Ribbon Mic Blown the Hell Up!

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MikoKensington

Well-known member
Joined
Jun 3, 2004
Messages
1,371
Location
Detroit
I blew up my Okava ML52. I plugged into the wrong channel of my snake. Instead of plugging into my Ampex pre, I went into my board with phantom juicing away. There's no output at all.

My question is what kind of damage do you think I did? Severed ribbons? Fried Trafo? Who can help? :cry:
 
You know, I feel dumb asking this, but it is related. I was under the impression that transformers blocked DC, and therefore offered protection from this sort of problem. I have always adhered to the no-phantom-on-ribbons guideline(obviously for good reason), but am curious as to the mechanics(as it were) of this problem. After 10 years engineering and techin' I feel like I should know this, but damned if I don't! :oops:

Sorry Miko, I'm not trying to jack your thread.

Zach
 
We must have had a twenty page dialog on this exact same topic at the ol place.
I don't know if a solid conclusion was ever met.
Rafa, you got that thread backed up?
:thumb:
 
[quote author="SPG"]You know, I feel dumb asking this, but it is related. I was under the impression that transformers blocked DC, and therefore offered protection from this sort of problem. I have always adhered to the no-phantom-on-ribbons guideline(obviously for good reason), but am curious as to the mechanics(as it were) of this problem. After 10 years engineering and techin' I feel like I should know this, but damned if I don't! :oops:

Sorry Miko, I'm not trying to jack your thread.

Zach[/quote]

I believe that if the 48v applied to the secondary of a mic transformer was ramped up very slowly, you might be fine. Problem is that phantom is usually just switched on. The first impulse is passed by the transformer (I would believe). I would hazard a guess that the first pulse of 48V (or more/less depending on xformer ratios) would kill the ribbon.

Ian
 
I remenber that thread on the old place. Stephen Paul insisted in saying that it was IMPOSSIBLE to fry a ribbon mic with phantom. I will see if I can find it on my back-up files, but I don´t think I´ve saved it...

I´m still searching for a ramped phantom schem.
 
surely a transformer as we know them cannot pass DC ( disregarding applying thousands of volts to break down the insulation ). The thing to remember is that if you go from 0V to say +48V in a matter of micoseconds ( ie throwing a switch ) then you are doing more than applying DC, you are also applying a "cluster" of AC components ie all the odd harmonics from 1 Hz up. ( Do a spectal analysis of a DC pulse sometime ). These AC components will be passed by a transformer to varying degrees. As to wether they would kill a transformer coupled ribbon mic depends on lots of factors. Certainly if the +48V is coupled after the switch by a series resistor say 100R and a large cap say 220uF then the rise time will be slowed and should render the phantom power safe for a ribbon mic. The real danger lies with a ribbon mic that doesn't use a transformer and a short occurs between one leg and ground ( ie inserting a jack in a jackfield or a faulty cable ) then the 48V is applied directly across the ribbon. In fact the same applies if one leg is open circuit.
 
[quote author="kvintus"]
Do you have DC coming out of the wall?[/quote]

:green: love it! :thumb:

Any unbalanced connection will apply the 48V to the secondary, which will step down back to the primary as low voltage, but with colossal current. Just the sudden connection will kill it.

The 48V has to be applied across the coil though, so a properly wired phantom supply should still not kill the vast majority of ribbon mics.

Keith
 
On a similar subject, and in terms of trouble shooting, even putting a multimeter across the secondary can kill or at least stretch a ribbon. You can hear it go 'click'. So resist the temptation of checking your transformer for continuity (unless you have disconnected the ribbon element from the primary first).

The ML52 comes apart easiliy enough. You should be able to see if the ribbons are bust or not (I think this is a dual ribbon?). Check all the connections and wires. If they look fine & the ribbons are intact, carefully unsolder the xfo and test it.
 
If it makes you feel any better Miko, I have exactly the same mic, and a while ago I plugged it into the wrong patch point, and then sat comping tracks with an artist in the control room for an hour. She kept asking me where the "buzzing bee" sound was coming from, and I told her she was imagining things. Well, it was the Oktava ML52 on a mic stand behind her in the control room PLAYING THE MIX BUSS like a little speaker for over an hour. I couldn't hear it from where I was sitting, but it was driving her nuts.

I had only just bought it, so I coughed up blood for a while, but oddly it still works, and the parts of the ribbon I can see behind the soldered on shield look fine. Sounds the same too, even though it was loudly playing music in the room.

Spot the loser...

I do want to get it re-ribboned, anyone know who I can send it too? (Definately not my style of DIY).

So go on everyone, have a cheap little laugh at my expense.
 
Hi Miko,

Sorry to hear about your mic problem- the most probable fault is that the ribbon has either been broken or dislodged.

I do remember that thread in The Other Place, and I got caught up in it. If I remember correctly SP connected a 10A 48V PSU across the input to an RCA77 and left it on all day, therefore proving that ribbons are indestructable...

However, The counter to this was that most accidental connection of a ribbon mic to a source of phantom power is plugging the mics XLR into a phantom-powered mic socket. Assuming that the XLR cable is wired correctly, the +Ve voltage should be present at pins 2 and 3, and therefore an equal voltage is across the secondary. If the cable is wired incorrectly with pins 1 and 2 or 1 and 3 reversed, then this will pass a pulse of DC to the mic. This constitutes a fast-rising "step" of DC which will be coupled to the ribbon via the mics impedance matching transformer.

One assumption is that the DC is applied at exactly the same moment to both sides of the transformer secondary- but this may not be the case. Plugging an XLR plug into a socket at a slight angle will ensure that one side of the winding sees +48V before the other, and it may actually be the leakage capacitance from the momentarily open ended winding to the laminations of the transformer (which will be grounded to pin 1 a.k.a 0V for the phantom power) which allows a brief pulse to be passed from primary to secondary.

Two facts to prove this fault mechanism could kill a ribbon mic-

1. A Switched Mode Power supply chops DC (usually the rectified mains supply) and applies it at a high frequency to a transformer- this is essentially a square wave. Slow this right down, and observe one cycle of a squarewave from its lower crest (0V) to it's upper crest (+Ve) and this will pass through the transformer.

2. The second argument is that phantom power supplies such a small current through the 2 6k8 resistors (Thevenises to +48V supply in series with 3k4 resistor = 14mA full short. But don't forget the transformer which acts upon voltage also acts upon current...An electric arc welder is a good case. Most arc welders have a mains input going to a variable tapped (or adjustable cored) transformer which steps the voltage down to 12 or 24V. This is an extreme example, but shows the theory of voltage step down/current step up. If we look at a power transformer in the UK this would mean that for an input voltage of 240V and an input power of 240W, the input current in the primary is 1A. The transformer with a 24V secondary has a 10:1 step down ratio. Therefore (discounting losses in the windings- assume "ideal transformer"!) the power from the secondary must also be 240W, in other words the current available from the winding is stepped up. So 10A is theoretically available from the secondary.

Applying this to our ribbon mic, it's easy to see that even a limited current from the supply via the step up transformer from the ribbon (viewed from reverse to the phantom surge- a voltage step down/current step up.

As Stewart mentioned above, if you've ever accidentally connected a multimeter set to Ohms range to the secondary winding of a ribbon mic, you can here the ribbon "flap" about momentarily. And the peak voltage of most DMMs is 1.5V or 9V.

I know of no ribbon mic manufacturer who doesn't specifically state in their handbook to keep their product away from a phantom power source.

Just my £0.01's worth. Ignite blue touchpaper and retire...

Miko, if you're not keen on opening up your mic, get someone who's done it before to have a look- the ribbon may have just been displaced slightly. A visual inspection will soon tell what's wrong. Hope you sort it!

:thumb:

Mark
 
I have been using ribbon mics nearly every day for the last 2-3 years..I never care to switch ph-power on, or off..NEVER had a problem to this day..if your wireing is okay, and your mic is okay..everything should be just fine and dandy in my book. This is only seen from the "working day" practical side of the show...somebody might just find a "mind juice" way of judging on these matters..*GGG*

Kind regards

Peter
 
Hey Miko,

No worries, you have two possibilities--it is whether trafo or ribbon. Actually, both are for the best, as if it is the trafo, then the mic will greatly benefit from changing it for Lundahl; and if it is the ribbons.... then it is just fate to use some 1.5um foil.
Send it to me and I will check and fix it for you.
 
I cracked it open before I started this thead. The ribbon elements are well guarded, indeed. Only the points of attachment are in plain sight. They may be broken at the middle from some hefty excursion. The transformer does not look to be center-tapped. At least, I'm guessing because there is only 4 leads. Great machining on this mic case. Some new guts would probably make it rock. I'll PM you Marik.
 
Hi Rafa

I´m still searching for a ramped phantom schem.

http://www.welbornelabs.com/ps3.htm

This PSU will ramp according to the values of R104 & R105. The standard values in the .pdf give a rise of 15sec or so, altho' this was to +250v.

I have a PCB layout if you want.

Peter
 
A transformer won't pass DC. That includes the transformer-coupled 300 Watt Ampeg SVG head that blew the cones out of my Kustom speaker cabinet.

Obviously a transformer can pass a heck of a thump, and a big enough thump will always blow dynamic transducers. No difference whether sold for speakers or microphones, except mikes die with smaller thumps.

If the connection is made properly, both "hot" wires get the same voltage and the mike sees zero voltage, no thump.

If the mike's internal transformer is ungrounded, even a one-pin connection "should not" pass any thump into the mike. Since all recent ribbons are ungrounded, it is "usually safe".

Ramping the Phantom helps only if the ramp is MUCH slower than the transformer. Maybe 10 seconds. And that still does not help if you plug-in "live".

I don't know how Miko blew-up a reasonably modern ribbon. It is possible the mike was fine after Phantom, and blown-up by panicked multi-metering. Also possible it got on an OUTput. While a ribbon may survive a weak line-level audio signal, this is clearly a bad thing.

Back in RCA's day, there was no Phantom or other dangerous voltage on mike lines. There was however often a 50KW AM radio transmitter in the building. RCA ribbons usually have a grounded center-tap transformer to knock-down some of the radio waves beating on the mike lines. If one pin hits before the other, there is a 8mA "thump" in the transformer primary. The 1:40 transformer steps this up to a 320mA thump in the mike. The ribbon moves. How much depends on many factors. In modern studio work, RCA ribbons should probably be de-grounded, or grounded through a 0.1uFd cap if RF is an issue.

> a ribbon mic that doesn't use a transformer and a short occurs between one leg and ground

Shorting one leg to ground IS a big risk (and probably the reason no ribbon-maker wants to hear about their mikes on Phantom power).

I don't think there are any ribbon mikes with no transformer. But that makes little difference. If the transformer has good bass response, then it will pass a large thump and that is all it takes to jump a ribbon right off its mounts.
 
Holy crap! :shock: I feel like I've been hit over the head with the learnin' stick! In a good way, of course. I didn't think my question would generate that kind of response. Thanks for the info everybody, and sorry again to Miko for jackin' his thread.

Zach
 
There isn't any thread jacking going on here. This is a community that questions and a community that answers.

I'm really beginning to doubt that it was phantom that killed my mic. I do recall some funny behavior from it before it died. Perhaps the phantom put it over the edge?

I had a "Phantom Loaf" today. I chuckled.
 
>> A transformer won't pass DC.

> under normal operating conditions.

I stick to my claim. Transformers don't pass DC. (Unless they have gone bad, interwinding short; unlikely but not impossible.)

Transformers pass THUMPS. The damage, if any, is done in around 10 milliSeconds. By definition, the bottom of the audio band. (Because the transformer passes audio, but not sub-audio. The exact definition of "audio" is up to the transformer designer, but "DC" is not a possibility.)

> According to Royer; ...a serious voltage spike can blow the ribbon...

Note: Royer's word "spike", not "DC". Spike, thump, call it what you will: it is both. I'll sketch some pictures to show. I feel that the "thump" aspect is what blows the ribbon. Royer is mostly trying to stop you from having Phantom accidents, and "spike" may be more frightening which is what he wants. But since we may already have had a Phantom accident, I think it is important to see what really happens, to clarify diagnosis and possible preventions.

> according to Eddie Ciletti in Mix Magazine; "...phantom power is invisible to dynamic and ribbon mics. However, if pin-1 and pin-2 (or pin-1 and pin-3) are reversed ?...? 48 volts would be applied either across the mic's transformer or across the capsule itself."

Here it is possible to confuse dynamic with ribbon. Many dynamics have no transformer, so the balanced (or UN-balanced) Phantom is applied across the capsule. Some dynamics and all ribbons have a transformer: the transformer is subject to 48V DC, but what passes to the capsule is audio band only.

Look, I need to get to bed but will return to this when I can draw you a picture. Meantime, it might be helpful if someone has pix of a low-Z transformer passing square waves of 20Hz and 20KHz. From pix of normal transformer action, we can derive what happens in the normal and abnormal Phantom situations.

BTW: all this makes me think ribbon mikes SHOULD all "be Phantom". Not that they really need the first amplifier stage moved up into the mike, but because having a phantom power stage inside the case isolates the ribbon from the now ubiquitous always-on Phantom supply.
 
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