Phantom power current through input transformer?
Renee Rober
I have a Neutrik NTM4 input transformer sitting around. It has
1 to 4 ratio and the primary winding is center tapped.
Can I feed the phantom power to the center tap? I expect that
the current splits into two parts. Therefore the two magnetic
fields should cancel each other.
Is this _a_ way to go? I don't like using two matched 6k8
resistors when I have a center-tapped transformer. Could the
primary windings stand the current to supply an ordinary
microphone - say my TLM103?
thanks in advance
Renee
--
Renee Rober
Wikingerstraße 4
D-24143 Kiel
Scott Dorsey
>A little tech question:
>
>I have a Neutrik NTM4 input transformer sitting around. It has
>1 to 4 ratio and the primary winding is center tapped.
>
>Can I feed the phantom power to the center tap? I expect that
>the current splits into two parts. Therefore the two magnetic
>fields should cancel each other.
This depends on how well the two halves are matched, and what kind of
DC offset the transformer can tolerate.
With the Manley transformers, there is no problem doing this, because
they are bifilar wound and have very good matching.
With the old G-series UTC transformers, there is no problem either,
because although the two legs aren't perfectly matched, the core is
huge and lossy and so it never saturates.
With the Neutrik, it should be listed on the data sheet what the
winding matching tolerances and allowable DC offset are.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Wddbl
supply feeding the centertap. This will limit the current thus protecting the
supply and the transformer should anything get shorted out.
Regards,
Doug
>Subject: Phantom power current through input transformer?
>From: Renee Rober re...@hobo.toppoint.de
>Date: 2/1/00 4:52 PM Eastern Standard Time
>Message-id: <389755B7...@hobo.toppoint.de>
>
>A little tech question:
>
>I have a Neutrik NTM4 input transformer sitting around. It has
>1 to 4 ratio and the primary winding is center tapped.
>
>Can I feed the phantom power to the center tap? I expect that
>the current splits into two parts. Therefore the two magnetic
>fields should cancel each other.
>
>Is this _a_ way to go? I don't like using two matched 6k8
>resistors when I have a center-tapped transformer. Could the
>primary windings stand the current to supply an ordinary
>microphone - say my TLM103?
>
>thanks in advance
>Renee
>-- =
>
>Renee Rober
>Wikingerstra=DFe 4
>D-24143 Kiel
>
>
>
Georg Ritter
do you know any good reasons why to use a transformer?
I did some measurements (with AP-SystemI) and found the transformer I tested
to have pretty bad results (even it was no a al-cheapo one). A discrete pre amp
from dbx based on a SSM2013 had much better performance in all parameters.
Especially the CMR (what I thought was the strongest argument for using a
transformer)
I cannot imagine, that LL, Jensen or any state of the art transformers have a
much better performance (didn't test them). Perhapse the CMR is a bit better
because capacitive coupling could be better damped by tricky shieldings.
Greetings,
Georg
Scott Dorsey wrote:
>
> Renee Rober <re...@hobo.toppoint.de> wrote:
> >A little tech question:
> >
> >I have a Neutrik NTM4 input transformer sitting around. It has
> >1 to 4 ratio and the primary winding is center tapped.
> >
> >Can I feed the phantom power to the center tap? I expect that
> >the current splits into two parts. Therefore the two magnetic
> >fields should cancel each other.
>
Scott Dorsey
Better common mode rejection, better RF noise rejection, and (in some cases
but not necessarily) better noise performance. There was a very good paper
from Marshall Leach in the JAES a decade ago about noise performance in
transformer and active front ends.
>I did some measurements (with AP-SystemI) and found the transformer I tested
>to have pretty bad results (even it was no a al-cheapo one). A discrete pre amp
>from dbx based on a SSM2013 had much better performance in all parameters.
>Especially the CMR (what I thought was the strongest argument for using a
>transformer)
Maybe you should be using a better transformer. The reason why most of
the manufacturers have gone to active input stages is because good transformers
are very expensive and still not all that wonderful.
>I cannot imagine, that LL, Jensen or any state of the art transformers have a
>much better performance (didn't test them). Perhapse the CMR is a bit better
>because capacitive coupling could be better damped by tricky shieldings.
CMRR should be a lot better, and don't forget interwinding capacitance issues.
Personally, I don't like transformers, and I would rather do without them,
but occasionally they turn out to be necessary. The best sounding preamp
I ever heard was transformerless, but I have heard some transformer input
units that were close.
Arny Krüger
"Georg Ritter" <georg....@uibk.ac.at> wrote in message
news:38984519...@uibk.ac.at...
> Hi,
>
> do you know any good reasons why to use a transformer?
>
transformer I tested
> to have pretty bad results (even it was no a al-cheapo one). A
discrete pre amp
> from dbx based on a SSM2013 had much better performance in all
parameters.
> Especially the CMR (what I thought was the strongest argument for
using a
> transformer)
>
transformers have a
> much better performance (didn't test them). Perhapse the CMR is a
bit better
> because capacitive coupling could be better damped by tricky
shieldings.
The place where transformers shine w/r/t common mode rejection is in
terms of dynamic range.
With many if not most op-amp balanced input designs, the common mode
voltage is not summed out until the second stage. If the common mode
voltage causes clipping in either of the first stages, then the
common mode rejection goes down the drain. There can be second-order
effects as well.
I recently had a case where a relatively high common mode 60 Hz AC
voltage from a balanced output to a balanced input caused audible
hum. This was a situation where part of the system was on one leg of
house 60 Hz power, and the load was on another. The solution was to
run an extension cord from the AC power source that served the load
(an amplifier) to the source equipment. There was still obviously
some common-mode voltage, but it was within the common-mode dynamic
range of the equipment.
You don't often see common mode dynamic range specs do you? For
op-amp type equipment it will probably be less than 10 or 15 volts
AC. If the input stages have gain, far less. For transformers, it is
usually set by the voltage limits of the insulation between the
windings, usually at least 220 volts.
schuyler
either blow a fuse or roast your transformer (or both). +48 volts goes
to the transformer winding on the mic. input side, -48 volts goes to the
shield/ground. Stated another way: Phantom + is connected to both high
and low inputs, and phantom - is connected to the shield.
Schuyler
schuyler
subjective thing, though mainly in terms of limiting bandwidth a bit...
Anything less than a great transformer will roll of the high end and/or low
end, diminish transient response, sometimes introducing gradual resonant
'bumps' in some portion of the spectrum too... Make sure your impedance's
are matched okay and you'll minimize such adverse qualities, or just don't
use transformers. I usually only employ transformers in PA applications
where RF or other types of AC interference is a problem. Some folks swear by
them though. I've often wondered as to whether there isn't some pleasing
effect that results from hysteresis in some situations, which would make
them a good choice for subjectively improving recordings where you're going
to a digital medium...
Schuyler
Monte P McGuire
Galvanic isolation and sometimes the sound.
>I did some measurements (with AP-SystemI) and found the transformer I
>tested to have pretty bad results (even it was no a al-cheapo one). A
>discrete pre amp from dbx based on a SSM2013 had much better
>performance in all parameters. Especially the CMR (what I thought was
>the strongest argument for using a transformer)
Cheap transformers will sound bad and perform poorly... Good ones
however are really quite clean as long as you don't overload them.
>I cannot imagine, that LL, Jensen or any state of the art transformers
>have a much better performance (didn't test them). Perhapse the CMR is
>a bit better because capacitive coupling could be better damped by
>tricky shieldings.
Well, they do perform better. A good transformer's distortion will
fall with rising frequency and will be around the 1ppm level by 500Hz
or 1KHz. It should not be measureable above 1KHz or so. I have
verified this on my own AP2 and found that the good Jensens and some
older UTC transformers are really quite clean. They do distort at low
frequencies, especially at high levels, but over the rest of the band,
they can be extremely clean.
As for shielding, a good transformer will also have better shields and
this could improve CMRR as well.
The final reason for using a transformer is for the sound. The sound
of iron is low order distortion only at low frequencies, along with
possibly an HF bump and a slight highpass effect. Some people like
that on certain signals as it 'beefs it up' a bit. If you want
absolute cleanliness, transformers are only useful if you're fighting
ground problems and need the isolation. If not, a high quality
transformerless circuit will be cleaner. Sometimes though, engineers
don't want a clean sound and transformers can help to add dirt.
Regards,
Monte McGuire
mcg...@world.std.com
Monte P McGuire
Renee Rober <re...@hobo.toppoint.de> wrote:
>I have a Neutrik NTM4 input transformer sitting around. It has
>1 to 4 ratio and the primary winding is center tapped.
>
>Can I feed the phantom power to the center tap? I expect that
>the current splits into two parts. Therefore the two magnetic
>fields should cancel each other.
They will cancel, but not at the moment when you plug the mike in.
Pin 2 or pin 3 will connect first and the mike will get charged up
through only one half of the transformer, probably magnetizing the
core. The same thing could happen if you unplug a mike that's still
powered.
If you _always_ plug a mike in when phantom is off and vice versa,
then it's safe. However, sooner or later, someone will hot plug a
mike and your transformer will get zapped.
Hot plugging a preamp with a transformer and two 6K8 resistors is not
completely safe either, but the unbalanced current should be a bit
lower than if you use the center tap only.
>Is this _a_ way to go? I don't like using two matched 6k8
>resistors when I have a center-tapped transformer. Could the
>primary windings stand the current to supply an ordinary
>microphone - say my TLM103?
Stick with the pair of 6k8 resistors and you'll be safer. It's not
hard to match resistors these days...
Regards,
Monte McGuire
mcg...@world.std.com
John W. Hardy
> do you know any good reasons
> why to use a transformer?
>
> I did some measurements (with AP-SystemI)
> and found the transformer I tested
> to have pretty bad results (even it was
> no a al-cheapo one). A discrete pre amp
> from dbx based on a SSM2013 had much better
> performance in all parameters.
> Especially the CMR (what I thought was
> the strongest argument for using a
> transformer)
>
> I cannot imagine, that LL, Jensen or any
> state of the art transformers have a
> much better performance (didn't test them).
> Perhapse the CMR is a bit better
> because capacitive coupling could be
> better damped by tricky shieldings.
You should test the Jensen JT-16-B mic-input
transformer. It is their best mic-input model, and it
is the one that I use in all of my mic preamps, and the
results are, in my opinion, excellent. Please do not
base your opinion of ALL mic-input transformers on your
experience with just one model. What model was it that
you tested?
John Hardy
schuyler
misread your post originally... Sorry.
Use the 6.8k resistors, carefully matching them, to be on the safe side. It
won't hurt the performance of the circuit and will reduce the possibility of
an imbalance in the transformer, or a chance of burning up the winding if
the phantom supply gets shorted across the input. Resistors are cheap. Good
transformers aren't, so the current limiting that results from using the
resistors are a good choice from this angle.
Schuyler
Scott Dorsey
>Georg Ritter wrote:
>> do you know any good reasons why to use a transformer?
>
>night when I was trying to use a Zefiro InBox (a miniaturized
>preamp and converter that is not too awful) with a pair of
>phantom-powered microphones. The preamp does not have
>its own phantom powering built in, so I brought an outboard
>supply, but I got no signal back from the microphones at all.
>
>With a tone generator I verified that the cables were OK and
>that the preamp itself was working. But as soon as I turned
>on the phantom power, the mike preamp generated nothing
>but noise. Fortunately there was someone else there who
>was able to take over and make some kind of recording.
>
>It turns out that this (*&^& little preamp not only does not
>have phantom powering, it also can't _tolerate_ phantom
>powering at its inputs. It apparently has no adequate form
>of DC blocking at all, neither capacitors nor transformers.
>As a result the +48 V, in common mode, saturated its input
>stage and blocked all the useful signal from being amplified.
Your problem is not with the preamp, but the phantom supply, which
ought to have blocking capacitors in it to prevent this. The preamp's
behaviour is completely normal for an electronically balanced input.
Rag on the manufacturer of the supply (and put some caps in there).
Scott Dorsey
>This is interesting... I've noticed problems with transformer secondaries
>not being loaded right at the root of freq response problems in a preamp I
>was building once. Eventually I figured it out... Transformers used to be
>everywhere in audio and telephony, but now that their applications are
>more esoteric. Somewhat archaic, albeit simple stuff like this is not so
>well known by the average electronics guy like me. :-)
Yes. In AC circuits class they tell you that the transformer has a fixed
ratio of impedances and that whatever impedance is presented to one side
is reflected to the other in proportion to that ratio. In E-mag class you
find out that's a hasty generalization and that real-world transformers
only work on a small range of load impedances.
Better transformers can handle a wider range of load impedances without
ringing. The old UTC output transformers needed a termination resistor
going into a high-Z load, but the modern Jensens don't care so much.
David Satz
>It turns out that this (*&^& little preamp not only does not
>have phantom powering, it also can't _tolerate_ phantom
>powering at its inputs. It apparently has no adequate form
>of DC blocking at all, neither capacitors nor transformers.
>As a result the +48 V, in common mode, saturated its input
>stage and blocked all the useful signal from being amplified.
Scott Dorsey wrote:
> Your problem is not with the preamp, but the phantom supply, which
> ought to have blocking capacitors in it to prevent this. The preamp's
> behaviour is completely normal for an electronically balanced input.
> Rag on the manufacturer of the supply (and put some caps in there).
I strongly disagree, just for the record. An outboard phantom power
supply doesn't have to block DC at its output; it's legit if it does, and
legit if it doesn't, as long as the user knows which way it is. I don't
think it's legitimate for a "balanced input" to fail when this most
common, standard microphone powering system is applied.
Most of my preamps have either blocking capacitors at their inputs
(because they themselves supply phantom powering) or they have
input transformers. If I have to use an external phantom supply I
don't want two DC blocking components in series on each channel.
--best regards
Ben Bradley
>In article <389755B7...@hobo.toppoint.de>,
>Renee Rober <re...@hobo.toppoint.de> wrote:
>>I have a Neutrik NTM4 input transformer sitting around. It has
>>1 to 4 ratio and the primary winding is center tapped.
Do you have some specs on it (specifically DC resistance of the
primary)? I fount this <http://www.neutrik.com/Transformer.htm>, but
there isn't nearly as much as Jensen provides. I'm assuming the
Neutrik has a DC resistance value similar to Jensen's. There's no good
reason to believe otherwise.
>>Can I feed the phantom power to the center tap? I expect that
>>the current splits into two parts. Therefore the two magnetic
>>fields should cancel each other.
>
>They will cancel, but not at the moment when you plug the mike in.
>Pin 2 or pin 3 will connect first and the mike will get charged up
>through only one half of the transformer, probably magnetizing the
>core. The same thing could happen if you unplug a mike that's still
>powered.
>
>If you _always_ plug a mike in when phantom is off and vice versa,
>then it's safe. However, sooner or later, someone will hot plug a
>mike and your transformer will get zapped.
>
>Hot plugging a preamp with a transformer and two 6K8 resistors is not
>completely safe either, but the unbalanced current should be a bit
>lower than if you use the center tap only.
It seems that if there's a possibility of DC going into the primary
in either case, then either one could magnetize the core. And for sake
of argument, in the center-tap-powered case, the current is only going
through half the winding to a 3.3k resistor (assuming you do what I
describe below), instead of through the whole winding to a 6.8k
resistor, so to me it looks like a wash.
>>Is this _a_ way to go? I don't like using two matched 6k8
>>resistors when I have a center-tapped transformer. Could the
>>primary windings stand the current to supply an ordinary
>>microphone - say my TLM103?
>
>Stick with the pair of 6k8 resistors and you'll be safer. It's not
>hard to match resistors these days...
Gee, it almost seems the transformer is made for this, and since a
'hot plugged' mic would magnetize the core either way, it seems that
centertap powering (properly, through a resistor as I describe below)
might be a good way to go.
I just got the JT-16A data sheet from the Jensen Transformers site,
and as I suspected, the DC resistance of these windings is Really Low
(20 to 200 ohms). You absolutely do need a resistance between the
+48V and the center-tap to properly use the transformer without
damaging something expensive, such as any phantom-powered microphone
you might plug into it.
If you power through the center tap, put a 3300 ohm resistor in
between it and +48V. This will give virtually the same current into
the mic as will 6800 ohm resistors from +48v to each of the signal
lines.
OTOH, I don't see any transformers with center tapped input
windings at the Jensen site
<http://www.jensen-transformers.com/mic_in.html> (though I wasn't
exhaustive). The JT-16-A and JT-16-B (pdf files) are shown with
schematics that have no phantom power, but the JT-MP-CA does, and it
shows two 6.8k resistors from the primary terminals to a switched +48V
source. So at least you know this works.
-----
http://listen.to/benbradley
To reply by email, press the 'reply by email' button on your newsreader.
Monte P McGuire
Ulysses <mors...@tc.umn.edu> wrote:
> I've got some military-spec Triad
>transformers that, despite being relatively compact, have signifigantly
>better frequency & phase response than some decent transformerless preamps
>I have. They can't handle extremely high levels of low frequencies
>without distortion, but that distortion sounds good.
Which model is this? Iron hounds need to know!!! ;-)
I know that Triad's HS series is pretty nice and pretty compact
too... is this the series you were using? I once tried an HS-11,
which is 600:~80K, as a substitute for the Rolls RP220 input tranny.
While it sounded nice, the ratio was too high and it simply had too
much gain for my uses. It sounded sweet though...
Regards,
Monte McGuire
mcg...@world.std.com
Mike Rivers
In article <u8WZcQdb$GA.70@cpmsnbbsa02> DS...@msn.com writes:
> It turns out that this (*&^& little preamp not only does not
> have phantom powering, it also can't _tolerate_ phantom
> powering at its inputs.
> I wish I had known this would occur. I have a pair of Jensen
> 1:1 transformers mounted in a little steel box--Jensen actually
> sells this as a product, the ISO-MAX model MS-2XX. When
> I got home from the failed recording I recreated the circuit
> with those transformers in the line--and everything worked.
You need to learn the Rules of Remote Recording:
#1 - Learn where the nearest Radio Shack is
#2 - Don't go on a remote without a brownie (the cake kind) - it might
be all you get to eat for the whole night.
#3 - Don't leave home without your transformers
--
I'm really Mike Rivers (mri...@d-and-d.com)
Ulysses
> Most of my preamps have either blocking capacitors at their inputs
> (because they themselves supply phantom powering) or they have
> input transformers. If I have to use an external phantom supply I
> don't want two DC blocking components in series on each channel.
I ran into a similar problem once and I came to the (possibly erroneous)
conclusion that the input transformers on the preamps were at fault. If
the +48 is going into the preamp transformer, and the center tap on that
transformer was grounded, wouldn't that suck all the +48 away from the
mic? It seems unfair to blame the preamp for this, since it may have been
built before phantom power was invented. That's why I side with Scott in
blaming the phantom supply.
By the way, the preamp in question in my case was a Neve 33114 or 33115 in
a console. It's possible that whoever installed the console grounded pin
1 somewhere between the studio and the console, since (for the above
reason) Neve would not be likely to ground the center tap on the inpuit
primary...right?
Justin Ulysses Morse
Roll Music Studios
Minneapolis, MN
(612)379-3255
www.rollmusic.com
Scott Dorsey
>
>> Your problem is not with the preamp, but the phantom supply, which
>> ought to have blocking capacitors in it to prevent this. The preamp's
>> behaviour is completely normal for an electronically balanced input.
>> Rag on the manufacturer of the supply (and put some caps in there).
>
>I strongly disagree, just for the record. An outboard phantom power
>supply doesn't have to block DC at its output; it's legit if it does, and
>legit if it doesn't, as long as the user knows which way it is. I don't
>think it's legitimate for a "balanced input" to fail when this most
>common, standard microphone powering system is applied.
>
>(because they themselves supply phantom powering) or they have
>input transformers. If I have to use an external phantom supply I
>don't want two DC blocking components in series on each channel.
Blocking caps on the input are bad, because they are additional junk in
the signal path that doesn't need to be there in normal operation with
a microphone signal. The phantom supply should be putting a microphone
signal out on the output, not a microphone signal with 48V of DC offset.
But I bet there is an AES standard for this. Josephson, care to comment?
Georg Ritter
> ...
>
> You should test the Jensen JT-16-B mic-input
> transformer. It is their best mic-input model, and it
> is the one that I use in all of my mic preamps, and the
> results are, in my opinion, excellent. Please do not
> base your opinion of ALL mic-input transformers on your
> experience with just one model. What model was it that
> you tested?
It was Experience electronics E-1420. A 1:2+2 transformer.
With specs: <10...>45kHz -0.5dB, 0.01 % THD. (@+6dBm, and
30KOhm sek.)
Greetings,
Georg
Georg Ritter
does Jensen have a webpage? Anyone knows the link?
Greetings,
Georg
Dave Matthews
http://www.jensen-transformers.com
The Jensen web site is a fantastic resource of information, and the
Jensen transformers are excellent, wonderful, great. I have nearly
completed my homebrew preamp which uses a JT-16-A input transformer with
a JT-11-DM output transformer (and OLI-3 output isolators, thanks
Dale!). Sounds much better to me than the transformerless version I
built first. I have decided that I am one of those who likes iron.
Read the application notes, they are full of great information. I have
also had the pleasure of receiving an education from the Jensen
technical support regarding the proper use of their transformers, and
transformers in general. They are extremely helpful and knowledgeable
people.
Thanks for requesting the link, it gave me to opportunity to rave about
Jensen!
--
Dave Matthews, ( no, not *that* one! )
http://www.lostfrogs.com
David Satz
> conclusion that the input transformers on the preamps were at fault. If
> the +48 is going into the preamp transformer, and the center tap on that
> transformer was grounded, wouldn't that suck all the +48 away from the
> mic? It seems unfair to blame the preamp for this, since it may have been
> built before phantom power was invented. That's why I side with Scott in
> blaming the phantom supply.
You're right that grounded center taps would short out phantom
powering. These have to be lifted where they are found. In my
case there were no input transformers, however.
--Best regards
P.S.: to save bandwidth, I'll say here that I completely agree with
Mike Rivers' three rules of remote recording. And it's not just
remote recording--RCA Studios in NYC used to be across 6th
Avenue from a Radio Shack, and I'm embarrassed to tell you
how many times a session was rescued by something we had
to run across to the Shack to pick up.
David Satz
> Blocking caps on the input are bad, because they are additional junk in
> the signal path that doesn't need to be there in normal operation with
> a microphone signal.
We agree completely about that. It's for that precise reason, I think,
that we disagree about what follows.
> The phantom supply should be putting a microphone
> signal out on the output, not a microphone signal with
> 48V of DC offset.
Umm, how do you expect a phantom supply to block the DC
voltage without using either a capacitor or a transformer in its
output leads?
We just agreed that having more of that stuff than you need is
a Bad Thing. And most preamps already have one or the other
of those blocking devices built in. Therefore, I say it is not a bad
thing, and is quite possibly a very good thing, for a phantom
supply _not_ to have a transformer or capacitor on its outputs.
Otherwise, the engineer is forced to put two such devices in
series with one another.
The worst case is when the preamp has input capacitors that
are conventional, polarized electrolytics, which (if DC is being
blocked before it reaches them) may be left without a bias across
them. This greatly increases their small-signal distortion levels.
Just for clarity: in the recording incident I described, I hold
_myself_ responsible above all for not having tested the preamp
beforehand to see how it would react to the presence of a phantom
voltage. Only after blaming myself do I then turn around and
blame Zefiro/Denecke for the preamp's shoddy behavior.
> But I bet there is an AES standard for this.
No, I don't believe that there is any such standard, but
would agree that if anyone here knows best, it's David J.
I always welcome his comments--he's one of the voices
of sanity on the newsgroup. (Not that sanity is the only
thing to be valued here, but I prefer that there be at least
a thin layer of sanity that can be reached occasionally.)
John W. Hardy
Jensen has two web addresses:
www.jensentransformers.com
www.jensen-transformers.com
Look there for data on their JT-16-A or JT-16-B mic
input transformers (their best mic-input models, same
coil but different packages), the JT-11-BM (or
JT-11-BMCF) line output transformers, and (if they show
it) the JT-11-BMQ. These JT-11 models are their best
line output models, and all of the above transformers
are probably MUCH better than the one you were testing.
Jensen has .pdf files on many interesting things. I
believe that Jensen makes the absolute best audio
transformers, and I use them exclusively. Thank you.
John Hardy
John W. Hardy
> I have nearly
> completed my homebrew preamp which uses a JT-16-A
> input transformer with a JT-11-DM output
> transformer (and OLI-3 output isolators, thanks Dale!).
HEY - that sounds alot like my M-1 mic preamp!
> I have decided that I am one of those who likes iron.
Just to nit-pick: The JT-16-A and the JT-11-BM have a
core that is 80% nickel, not the more common 97% iron
that many other companies use. I know it is an old
saying, "iron", but think "nickel"!
John Hardy
Scott Dorsey
>P.S.: to save bandwidth, I'll say here that I completely agree with
>Mike Rivers' three rules of remote recording. And it's not just
>remote recording--RCA Studios in NYC used to be across 6th
>Avenue from a Radio Shack, and I'm embarrassed to tell you
>how many times a session was rescued by something we had
>to run across to the Shack to pick up.
I carry 600:600 transformers, 600:10k transformers, and also ferroresonant
isolation transformers (little baby 60W ones) in the big field kit. You
never know when you'll need them. And I don't even like transformers.
Dave Matthews
>
> Dave;
>
> > I have nearly
> > completed my homebrew preamp which uses a JT-16-A
> > input transformer with a JT-11-DM output
> > transformer (and OLI-3 output isolators, thanks Dale!).
>
> HEY - that sounds alot like my M-1 mic preamp!
In transformer description only I'm sure! I've had the pleasure to
experience the quality of your preamps at a local studio last year, that
was one of the things that started me on this quest for what makes that
kinda sound. My homebrew has progressed in stages of improvement over
the years, and it will probably be a number of years more before it can
hold a candle to the likes of yours. But I'm doing it for the fun and
education, next time I find an artist with great songs and great talent
I will invest in something along the calibre of the M-1!
> > I have decided that I am one of those who likes iron.
>
> Just to nit-pick: The JT-16-A and the JT-11-BM have a
> core that is 80% nickel, not the more common 97% iron
> that many other companies use. I know it is an old
> saying, "iron", but think "nickel"!
Thank you for that...it will add a little interest for me in future
conversations to use that, so know I have decided that I am one of those
who likes nickle. hee!
John W. Hardy
Best of luck with your on-going home-brew mic preamp
project! I'm sure it will be educational, rewarding,
and somewhat insane.
John Hardy
Geoff Tanner
mors...@tc.umn.edu (Ulysses) wrote:
> In article <#UFF26pb$GA.241@cpmsnbbsa02>, "David Satz"
<DS...@msn.com> wrote:
>
> > Most of my preamps have either blocking capacitors at their inputs
> > (because they themselves supply phantom powering) or they have
> > input transformers. If I have to use an external phantom supply I
> > don't want two DC blocking components in series on each channel.
>
erroneous)
> conclusion that the input transformers on the preamps were at fault.
If
> the +48 is going into the preamp transformer, and the center tap on
that
> transformer was grounded, wouldn't that suck all the +48 away from the
> mic? It seems unfair to blame the preamp for this, since it may have
been
> built before phantom power was invented. That's why I side with
Scott in
> blaming the phantom supply.
>
33115 in
> a console. It's possible that whoever installed the console grounded
pin
> 1 somewhere between the studio and the console, since (for the above
> reason) Neve would not be likely to ground the center tap on the
inpuit
> primary...right?
>
> Justin Ulysses Morse
> Roll Music Studios
> Minneapolis, MN
> (612)379-3255
> www.rollmusic.com
Hi Justin
The 33114 and 33115 Channel amplifiers use the same 10468 input
transformer as virtually all the other Neve Channel Amplifiers. There
is no provision to ground the centre tap of that transformer nor any
reason at all for doing so. . . it defeats the whole concept of the
transformer's isolation from ground!
I had to fit 48v phantom power resistors to an RCA tube mic pre a while
ago and that has the centre tap of the input transformer grounded. I
got around that by fitting a Neve transformer in front of it, wired for
1:1 and then fitting the phantom power resistors to that. It worked OK,
with a nice square wave, etc.
--
Geoff Tanner
phoeni...@earthlink.net
http://www.phoenixaudio.com
phone: 323 462 4373
Sent via Deja.com http://www.deja.com/
Before you buy.
Ulysses
<phoeni...@earthlink.net> wrote:
> The 33114 and 33115 Channel amplifiers use the same 10468 input
> transformer as virtually all the other Neve Channel Amplifiers. There
> is no provision to ground the centre tap of that transformer nor any
> reason at all for doing so. . . it defeats the whole concept of the
> transformer's isolation from ground!
I figured as much. This was at a college radio station which was probably
installed by somebody who thought it would be a good idea to install a big
ground buss and connect all the shields to it.
> I had to fit 48v phantom power resistors to an RCA tube mic pre a while
> ago and that has the centre tap of the input transformer grounded. I
> got around that by fitting a Neve transformer in front of it, wired for
> 1:1 and then fitting the phantom power resistors to that. It worked OK,
> with a nice square wave, etc.
Why not just list the ground on the original transformer's CT?
By the way, you said the 10468 input transformers were used on "virtually
all the Neve channel amplifiers." Do you mean all of them from that time
period, or did they use the same transformers as the old stuff (and new
stuff)?
Geoff Tanner
mors...@tc.umn.edu (Ulysses) wrote:
Hi Justin
> In article <87g6qs$agb$1...@nnrp1.deja.com>, Geoff Tanner
> <phoeni...@earthlink.net> wrote:
>
> > The 33114 and 33115 Channel amplifiers use the same 10468 input
> > transformer as virtually all the other Neve Channel Amplifiers.
There
> > is no provision to ground the centre tap of that transformer nor any
> > reason at all for doing so. . . it defeats the whole concept of the
> > transformer's isolation from ground!
>
> I figured as much. This was at a college radio station which was
probably
> installed by somebody who thought it would be a good idea to install
a big
> ground buss and connect all the shields to it.
>
Sounds like another of those "I know better than those tech's at Neve"
types!
> > I had to fit 48v phantom power resistors to an RCA tube mic pre a
while
> > ago and that has the centre tap of the input transformer grounded. I
> > got around that by fitting a Neve transformer in front of it, wired
for
> > 1:1 and then fitting the phantom power resistors to that. It worked
OK,
> > with a nice square wave, etc.
>
> Why not just list the ground on the original transformer's CT?
>
Because the transformer formed the input bias for the grid of the first
tube and was not happy with my lifting the ground on the other winding.
I didn't want to mess with the RCA circuit so stuck a 1:1 in front of
it!
> By the way, you said the 10468 input transformers were used
on "virtually
> all the Neve channel amplifiers." Do you mean all of them from that
time
> period, or did they use the same transformers as the old stuff (and
new
> stuff)?
The 10468 hung around at Neve from its introduction from Marinair C1968
to the mid 70's whereby the Belclere mu-metal can type started to creep
in. You'll find these in the 31102 of the 8058/68 albeit the 8078's
31105 retained the 10468 because the BA382 motherboard was also used on
the earlier 1091, 1093 and 1095 channel amplifiers.
So, generally, you will find the 10468 in practically all the 10**
channel amplifiers, most of the 35mm channel amplifiers (33114,5,6,7
etc), and most of the line amplifiers 1272, 3402, 3405, 33415, etc.
I was not all together impressed with the Belclere's as they seemed to
need a lot of "frigging" compared to their predecessors which were just
plonked in and got on with their job! If you look at the Belclere line
input transformer (the 8078 has a few!) you'll note that the normal
10Kohm input Z configuration you would expect with a 31267 is replaced
by a Belclere where you have to load the secondary at twice the regular
impedance (which reflects twice the impedance over to the primary) and
then 20Kohm is wired in parallel to the primary to bring the impedance
back to 10Kohm!
For a while, the Belclere mic input transformers fitted to 54 series
consoles suffered horrendous microphony problems, especially when the
stepped gain switch was turned. Again, the fix was unusual in that the
transformer was mounted upside down in a custom metal cradle (requiring
that leads be soldered to its pcb pins and extended to reach the pcb)
and then the transformer was suspended in foam. I have never seen that
done on a 10468 but, to be fair, they were not mounted so closely to
the input switch as the transformer on the 54.
These are all problems relating to a period around 15 + years ago and
the Belclere and Willesden (?) transformers fitted to current
equipment (33609, etc) are a long way down the road from that era. You
can probably imagine my puzzlement though, when new transformers were
produced, that the consoles were modified to suit the transformers
rather than the other way around!
BOB URZ
David Satz wrote:
> I wrote:
>
> >It turns out that this (*&^& little preamp not only does not
> >have phantom powering, it also can't _tolerate_ phantom
> >powering at its inputs. It apparently has no adequate form
> >of DC blocking at all, neither capacitors nor transformers.
> >As a result the +48 V, in common mode, saturated its input
> >stage and blocked all the useful signal from being amplified.
>
> Scott Dorsey wrote:
>
> > Your problem is not with the preamp, but the phantom supply, which
> > ought to have blocking capacitors in it to prevent this. The preamp's
> > behaviour is completely normal for an electronically balanced input.
> > Rag on the manufacturer of the supply (and put some caps in there).
>
> I strongly disagree, just for the record. An outboard phantom power
> supply doesn't have to block DC at its output; it's legit if it does, and
> legit if it doesn't, as long as the user knows which way it is. I don't
> think it's legitimate for a "balanced input" to fail when this most
> common, standard microphone powering system is applied.
>
> Most of my preamps have either blocking capacitors at their inputs
> (because they themselves supply phantom powering) or they have
> input transformers. If I have to use an external phantom supply I
> don't want two DC blocking components in series on each channel.
>
Caution is the word here. I had a high Dollar 4 channel battery operated
mixer a while back that would fail with external
battery 48 volt phantom applied to its inputs. Opened it up, it was
electronically balanced and HAD input caps on both the +,- op amp inputs. It
also had 25 volt breakdown voltage on the N.P. caps. The caps turned into
short circuits with 48 volt Phantom on them. Replaced the caps with 50 volt
units and all was fine.
BOB
>
> --best regards
David Satz
> Caution is the word here. I had a high Dollar 4 channel battery operated
> mixer a while back that would fail with external
> battery 48 volt phantom applied to its inputs. Opened it up, it was
> electronically balanced and HAD input caps on both the +,- op amp inputs.
It
> also had 25 volt breakdown voltage on the N.P. caps. The caps turned into
> short circuits with 48 volt Phantom on them. Replaced the caps with 50
volt
> units and all was fine.
Thanks for the note, and your point is well taken. I don't have a
schematic for the mike preamp/converter unit (Zefiro "InBox"),
but it is easy to open up, so I'll have another look at the circuit.
By the way, the preamp does not seem to have suffered any
harm from this incident. I used it on a recording yesterday
afternoon; it sounded fine.
Denecke, who actually manufactures this preamp/converter,
also sells a (rather nice) 48 V phantom power supply that runs
off of a single 9V battery. It has capacitors at the outputs to
block the phantom voltage from reaching a preamp, and it's
clear that they made the two pieces to work together. So
another option is to pay the extra bucks and buy their supply
to use with their preamp.
But I still think that the phantom powering belongs in a
preamp, and that if a preamp doesn't provide phantom
powering, it should at least be able to function with it.