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[Oh! My Sea Captain!]

So, I'm trying to locate The Wind Noise on my friends Twin Reverb. I isolate the problem at V4, a 12ax7, which is the reverb recovery tube. Figuring it's a failing component or solder joint, I go through the steps to rid the amp of the noise. After finding that no component in the input circuit of this tube is seriously out of spec, failing or otherwise not working correctly, the noise is still there. So on a whim I replaced the wire connecting the grid to the load resistors etc, and the noise was gone. To make sure I wasn't missing something, I put the old wire back, and sure enough the noise was back. When I tapped on the wire, it was like tapping on a microphonic tube.

So, am I missing something? I'm young and inexperienced, so be kind if this is common, but I've never seen this before. What would cause a wire to behave like this?

[ssltech]

Typically an insulator can build up a sort of 'electret' characteristic, and then when you tap it it acts microphonically. It's wierd as hell when it happens, but it DOES happen, and people often don't believe you when you tell them, so get ready for non-believers!

The alternative is usually inductive/magnetic pickup, but ANY conductor will be prone to that, in a concentrated magnetic field, so replacing the wire won't fix it.

The 'electret' scenario occurs usually only in ultra-high impedance situations, so yours might fit the bill perfectly in this situation.

Anyone else ever seen this?

Keith

[bruno putzeys]

Triboelectric noise can attain stupendous levels in open-circuited cables (tens of mV when you actually tap it), but I understand this is a piece of hookup wire. Is it? My bet would then be a corroded solder joint or a break somewhere.

[johnR]

I encountered this a couple of months ago for the first time. I built a transformer coupled line driver for a customer, and when I tested it, it was microphonic. After resoldering everything and replacing components, it turned out to be the silicone sleeving I'd put on some of the wires. Replacing it with PVC cured it. I had a hard time believing it, but now I'm aware of the problem I'll be avoiding silicone sleeving in audio circuitry. I'm guessing that the high insulation resistance of the silicone combined with the loose fit on the wires was causing some kind of electrostatic effect.

[Ted Olausson]

ssltech wrote on Sun, 06 January 2008 20:17

The 'electret' scenario occurs usually only in ultra-high impedance situations, so yours might fit the bill perfectly in this situation. Anyone else ever seen this? Keith

I once had a guitarrist on stage that everytime he pulled the cable from the guitar got feedback in the marshall amp.
We didnt understand what it was until we moved the cable away from the speakers.
-We also found out that it was possible to speak into the cable and hear it in the speakers.

[ssltech]

Yes, in coax, the capacitance between the core and the shield is a VERY common source of microphonics. Here however, the problem is perhaps more of a single conductor with no shield...

...at least that's how I was reading it.

Into hich impedance inputs, very many 'hard' dielectric AES/EBU digital cables are microphonic.

Keith

[MagnetoSound]

Yes, I've encountered this phenomenon in old Fender amps. I believe the insulation on the wire is prone to picking up moisture, and the resultant capacitive contamination makes the wire microphonic. It's more likely in cloth-covered wire, of course, but the impedances are high enough in the reverb circuit for the slightest contamination to be potentially problematic ....

[Ted Olausson]

Sorry Keith  

[ssltech]

Hey no biggie... it's just that lots of shielded cable (be it twisted pair or coax) becomes microphonic with a high-ish impedance source into a high-impedance load, and people never notice until it's too late. Try tapping of 'flicking' guitar cords (particularly any 'stiff' guitar cords) while they're unplugged from the guitar, but plugged into a live amp... you'll be surprised how many ARE slightly microphonic.

But when they STAY microphonic when they're plugged into a guitar with the volume control open, it's time to toss them!

Keith

[Michael Brebes]

I used to work for a vibration monitoring equipment manufacturer.  All cable can have microphonic tendencies.  In the case of shielded cable, there is an inherent capacitance between conductors and/or shield.  When the cable is moved or vibrates, depending on how tight the cable is made, the various conductors can move in relationship to each other which changes the capacitance and, with a high impedance termination, can act just like a condensor microphone which works on changing capacitance.

The only way we found to reduce the microphonic tendencies in our measuring cables, which were in very harsh conditions, was to have a twisted pair cable made with two teflon jacketed wires that were bonded together with a special carbon tape that is melted together.  Then it had two stainless steel braids, then covered by another teflon jacket.  Trying to get off the carbon material was a bear, but it was definitely the quietest cable I've ever tested.  Too stiff to use for microphones.

A single wire in a guitar amp is going to have stray capacitance.  With it in a high impedance location, the vibration of the wire could definitely inject noise into the circuit.  You could also replace that wire with a shielded cable with the shield tied to a ground at one end only.

[bruno putzeys]

Teflon in its own right is horrendous when it comes to microphony, especially combined with silver. The most microphonic cable I've ever tested was of the teflon/silver variety. That said, teflon can be made to bond with silver and then it suddenly becomes the ticket. I've got some incredibly quiet coax lying about but I simply can't find who manufactured it. I think it was used in telephone exchanges and I doubt that low microphonics was a requirement there but I'd be pretty happy if I found out where to get it. Should anyone ever bump into it: it has a thin silvered copper conductor (one strand) with extruded teflon around it (very tough to strip), then some translucent soft plastic that melts easily and then copper braid that is laid on extremely tightly (once again hard to unravel). The whole thing is about 2.5mm thick.

@mbrebes: I think I know the carbon tape you're speaking of. A carbon-loaded kapton coated with teflon which fuses with the teflon insulation of the conductors. Du Pont sells this stuff in exchange for, I believe, an arm and a leg.

[Larrchild]

Druflon is only an arm, not a leg, also, if that helps.
http://www.druflon.com/coaxial.html

[bruno putzeys]

Yes but that kind of cable is microphonic, and spectacularly so. The arm and leg referred to teflon-coated kapton tape that one can use to make fused-together pairs.

[Larrchild]

Oh, sorry Bruno, I may have misread. Their special "Low-Microphonic" coax for transducers and such seemed to have something similar to that inside.

[bruno putzeys]

Larrchild wrote on Wed, 09 January 2008 21:26

Their special "Low-Microphonic" coax for transducers and such seemed to have something similar to that inside.

Aha didn't see that, thx for pointing out. The web site says that the insulation is tape that's sintered (annealed) so I suppose that's indeed the same trick. I had already been wondering whether the carbon was really necessary because carbon only helps once triboelectric charge is actually produced, that is when the conductors and insulators can rub together which they obviously can't when the whole thing is bonded together.