Re: Ribbon Mic - RCA 77D - worth messing with????


Rob Reedijk wrote:
> (Doug's initial reply)
> I note the comments below, but I
> am not clear on why there would be a voltage imbalance between pins 2 and 3,
> even momentarily. I suppose it could be argued that a capacitance exists
> between each audio lead and ground, and that if this capacitance was not
> exactly the same between each lead and ground, an imbalance could occur
> while the RC circuit is charging, but I think this would be a reach.

As long as the cable doesn't have a short between one of the signal
pins and pin 1, or the mic doesn't have a grounded center tap, it's
usually safe to turn on phantom power once the mic is connected. As
Doug suggests, if there is an imbalance between the voltage applied to
the two signal pins, it would be momentary and very small (assuming the
phantom power supply is properly designed and one of the resistors in
series with the phantom supply voltage hasn't changed value) and would
likely not damage the mic. However, there is greater risk (as I
mentioned in another message, I think) if the mic is plugged in to a
source of phantom power. If the two signal pins don't make contact at
the same time, current can flow until the second pin contacts the
socket.

> Having said this, it seems to me that the prudent thing to do when using a
> ribbon microphone is to ensure that phantom power is off.

I agree. If it's necessary to turn it on (for instance the mic is
connected to a mixer with a single phantom power switch and other mics
need power) the prudent thing is to turn it off until the ribbon mic is
plugged in, and then turn it on.


> (Rob realizes that the more he knows the less he knows)
> > I guess I should have asked this earlier. I thought a transformer blocks
> > DC. But I guess it doesn't, right? Seeing as if you can have DC across
> > the secondary, you are saying that this will induce DC across the the
> > primary. Can I just clarify this? I guess it shows how little I know...

It blocks DC, but it passes AC (obviously - otherwise the microphone
wouldn't work). In reality, you don't have DC unless it's been on since
the beginning of time and is never turned off. So when you go from 0V
to 48V, until the voltage stabilizes at 48V, you have AC, and that's
what goes through the transformer.

Incidentally, Doug's useage of "primary" and "secondary" could be a
little confusing. Conventially, the input side of a transformer is
considered the primary. So, as he uses the term "primary" it's relative
to the microphone operating as a microphone. The windings connected to
the ribbon are indeed the primary. But when explaining what happens
when the mic is connected to some voltage souce, the OTHER winding
becomes the primary. Are you properly confused yet?

So what are you going to do?

.

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