Low end rumble after replacing the stock capsule of an MXL V69ME

[fripholm]

Hello everybody,

after finishing my U87 clone and replacing the 67-type capsule (which micandmod.com wrongly sent to me) with a real 87-type from microphone-parts.com I wanted to swap out the stock capsule of my MXL V69 with the remaining 67-type.

Installation was no problem but now there's this excessive amount of low end "noise" in the output of the mic - see attached frequency plot. Prior to installing the new capsule the mic didn't have this problem. Apart from this it sounds good when using a hi-pass filter but the rumbling eats up a lot of available headroom.

For troubleshooting I swapped back in the original capsule and replaced the NOS Mullard tube I had in there by the stock tube but the problem still persists. Fortunately I can rule out a defective capsule or tube but I'm afraid something in the circuit or even the PSU got damaged. I've tried to search the forum and the internet but didn't find something helpful.

Any ideas where to look for possible faults?

 

[RuudNL]

Looks like high humidity in the capsule.
(Or some semi conductive pollution on the high-Z wires/connections...)

 

[fripholm]

Thanks Ruud.

I think I can rule out humidity as both capsules are affected - both were working before they were swapped and the conditions they were in haven't changed ever since.

What do you mean by semi conductive pollution? All I did was cut the cables from the old capsule(s), soldered the new ones in and shrink-wrapped it (did that twice for troubleshooting). I didn't touch anything else and the mic was working fine before the swap...

Anyone else?

 

[RuudNL]

I noticed that sometimes after soldering the capsule wire to the teflon isolated terminal, some residue was left behind, causing this low noise rumble. Remember that we are working with very high impedances at this point!
After cleaning the teflon isolator with alcohol, the rumble disappeared.
(The stuff that was left behind might not have been really conductive, but it was enough to cause trouble!)

 

[fripholm]

Yes, when looking at the PCBs I noticed a lot of solder flux and dirt leftover from manufacturing. Even the original solder points themselves are in a pretty miserable state - almost as if I had soldered them. I didn't solder anything myself to these boards but maybe me touching them introduced something conductive there.

I'll try to clean the PCBs and report back. Thanks.

 

[fripholm]

Okay, I've just cleaned both sides of the boards with alcohol. They're looking like new now - but nope, didn't work  :'(

Same problem. Any other ideas guys?

 

[pucho812]

how does the capsule sound other wise? it might have condensation trapped in it and needs to dry out

 

[RuudNL]

It seems the strange noise occurs on both capsules, so IMHO it has nothing to do with the capsule itself.
The noise must be coming from the electronic circuit.

 

[pucho812]

It seems the strange noise occurs on both capsules,

I missed that part :-[

 

[okgb]

Try Deflux spray ?

 

[soapfoot]

are we positively sure the tube is not microphonic?

 

[mjrippe]

Clean tube socket and tube pins as well!

 

[bruce0]

Really interesting noise graph.

If it was a PSU issue, I would expect noise centered on 60Hz (or 50Hz if on that side of the pond), so I wouldn't start there.

If it was a shielding problem (loss of good ground contact on mesh or capsule body) i would expect radio, but I suppose this is possible depending upon what you have radiating in your area.

I suppose it could be some HF oscillation that has low order harmonics (this is a really lound noise, am I correct it is -12dbu noise?)


Did you make any other changes to the mic?  Mesh changes?  Body grounding? Cabling changes for a switchable pattern?

Knowing nothing else my place to start would be to check voltages make sure they are what I expect to eliminate a grounding problem or something wired up backwards, then put an oscilloscope on the output and see if there is an oscillation of some kind.

 

[dmp]

You could try replacing the capsule with a ~50pf capacitor and see if you still have the rumble.

 

[fripholm]

Thanks guys for all the suggestions!

Apart from the low frequency noise the output of the mic sounds pretty clean, there's no other disturbing noise. I think when using a steep HPF when (or after) recording the signal quality could almost be usable. Unfortunately the noise has a very high level. When I took the screen grab of the graph, the pre amp gain on my Fireface UFX was set as low as -40 dB but the noise already averaged around -10 dB dBFS with the occasional clipping, i.e. there's no headroom left.

Other than swapping the capsule and (for troubleshooting) the tube I did not make any changes. I'd love to check voltages but without knowing how they should be in certain places this might be difficult. I couldn't find anything in the internet about that for this particular mic.

First I'll try the suggestion with replacing the capsule with a capacitor...

 

[pasarski]

Could be also a fingerprint or something on the tube socket.

 

[fripholm]

Okay, I've tried replacing the capsule by a capacitor - no difference, still rumbling.

Next I've cleaned the tube and its socket with alcohol and put the Mullard tube and the 67-type capsule back in. The rumble is still there. 

I maybe wrong but after all that I've noticed a somewhat hotter level for the useful signal, so (despite its current state) this mic might still be useable for loud sources. That noise's frequency is so low I would filter it out anyway on most sources.

 

[bruce0]

I know this is unlikely , but you don't by any chance have the output you are measuring being played from a speaker do you?  I mean this isn't just a low frequency feedback situation?

Innocent question, not saying you did this.

 

[fripholm]

Nope - no speaker involved...  ;D

 

[bruce0]

Nope - no speaker involved...  ;D

Sorry for even suggesting it, but sometimes the obvious .... isn't

Also, I just can't get over the shape of that noise curve.  It is almost like you have "accidentally" built a high boost EQ into the circuit.  It is a beautiful low boot curve.

Even runs 8dB per octave, like a passive filter.