I was using Ebtech Line Level Shifters in my synth rack to eliminate
ground loops, but was unhappy with the tonal shift the transformers
imparted to the sound.

I am an instrumentation technician with over 20 years experience, and
decided to design and build a unit to satisfy my needs after finding
that there are only a couple of these type of units available

I have my synthesizers analog outs connected to a MOTU 1224, and after
connecting through my custom box, not only are the synth signals
quieter because of elimination of ground loops, they are more open
sounding and have a higher signal level in general.

This is because almost all AD boxes (MOTU, Lucid, Apogee etc) have an
input impedance at around 10k ohms. This can "pull down" the signal
coming from most synths with unbalanced outputs and make them
decidedly more "muddy" sounding.

The input impedance of my box is around 30k ohms and adds no gain, but
the signals are generally 6 to 7 db hotter just from signal being
"recovered" by a higher input impedance.

The design is based on the Analog Devices balanced line driver IC. It
can easily drive 600 ohm loads and the outputs emulate transformers
for easy connectability.

The noise floor is at -99 db with the inputs shorted.

Here are some links to pictures of my prototype:

http://www.electronicscene.com/m1/17...media/4746.jpg
http://www.electronicscene.com/m1/17...media/4747.jpg
http://www.electronicscene.com/m1/17...media/4748.jpg

And some audio files showing the difference in the sound:

http://www.electronicscene.com/artist_files.cfm?a=1120

The pics will load automatically, but you will need to hold down the
mouse button and select "save to disk" or equivalent to download the
audio files.

There are 4 audio files he
1-Orchestral with
2-Orchestral without
3-With
4-Without .

The files were recorded by splitting an output to a Lucid AD9624 with
one leg going through the balanced line driver. I then split the audio
files into 2 mono files for comparison. The format is 24 bit / 44.1khz
, but the files are pretty small and load reasonably fast even with a
dialup connection.

I am currently re-designing the circuit boards to have all the parts
except power supply on the board to eliminate hand wiring. The power
supplies I am using are medical certified external units.

Any input would be appreciated, and I am thinking of having a unit
available for review and testing by a professional audio engineer.

Thank you

Roger A. Foote

"Roger W. Norman" wrote in message ...
I don't know but most of us would simply pop the synths into a DI and run
balanced cable back to a mixer, but I guess people are now using their
computers for direct inputs sans mixer, so your device should help
significantly. Unless I'm missing something here, though, there are synth
outputs and then there are synth outputs. How would you balance the inputs
level across a disparate set of synths? Sometimes noise isn't all in the
ground loops.

--


Roger W. Norman
SirMusic Studio

301-585-4681

I have the following synths connected:

Yamaha EX5R
Roland JP8080
Nord Modular
Yamaha FS1R
Nord Lead Rack
Yamaha VL70m

Most of the noise I was getting was ground loop because of all those
synths' signal common connected together at the AD box. And yes the
outputs vary in their robustness, but all of them had a higher output
level with this box. The key here is that there is a breakpoint in
impedance that most synth outputs are "comfortable" with.

Some mixers have input impedances of 20k - 30k ohms on their line
inputs, while most AD boxes like the Apogee 8 channel ,the Rosetta,
MOTU 1224 or the Lucid AD9624 have a 10k ohm input impedance... That
WILL bog down most synth outputs. The other thing is that if the
output is loaded heavily, the frequency response on *some* even high
dollar synths will start accentuating the 250- 400 hz range more than
if they have an easier input structure to deal with.

With all analog output stages, if the load they are connected to
causes a drop in amplitude of 6 db or so, the output's ability to
swing the signal across the supply rails gets compromised, which means
less dynamic range. That's my main reason for using it, since I want
to hit the AD as hard as possible without amplification.

As far as disparate output levels, that seems a lot less prevelant
now, at least with the synth rack listed above. It is now much easier
to get a balanced level out of them than before using the box, if I
want a balanced level.

The other nice thing about having the shield disconnected at the
receiving end is that you get a lot less of those head scratching
sessions when an "alien" piece of gear comes into the picture and
upsets the grounding balance in your rig.

Thank you for your thoughts!

Roger A Foote