Lexicon
12
Using the 20/20 AD
Make sure the BALANCE switches located on the rear panel are set to match
the type of input. (OUT=balanced , IN=unbalanced). The red UNBAL lights on
the front panel will indicate an error if there is a mismatch between the switch
setting and the signal type.
Note that when an UNBAL light goes on, there is a good chance that the input
signal has been audibly clipped, even if the level meter does not indicate an
overload.
If it is necessary to mismatch the switch settings to the signals (for example,
when switching quickly between balanced and unbalanced inputs) you can
avoid clipping by setting the peak input levels about 3dB below full scale.
Under some conditions, the UNBAL light may remain on, even though the source
and the 20/20 AD are both set for unbalanced operation.
1. The source has a pin-3-high unbalanced signal. Try swapping pins 2 and 3.
2. The source is floating (transformer-isolated). Tie pin 3 to pin 1 at either end
of the cable.
The UNBAL light may also flash intermittently if the source is balanced and
transformer-isolated, and the 20/20 AD shares the source with another con-
verter or an amplifier. The other box may be unequally loading the positive and
negative source signals, causing them to become unbalanced. Some solutions
to this problem are:
1. Drive the 20/20 AD by itself. This will ensure a balanced signal and minimize
common mode noise.
2. If the source's isolation transformer has a center-tapped output, ground the
center tap.
3. Reduce the signal level into the 20/20 AD and make it up with the front panel
gain control(s).
4. Remove the power cord, open the 20/20 AD, and short out R4, R28, R54 and
R95. These are 10 K resistors (brown, black, black, red) located near the input
relays. Note that this change may degrade the 20/20 AD's CMRR.
5. Equalize the load resistances of the other box with the following procedure:
a) With the 20/20 AD and the other box connected, send a high level tone
through the source so that the 20/20 AD's UNBAL light goes on.
b) Connect a 1-Meg potentiometer (set at maximum resistance) between the
pin 2 signal and ground.
c) Gradually reduce the resistance until the UNBAL light goes out. (If the
UNBAL light does not go out, reconnect the pot between pin 3 and ground,
and repeat.)
d) Once the UNBAL light goes out, continue reducing the resistance until the
light comes on again. Set the pot so that it is in the middle of the range where
the UNBAL light is out.
e) Remove the pot and measure the resistance. Solder a resistor of that value
between the correct pin and ground.
Balanced and
Unbalanced
Input Signals
