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?
> Before you close the case Tom something else for you to consider. To drive
> a GG Amp you place watts on the cathode. To drive a Grounded Cathode Amp
> you place volts on the grid. This provides you with a cheap and easy way
> to increase the gain of any stage. Simply insert a voltage transformer in
> the input. With a 1 to 9 unit on the input of the 4CX250 stage you should
> be able to chop out the 6146 and drive it directly with the OPA-695. The
> draw back of this method is the available operating bandwidth. Seeing as
> you intend to use this in a monoband application this should not pose a
> problem. The other thing to consider is the distortion generated by
> saturating the transformer core. Hardly a problem worth considering at
> that power level.
Amps mailing list