Motor Boat Noise Issue - LDC

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GraemeWoller

Well-known member
Joined
Mar 6, 2017
Messages
143
Location
New Plymouth, New Zealand
So, I have this weird noise that one of my LDCs has been doing for a while now. It sounds like a strange motor boat sort of oscillation that seems to stop when the mic has been powered for a while, but occasionally comes back later...

The crazy stuff you can see on the spectrogram appears to be coming from my Element 88 and doesn't seem to be the mic, but the other noise is a bit of a pain in the arse.

Attached is the spectrogram & here's a link to the audio: https://goo.gl/LydDvX ... Just ignore the background sounds...

Anyone got any ideas because I don't?  ???
 

Attachments

  • Mic %22A%22 Spectrogram - Small.jpg
    Mic %22A%22 Spectrogram - Small.jpg
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Hi Ruud,

I'd rather not say who made it because I feel like I've been wrapped across the knuckles by GroupDIYers for "complaining" about small manufacturers.  That's not ever the case, but it's been perceived that way. I don't mean that to sound obtuse, I'm just after an unbiased conversation about the specific issue.  :)

The circuit is a KM84 circuit being driven by a edge terminated capsule.

Have you ever come across a sound like that?
 
Well if it's only in that mic then it sounds like a dodgy cap. Check that there aren't any dry solder joints sometimes just putting the solder iron on all the solders points to reset the solder is enough, but usually that type of noise means dried out filter cap. Unless you got it near s fan or something ;-)
 
Thanks Timjag,

Damn, I guess I should take it out of the air conditioner then... LOL.

Yeah, I wondered if it was a capacitor somewhere doing some sort of weird charge/discharge thing, but I thought it might be good to come to the experts. I'll check that out and see what I come up with. :)
 
My first idea would be a fluctuating polarisation voltage.
Sometimes it is caused by the DC/DC converter, but since this is a KM84 circuit, this can't be the case.
And yes, a defective capacitor can also cause this.
 
Not quite sure if / how related this is to your issue, but a couple years ago i bought a Sennheiser e914 as faulty, the issue being it going "tick" periodically, about 1-2 times a second, on the first of the two high-pass settings. After lifting the schematic and identifying the potential culprit (an SMD cap, can't recall if film or ceramic), i measured it, replaced with a similar-value styroflex, and it's been working just fine ever since.
 
Righto,

So, I've had it on +48 overnight, it stopped motor-boating after about 20 min or so and didn't seem to start up again by the morning. I've still gone thru and touched an iron to each of the solder joints just to be sure, plugged her back in and everything still seems to be running nicely, which is brilliant.

An interesting side effect of this has been that the noise floor of the circuit has dropped... It still needs a good isopropyl clean to get rid of the flux (it's always been dirty since I got her), but I can't put my finger on why the noise floor would've dropped.

I'm not complaining AT ALL, I'm super-happy if the noise floor is lower!  8)

My next question about the circuit, if you don't mind, is C1 is specified at 470pf but the cap that's in there is marked as 392. What effect will a difference like have on the circuit and the sound?

Cheers for all your great help!
 
Just so you know, "C1" is quite meaningless, it's just a component designation, with NO connection to its role in the schematic ;)

But judging by its value, it's the coupling cap between the capsule and the JFET gate, i'd reckon?

In conjunction with the 1G gate resistor, 470pF would give a 0.34Hz (-3dB) cut-off frequency for the resulting high-pass.

3.9nF (39 and two 0's, ie. 3900pF) is interestingly large a value, i don't recall seeing or hearing of anything over 1nF in that position. That being said, it definitely won't harm anything, although whatever interactions there may be between that, and the input capacitance of the JFET, is up for debate.

But then again, with only a "name" and a value to go by, this is all just pure speculation...  ::)
 
Oh god, yeah sorry!

I was supposed to attach the schematic but my six year old was right in my ears and I completely forgot to do it!!

You're right, it's the coupling cap with cap and a 1G resistor on one side and a 1G and the FET on the other side.

I'll attach the schematic and a photo of the cap in question too.

Sigh, the simple things make a sane person look crazy. ;)
 

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  • KM84 - Circuit Diagram.pdf
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Well, on the bright side, it IS a film cap (unlike the ceramic you usually see), but other than that, i wouldn't worry.
 
Well, I've had the mic on the bench cold for 12 hours or so. Just plugged it in and she's running quiet as, so that's quite lovely!!

So far it seems that the soldering iron touch-up job has been the winner on the day, at best guess. I just need to give it an isopropyl massage and she'll be as clean as the day she was hatched.

Thanks so much for all the great suggestions! I'll keep an eye on it and hopefully you'll never hear from me again about this particular mic!!  ;D
 
Good news! - the noise floor has gone down because you got a good solid connection now.

Someone recently asked me to service  a brand new Chinese SDC that they bought off eBay (didn't want to send it back and wait) it was noisy and kept cutting out, when I opened it none of the components were soldered in just held in place by some flux  - I'd never seen anything like it like the builder had soldered it in and then desoldered with an desolder pump, then put it back in the body. I'm guessing he cocked it up, desoldered, then went to lunch came back forgot and put it back together.

Anyhow a quick go round with an iron and fresh solder and she worked like a dream.
 
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