R.G.
9/25/98 8:06 AM


Relays - all relays - can pop from the
normal switching problems, as well as one
of their own. Any metal-contack switch
will pop if the DC levels between open
and closed are not the same. The pop is the
current pulse as the voltages equalize. 

Relays have the additional problem that the
change of voltage on the relay coil will
induce pops into the audio path by
capacitive coupling of the coil voltage to
the signal path. This is especially bad if
the designer simply allows the coil voltage
to fly or be clamped with a diode when the
coil is turned off. This causes a flyback
voltage on the coil that is -very- fast, and
couples easily to the audio path. The
coupling is dependent on the amount of
capacitance to the audio path and the
impedance of the "out" side of the audio
path. If this is low, the capacitance may not
be able to make it pop much. If this is the
input of a tube, even small coupling
capacitances will cause bad pops. 

The solutions to this problem are two:
slow down the voltage transition on the
coil by ramping the voltage to turn it on
and off, and/or shield the signal path inside
the relay from the coil. 

Reed relays were invented and developed
by the Bell Labs for switching audio in
telephone applications. They will also pop
if they are not shielded or coil-ramped. 

Reeds are fast, low power, and have low
contact distortion and thermal noise
voltages. Get them with shields.