A
PP
EN
D
IX
–
Le
ve
l P
ro
bl
em
s
Level Problems
No Signal
This problem might be caused by balanced wiring. If the signal almost completely drops when the
alpha compressor is activated, it is probable that a pin of the XLR connector at the input is not con-
nected. As the input stages feature transformers, both pins have to be connected. The classic case
for this problem would be a balanced XLR connection that is connected to an unbalanced output
that only uses ground and pin 2. Connecting pin 3 to ground should solve this problem.
Slight Level Jump
Under certain circumstances, pushing the active switch may cause a small change of level. If there
is no noticeable difference between bypass and active when in stereo mode with activated di-
rect buttons and mix controllers fully turned counter-clockwise, everything is OK. But if there is
a small change of level somewhere between 0.5 - 1 dB, the input and output impedances of the
devices preceding and succeeding the alpha compressor might be the reason. A high output im-
pedance (approx. 1 kOhm) of the preceding device and a low input impedance (<4700 Ohm) of
the succeeding unit are especially critical. If the compressor is in bypass, the preceding device
will be loaded with the input impedance of the alpha compressor itself plus the input impedance
of the succeeding unit. This cumulative load might result in a decrease of level. If now the alpha
compressor is activated, the preceding device only “sees” the alpha compressor and no longer the
following unit. Therefore the load decreases and the level rises as a consequence. Especially tube
equipment tends to produce this phenomenon. Generally speaking, output impedances smaller
than 150 Ohm are recommended for a trouble-free signal chain.
Large Level Jump
The output stages of certain audio processors are designed in a way that the level will always
stay the same – no matter if they are hooked up with a balanced or an unbalanced connection. If
pin 3 is connected to ground, for example, the level at pin 2 will automatically become twice as
loud as before. These kinds of output stages are usually unproblematic. But there are also stages
that cannot compensate for that. In these cases the level at pin 2 stays as it is, even if pin 3 is con-
nected to ground. If the alpha compressor is placed between a device with that kind of output
stages and a device with unbalanced inputs and with pin 3 connected to ground, it is possible that
the level jumps up by 6 dB when the compressor is activated. As a general rule, input stages with
balanced connectors are always the best choice. In case these are not available, the first attempt
to solve this problem should be to disconnect pin 3 at the XLR inputs of the alpha compressor
and then connect it to ground. This generates an unbalanced signal that should not shift the level
anymore.
APPENDIX
