Help with preamp boost circuit?

[earthsled]

Hi all!

So, I'd like to add a gain boost switch to the input of my REDD-47 clone and I need some help with the best way to implement the circuit, isolate it, and bypass it properly.

I'm thinking about something close to PRR's transistor-based circuit for an active ribbon mic...
The 6V version is shown here: http://groupdiy.com/index.php?topic=5743.msg72416;topicseen#msg72416
The 48V phantom-powered version is here: http://groupdiy.com/index.php?topic=5743.msg73762;topicseen#msg73762

My build has either 6V or 48V regulated DC rails to pull from. First off, I'm wondering if it would be better to have the boost circuit pre- input transformer and somehow isolate it from phantom power, or use an unbalanced version of the circuit after the IT and before the grid of the first tube.

Ether way, I'd like to keep the boost circuit independent of phantom power, so that P48 condenser mics could use the gain boost (in addition to dynamics, ribbons, and tube condensers).

I've attached a schematic of the preamp for reference.

Thanks for your help!

 

[moamps]

I would add one additional gain stage using EF86 between the input transformer and the first tube in the REDD.
You already have all rails you need.

 

[CJ]

you could also use another input transformer with a higher ratio,

less noise,  if well shielded,

 

[earthsled]

I would add one additional gain stage using EF86 between the input transformer and the first tube in the REDD.
You already have all rails you need.

I like this idea and I think it would be great to try for my next build. Unfortunately, I'm running short on space in the chassis and I don't think I could fit another tube.

you could also use another input transformer with a higher ratio,

less noise,  if well shielded,

This is a great idea too, but I've already purchased the input transformers with ratios similar to the original design.

I greatly appreciate the help!

I've attached a drawing of what I had in mind using PRR's 6V circuit...

I'm confused about the ground relationship in the 6V version of the circuit. It almost seems like the circuit would need a negative 6V supply (rather than positive) if the ground were common with the preamp circuit. Also, PRR's circuit was designed to come before the phantom power circuitry. In my case, I'd rather have the boost be independent of the phantom power, but I'm not sure if the caps in the input and output would be appropriate for isolation. I would think the orientation of some of the caps may need to change.

Thanks again for your ideas!