Adding a DI to a preamp

[Rmanen]

Hi there,

I've been busy getting all the parts together for a F2B/Fender bass pre, and though I love the original, there are a few things missing. I'm planning on adding a tuner out, by simply wiring a opamp buffer to the input jack. The one thing I simply cannot find on google, is how do I add a good, balanced, impedance matched DI for hooking up to the FOH?

Peace

  Rob van Manen

[Rmanen]

Sooooo..... is it usually this quiet...

[PerroGrande]

Just so I correctly understand what you're asking...

You are adding a tuner output via a simple op-amp. No issue.

You want to add a BALANCED output to act as a DI connection for the house sound system (rather than using an external DI box). 

Correct?

[PerroGrande]

This article provides some very useful information on balanced and unbalanced connections:

www.jensen-transformers.com/an/an003.pdf


and this project might have some interesting tidbits to point you in the right direction...

http://sound.westhost.com/project35.htm

[Rmanen]

Hmm so basically its just adding a 1:1 transformer?? Wonder why i couldn't find anything.
As for the tunerout, yeah thats just going to be a opamp buffer, since it will never pass any signal I have to listen to 

Cheers

PS: Any suggestions for a better DI or transformer suggestions are ofcourse welcome...

[PerroGrande]

Well, you do have several options on the conversion.   

A transformer is only one of several options, and if you go with a Jensen model, certainly not the cheapest.  You can use two op-amps to produce the differential signal.  If you used a single quad op-amp, you could do your tuner output and your active DI on one chip.  However, I get the distinct impression that you don't care for the sound of an op-amp.   

You may end up stuck needing some form of buffer stage even if you use a transformer.  You're looking at a 600-ohm input impedance on that transformer. Depending on where you get the signal, loading could be an issue.

[Rmanen]

Its not that i don't like it, its just that I don't see the point of having an all tube pre and adding a Opamp. It just sounds icky...

I would be taking the signal from the output, it looks like it would be a 1M output impedance.

Rob

[sverige_cruz]

Hey Rob,

You might want to check out this:

http://web.telia.com/~u31617586/#active%20DI-box%20my%20work%20horse%20from%201975

Also, at the prodigy-pro "lab" forum, there are some conversations about it being very good, with several transformer options from the designer, Bo Hansen

http://www.prodigy-pro.com/forum/viewtopic.php?t=5974

It's on my list of "to-do"... but aren't those types of lists usually pretty long  Looking at your preamp of choice, I seem to recall that also being on the list.... Anyhow, good luck with the build. If you do make a DI, you may want to think about making it separate, so you can use it by itself, if you wanted. I know it'd be another box, but it leans towards versatility.

Cheers,
Jesus

EDIT: I just re-read your post, that you'd be taking from the output. Don't know if what I just typed will work for you (it's late... I skipped a few lines ) . I'd probably try the Jensen alternative with the following:

http://www.jensen-transformers.com/as/as066.pdf

If you've got a 1M output from the preamp, it seems like it should work.
Happy soldering!

[PerroGrande]

The Application Schematic from Jensen is, I think, spot on for what you want to achieve.   You'll get a DI output with a ground lift (a nice option to have) and no op-amps in the actual signal chain.