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Author Topic: KM86 capsules not quite right. Options?  (Read 5658 times)

BobSchwenkler

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KM86 capsules not quite right. Options?
« on: June 08, 2011, 03:48:46 am »

I have a pair of KM86s that I've had for a while now. They sound fantastic, but at the same time there's always been a little something missing from their sound. I borrowed another 86 from the studio I manage and did a comparison against one of my mics (my pair sound very similar to each other). The result is that the studio's 86 sounds better. It sounds like I feel like my mics should and can sound like. After some swapping I determined that the sonic differences followed the capsules.

The main things I'm noticing in my mic compared to the borrowed mic are a less present high end and a more closed, pinched, or nasally mid/upper midrange.

A possibly relevant tidbit: My capsules have gathered a little bit of what looks like it might be deteriorated foam of some sort. It looks like it might possibly have come from the foam bit that holds the capsule assembly against the top of the head basket, but I couldn't really be sure about that. The screen over the capsule looks fine enough to keep out any of what I'm seeing, but perhaps not?

How possible is it that what I'm hearing will be resolved by having the capsules cleaned? How possible is it that what I'm hearing is the result of unrepairable stress or damage to the diaphragms? Can I get a recommended first course of action?

While I'm asking questions, I've got one more quick one. The studio's mic has a blue dot at the base and mine have a red dot. The studio's is a later serial number than my pair and there are some noticeable parts and construction differences. Do the dots just signify different revisions? Something else?

Thanks!

David Satz

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Re: KM86 capsules not quite right. Options?
« Reply #1 on: June 08, 2011, 08:42:50 am »

The colored dots refer to the way the microphone's output circuit was set up at the factory. The output transformers of most Neumann "fet 80"-series microphones have secondary windings that can be wired either in series (the default for microphones sold in Europe), with a 200-Ohm output impedance, or in parallel, giving a 50-Ohm output impedance.

The 200-Ohm setting gives the highest output levels and is often preferred today. To my knowledge the microphones set up this way never carried any special paint marking. The 50-Ohm setting was/is sometimes preferred on the other hand because it has a 6 dB lower output voltage for a given level of sound pressure, which reduces the risk of overloading the preamp, mixer or recorder to which the microphone is connected. (The noise level of the microphone is also 6 dB lower, so the s/n ratio stays the same unless you have an unusually noisy preamp.) The 50-Ohm setting also makes the output circuit less subject to cable capacitance, thus avoiding high frequency losses and decreased high-frequency headroom. Personally this is the option I prefer, but I'm in something of a minority here.

Along with the parallel (50-Ohm) winding arrangement there is a further option to let a pair of built-in, ca. 50-Ohm resistors become part of the circuit. Doing so reduces the output level even further, while raising the output impedance to 150 Ohms. That arrangement was chosen by Gotham Audio (exclusive U.S. distributor for Neumann microphones at that time) as the default for microphones that they imported, and those microphones were marked with a dot of colored paint. In the U.S. many professional consoles still had microphone inputs that had been designed for ribbon or other dynamic microphones; the output levels of condenser microphones were typically 20 dB higher, which could overload those inputs. Also, some transformer-equipped input circuits needed the microphone to have an output impedance in the 150-200 Ohm range or the resulting high-frequency response would either peak or roll off. These problems still exist with some equipment, though less commonly nowadays.

So there are three possible settings: 50, 150 and 200 Ohms. From an "objective engineering" standpoint, however, the 150-Ohm setup is a rather poor choice. For best immunity to interference and lowest losses in the cable, including frequency-dependent losses due to capacitance, the output impedance of the microphone should be as low as possible. That would argue, as I see it, for the 50-Ohm setting unless you have a preamp that deviates from flat response with so low an impedance. In that (relatively rare) situation, a well-matched pair of 50-Ohm series resistors built into an adapter at the input of the preamp would be far better than using the resistors in the microphone, and the resulting impedance of 150 Ohms should bring the frequency response back to flat.

I can never remember which color of paint dot is which, since the different manufacturers had different color codes, so maybe Klaus or someone else here can fill that information in. If I try to do it, I will probably get it wrong. But many Neumann microphones sold by Gotham with the 150-Ohm setup were later reset by users to 200 Ohms while the paint dot was left undisturbed, so it isn't a reliable guide anyway.

--best regards
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radiovinhet

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Re: KM86 capsules not quite right. Options?
« Reply #2 on: June 08, 2011, 03:05:06 pm »

"""How possible is it that what I'm hearing will be resolved by having the capsules cleaned? How possible is it that what I'm hearing is the result of unrepairable stress or damage to the diaphragms? Can I get a recommended first course of action?""

My advice: don't try to clean the capsules.
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klaus

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Re: KM86 capsules not quite right. Options?
« Reply #3 on: June 08, 2011, 03:15:35 pm »

As David said, 50-Ohm versions of Neumann FET80 mics were identified with a colored dot next to the serial number. The dot changed from red to light blue/turquoise around 1980, but the function of the dot, regardless of color, stayed the same: an identifier for a special version of the mic with reduced output.

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Klaus Heyne
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BobSchwenkler

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Re: KM86 capsules not quite right. Options?
« Reply #4 on: June 08, 2011, 06:14:33 pm »

"""How possible is it that what I'm hearing will be resolved by having the capsules cleaned? How possible is it that what I'm hearing is the result of unrepairable stress or damage to the diaphragms? Can I get a recommended first course of action?""

My advice: don't try to clean the capsules.

At all? I'm certainly not going to do it myself if that's what you're suggesting I not do.

klaus

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Re: KM86 capsules not quite right. Options?
« Reply #5 on: June 08, 2011, 08:06:59 pm »

Speaking from experience, I doubt that the cleaning of the capsule elements alone would change the timbre of the mic to your satisfaction.

If indeed the bad sounds follows the capsules, the next thing to do is make a front/rear side analysis: set the mic to figure eight, and speak into the front, then rear (phase of your headphone channel reversed, of course during rear test.) If you find a more pleasant timbre in the rear, you can always reverse the two capsule heads. Then test again (with the new front side) in cardioid. If the timbre remains unpleasant, stronger medicine is called for: either a re-calibration of the two capsules, or a replacement of same.
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Klaus Heyne
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BobSchwenkler

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Re: KM86 capsules not quite right. Options?
« Reply #6 on: June 08, 2011, 08:20:59 pm »

I'll do some more listening to both sides, but I did a little bit of a test a while back and recall everything sounding pretty similar.

If I do swap capsules from front to back, do the pins that hold them to the capsule stand/mount simply pull out or is there some other trick to it?

Would Neumann/Sennheiser be able to recalibrate the capsules or is this something I'd need to find a 3rd party tech to do?

klaus

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Re: KM86 capsules not quite right. Options?
« Reply #7 on: June 09, 2011, 12:44:23 pm »

To my knowledge, Neumann/Sennheiser USA does not perform component-level calibration or tuning. The KK86 capsule heads are screwed in to their place. By unscrewing each one ccw, the whole unit, including contact needle, will come out , just as in a KK83, KK84, or KK85.
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Klaus Heyne
German Masterworks®
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BobSchwenkler

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Re: KM86 capsules not quite right. Options?
« Reply #8 on: June 09, 2011, 01:47:03 pm »

I see. I couldn't quite tell and didn't want to just start twisting capsules. Thanks everyone, for your input.

sheltsound

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Re: KM86 capsules not quite right. Options?
« Reply #9 on: July 28, 2011, 01:35:34 am »

I'm puzzled by klaus' statement that when testing a figure eight pattern on a condenser mic that the rear capsule would be out of phase necessitating a headphone phase reversal. I can understand that would happen on a ribbon mic as the ribbon would be moving in the opposite direction when addressed from behind creating a phase reversal but not on a condenser.... a positive pressure on either capsule would create the same shift in capacitance...no???

Steve Addabbo
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klaus

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Re: KM86 capsules not quite right. Options?
« Reply #10 on: July 28, 2011, 02:20:54 am »

In omni, both backplates receive polarization voltage.
In figure eight, the polarization voltage for the rear capsule (and consequently its audio polarity) is reversed: instead of the backplate, the diaphragm is polarized.
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Klaus Heyne
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sheltsound

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Re: KM86 capsules not quite right. Options?
« Reply #11 on: July 28, 2011, 11:16:29 pm »

I love to use figure 8 when tracking two backing vocalists together...I should have some reverse phase headphones for the occasion..This bit of knowledge had passed me by..thank you Klaus

Steve Addabbo
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klaus

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Re: KM86 capsules not quite right. Options?
« Reply #12 on: July 29, 2011, 01:54:10 pm »

All you need to do to get polarity reversal for an in-polarity rear-capsule experience: grab a pair of most AKG headphones- 240s in particular.

AKG does not support the standard headphone polarity, as adopted by Sennheiser, Audio-Technica, Beyer etc. but has delivered many of its most popular studio headphones with reverse polarity for decades.

This is to be understood as a joke, of course. The proper way to listen in-polarity to the rear of any dual diaphragm mic: flip the polarity switch on your mic pre.
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Klaus Heyne
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Randyman...

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Re: KM86 capsules not quite right. Options?
« Reply #13 on: July 31, 2011, 09:38:51 pm »

You could also use a dedicated HP feed to the "out of phase" vocalist and invert the phase on that output (or mult the mic to a second input channel, and flip the polarity there, then feed that inverted channel to the second HP Mix).  I do this in software (each HP mix has a dedicated output channel with full channel-strip functionality), but sounds like you might be monitoring through an analog path (I'm jealous!)...

AKG seems to follow JBL's "inverted" polarity for loudspeakers (positive voltage on a JBL driver's "+" terminal results in negative pressure/rarefaction).  Never understood why they chose that standard???

 8)
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Randy Visentine
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BobSchwenkler

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Re: KM86 capsules not quite right. Options?
« Reply #14 on: August 26, 2011, 02:43:37 pm »

As a follow up to the possibility of re-calibration for these capsules, can anyone direct me toward techs who perform this sort of work (aside of course, from Klaus, who I may give a call to but who also seems to be very booked)?
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