250 driver
will mean a 20db reduction in gain. I suppose a lot depends upon the gain
of the 4CX-250 driver and the transformer. This is where I am not sure how
to calculate it and proceed. [20 Log db for voltage, etc.]
ie, Will an OPA-695 putting out 500mw into an X? step up ratio transformer
that drives a 4CX-250, that drives 300v into a 4CX-XXX X final with 20db of
negative feedback have enough system gain? And, will the 9:1 voltage step
up transformer have absolutely no affect on the low distortion goal... even
if a small space charge current is pulled? What if we need more than 9:1
step up, can it be done with cascaded transformers without ill effects?
BTW, I'm curious why you feel the transformer will have bandwidth
limitations... are we talking of a broadband unun torroidal type?
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