Scott Dorsey wrote:
Rob Reedijk wrote:
Agent 86 wrote:
On Wed, 07 Sep 2005 17:14:59 -0400, danger wrote:

Sorry I have about 15from 44a and 77a up and have never blown one please
explain to me how the dc is going to fly over the transformer windings.

Short answer: It takes a few milliseconds to rise from 0V
to +48V. The tranny won't recognize it as DC until it gets to +48 and
STAYS THERE. Meanwhile, your potentially very cool microphone has become
a paperweight.

I am going to get into touch with Doug Walker and see what he has to
say about this issue.

He'll tell you to lift the center tap on those few RCA mikes that have
them, and stop worrying so much.

Hey! I AM NOT the one who was worrying. I was one who was saying
it likely wasn't a problem!

In the meantime, it's the ribbon that gets cooked, right, not the
transformer? So in the end we are talking about a $100 mistake.
Not quite paperweight. Except mics like B&Os where the ribbons don't
exist anymore. (Actually I accidently phantom powered my B&O once
and nothing happened. But the mic was already connected so it
only would have been a factor if the output transformer was centre-
tapped.

Right, but a $100 mistake is still a lot of money for something silly
that could easily have been avoided. Especially if you're also out
of use of your favorite mike for a couple weeks while it's being fixed.
--scott

True. But I was just pointing out that this is not catastrophic.
It is a survivable error. In fact, I was saying, "stop worrying
so much!"

Rob R.

Mike Rivers wrote:

Rob Reedijk wrote:

I am going to get into touch with Doug Walker and see what he has to
say about this issue.

You don't believe me, or John, or Wes? Trust who you choose, but be
careful.

I believe lots of people including you for many things.

When it comes to microphones, I think Doug is at the top of the
list of experts. He is one of three guys (Wes is also) when it
comes to this in North America. The only problem is that now
that Stephen Paul is no longer with us, there is so much demand for
Doug's time in building and rebuilding condensor capsules that he
does not have as much time for ribbons.

Anyway, Doug has already sent me a response and I just am waiting for
one clarification and his permission to post it.

Rob R.

Been back to lurking mode for a while....& read thru a bunch of
responses on this thread. I'll throw in my 2 pennies of experience on
the RCA 77dx - I own and use 2 of them occasionally.

- I blew the ribbon on one when I thought phantom was off (it wasn't)
and plugged the cable on the mic end - turning the XLR until it clicked
in! My bad. These days I plug in the mic cable first - then go to the
mic pre to be sure the phantom switch is off before powering on the
pre. It's a habit kinda thing that works for me.

- If ribbons cost $100, replacing one might be a bit more. Clarence
Kane charged over $300 cuz his expertise is worth something. Then
there's the FEAR of shipping.

- I've used my 77's on Great River MP2-MH as well as GR NV. Both work
great. For the sources I usually mic, I can't get enuf gain out of my
Peavey VMP-2 to satisfy. But the OP (or others) might be interested in
trying their 77's on a Studio Projects VTB-1 mic preamp (cheap) but a
*suprisingly* good match for the 77 series. Personally, I keep the
"Tube Blend" very low or off. Learned about this one from The Pope.

===================
Note the new e-address:
Rich-at-
StolenMomentsAudio-dot-com
===================

Best...
Rich

with the value of 77's now days id put couple hundred into one any day
unless you want to sell it as-is

dnw


In article .com,

Been back to lurking mode for a while....& read thru a bunch of
responses on this thread. I'll throw in my 2 pennies of experience on
the RCA 77dx - I own and use 2 of them occasionally.

- I blew the ribbon on one when I thought phantom was off (it wasn't)
and plugged the cable on the mic end - turning the XLR until it clicked
in! My bad. These days I plug in the mic cable first - then go to the
mic pre to be sure the phantom switch is off before powering on the
pre. It's a habit kinda thing that works for me.

- If ribbons cost $100, replacing one might be a bit more. Clarence
Kane charged over $300 cuz his expertise is worth something. Then
there's the FEAR of shipping.

- I've used my 77's on Great River MP2-MH as well as GR NV. Both work
great. For the sources I usually mic, I can't get enuf gain out of my
Peavey VMP-2 to satisfy. But the OP (or others) might be interested in
trying their 77's on a Studio Projects VTB-1 mic preamp (cheap) but a
*suprisingly* good match for the 77 series. Personally, I keep the
"Tube Blend" very low or off. Learned about this one from The Pope.

===================
Note the new e-address:
Rich-at-
StolenMomentsAudio-dot-com
===================

Best...
Rich

Rob Reedijk wrote:
Mike Rivers wrote:

Rob Reedijk wrote:

I am going to get into touch with Doug Walker and see what he has to
say about this issue.

You don't believe me, or John, or Wes? Trust who you choose, but be
careful.

I believe lots of people including you for many things.

When it comes to microphones, I think Doug is at the top of the
list of experts. He is one of three guys (Wes is also) when it
comes to this in North America. The only problem is that now
that Stephen Paul is no longer with us, there is so much demand for
Doug's time in building and rebuilding condensor capsules that he
does not have as much time for ribbons.

Anyway, Doug has already sent me a response and I just am waiting for
one clarification and his permission to post it.

Rob R.

Below is the email that Doug sent me when I forwarded my questions to
him concerning worries about phantom powering ribbons. He appears to
follow a better-safe-than-sorry philosophy. I have also included my
follow up questions and his responses. I hope this is helpful:



(Doug's initial reply)
A ribbon can be badly stretched or destroyed when a current flows through
the secondary of the microphone's output transformer, and is then
inductively coupled to the primary side, causing a current to flow through
the ribbon. As you know, phantom power places 48 VDC on the audio pair (pin
2 and 3) with respect to ground (pin 1) through very closely matched
resitors. When phantom power is turned on, it is true that 48 volts will
appear at each terminal of the secondary of the transformer, but there
should be no potential difference between these terminals which would cause
a current to flow through the secondary. 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.

Regarding ribbon microphones and phantom power, I just reviewed the
schematics for the 44BX, 77DX, and the BK5. From this examination, there
seems to be no indication, for these particular microphones, that having
the phantom power on would cause a problem, unless there was a fault causing
a short to ground in the secondary side of the circuit. Then current could
flow through the secondary causing possible ribbon damage.

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. Some of my
clients use dedicated pre's for their ribbon mics, and these pre's either do
not have phantom power at all, or if the pre has phantom power, it is
permanently disabled.

(Rob clarifies the connecting hot to phantom fear)
The reason that some people are worried about a voltage imbalance between
pin 2 and pin 3 is if phantom power is already on, but the mic has not yet
been connected, they worry that since both pins may not connect at the
same time as the mic cable is hooked up to the mic there may be for a
brief moment 48v on one side before the other pin goes live.

Is this long enough for it to matter? Who knows.

(Doug's response)
In the situation described above, there would still need to be a current
path through the secondary to ground, and absent a grounded secondary centre
tap or a short somewhere in the secondary circuit to ground, the only thing
that I could think of that would cause a current flow in the secondary would
be the charging of stray capacitance within the secondary circuit to ground.
Depending on the values you assume for the equivalent circuit, we could be
talking about a pulse of a few milliamps, lasting for a fraction of a
microsecond. I doubt that this would cause much, if any, ribbon movement.

A greater concern when using ribbon microphones, I believe, is the likelihood
of a damaged cable, or a shorted or improperly wired connector, causing a
A greater concern when using ribbon microphones, I believe, is the likelihood
of a damaged cable, or a shorted or improperly wired connector, causing a
constant current flow in the secondary. This is much more likely to damage
the ribbon.

As I said earlier, it is prudent when using a ribbon microphone, to ensure
that phantom power is off, and never turned on.

(Rob realizes that the more he knows the less he knows)
One more stupid question.

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...

(Doug's response)
When I said that a current would flow in the primary, I certainly didn't
intend to imply that it would be a continuous current in one direction (as
in DC), but rather a momentary current, or pulse. A change from 0 volts to
some DC value applied to the secondary of a transformer won't induce a DC
current into the primary, but it can induce a momentary current flow in the
primary, the magnitude of which will depend on the rate of change and
magnitude of the current in the secondary, a host of other properties of
the transformer such as the turns ratio, and the impedance of the 'load' on
the primary, in this case, the ribbon.

(End of Doug's responses to Rob)

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?