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Author Topic: KM84 First Generation - Low Output  (Read 1527 times)

NVS

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KM84 First Generation - Low Output
« on: October 09, 2022, 01:16:01 PM »

Does anyone have any knowledge/comments on this mic? It only says KM84 us 62, no serial, unless us 62 is the serial. It doesn't have a -10dB switch, but it has an XLR, not Tuchel connector. It has a fairy low output compared to a newer KMi 84 that I have, but when I engage the -10dB switch on the newer one, they sound very similar. I swapped the capsules on them, but the output on the old one was still low, so I assume the capsule is ok. Does anyone know is the old ones had a lower output than the newer ones?

Thanks
Narve Vik-Strandli
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Kai

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Re: Old KM84
« Reply #1 on: October 10, 2022, 05:43:39 AM »

According to the catalog Neumann Archive offers, there’s an early version without the -10 dB switch mentioned.
It’s the first one up left:
https://en-de.neumann.com/file-finder?product=KM%2084&category=historical&area=84

It has a 6 dB lower output of 0,5 mV / µbar (= 5 mV / PA) compared to the later version’s 10 mV / Pa.

µbar (Micro-Bar) is the old unit for (sound-)pressure, before today’s Pa (Pascal) was used.
10 µbar = 1 Pa.

The use of the old unit even shows the catalog refers to an earlier version.
If you don’t get noise from the mic compared to the newer one, everything is OK.
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klaus

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Re: Old KM84
« Reply #2 on: October 10, 2022, 09:36:13 AM »

Looking at the schematic documentation for the first edition KM84 (930-0, 1967), I see how the circuitry in your mic would account for modestly lower output, compared to all later versions: feedback C2 was 4.7pf, whereas later versions (those with -10dB switch) used 3.3pf.

The transformer of your mic did not yet have the option for switching between 50Ω and 200Ω. It also lacked the pads that were introduced in the next version (with -10dB switch).

P.S.: I corrected my earlier version of this post after discovering new information in an official schematic/data sheet for the first generation KM84.
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Klaus Heyne
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NVS

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Re: KM84 First Generation - Low Output
« Reply #3 on: October 13, 2022, 03:13:39 PM »

My apologies for my late reply, life happens every day.

Thank you very much for your excellent answers, they explained a lot.

However, I was wrong on one thing. The first testing of the mics, like I described above, was done quickly, and I obviously did not test well enough. Turns out that the mic in question, has a very poor low end. It’s not the capsule, because swapping capsules didn’t help. There’s something going on in the electronics of the mic. I have heard that old tantalum capacitors can go bad when they get old, and was thinking maybe that could be the reason, but I do not know if this is true or not. If I’m not mistaking, the rest of the circuit is just poly and film caps, resistors and the transformer, which rarely fail, or am I wrong?

I have been unable to find the schematic for this mic, and there’s no value written on the Tantalum caps.

The mic has a BV10 transformer, by the way.

Thanks
Narve Vik-Strandli
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klaus

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Re: KM84 First Generation - Low Output
« Reply #4 on: October 13, 2022, 06:07:58 PM »

Lift one leg of each of the two Tantalums, and measure capacitance: the coupling cap should read around 1mfd, the source cap 4.7mfd. If lower by 20% or more, replace with a modern polarized electrolytic like Nichicon or Panasonic with a 50V minimum rating.
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Klaus Heyne
German Masterworks®
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NVS

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Re: KM84 First Generation - Low Output
« Reply #5 on: October 13, 2022, 06:42:35 PM »

Excellent, Klaus, I will look at it this weekend and post a reply as soon as I know more.

Thanks
Narve Vik-Strandli
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NVS

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Re: KM84 First Generation - Low Output
« Reply #6 on: May 30, 2023, 03:03:25 PM »

Hello again.

Well, this took a while! I have just finished servicing this mic, turns out it had a bad transformer.

Found someone to rewind it and that worked, but the work they did was a little sloppy, so one of the leads came loose when I installed it in the mic (I tested it before cutting the wires to size).

Since the whole thing had taken so long, I was a little out of time, so I decided to try the BV107 copy made by Moby Transformers in Serbia. This transformer is really high quality and the mic now sounds great.

I also recapped the mic with electrolytics, as you recomended, Klaus.

Thank you very much for the help!

Narve

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klaus

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Re: KM84 First Generation - Low Output
« Reply #7 on: May 30, 2023, 10:14:32 PM »

Your photo of the inside finally revealed the cause of the low output:
Change the feedback capacitor from currently 7pf to the standard 3.3pf (3pf or 4pf if you cannot find that exact value).
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Klaus Heyne
German Masterworks®
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NVS

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Re: KM84 First Generation - Low Output
« Reply #8 on: June 01, 2023, 10:55:53 AM »

Ok, I will look into this, but the mic dosen’t really have a low output anymore. It really did have a bad transformer. It sounds full and good, now. I did not immediately hear any difference in level when quickly comparing it with my newer fully functional KMi84, though the meters told me it was slightly lower, but very very little.

By the way, what can cause transformers like this to fail? I've heard that phantom power can destroy the secondary side if a mic is connected with the phantom power already on - and that this can cause large DC spikes. And, that a bad coupling capacitor can destroy the primary side - but I guess this is most common in a tube mic...?
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klaus

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Re: KM84 First Generation - Low Output
« Reply #9 on: June 01, 2023, 12:50:05 PM »

You will be down ~ 3dB with the 7pf NFB.

This is the first time I have heard of a Neumann transformer malfunctioning due to oversupply of phantom power (whether momentary, spike or constant). One of the central features of Phantom Power is its ability to hot plug any condenser mic without fear of destroying components. So you are fine there.

Regarding turning Phantom Power off when plugging in your mic, before back on: consider that Phantom Power in many pres or boards cannot easily and quickly be activated for individual channels. Some boards have no switching for that at all.

Why do transformers fail? I will let Bryan Sours, a prominent boutique transformer designer, answer that.
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Klaus Heyne
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Kai

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Re: KM84 First Generation - Low Output
« Reply #10 on: June 01, 2023, 05:45:44 PM »

Most transformer failures (besides overload of power transformers) are caused by oxidization, usually close to the solder joints.
Even strains due to temperature variations do their part.

Phantom power doesn’t supply enough current to harm an audio transformer.
Specially as the relevant windings (secondary in the mic, primary in the preamp) are built from thicker wire due to their lower 200 Ohm impedance.
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gtoledo3

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Re: KM84 First Generation - Low Output
« Reply #11 on: June 01, 2023, 08:59:45 PM »

If shield and primary don’t connect to the 48 volts at the same time, or if one leg of the line connects before the other, it would seem as though it could cause problems in some cases.
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bryansours

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Re: KM84 First Generation - Low Output
« Reply #12 on: June 01, 2023, 10:37:23 PM »

Hey all,
Klaus asked me to pop in and kick some thoughts around about how that KM84 transformer might have failed.  At my shop we predominantly do paper layer winding with the exception of a few bobbin wound designs, but I've torn down enough BV-type transformers (as used in Neumann mics) to have a good feel for what makes them special and how they're likely to fail.

Phantom power generally shouldn't be an issue with them as Klaus mentioned.  One of the functions of the floating secondary is to isolate external dc voltage systems from the primary side preamp circuit, power supply, and capsule. 

DC Voltage on the secondary winding (with no leakage to ground and no tap/CT to ground) is effectively unseen by the transformer.  The DC is on the winding, but with no path to ground. 

But DC current is a different story. If one side of the secondary sees any path to ground, the P48 voltage present will draw DC current through the secondary winding. It’s current limited by the 6800Ω usually present in the preamp (not always 6800Ω unfortunately), but because the wire in the secondary is so small (doesn't need to be big - its AC current spec is minuscule), even small levels of DC current can cause the winding to go open. 

Plug in a cable with a connector pinned incorrectly, or frayed wire shorting a pin, and it is possible to eventually cause an open coil in the transformer secondary with Phantom Power.  It won’t happen right away, and it probably won’t happen with a properly designed preamp, but it is possible given all the wrong circumstances. 

You would most likely hear that issue happening as well: Current limited P48 (6800Ω) is something like 7mA DCI which would draw through the secondary of the OT, which would bump the flux density in the core up, and with the high permeability Permalloy laminations in there, your saturation point is very very low. That means you would surely hear your low end turn to garbage if there was standing current in the secondary.

I agree also that voltage spikes are to be avoided (with all microphones).
These types of transformers (high permeability core, high inductance, low leakage inductance) couple very low and very high frequency information between the primary and secondary, so although the primary-secondary isolation aspect of the design is there, the spike transient can reflect back to the primary via the turns ratio, which is relatively high in these types of transformers. 

For this reason there's usually an RC ramp on the Phantom Power switch of mic preamps. The failure mode in this sort of case is most likely going to be primary-to-secondary breakdown. Tesafilm, Mylar, or polyester (tape mostly - haven't seen anyone use sheet yet) was generally used as inter-winding insulation in these, so the voltage breakdown rating is really high (that was their intention along with low LL I'm sure), but repeated spikes could breakdown the material (and the wire insulation). This becomes exasperated by the primary to secondary capacitance, since the interwinding insulation is so thin there’s a lot of C between the two sections.  May not cause a failure the first time, or even the 20th time, but eventually is could break down.  Klaus- that’s basically how your Marshall 1974X OT failed (corona burst through the polyester tape).  Poly insulation isn’t perfect and it can (and does) fail.

In my opinion, the biggest source of failure in this type of transformer is physical. You're random-winding insanely tiny wire (42-45 awg) on a bobbin usually made of PET or (modern builds) nylon. If you've ever grabbed a thicker sheet of PET it's really sharp on the edge - and brittle.  Even nylon bobbins can have sharp edges, and with how small that wire is, even the slightest burr on the bobbin can cause a break eventually. 

With that smaller wire, winding tension is absolutely critical as well. If your entrance angle to the bobbin isn't just right, tension will increase more than expected as the winding builds up, and by the end the strain on the wire is much higher than when you started. If you've got a hard edge under the wire at the starting layer, you've now got even more downward pressure on it. Then add a secondary on top of that primary winding. Problem compounded.  The output transformer has nested leads in it, so wire build up will be irregular and this decreases production consistency in the transformers, in addition to the random winding. That means some transformers might eventually have a problem. 

When it gets installed at the factory the leads get moved around and probably yanked a bit, this likely filters out most of the output transformers with immediate manufacturing defects, so any that will exhibit physical problems will be long out of a warranty period. 
Additionally these transformers typically aren't vacuum pressure impregnated (there's exceptions to that, I believe), so wire does move internally despite a very tight wind. Combined with the potential issues involved with winding it in the first place, a wire break can easily happen when the microphone is of an older age (they're just like us!).

Long post barely scratching the surface on this stuff, but fun to wonder how the thing broke.  Glad you fixed it and can get back to making music with it!
-Bryan Sours // Soursound

Because of the topic's relevance and Bryan Sour's excellent, detailed explanations I will make this post sticky.

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