Tube Amps / Music Electronics
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|previous: john b "but heard from people here the rea... -- 6/29/2003 10:59 AM||View Thread|
|6/29/2003 9:01 PM|
|Steve A.||Re: relay popping|
The diode goes right across the relay coil contacts. Here's the text from an old post by 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.
And here's the text from a post by RE:
First off, if you input impedances are high enough I would couple
both preamps together on the input side and select the output of
either stage. This will eliminate the input side switch being amplified
through the preamp(s). If you have bleed problems from a
distortion stage short the output of the first stage of that channel to
ground, after the coupling cap. Make sure you have coupling caps
& ground ref. resistors on the outputs you are switching. You want
to eliminate any dc potential shifts when switching between
channels. This goes for the output side of the switch also. Capacitor
couple and use a ground resistor to drain the cap. on the switch
side. If you must switch the inputs also, capacitor couple them the
same way. Any capacitor that may be switched must have a dc
path to ground or it will build a charge that will give you a pop
when switched. Also on the relay itself - add a reverse-baised
diode and a forward biased (charged) capacitor across the relay
coil to reduce spikes. I use a 22uF cap. on 5V relays in most
situatioons. Use a well regulated voltage to the relay and switch the
relay coil to ground. With a 5V 50 ohm relay coil you can draw
enough current across the footswitch to light up an LED without
closing the relay contacts. Then when you short the footswitch the
LED goes out and the relay energizes. All this with just two wires
to the footswitch. Neat trick eh?
Hope this helps.
|john b thanks for the info steve,|