Bleeding me softly (but quickly!)

[Ethan]

I've been playing with a switch-off bleeder (the bleeder resistor isn't in the circuit until switched "off"), and I can't come up with a clever way to bleed the voltage quickly without getting a decent *pop*. Using a higher resistance would solve the problem but then it takes longer to bleed. Anyone have any thoughts on how to go about this ASIDE from just using a permanent bleeder resistor in the circuit?

 

[Gus]

That is hard to do sometimes. The following is not what you asked for but something that might be of use.

You could add another very high value resistor between the switch terminals. The added high value would alway be in series with the bleeder you want to switch in.

Not perfect but helps to reduce pops. If you use this the lowest value that does not effect what every you are trying to do will give the smallest pop.

 

[PRR]

> a clever way to bleed the voltage quickly without getting a decent *pop*.

Depends on the circuit. Some audio circuits don't pop at power-down. Perfectly symmetrical push-pull won't, or not much. But in many simple circuits, say +24V rail, the internal point is at +12V when active, and has to get to zero V when the power goes away. That's a 12V change. If done quickly, it is a 12V audio signal, obviously much bigger than normal signals. Doesn't really matter if the bleeder is permanent or only at switch-off.

Of course it will POP if that turn-off bleeder returns to "ground" somewhere near the input.

Why do you need quick-bleed?

 

[Ethan]

Well I'm trying to modify this phantom supply so that it doesn't pop (or pops at a lower level) when switched off. By quickly I just mean <5 seconds.

Doesn't really matter if the bleeder is permanent or only at switch-off.

I was hoping for another way to do this but wouldn't permanently adding a resistor across the filter cap lessen the pop when switching off rather than having it break contact and switch to the bleeder resistor? The real annoyance I'm trying to get rid of is popping in adjacent channels.

 

[bcarso]

If your phantom circuits are perfectly balanced and you are feeding a non-powered mic then you should be pretty immune to phantom power rail transients. And departures from this balance may be alleviated by a slower but uniform ramp, which could be achieved in a variety of ways.

But if the mic itself is powered and you are powering down then its circuitry could be getting unhappy and generating lots of noise. Not much to be done about that. In these situations there is a tendency to think that if the voltage ramped more smoothly or slowly it would prevent the circuitry from making noise---but this is usually not the case. In fact the slower ramp may prolong the time the phantom-powered circuit spends in limbo, where it may be oscillating etc.

Another question would be whether the pop is really from the 48V changing or instead a prompt transient from a switch contact arc that is spraying r.f. around and that noise being detected and amplified in the preamp or other nearby circuits. You can figure this out by noting on a 'scope when the 48V is really moving, vs. when you hear the pop.