A 'fix' for microphonic polystyrene caps.

[k brown]

In my continued 'messings-about' with the Behringer B-5, I noticed that one of my older, through-hole versions produced a very loud, sustaining pinging noise when tapped (quite distinct from mic-body ringing), the other didn't do this at all. Nor did either of my later examples with the SMD board.

Then I remembered that I'd swapped out it's nasty little ceramic capsule cap with a styrene, and hadn't done it yet with the other. So I tucked a small blob of Coax-Seal between the styrene cap and the board, and also a tiny noodle of it on the lead connected to the capsule. Pinging gone!

https://www.amazon.com/Universal-Electronics-Coax-Seal-Moisture-Sealing/dp/B004FVMALW/ref=sr_1_8?crid=3MD5S8COCRGP9&dib=eyJ2IjoiMSJ9.8cLRCTzjbDEeiZLQTKrblwHUBz7UuVzakZFNg5f4Q5hpH_bTZqCfyPdUNhXso2KDgy8C1GzQ81YFzK2Wh-fRnTBFbCNqXJkVuCiPgzexD84727M0HPkchySYeBJGPP_e6Wv8zCqHenvHhTX-OjhKVPrSZpfUfZTV2Auc6qjxPdv8B-fb5EwZXAESlbI-oj71HWDXtnwiGmNHf4dxcKaMPoQT81UJ0bOI8z_XExXwCEA.neylVKrpqYcZPrFivGuLuCke7mGLPM0QKzPJILtNc0g&dib_tag=se&keywords=coax-seal&qid=1720551176&sprefix=coax-seal,aps,208&sr=8-8

The noise was so spring-like, that I thought there was something about the spring in the original model's capsule connection pin that was the culprit!

Probably the reason this pinging was so pronounced was that I hadn't soldered the styrene into the original cap's PCB holes, but connected it 'in air' between the capsule connector pin and the board.

The newer, SMD boards come with a nice little COG ceramic for the capsule connection.

 

[JohnRoberts]

I don't think I ever encountered a microphonic polystyrene cap but I guess anything is possible.

A NPO/COG ceramic should be fine.

JR

 

[ChrisMM]

Yes, this is exactly my experience too. At the front end of a microphone, every part becomes microphonic no matter what it is. Any stray capacitance around there becomes its own "capsule" when the parts physically vibrate for any reason, which the circuit then happily amplifies.

I was so surprised to discover this the first time I tapped on a mic circuit with a spudger. I recently replaced a 1000pF mica cap with a physically smaller polypropylene for this reason. I thought the mica cap would be superior as a coupling cap, and maybe this is true in theory. But in practice, the cap was mounted in the air off a pair of resistors, and it is relatively huge compared to other options, so it was going to vibrate more than a smaller, lighter part, which almost certainly negated any advantages it may have had.

Nowadays I keep all front-end components as physically small as possible (preferring C0G/NPO ceramics) and I use silicone-insulated wiring to the capsule to minimize stiffness which could lead to ringing. I make exceptions to this when the goal is something other than absolute accuracy of sound capture.

Another funny story: I built a 251 clone that had a problem where the rear diaphragm sounded much quieter than the front. After much troubleshooting and worrying that I might've ruined the capsule somehow, I noticed the RD wire was run very close to the backplate wire, almost parallel to it. Whoops, unintentional capacitor! I moved the RD wire to be as far away from the backplate wire as possible. Immediately the RD came up to the correct level!

Chris

 

[k brown]

I don't think I ever encountered a microphonic polystyrene cap but I guess anything is possible.

A NPO/COG ceramic should be fine.

JR

It may indeed have been the leads themselves ringing, and not the fact that they were styrenes (?).

 

[klem]

I wonder if neutral cure silicone would be a good or even superior solution as well? Insomuch as less contaminants for stray capacitance?
Hi-z area cleaning and pinging has been front and center for me lately as well.

 

[soliloqueen]

Yes, this is exactly my experience too. At the front end of a microphone, every part becomes microphonic no matter what it is. Any stray capacitance around there becomes its own "capsule" when the parts physically vibrate for any reason, which the circuit then happily amplifies.

I was so surprised to discover this the first time I tapped on a mic circuit with a spudger. I recently replaced a 1000pF mica cap with a physically smaller polypropylene for this reason. I thought the mica cap would be superior as a coupling cap, and maybe this is true in theory. But in practice, the cap was mounted in the air off a pair of resistors, and it is relatively huge compared to other options, so it was going to vibrate more than a smaller, lighter part, which almost certainly negated any advantages it may have had.

Nowadays I keep all front-end components as physically small as possible (preferring C0G/NPO ceramics) and I use silicone-insulated wiring to the capsule to minimize stiffness which could lead to ringing. I make exceptions to this when the goal is something other than absolute accuracy of sound capture.

Another funny story: I built a 251 clone that had a problem where the rear diaphragm sounded much quieter than the front. After much troubleshooting and worrying that I might've ruined the capsule somehow, I noticed the RD wire was run very close to the backplate wire, almost parallel to it. Whoops, unintentional capacitor! I moved the RD wire to be as far away from the backplate wire as possible. Immediately the RD came up to the correct level!

Chris

this is the primary reason why my capsules have shipped with silicone-insulated wires for a few years now. it annoys the shit out of me.

 

[cyrano]

I have seen silicone props at the end of the capsule, inside the housing before. I just can't remember what brand mics they were.

 

[jp8]

To my experience, with a piece of foam on both sides of the PCBA of an SDC mic, microphonics can be reduced significantly. Maybe this is also the reason the KM184 has this foam inside? See @RuudNL 's post in https://groupdiy.com/threads/neumann-km-184-repair.76336/post-969668

Or maybe they just added it to prevent MLCC cracking. It's anyone's guess, but these seem the most plausible reasons to me.

Jan

 

[k brown]

To my experience, with a piece of foam on both sides of the PCBA of an SDC mic, microphonics can be reduced significantly. Maybe this is also the reason the KM184 has this foam inside? See @RuudNL 's post in https://groupdiy.com/threads/neumann-km-184-repair.76336/post-969668

Or maybe they just added it to prevent MLCC cracking. It's anyone's guess, but these seem the most plausible reasons to me.

Jan

I wonder if those foam pieces are original; they don't look very 'vintage' to me.

 

[jp8]

Maybe @RuudNL can tell? I've never had one myself. I guess RuudNL has seen more of them for repair in his shop than the one pictured. As Ruud said, this looks quite 'amateuristic'. But I've seen pictures of other SDC PCBAs with half-circle foam pieces to prevent PCB vibrations. Just don't remember which ones they were.

Jan

 

[Khron]

But I've seen pictures of other SDC PCBAs with half-circle foam pieces to prevent PCB vibrations. Just don't remember which ones they were.

Something like these, perhaps?

https://khronscave.blogspot.com/2024/07/86-condenser-cavalcade-akg-perception.html

 

[k brown]

Something like these, perhaps?

https://khronscave.blogspot.com/2024/07/86-condenser-cavalcade-akg-perception.html

Do those 'half-rounds' press against the body when assembled, or just hug the boards?

 

[Khron]

Do those 'half-rounds' press against the body when assembled, or just hug the boards?

It's not a terribly tight fit, so they may well not dampen the body ring much, but at least the pcb isn't "rattling" freely in there.

 

[ricardo]

I don't think I ever encountered a microphonic polystyrene cap but I guess anything is possible.

A NPO/COG ceramic should be fine.

I have some correspondence with the late Guru Wurcer on caps for LN microphone designs who picked this up. I encountered this late 70s when I was at Calrec.
There was an occasion when we replaced every single small polystyrene on a brand new 70 channel Broadcast desk with NPO ceramics. I can't remember if was for some other reason though it was about the same time.
The microphony might be maker dependent but I wouldn't use a polystyrene if a NPO/COG ceramic was available. Not sure I like the use of gunk on the Hi Z end of a capacitor mike.
Another of Guru Wurcer's findings was that Mica caps have a strange low level LF/Mid noise.

 

[Tubetec]

Theres no doubt axial mounted components can have resonances related to leadouts and mounting
A long thin cap supported close to either end Ive found best ,

I note in the Sony C80 condenser mic , they use a floating internal mounting for the electronics and its not even a tube mic .
They quote a direct lineage from the C800G in this respect and it contains the same capsule as the C-100's LDC section .

At the price Im considering the C80 as a donor body and capsule for a CF based tube mic ,

 

[k brown]

I have some correspondence with the late Guru Wurcer on caps for LN microphone designs who picked this up. I encountered this late 70s when I was at Calrec.
There was an occasion when we replaced every single small polystyrene on a brand new 70 channel Broadcast desk with NPO ceramics. I can't remember if was for some other reason though it was about the same time.
The microphony might be maker dependent but I wouldn't use a polystyrene if a NPO/COG ceramic was available. Not sure I like the use of gunk on the Hi Z end of a capacitor mike.
Another of Guru Wurcer's findings was that Mica caps have a strange low level LF/Mid noise.

The 'gunk' didn't increase self noise any; I don't know what other concern there'd be.

 

[JohnRoberts]

I have some correspondence with the late Guru Wurcer on caps for LN microphone designs who picked this up. I encountered this late 70s when I was at Calrec.
There was an occasion when we replaced every single small polystyrene on a brand new 70 channel Broadcast desk with NPO ceramics. I can't remember if was for some other reason though it was about the same time.
The microphony might be maker dependent but I wouldn't use a polystyrene if a NPO/COG ceramic was available. Not sure I like the use of gunk on the Hi Z end of a capacitor mike.
Another of Guru Wurcer's findings was that Mica caps have a strange low level LF/Mid noise.

I don't recall having npo/cog caps available back in the 70s, while I did like the fact that polystyrene capacitors were really inexpensive (I'm cheap).
====
I haven't retold this story lately. Back in the 80s my business partner trashed a whole production run of studio efx (Loft delay lines), when he used a high pressure air hose to blow wash water off the PC boards. He managed to blow water inside the polystyrene caps ruining them. Polystyrene is a nice dielectric but not very mass production friendly.

JR

 

[klem]

101 question - approx when did C0G NP0 and also polystyrene come onto market?

Am guessing this has been discussed before, but I am wondering if some of the canonical microphones would have used styrene if C0G was available, maybe I have dates flipped though...

 

[JohnRoberts]

101 question - approx when did C0G NP0 and also polystyrene come onto market?

I don't know... I will speculate that there was some NP0/COG usage that long ago on military spec designs where cost was no object.

JR

Am guessing this has been discussed before, but I am wondering if some of the canonical microphones would have used styrene if C0G was available, maybe I have dates flipped though...

 

[ricardo]

I don't recall having npo/cog caps available back in the 70s, while I did like the fact that polystyrene capacitors were really inexpensive (I'm cheap).

At that time (late 70s), I knew them as NPO ceramics. Didn't know what COG was until this Millenium. They were cheaper than small polystyrenes and at first, we looked down on them until we discovered the microphony and production problems with polystyrenes.