Transformer DI for modulars

[Magnus Danielson]

From: DCMagnuson at aol.com
Subject: Transformer DI for modulars
Date: Sat, 23 Dec 2000 13:38:33 EST

> Hi list,
> 
> I'm considering making a DI module for the output of my modular synth.  
> Basically, I'd like to have a small mixer feed into a transformer so I can 
> provide a balanced output to my mixer.  I have a few questions regarding this:
> 
> 1) Can anyone recommend a suitable transformer?  I imagine I'd want a high 
> input impedance, and around 600 ohm balanced output.  I hear good things 
> about Jensen trannys... they are supposedly quite clean.  Any other 
> manufacturers that I should look at?  

Haven't used Jensen transformers myself, but I have heard good things
about them. If I where to get a good transformer, then I would fetch a
Lundahl transformer (http://www.lundahl.se/). Please check their site
out since it contains possibly interesting material.

You don't require a transformer in all cases though, you could use
op-amp setups to generate the diffrential signal.

> 2)  I figure I'll need some attenuation to drop the signal levels from 10V 
> p-p to standard line level.  Should this pad be put before or after the 
> transformer?  I was figuring that putting the pad after the tanny will allow 
> me to saturate the transformer a little, and "warm" things up a bit with some 
> distortion.  Is this a good idea?  Maybe a selectable pad before and after 
> the transformer would be best?

Put it at the end of the whole thing, preferably in the receiving
mixer. By keeping a high level over the diffrential signalpath you can
get better signal/noise level than if you run a low level. So, if you
are up to high levels, this is not necessarilly bad, that is, as long
as you dont look into 100 V signals from those glasfets ;)

> 3) What precautionary measures should be taken to avoid damage from phantom 
> power?  My mixer has globally selectable phantom power, so I need to protect 
> the DI from damage.  Will the current limiting resistors in the mixer provide 
> enough protection to avoid smoking something?  Transformers (inductors) are 
> seen as shorts at DC, correct?  I just want to make sure I'm not going to 
> kill either the transformer or my phantom power supply by drawing too much 
> current.

Since you apply the phantom power (say +48 V) to BOTH the polarity
signals, these will have the same DC offset. The phantom supply those
elivate the signal leads with an DC offset to the common ground
signal. Now, if you have a transformer it will only see the
diffrential signal and isolate out the common signal. If you have a
middle tap of the cable side of the transformer, you can use this to
take out the phantom power to the curcuit if you so wishes. If you
only know how it works, there is no magic behind handling phantom
power safely.

I hope this gave you sufficient insight so that you can build a good
DI box.

> Thanks everyone, and happy holidays

Merry Christmas and a Happy New Year everyone!

Cheers,
Magnus