Transformer DI for modulars

[Tony Allgood]

Hi all,

>The use of positive feedback in the classic three-opamp approach makes
me a little skeptical -- I wonder what situations would compromise the
stability.

Me too. But I have never yet found it to be unstable, unless you don't
fit the little pF capacitors. And it used in many commercial units too.

>Check out the Analog Devices SSM-2142.

I am getting very distrustworthy of AD devices in general, especially
those with the SSM moniker. It seems that supply in Europe is sketchy at
the best. And it is more expensive that three op-amps, although you do
lose the trimmer.

>So you can't just use, say, the positive output of the chip and leave
the negative output floating, you also need to connect the negative
output to ground.

This is true, but no problem if you use the simple rule of TRS jacks for
balanced and mono jacks for unbalanced as you say. But isn't this true
of transformer outputs too?

>Also, I figure a balanced driver stage would have seriously degraded
performance driving a phantom power input; both outputs would slide up
toward the positive rail.

This is interesting indeed. No, I don't think it would be a good idea at
all. There are capacitors protecting the op-amp's output from DC
although they are not usually rated at 63V. But it is unlikely that you
would be using a mic input. Most desks do isolate the 48V supplied to
the mic XLR from the TRS-jack input sockets.

Regards,

Tony Allgood  Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan