Servo assisted CF LCR-filter

[revintage]

After having read SY´s "A Heretical Unity Gain Line Stage part III" I thought that this might be applicated to my planned crossover.
As I want to get rid of the coupling capacitor for the LP-part I want to implement a servo there.

I think that biasing through the input transformer secondary is the way to go.

The problem though, is that I do not have a clue how a servo works.

The filter configuration is no problem as I have used it before. In the schematic the negative voltage is unnecessary high, I think 24V will be enough. But there is just guesswork around the servo:

http://www.revintage.se/filter.pdf

Anyone who cares to help me with that part? And don´t mind the component values in the filter part, they are not the ones that will be used.

 

[revintage]

Ooops! Found out that at least 10Meg is needed at the output of the integrator make it work. Also I am stuck with the big 10uF cap at the downside of the tranny. So maybe my idea with biasing through the transformer wasn´t so good anyway.

 

[revintage]

Found Bernings site and copied the servo from there.

 

[ErikdeBest]

Hi Revintage

I can't help you with this one, but maybe you can use Broskie's implementation of a DC coupled cathode follower using the bipolar rails schematic.
http://www.tubecad.com/2007/04/blog0102.htm
(scroll down a bit).

With this idea you can attach a servo circuit to each of the cathodes, without having to worry about larger servo loops. Also enhancement mosfets are prefered over depletion ones, as the DN2540 in your schematic. (Enhancement ones are cheaper and easier to source, but maybe you already got the DN2540...)

It is just an idea! Erik

 

[SY]

The disadvantage I see with biasing up through the transformer is the the servo output is effectively in series with the signal. One way around that is to divide the output down. Let's say you need a maximum of 0.5V of "correction." With 15V rails, the servo can swing (about) 14.5V, so the output can be divided down by a factor of 29. With a 100 ohm resistor between the transformer and ground, the servo output can be taken through a 2k9 (practically speaking a 2k7) resistor. Yes, there will be a slight loss because of the 100R in series with the secondary, but it's pretty small.

This method works best if you can design the CF to have pretty low offset to begin with, which allows a high divider ratio.

 

[revintage]

Did a total rethink after input from another person. This is what I ended up with. Simpler and without caps in the LP-chain. With 600ohm filters the chokes will be small enough to work properly.