A new simple project from Fabio (discreet DI)

BR · Oct 20, 2008

Hey guys,

Since I had a few BFB (fet bloaks) laying around, I asked Fabio do design a discreet DI circuit based on it and this is what he came up with:
I asked him to share this with the group and off course he agreed.

http://www.braudio.com/forumfiles/DI_BFB.pdf

Let me know what your thoughts are.

I'm designing a PCB for a project and could morph it towards a DI box design.
I just have to figure out how to get power to it, and if it would run OK from phantom.

btw.. Does anyone know how to display pdfs on the thread?

take care
Gil

PRR · Oct 21, 2008

> how to display pdfs on the thread?

Not supported here. Not particularly wise with over-size schematics.

What you did, post a link, is fine. It opens in new window/tab, it can be right-clicked and saved, etc.

If you must: use a screen-grab tool to get the image, crop/shrink/contrast, post that to www.tinypic.com or other clean host, and use the {IMG} tags.

Svart · Oct 21, 2008

Something is not right with the TRS jack and the associated relay control circuit. The ring is shown to connect to the NI input when it *should* be the tip that connects to the NI input. The ring contact should be used for the current sink to turn the PNP "on" since this would most likely be used with a TS plug, not a TRS plug. The TS plug's shaft will contact the ring contact in this case.

EDIT: I've also found the FETblokes to work best with around 15k of Rfb but I do think 10k should work fine too.

EDIT2: If you use the darlington output parts you can drive a tranny.. :wink:

EDIT3: I'm also not sure if the fetbloke would like to be used on a single supply.. most CFB opamps will balk at the single rail attempt.. I've never tried it with this design though, it's worth a shot.

clintrubber · Oct 21, 2008

Nice :thumb:

Since the schematic goes for an impedance-balanced out, here the suggestion to make the story complete and give the neg-pol output a 51 Ohms series resistor as well, just like R5 for the pos-pol output.

Or perhaps even a bit bigger, dunno what the output-impedance of the opamp over frequency does.

When going for imped.-balanced that now missing resistor is in fact more important than the two already present components C4,R7.

Bye,

Peter

BR · Oct 21, 2008

Hey Svart,

I had noticed the TRS wiring problem too, but figured it would be an easy one to deal with.

EDIT1: That should be really easy to try.

EDIT2: cool, but i'll leave that for the smarts like you. I just want to try it as a small DI circuit without trannies for now, but everyone is welcome to experiment, off course.

Hey Peter,

That would be something to try also. Since this wasn't tested yet, I'm sure there will be a few things to tinker with.

If anyone is serious about messing with this little circuit, I could donate a BFB for the cause.

take care
Gil

Svart · Oct 21, 2008

the "big" version of the bloke uses the darlington output already so it would be simple to hang a tranny on the output and get balanced outputs. If Fabio hasn't layed this out yet I would ask that you put jumpers on the PCB so that when you stuff the board you can choose to use a tranny or not.

other than that, I still suggest to try using the fetbloke single ended before you design the board like that.